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CHAPTER 4

The class File Format


This chapter describes the Java Virtual Machine class file format. Each class file contains one Java type, either a class or an interface. Compliant Java Virtual Machine implementations must be capable of dealing with all class files that conform to the specification provided by this book.

A class file consists of a stream of 8-bit bytes. All 16-bit, 32-bit, and 64-bit quantities are constructed by reading in two, four, and eight consecutive 8-bit bytes, respectively. Multibyte data items are always stored in big-endian order, where the high bytes come first. In Java, this format is supported by inter-faces java.io.DataInput and java.io.DataOutput and classes such as java.io.DataInputStream and java.io.DataOutputStream.

This chapter defines its own set of data types representing Java class file data: The types u1, u2, and u4 represent an unsigned one-, two-, or four-byte quantity, respectively. In Java, these types may be read by methods such as readUnsignedByte, readUnsignedShort, and readInt of the interface java.io.DataInput.

The Java class file format is presented using pseudostructures written in a C-like structure notation. To avoid confusion with the fields of Java Virtual Machine classes and class instances, the contents of the structures describing the Java class file format are referred to as items. Unlike the fields of a C structure, successive items are stored in the Java class file sequentially, without padding or alignment.

Variable-sized tables, consisting of variable-sized items, are used in several class file structures. Although we will use C-like array syntax to refer to table items, the fact that tables are streams of varying-sized structures means that it is not possible to directly translate a table index into a byte offset into the table.

Where we refer to a data structure as an array, it is literally an array.


4.1 ClassFile

A class file contains a single ClassFile structure:


    ClassFile {
    	u4 magic;

    	u2 minor_version;

    	u2 major_version;

    	u2 constant_pool_count;

    	cp_info constant_pool[constant_pool_count-1];

    	u2 access_flags;

    	u2 this_class;

    	u2 super_class;

    	u2 interfaces_count;

    	u2 interfaces[interfaces_count];

    	u2 fields_count;

    	field_info fields[fields_count];

    	u2 methods_count;

    	method_info methods[methods_count];

    	u2 attributes_count;

    	attribute_info attributes[attributes_count];

    }


The items in the ClassFile structure are as follows:

       magic

The magic item supplies the magic number identifying the class file format; it has the value 0xCAFEBABE.

       minor_version, major_version
The values of the minor_version and major_version items are the minor and major version numbers of the compiler that produced this class file. An implementation of the Java Virtual Machine normally supports class files having a given major version number and minor version numbers 0 through some particular minor_version.

If an implementation of the Java Virtual Machine supports some range of minor version numbers and a class file of the same major version but a higher minor version is encountered, the Java Virtual Machine must not attempt to run the newer code. However, unless the major version number differs, it will be feasible to implement a new Java Virtual Machine that can run code of minor versions up to and including that of the newer code.

A Java Virtual Machine must not attempt to run code with a different major version. A change of the major version number indicates a major incompatible change, one that requires a fundamentally different Java Virtual Machine.

In Sun's Java Developer's Kit (JDK) 1.0.2 release, documented by this book, the value of major_version is 45. The value of minor_version is 3. Only Sun may define the meaning of new class file version numbers.

       constant_pool_count
The value of the constant_pool_count item must be greater than zero. It gives the number of entries in the constant_pool table of the class file, where the constant_pool entry at index zero is included in the count but is not present in the constant_pool table of the class file. A constant_pool index is considered valid if it is greater than zero and less than constant_pool_count.

       constant_pool[]
The constant_pool is a table of variable-length structures (§4.4) representing various string constants, class names, field names, and other constants that are referred to within the ClassFile structure and its substructures.

The first entry of the constant_pool table, constant_pool[0], is reserved for internal use by a Java Virtual Machine implementation. That entry is not present in the class file. The first entry in the class file is constant_pool[1].

Each of the constant_pool table entries at indices 1 through constant_pool_count-1 is a variable-length structure (§4.4) whose format is indicated by its first "tag" byte.

       access_flags
The value of the access_flags item is a mask of modifiers used with class and interface declarations. The access_flags modifiers are shown in Table 4.1.

Flag Name Value Meaning Used By
ACC_PUBLIC 0x0001 Is public; may be accessed from outside its package. Class, interface
ACC_FINAL 0x0010 Is final; no subclasses allowed. Class
ACC_SUPER 0x0020 Treat superclass methods specially in invokespecial. Class, interface
ACC_INTERFACE 0x0200 Is an interface. Interface
ACC_ABSTRACT 0x0400 Is abstract; may not be instantiated. Class, interface


An interface is distinguished by its ACC_INTERFACE flag being set. If ACC_INTERFACE is not set, this class file defines a class, not an interface.

Interfaces may only use flags indicated in Table 4.1 as used by interfaces. Classes may only use flags indicated in Table 4.1 as used by classes. An interface is implicitly abstract (§2.13.1); its ACC_ABSTRACT flag must be set. An interface cannot be final; its implementation could never be completed (§2.13.1) if it were, so it could not have its ACC_FINAL flag set.

The flags ACC_FINAL and ACC_ABSTRACT cannot both be set for a class; the implementation of such a class could never be completed (§2.8.2).

The setting of the ACC_SUPER flag directs the Java Virtual Machine which of two alternative semantics for its invokespecial instruction to express; it exists for backward compatibility for code compiled by Sun's older Java compilers. All new implementations of the Java Virtual Machine should implement the semantics for invokespecial documented in Chapter 6, "Java Virtual Machine Instruction Set." All new compilers to the Java Virtual Machine's instruction set should set the ACC_SUPER flag. Sun's older Java compilers generate ClassFile flags with ACC_SUPER unset. Sun's older Java Virtual Machine implementations ignore the flag if it is set.

All unused bits of the access_flags item, including those not assigned in Table 4.1, are reserved for future use. They should be set to zero in generated class files and should be ignored by Java Virtual Machine implementations.

       this_class
The value of the this_class item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Class_info (§4.4.1) structure representing the class or interface defined by this class file.

       super_class
For a class, the value of the super_class item either must be zero or must be a valid index into the constant_pool table. If the value of the super_class item is nonzero, the constant_pool entry at that index must be a CONSTANT_Class_info (§4.4.1) structure representing the superclass of the class defined by this class file. Neither the superclass nor any of its superclasses may be a final class.

If the value of super_class is zero, then this class file must represent the class java.lang.Object, the only class or interface without a superclass.

For an interface, the value of super_class must always be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Class_info structure representing the class java.lang.Object.

       interfaces_count
The value of the interfaces_count item gives the number of direct superinterfaces of this class or interface type.

       interfaces[]
Each value in the interfaces array must be a valid index into the constant_pool table. The constant_pool entry at each value of interfaces[i], where 0 £ i < interfaces_count, must be a CONSTANT_Class_info (§4.4.1) structure representing an interface which is a direct superinterface of this class or interface type, in the left-to-right order given in the source for the type.

       fields_count
The value of the fields_count item gives the number of field_info structures in the fields table. The field_info (§4.5) structures represent all fields, both class variables and instance variables, declared by this class or interface type.

       fields[]
Each value in the fields table must be a variable-length field_info (§4.5) structure giving a complete description of a field in the class or interface type. The fields table includes only those fields that are declared by this class or interface. It does not include items representing fields that are inherited from superclasses or superinterfaces.

       methods_count
The value of the methods_count item gives the number of method_info structures in the methods table.

       methods[]
Each value in the methods table must be a variable-length method_info (§4.6) structure giving a complete description of and Java Virtual Machine code for a method in the class or interface.

The method_info structures represent all methods, both instance methods and, for classes, class (static) methods, declared by this class or interface type. The methods table only includes those methods that are explicitly declared by this class. Interfaces have only the single method <clinit>, the interface initialization method (§3.8). The methods table does not include items representing methods that are inherited from superclasses or superinterfaces.

       attributes_count
The value of the attributes_count item gives the number of attributes (§4.7) in the attributes table of this class.

       attributes[]
Each value of the attributes table must be a variable-length attribute structure. A ClassFile structure can have any number of attributes (§4.7) associated with it.

The only attribute defined by this specification for the attributes table of a ClassFile structure is the SourceFile attribute (§4.7.2).

A Java Virtual Machine implementation is required to silently ignore any or all attributes in the attributes table of a ClassFile structure that it does not recognize. Attributes not defined in this specification are not allowed to affect the semantics of the class file, but only to provide additional descriptive information (§4.7.1).


4.2 Internal Form of Fully Qualified Class Names

Class names that appear in class file structures are always represented in a fully qualified form (§2.7.9). These class names are always represented as CONSTANT_Utf8_info (§4.4.7) structures, and they are referenced from those CONSTANT_NameAndType_info (§4.4.6) structures that have class names as part of their descriptor (§4.3), as well as from all CONSTANT_Class_info (§4.4.1) structures.

For historical reasons the exact syntax of fully qualified class names that appear in class file structures differs from the familiar Java fully qualified class name documented in §2.7.9. In the internal form, the ASCII periods ('.') that normally separate the identifiers (§2.2) that make up the fully qualified name are replaced by ASCII forward slashes ('/'). For example, the normal fully qualified name of class Thread is java.lang.Thread. In the form used in descriptors in class files, a reference to the name of class Thread is implemented using a CONSTANT_Utf8_info structure representing the string "java/lang/Thread".


4.3 Descriptors

A descriptor is a string representing the type of a field or method.

4.3.1 Grammar Notation

Descriptors are specified using a grammar. This grammar is a set of productions that describe how sequences of characters can form syntactically correct descriptors of various types. Terminal symbols of the grammar are shown in bold fixed-width font. Nonterminal symbols are shown in italic type. The definition of a nonterminal is introduced by the name of the nonterminal being defined, followed by a colon. One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. A nonterminal symbol on the right-hand side of a production that is followed by an asterisk (*) represents zero or more possibly different values produced from that nonterminal, appended without any intervening space.

4.3.2 Field Descriptors

A field descriptor represents the type of a class or instance variable. It is a series of characters generated by the grammar:

       FieldDescriptor:

FieldType

       ComponentType:

FieldType

       FieldType:

BaseType

ObjectType

ArrayType

       BaseType:

B

C

D

F

I

J

S

Z

       ObjectType:
L <classname> ;

       ArrayType:
[ ComponentType

The characters of BaseType, the L and ; of ObjectType, and the [ of ArrayType are all ASCII characters. The <classname> represents a fully qualified class name, for instance, java.lang.Thread. For historical reasons it is stored in a class file in a modified internal form (§4.2).

The meaning of the field types is as follows:



 B	      byte			signed byte 
C char character
D double double-precision IEEE 754 float
F float single-precision IEEE 754 float
I int integer
J long long integer
L<classname>; ... an instance of the class
S short signed short
Z boolean true or false
[ ... one array dimension


For example, the descriptor of an int instance variable is simply I. The descriptor of an instance variable of type Object is Ljava/lang/Object;. Note that the internal form of the fully qualified class name for class Object is used. The descriptor of an instance variable that is a multidimensional double array,

    double d[][][];

is

    [[[D

4.3.3 Method Descriptors

A parameter descriptor represents a parameter passed to a method:

       ParameterDescriptor:

FieldType

A method descriptor represents the parameters that the method takes and the value that it returns:

       MethodDescriptor:

( ParameterDescriptor * ) ReturnDescriptor

A return descriptor represents the return value from a method. It is a series of characters generated by the grammar:

       ReturnDescriptor:

FieldType

V

The character V indicates that the method returns no value (its return type is void). Otherwise, the descriptor indicates the type of the return value.

A valid Java method descriptor must represent 255 or fewer words of method parameters, where that limit includes the word for this in the case of instance method invocations. The limit is on the number of words of method parameters and not on the number of parameters themselves; parameters of type long and double each use two words.

For example, the method descriptor for the method

    Object mymethod(int i, double d, Thread t)

is

    (IDLjava/lang/Thread;)Ljava/lang/Object;

Note that internal forms of the fully qualified class names of Thread and Object are used in the method descriptor.

The method descriptor for mymethod is the same whether mymethod is static or is an instance method. Although an instance method is passed this, a reference to the current class instance, in addition to its intended parameters, that fact is not reflected in the method descriptor. (A reference to this is not passed to a static method.) The reference to this is passed implicitly by the method invocation instructions of the Java Virtual Machine used to invoke instance methods.


4.4 Constant Pool

All constant_pool table entries have the following general format:


    cp_info {
    	u1 tag;

    	u1 info[];

    }


Each item in the constant_pool table must begin with a 1-byte tag indicating the kind of cp_info entry. The contents of the info array varies with the value of tag. The valid tags and their values are listed in Table 4.2

Constant Type Value
CONSTANT_Class 7
CONSTANT_Fieldref 9
CONSTANT_Methodref 10
CONSTANT_InterfaceMethodref 11
CONSTANT_String 8
CONSTANT_Integer 3
CONSTANT_Float 4
CONSTANT_Long 5
CONSTANT_Double 6
CONSTANT_NameAndType 12
CONSTANT_Utf8 1


. Each tag byte must be followed by two or more bytes giving information about the specific constant. The format of the additional information varies with the tag value.

4.4.1 CONSTANT_Class

The CONSTANT_Class_info structure is used to represent a class or an interface:


    CONSTANT_Class_info {
    	u1 tag;

    	u2 name_index;

    }


The items of the CONSTANT_Class_info structure are the following:

       tag

The tag item has the value CONSTANT_Class (7).

       name_index
The value of the name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info (§4.4.7) structure representing a valid fully qualified Java class name (§2.8.1) that has been converted to the class file's internal form (§4.2).

Because arrays are objects, the opcodes anewarray and multianewarray can reference array "classes" via CONSTANT_Class_info (§4.4.1) structures in the constant_pool table. In this case, the name of the class is the descriptor of the array type. For example, the class name representing a two-dimensional int array type;

    int[][] 

is

    [[I

The class name representing the type array of class Thread;

    Thread[] 

is

    [Ljava.lang.Thread;

A valid Java array type descriptor must have 255 or fewer array dimensions.

4.4.2 CONSTANT_Fieldref, CONSTANT_Methodref, and CONSTANT_InterfaceMethodref

Fields, methods, and interface methods are represented by similar structures:


    CONSTANT_Fieldref_info {
    	u1 tag;

    	u2 class_index;

    	u2 name_and_type_index;

    }



    CONSTANT_Methodref_info {      u1 tag;      u2 class_index;      u2 name_and_type_index;     }

    CONSTANT_InterfaceMethodref_info {      u1 tag;      u2 class_index;      u2 name_and_type_index;     }
The items of these structures are as follows:

       tag

The tag item of a CONSTANT_Fieldref_info structure has the value CONSTANT_Fieldref (9).

The tag item of a CONSTANT_Methodref_info structure has the value CONSTANT_Methodref (10).

The tag item of a CONSTANT_InterfaceMethodref_info structure has the value CONSTANT_InterfaceMethodref (11).

       class_index
The value of the class_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Class_info (§4.4.1) structure representing the class or interface type that contains the declaration of the field or method.

The class_index item of a CONSTANT_Fieldref_info or a CONSTANT_Methodref_info structure must be a class type, not an interface type. The class_index item of a CONSTANT_InterfaceMethodref_info structure must be an interface type that declares the given method.

       name_and_type_index
The value of the name_and_type_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_NameAndType_info (§4.4.6) structure. This constant_pool entry indicates the name and descriptor of the field or method.

If the name of the method of a CONSTANT_Methodref_info or CONSTANT_InterfaceMethodref_info begins with a '<' ('u003c'), then the name must be one of the special internal methods (§3.8), either <init> or <clinit>. In this case, the method must return no value.

4.4.3 CONSTANT_String

The CONSTANT_String_info structure is used to represent constant objects of the type java.lang.String:


    CONSTANT_String_info {
    	u1 tag;

    	u2 string_index;

    }


The items of the CONSTANT_String_info structure are as follows:

       tag

The tag item of the CONSTANT_String_info structure has the value CONSTANT_String (8).

       string_index
The value of the string_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info (§4.4.3) structure representing the sequence of characters to which the java.lang.String object is to be initialized.

4.4.4 CONSTANT_Integer and CONSTANT_Float

The CONSTANT_Integer_info and CONSTANT_Float_info structures represent four-byte numeric (int and float) constants:


    CONSTANT_Integer_info {
    	u1 tag;

    	u4 bytes;

    }



    CONSTANT_Float_info {      u1 tag;      u4 bytes;     }
The items of these structures are as follows:

       tag

The tag item of the CONSTANT_Integer_info structure has the value CONSTANT_Integer (3).

The tag item of the CONSTANT_Float_info structure has the value CONSTANT_Float (4).

       bytes
The bytes item of the CONSTANT_Integer_info structure contains the value of the int constant. The bytes of the value are stored in big-endian (high byte first) order.

The bytes item of the CONSTANT_Float_info structure contains the value of the float constant in IEEE 754 floating-point "single format" bit layout. The bytes of the value are stored in big-endian (high byte first) order, and are first converted into an int argument. Then:

    int s = ((bytes >> 31) == 0) ? 1 : -1;

    int e = ((bytes >> 23) & 0xff);

    int m = (e == 0) ?

    		(bytes & 0x7fffff) << 1 :

    		(bytes & 0x7fffff) | 0x800000;

Then the float value equals the result of the mathematical expression



.

4.4.5 CONSTANT_Long and CONSTANT_Double

The CONSTANT_Long_info and CONSTANT_Double_info represent eight-byte numeric (long and double) constants:


    CONSTANT_Long_info {
    	u1 tag;

    	u4 high_bytes;

    	u4 low_bytes;

    }



    CONSTANT_Double_info {      u1 tag;      u4 high_bytes;      u4 low_bytes;     }
All eight-byte constants take up two entries in the constant_pool table of the class file, as well as in the in-memory version of the constant pool that is constructed when a class file is read. If a CONSTANT_Long_info or CONSTANT_Double_info structure is the item in the constant_pool table at index n, then the next valid item in the pool is located at index n+2. The constant_pool index n+1 must be considered invalid and must not be used.1

The items of these structures are as follows:

       tag

The tag item of the CONSTANT_Long_info structure has the value CONSTANT_Long (5).

The tag item of the CONSTANT_Double_info structure has the value CONSTANT_Double (6).

       high_bytes, low_bytes
The unsigned high_bytes and low_bytes items of the CONSTANT_Long structure together contain the value of the long constant ((long)high_bytes << 32) + low_bytes, where the bytes of each of high_bytes and low_bytes are stored in big-endian (high byte first) order.

The high_bytes and low_bytes items of the CONSTANT_Double_info structure contain the double value in IEEE 754 floating-point "double format" bit layout. The bytes of each item are stored in big-endian (high byte first) order. The high_bytes and low_bytes items are first converted into a long argument. Then:

    int s = ((bits >> 63) == 0) ? 1 : -1;

    int e = (int)((bits >> 52) & 0x7ffL);

    long m = (e == 0) ?

    	(bits & 0xfffffffffffffL) << 1 :

    	(bits & 0xfffffffffffffL) | 0x10000000000000L;

Then the floating-point value equals the double value of the mathematical expression



.

4.4.6 CONSTANT_NameAndType

The CONSTANT_NameAndType_info structure is used to represent a field or method, without indicating which class or interface type it belongs to:


    CONSTANT_NameAndType_info {
    	u1 tag;

    	u2 name_index;

    	u2 descriptor_index;

    }


The items of the CONSTANT_NameAndType_info structure are as follows:

       tag

The tag item of the CONSTANT_NameAndType_info structure has the value CONSTANT_NameAndType (12).

       name_index
The value of the name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info (§4.4.7) structure representing a valid Java field name or method name (§2.7) stored as a simple (not fully qualified) name (§2.7.1), that is, as a Java identifier.

       descriptor_index
The value of the descriptor_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info (§4.4.7) structure representing a valid Java field descriptor (§4.3.2) or method descriptor (§4.3.3).

4.4.7 CONSTANT_Utf8

The CONSTANT_Utf8_info structure is used to represent constant string values.

UTF-8 strings are encoded so that character sequences that contain only non-null ASCII characters can be represented using only one byte per character, but characters of up to 16 bits can be represented. All characters in the range 'u0001' to 'u007F' are represented by a single byte:

0 bits 0-7


The seven bits of data in the byte give the value of the character represented. The null character ('u0000') and characters in the range 'u0080' to 'u07FF' are represented by a pair of bytes x and y:

x: 1 1 0 bits 6-10 y: 1 0 bits 0-5


The bytes represent the character with the value ((x & 0x1f) << 6) + (y & 0x3f).

Characters in the range 'u0800' to 'uFFFF' are represented by three bytes x, y, and z:

x: 1 1 1 0 bits 12-15 y: 1 0 bits 6-11 z: 1 0 bits 0-5


The character with the value ((x & 0xf) << 12) + ((y & 0x3f) << 6) + (z & 0x3f) is represented by the bytes.

The bytes of multibyte characters are stored in the class file in big-endian (high byte first) order.

There are two differences between this format and the "standard" UTF-8 format. First, the null byte (byte)0 is encoded using the two-byte format rather than the one-byte format, so that Java Virtual Machine UTF-8 strings never have embedded nulls. Second, only the one-byte, two-byte, and three-byte formats are used. The Java Virtual Machine does not recognize the longer UTF-8 formats.

For more information regarding the UTF-8 format, see File System Safe UCS Transformation Format (FSS_UTF), X/Open Preliminary Specification, X/Open Company Ltd., Document Number: P316. This information also appears in ISO/IEC 10646, Annex P.

The CONSTANT_Utf8_info structure is


    CONSTANT_Utf8_info {
    	u1 tag;

    	u2 length;

    	u1 bytes[length];

    }


The items of the CONSTANT_Utf8_info structure are the following:

       tag

The tag item of the CONSTANT_Utf8_info structure has the value CONSTANT_Utf8 (1).

       length
The value of the length item gives the number of bytes in the bytes array (not the length of the resulting string). The strings in the CONSTANT_Utf8_info structure are not null-terminated.

       bytes[]
The bytes array contains the bytes of the string. No byte may have the value (byte)0 or (byte)0xf0-(byte)0xff.


4.5 Fields

Each field is described by a variable-length field_info structure. The format of this structure is


    field_info {
    	u2 access_flags;

    	u2 name_index;

    	u2 descriptor_index;

    	u2 attributes_count;

    	attribute_info attributes[attributes_count];

    }


The items of the field_info structure are as follows:

       access_flags

The value of the access_flags item is a mask of modifiers used to describe access permission to and properties of a field. The access_flags modifiers are shown in Table 4.3.

Flag Name Value Meaning Used By
ACC_PUBLIC 0x0001 Is public; may be accessed from outside its package. Any field
ACC_PRIVATE 0x0002 Is private; usable only within the defining class. Class field
ACC_PROTECTED 0x0004 Is protected; may be accessed within subclasses. Class field
ACC_STATIC 0x0008 Is static. Any field
ACC_FINAL 0x0010 Is final; no further overriding or assignment after initialization. Any field
ACC_VOLATILE 0x0040 Is volatile; cannot be cached. Class field
ACC_TRANSIENT 0x0080 Is transient; not written or read by a persistent object manager. Class field


Fields of interfaces may only use flags indicated in Table 4.3 as used by any field. Fields of classes may use any of the flags in Table 4.3.

All unused bits of the access_flags item, including those not assigned in Table 4.3, are reserved for future use. They should be set to zero in generated class files and should be ignored by Java Virtual Machine implementations.

Class fields may have at most one of flags ACC_PUBLIC, ACC_PROTECTED, and ACC_PRIVATE set (§2.7.8). A class field may not have both ACC_FINAL and ACC_VOLATILE set (§2.9.1).

Each interface field is implicitly static and final (§2.13.4) and must have both its ACC_STATIC and ACC_FINAL flags set. Each interface field is implicitly public (§2.13.4) and must have its ACC_PUBLIC flag set.

       name_index
The value of the name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info (§4.4.7) structure which must represent a valid Java field name (§2.7) stored as a simple (not fully qualified) name (§2.7.1), that is, as a Java identifier.

       descriptor_index
The value of the descriptor_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8 (§4.4.7) structure which must represent a valid Java field descriptor (§4.3.2).

       attributes_count
The value of the attributes_count item indicates the number of additional attributes (§4.7) of this field.

       attributes[]
Each value of the attributes table must be a variable-length attribute structure. A field can have any number of attributes (§4.7) associated with it.

The only attribute defined for the attributes table of a field_info structure by this specification is the ConstantValue attribute (§4.7.3).

A Java Virtual Machine implementation must recognize ConstantValue attributes in the attributes table of a field_info structure. A Java Virtual Machine implementation is required to silently ignore any or all other attributes in the attributes table that it does not recognize. Attributes not defined in this specification are not allowed to affect the semantics of the class file, but only to provide additional descriptive information (§4.7.1).


4.6 Methods

Each method, and each instance initialization method <init>, is described by a variable-length method_info structure. The structure has the following format:


    method_info {
    	u2 access_flags;

    	u2 name_index;

    	u2 descriptor_index;

    	u2 attributes_count;

    	attribute_info attributes[attributes_count];

    }


The items of the method_info structure are as follows:

       access_flags

The value of the access_flags item is a mask of modifiers used to describe access permission to and properties of a method or instance initialization method (§3.8). The access_flags modifiers are shown in Table 4.4.

Flag Name Value Meaning Used By
ACC_PUBLIC 0x0001 Is public; may be accessed from outside its package. Any method
ACC_PRIVATE 0x0002 Is private; usable only within the defining class. Class/instance method
ACC_PROTECTED 0x0004 Is protected; may be accessed within subclasses. Class/instance method
ACC_STATIC 0x0008 Is static. Class/instance method
ACC_FINAL 0x0010 Is final; no overriding is allowed. Class/instance method
ACC_SYNCHRONIZED 0x0020 Is synchronized; wrap use in monitor lock. Class/instance method
ACC_NATIVE 0x0100 Is native; implemented in a language other than Java. Class/instance method
ACC_ABSTRACT 0x0400 Is abstract; no implementation is provided. Any method


Methods in interfaces may only use flags indicated in Table 4.4 as used by any method. Class and instance methods (§2.10.3) may use any of the flags in Table 4.4. Instance initialization methods (§3.8) may only use ACC_PUBLIC, ACC_PROTECTED, and ACC_PRIVATE.

All unused bits of the access_flags item, including those not assigned in Table 4.4, are reserved for future use. They should be set to zero in generated class files and should be ignored by Java Virtual Machine implementations.

At most one of the flags ACC_PUBLIC, ACC_PROTECTED, and ACC_PRIVATE may be set for any method. Class and instance methods may not use ACC_ABSTRACT together with ACC_FINAL, ACC_NATIVE, or ACC_SYNCHRONIZED (that is, native and synchronized methods require an implementation). A class or instance method may not use ACC_PRIVATE with ACC_ABSTRACT (that is, a private method cannot be overridden, so such a method could never be implemented or used). A class or instance method may not use ACC_STATIC with ACC_ABSTRACT (that is, a static method is implicitly final and thus cannot be overridden, so such a method could never be implemented or used).

Class and interface initialization methods (§3.8), that is, methods named <clinit>, are called implicitly by the Java Virtual Machine; the value of their access_flags item is ignored.

Each interface method is implicitly abstract, and so must have its ACC_ABSTRACT flag set. Each interface method is implicitly public (§2.13.5), and so must have its ACC_PUBLIC flag set.

       name_index
The value of the name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info (§4.4.7) structure representing either one of the special internal method names (§3.8), either <init> or <clinit>, or a valid Java method name (§2.7), stored as a simple (not fully qualified) name (§2.7.1).

       descriptor_index
The value of the descriptor_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info (§4.4.7) structure representing a valid Java method descriptor (§4.3.3).

       attributes_count
The value of the attributes_count item indicates the number of additional attributes (§4.7) of this method.

       attributes[]
Each value of the attributes table must be a variable-length attribute structure. A method can have any number of optional attributes (§4.7) associated with it.

The only attributes defined by this specification for the attributes table of a method_info structure are the Code (§4.7.4) and Exceptions (§4.7.5) attributes.

A Java Virtual Machine implementation must recognize Code (§4.7.4) and Exceptions (§4.7.5) attributes. A Java Virtual Machine implementation is required to silently ignore any or all other attributes in the attributes table of a method_info structure that it does not recognize. Attributes not defined in this specification are not allowed to affect the semantics of the class file, but only to provide additional descriptive information (§4.7.1).


4.7 Attributes

Attributes are used in the ClassFile (§4.1), field_info (§4.5), method_info (§4.6), and Code_attribute (§4.7.4) structures of the class file format. All attributes have the following general format:


    attribute_info {
    	u2 attribute_name_index;

    	u4 attribute_length;

    	u1 info[attribute_length];

    }


For all attributes, the attribute_name_index must be a valid unsigned 16-bit index into the constant pool of the class. The constant_pool entry at attribute_name_index must be a CONSTANT_Utf8 (§4.4.7) string representing the name of the attribute. The value of the attribute_length item indicates the length of the subsequent information in bytes. The length does not include the initial six bytes that contain the attribute_name_index and attribute_length items.

Certain attributes are predefined as part of the class file specification. The predefined attributes are the SourceFile (§4.7.2), ConstantValue (§4.7.3), Code (§4.7.4), Exceptions (§4.7.5), LineNumberTable (§4.7.6), and Local-VariableTable (§4.7.7) attributes. Within the context of their use in this specification, that is, in the attributes tables of the class file structures in which they appear, the names of these predefined attributes are reserved.

Of the predefined attributes, the Code, ConstantValue, and Exceptions attributes must be recognized and correctly read by a class file reader for correct interpretation of the class file by a Java Virtual Machine. Use of the remaining predefined attributes is optional; a class file reader may use the information they contain, and otherwise must silently ignore those attributes.

4.7.1 Defining and Naming New Attributes

Compilers for Java source code are permitted to define and emit class files containing new attributes in the attributes tables of class file structures. Java Virtual Machine implementations are permitted to recognize and use new attributes found in the attributes tables of class file structures. However, all attributes not defined as part of this Java Virtual Machine specification must not affect the semantics of class or interface types. Java Virtual Machine implementations are required to silently ignore attributes they do not recognize.

For instance, defining a new attribute to support vendor-specific debugging is permitted. Because Java Virtual Machine implementations are required to ignore attributes they do not recognize, class files intended for that particular Java Virtual Machine implementation will be usable by other implementations even if those implementations cannot make use of the additional debugging information that the class files contain.

Java Virtual Machine implementations are specifically prohibited from throwing an exception or otherwise refusing to use class files simply because of the presence of some new attribute. Of course, tools operating on class files may not run correctly if given class files that do not contain all the attributes they require.

Two attributes that are intended to be distinct, but that happen to use the same attribute name and are of the same length, will conflict on implementations that recognize either attribute. Attributes defined other than by Sun must have names chosen according to the package naming convention defined by The Java Language Specification. For instance, a new attribute defined by Netscape might have the name "COM.Netscape.new-attribute".

Sun may define additional attributes in future versions of this class file specification.

4.7.2 SourceFile Attribute

The SourceFile attribute is an optional fixed-length attribute in the attributes table of the ClassFile (§4.1) structure. There can be no more than one SourceFile attribute in the attributes table of a given ClassFile structure.

The SourceFile attribute has the format


    SourceFile_attribute {
    	u2 attribute_name_index;

    	u4 attribute_length;

    	u2 sourcefile_index;

    }


The items of the SourceFile_attribute structure are as follows:

       attribute_name_index

The value of the attribute_name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info (§4.4.7) structure representing the string "SourceFile".

       attribute_length
The value of the attribute_length item of a SourceFile_attribute structure must be 2.

       sourcefile_index
The value of the sourcefile_index item must be a valid index into the constant_pool table. The constant pool entry at that index must be a CONSTANT_Utf8_info (§4.4.7) structure representing the string giving the name of the source file from which this class file was compiled.

Only the name of the source file is given by the SourceFile attribute. It never represents the name of a directory containing the file or an absolute path name for the file. For instance, the SourceFile attribute might contain the file name foo.java but not the UNIX pathname /home/lindholm/foo.java.

4.7.3 ConstantValue Attribute

The ConstantValue attribute is a fixed-length attribute used in the attributes table of the field_info (§4.5) structures. A ConstantValue attribute represents the value of a constant field that must be (explicitly or implicitly) static; that is, the ACC_STATIC bit (§Table 4.3) in the flags item of the field_info structure must be set. The field is not required to be final. There can be no more than one ConstantValue attribute in the attributes table of a given field_info structure. The constant field represented by the field_info structure is assigned the value referenced by its ConstantValue attribute as part of its initialization (§2.16.4).

Every Java Virtual Machine implementation must recognize ConstantValue attributes.

The ConstantValue attribute has the format


    ConstantValue_attribute {
    	u2 attribute_name_index;

    	u4 attribute_length;

    	u2 constantvalue_index;

    }


The items of the ConstantValue_attribute structure are as follows:

       attribute_name_index

The value of the attribute_name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info (§4.4.7) structure representing the string "ConstantValue".

       attribute_length
The value of the attribute_length item of a ConstantValue_attribute structure must be 2.

       constantvalue_index
The value of the constantvalue_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must give the constant value represented by this attribute.

The constant_pool entry must be of a type appropriate to the field, as shown by Table 4.5.

Field Type Entry Type
long CONSTANT_Long
float CONSTANT_Float
double CONSTANT_Double
int, short, char, byte, boolean CONSTANT_Integer
java.lang.String CONSTANT_String


4.7.4 Code Attribute

The Code attribute is a variable-length attribute used in the attributes table of method_info structures. A Code attribute contains the Java Virtual Machine instructions and auxiliary information for a single Java method, instance initialization method (§3.8), or class or interface initialization method (§3.8). Every Java Virtual Machine implementation must recognize Code attributes. There must be exactly one Code attribute in each method_info structure.

The Code attribute has the format


    Code_attribute {
    	u2 attribute_name_index;

    	u4 attribute_length;

    	u2 max_stack;

    	u2 max_locals;

    	u4 code_length;

    	u1 code[code_length];

    	u2 exception_table_length;

    	{    	u2 start_pc;

    	      	u2 end_pc;

    	      	u2  handler_pc;

    	      	u2  catch_type;

    	}	exception_table[exception_table_length];

    	u2 attributes_count;

    	attribute_info attributes[attributes_count];

    }


The items of the Code_attribute structure are as follows:

       attribute_name_index

The value of the attribute_name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info (§4.4.7) structure representing the string "Code".

       attribute_length
The value of the attribute_length item indicates the length of the attribute, excluding the initial six bytes.

       max_stack
The value of the max_stack item gives the maximum number of words on the operand stack at any point during execution of this method.

       max_locals
The value of the max_locals item gives the number of local variables used by this method, including the parameters passed to the method on invocation. The index of the first local variable is 0. The greatest local variable index for a one-word value is max_locals-1. The greatest local variable index for a two-word value is max_locals-2.

       code_length
The value of the code_length item gives the number of bytes in the code array for this method. The value of code_length must be greater than zero; the code array must not be empty.

       code[]
The code array gives the actual bytes of Java Virtual Machine code that implement the method.

When the code array is read into memory on a byte addressable machine, if the first byte of the array is aligned on a 4-byte boundary, the tableswitch and lookupswitch 32-bit offsets will be 4-byte aligned; refer to the descriptions of those instructions for more information on the consequences of code array alignment.

The detailed constraints on the contents of the code array are extensive and are given in a separate section (§4.8).

       exception_table_length
The value of the exception_table_length item gives the number of entries in the exception_table table.

       exception_table[]
Each entry in the exception_table array describes one exception handler in the code array. Each exception_table entry contains the following items:

       start_pc, end_pc
The values of the two items start_pc and end_pc indicate the ranges in the code array at which the exception handler is active. The value of start_pc must be a valid index into the code array of the opcode of an instruction. The value of end_pc either must be a valid index into the code array of the opcode of an instruction, or must be equal to code_length, the length of the code array. The value of start_pc must be less than the value of end_pc.

The start_pc is inclusive and end_pc is exclusive; that is, the exception handler must be active while the program counter is within the interval [start_pc, end_pc).2

       handler_pc
The value of the handler_pc item indicates the start of the exception handler. The value of the item must be a valid index into the code array, must be the index of the opcode of an instruction, and must be less than the value of the code_length item.

       catch_type
If the value of the catch_type item is nonzero, it must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Class_info (§4.4.1) structure representing a class of exceptions that this exception handler is designated to catch. This class must be the class Throwable or one of its subclasses. The exception handler will be called only if the thrown exception is an instance of the given class or one of its subclasses.

If the value of the catch_type item is zero, this exception handler is called for all exceptions. This is used to implement finally (see Section 7.13, "Compiling finally").

       attributes_count
The value of the attributes_count item indicates the number of attributes of the Code attribute.

       attributes[]
Each value of the attributes table must be a variable-length attribute structure. A Code attribute can have any number of optional attributes associated with it.

Currently, the LineNumberTable (§4.7.6) and LocalVariableTable (§4.7.7) attributes, both of which contain debugging information, are defined and used with the Code attribute.

A Java Virtual Machine implementation is permitted to silently ignore any or all attributes in the attributes table of a Code attribute. Attributes not defined in this specification are not allowed to affect the semantics of the class file, but only to provide additional descriptive information (§4.7.1).

4.7.5 Exceptions Attribute

The Exceptions attribute is a variable-length attribute used in the attributes table of a method_info (§4.6) structure. The Exceptions attribute indicates which checked exceptions a method may throw. There must be exactly one Exceptions attribute in each method_info structure.

The Exceptions attribute has the format


    Exceptions_attribute {
    	u2 attribute_name_index;

    	u4 attribute_length;

    	u2 number_of_exceptions;

    	u2 exception_index_table[number_of_exceptions];

    }


The items of the Exceptions_attribute structure are as follows:

       attribute_name_index

The value of the attribute_name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be the CONSTANT_Utf8_info (§4.4.7) structure representing the string "Exceptions".

       attribute_length
The value of the attribute_length item indicates the attribute length, excluding the initial six bytes.

       number_of_exceptions
The value of the number_of_exceptions item indicates the number of entries in the exception_index_table.

       exception_index_table[]
Each nonzero value in the exception_index_table array must be a valid index into the constant_pool table. For each table item, if exception_index_table[i] != 0, where 0 £ i < number_of_exceptions, then the constant_pool entry at index exception_index_table[i] must be a CONSTANT_Class_info (§4.4.1) structure representing a class type that this method is declared to throw.

A method should only throw an exception if at least one of the following three criteria is met:

The above requirements are not currently enforced by the Java Virtual Machine; they are only enforced at compile time. Future versions of the Java language may require more rigorous checking of throws clauses when classes are verified.

4.7.6 LineNumberTable Attribute

The LineNumberTable attribute is an optional variable-length attribute in the attributes table of a Code (§4.7.4) attribute. It may be used by debuggers to determine which part of the Java Virtual Machine code array corresponds to a given line number in the original Java source file. If LineNumberTable attributes are present in the attributes table of a given Code attribute, then they may appear in any order. Furthermore, multiple LineNumberTable attributes may together represent a given line of a Java source file; that is, LineNumberTable attributes need not be one-to-one with source lines.3

The LineNumberTable attribute has the format


    LineNumberTable_attribute {
    	u2 attribute_name_index;

    	u4 attribute_length;

    	u2 line_number_table_length;

    	{  u2 start_pc;	     

    	   u2 line_number;	     

    	} line_number_table[line_number_table_length];

    }


The items of the LineNumberTable_attribute structure are as follows:

       attribute_name_index

The value of the attribute_name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info (§4.4.7) structure representing the string "LineNumberTable".

       attribute_length
The value of the attribute_length item indicates the length of the attribute, excluding the initial six bytes.

       line_number_table_length
The value of the line_number_table_length item indicates the number of entries in the line_number_table array.

       line_number_table[]
Each entry in the line_number_table array indicates that the line number in the original Java source file changes at a given point in the code array. Each entry must contain the following items:

       start_pc
The value of the start_pc item must indicate the index into the code array at which the code for a new line in the original Java source file begins. The value of start_pc must be less than the value of the code_length item of the Code attribute of which this LineNumberTable is an attribute.

       line_number
The value of the line_number item must give the corresponding line number in the original Java source file.

4.7.7 LocalVariableTable Attribute

The LocalVariableTable attribute is an optional variable-length attribute of a Code (§4.7.4) attribute. It may be used by debuggers to determine the value of a given local variable during the execution of a method. If LocalVariableTable attributes are present in the attributes table of a given Code attribute, then they may appear in any order. There may be no more than one LocalVariableTable attribute per local variable in the Code attribute.

The LocalVariableTable attribute has the format


    LocalVariableTable_attribute {
    	u2 attribute_name_index;

    	u4 attribute_length;

    	u2 local_variable_table_length;

    	{  u2 start_pc;

    	    u2 length;

    	    u2 name_index;

    	    u2 descriptor_index;

    	    u2 index;

    	} local_variable_table[

    			local_variable_table_length];

    }	


The items of the LocalVariableTable_attribute structure are as follows:

       attribute_name_index

The value of the attribute_name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info (§4.4.7) structure representing the string "LocalVariableTable".

       attribute_length
The value of the attribute_length item indicates the length of the attribute, excluding the initial six bytes.

       local_variable_table_length
The value of the local_variable_table_length item indicates the number of entries in the local_variable_table array.

       local_variable_table[]
Each entry in the local_variable_table array indicates a range of code array offsets within which a local variable has a value. It also indicates the index into the local variables of the current frame at which that local variable can be found. Each entry must contain the following items:

       start_pc, length
The given local variable must have a value at indices into the code array in the interval [start_pc, start_pc+length], that is, between start_pc and start_pc+length inclusive. The value of start_pc must be a valid index into the code array of this Code attribute of the opcode of an instruction. The value of start_pc+length must be either a valid index into the code array of this Code attribute of the opcode of an instruction, or the first index beyond the end of that code array.

       name_index, descriptor_index
The value of the name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must contain a CONSTANT_Utf8_info (§4.4.7) structure representing a valid Java local variable name stored as a simple name (§2.7.1).

The value of the descriptor_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must contain a CONSTANT_Utf8_info (§4.4.7) structure representing a valid descriptor for a Java local variable. Java local variable descriptors have the same form as field descriptors (§4.3.2).

       index
The given local variable must be at index in its method's local variables. If the local variable at index is a two-word type (double or long), it occupies both index and index+1.


4.8 Constraints on Java Virtual Machine Code

The Java Virtual Machine code for a method, instance initialization method (§3.8), or class or interface initialization method (§3.8) is stored in the code array of the Code attribute of a method_info structure of a class file. This section describes the constraints associated with the contents of the Code_attribute structure.

4.8.1 Static Constraints

The static constraints on a class file are those defining the well-formedness of the file. With the exception of the static constraints on the Java Virtual Machine code of the class file, these constraints have been given in the previous section. The static constraints on the Java Virtual Machine code in a class file specify how Java Virtual Machine instructions must be laid out in the code array, and what the operands of individual instructions must be.

The static constraints on the instructions in the code array are as follows:

The static constraints on the operands of instructions in the code array are as follows:

4.8.2 Structural Constraints

The structural constraints on the code array specify constraints on relationships between Java Virtual Machine instructions. The structural constraints are as follows:


4.9 Verification of class Files

Even though Sun's Java compiler attempts to produce only class files that satisfy all the static constraints in the previous sections, the Java Virtual Machine has no guarantee that any file it is asked to load was generated by that compiler, or is properly formed. Applications such as Sun's HotJava World Wide Web browser do not download source code which they then compile; these applications download already- compiled class files. The HotJava browser needs to determine whether the class file was produced by a trustworthy Java compiler or by an adversary attempting to exploit the interpreter.

An additional problem with compile-time checking is version skew. A user may have successfully compiled a class, say PurchaseStockOptions, to be a subclass of TradingClass. But the definition of TradingClass might have changed in a way that is not compatible with preexisting binaries since the time the class was compiled. Methods might have been deleted, or had their return types or modifiers changed. Fields might have changed types or changed from instance variables to class variables. The access modifiers of a method or variable may have changed from public to private. For a discussion of these issues, see Chapter 13, "Binary Compatibility," in The Java Language Specification.

Because of these potential problems, the Java Virtual Machine needs to verify for itself that the desired constraints hold on the class files it attempts to incorporate. A well-written Java Virtual Machine emulator could reject poorly formed instructions when a class file is loaded. Other constraints could be checked at run time. For example, a Java Virtual Machine implementation could tag runtime data and have each instruction check that its operands are of the right type.

Instead, Sun's Java Virtual Machine implementation verifies that each class file it considers untrustworthy satisfies the necessary constraints at linking time (§2.16.3). Structural constraints on the Java Virtual Machine code are checked using a simple theorem prover.

Linking-time verification enhances the performance of the interpreter. Expensive checks that would otherwise have to be performed to verify constraints at run time for each interpreted instruction can be eliminated. The Java Virtual Machine can assume that these checks have already been performed. For example, the Java Virtual Machine will already know the following:

Sun's class file verifier is independent of any Java compiler. It should certify all code generated by Sun's current Java compiler; it should also certify code that other compilers can generate, as well as code that the current compiler could not possibly generate. Any class file that satisfies the structural criteria and static constraints will be certified by the verifier.

The class file verifier is also independent of the Java language. Other languages can be compiled into the class format, but will only pass verification if they satisfy the same constraints as a class file compiled from Java source.

4.9.1 The Verification Process

The class file verifier operates in four passes:

Pass 1: When a prospective class file is loaded (§2.16.2) by the Java Virtual Machine, the Java Virtual Machine first ensures that the file has the basic format of a Java class file. The first four bytes must contain the right magic number. All recognized attributes must be of the proper length. The class file must not be truncated or have extra bytes at the end. The constant pool must not contain any superficially unrecognizable information.

While class file verification properly occurs during class linking (§2.16.3), this check for basic class file integrity is necessary for any interpretation of the class file contents and can be considered to be logically part of the verification process.

Pass 2: When the class file is linked, the verifier performs all additional verification that can be done without looking at the code array of the Code attribute (§4.7.4). The checks performed by this pass include the following:

Note that when it looks at field and method references, this pass does not check to make sure that the given field or method actually exists in the given class; nor does it check that the type descriptors given refer to real classes. It only checks that these items are well formed. More detailed checking is delayed until passes 3 and 4.

Pass 3: Still during linking, the verifier checks the code array of the Code attribute for each method of the class file by performing data-flow analysis on each method. The verifier ensures that at any given point in the program, no matter what code path is taken to reach that point:

For further information on this pass, see Section 4.9.2, "The Bytecode Verifier."

Pass 4: For efficiency reasons, certain tests that could in principle be performed in Pass 3 are delayed until the first time the code for the method is actually invoked. In so doing, Pass 3 of the verifier avoids loading class files unless it has to.

For example, if a method invokes another method that returns an instance of class A, and that instance is only assigned to a field of the same type, the verifier does not bother to check if the class A actually exists. However, if it is assigned to a field of the type B, the definitions of both A and B must be loaded in to ensure that A is a subclass of B.

Pass 4 is a virtual pass whose checking is done by the appropriate Java Virtual Machine instructions. The first time an instruction that references a type is executed, the executing instruction does the following:

The first time an instruction invokes a method, or accesses or modifies a field, the executing instruction does the following:

The Java Virtual Machine does not have to check the type of the object on the operand stack. That check has already been done by Pass 3. Errors that are detected in Pass 4 cause instances of subclasses of LinkageError to be thrown.

A Java Virtual Machine is allowed to perform any or all of the Pass 4 steps, except for class or interface initialization, as part of Pass 3; see 2.16.1, "Virtual Machine Start-up" for an example and more discussion.

In Sun's Java Virtual Machine implementation, after the verification has been performed, the instruction in the Java Virtual Machine code is replaced with an alternative form of the instruction (see Chapter 9, "An Optimization"). For example, the opcode new is replaced with new_quick. This alternative instruction indicates that the verification needed by this instruction has taken place and does not need to be performed again. Subsequent invocations of the method will thus be faster. It is illegal for these alternative instruction forms to appear in class files, and they should never be encountered by the verifier.

4.9.2 The Bytecode Verifier

As indicated earlier, Pass 3 of the verification process is the most complex of the four passes of class file verification. This section looks at the verification of Java Virtual Machine code in more detail.

The code for each method is verified independently. First, the bytes that make up the code are broken up into a sequence of instructions, and the index into the code array of the start of each instruction is placed in an array. The verifier then goes through the code a second time and parses the instructions. During this pass a data structure is built to hold information about each Java Virtual Machine instruction in the method. The operands, if any, of each instruction are checked to make sure they are valid. For instance:

For each instruction of the method, the verifier records the contents of the operand stack and the contents of the local variables prior to the execution of that instruction. For the operand stack, it needs to know the stack height and the type of each value on it. For each local variable, it needs to know either the type of the contents of that local variable, or that the local variable contains an unusable or unknown value (it might be uninitialized). The bytecode verifier does not need to distinguish between the integral types (e.g., byte, short, char) when determining the value types on the operand stack.

Next, a data-flow analyzer is initialized. For the first instruction of the method, the local variables which represent parameters initially contain values of the types indicated by the method's type descriptor; the operand stack is empty. All other local variables contain an illegal value. For the other instructions, which have not been examined yet, no information is available regarding the operand stack or local variables.

Finally, the data-flow analyzer is run. For each instruction, a "changed" bit indicates whether this instruction needs to be looked at. Initially, the "changed" bit is only set for the first instruction. The data-flow analyzer executes the following loop:

  1. Select a virtual machine instruction whose "changed" bit is set. If no instruction remains whose "changed" bit is set, the method has successfully been verified. Otherwise, turn off the "changed" bit of the selected instruction.
  2. Model the effect of the instruction on the operand stack and local variables:
  3. Determine the instructions that can follow the current instruction. Successor instructions can be one of the following:
  4. Merge the state of the operand stack and local variables at the end of the execution of the current instruction into each of the successor instructions. In the special case of control transfer to an exception handler, the operand stack is set to contain a single object of the exception type indicated by the exception handler information.
  5. Continue at step 1.
To merge two operand stacks, the number of values on each stack must be identical. The types of values on the stacks must also be identical, except that differently typed reference values may appear at corresponding places on the two stacks. In this case, the merged operand stack contains a reference to an instance of the first common superclass or common superinterface of the two types. Such a reference type always exists because the type Object is a supertype of all class and interface types. If the operand stacks cannot be merged, verification of the method fails.

To merge two local variable states, corresponding pairs of local variables are compared. If the two types are not identical, then unless both contain reference values, the verifier records that the local variable contains an unusable value. If both of the pair of local variables contain reference values, the merged state contains a reference to an instance of the first common superclass of the two types.

If the data-flow analyzer runs on a method without reporting a verification failure, then the method has been successfully verified by Pass 3 of the class file verifier.

Certain instructions and data types complicate the data-flow analyzer. We now examine each of these in more detail.

4.9.3 Long Integers and Doubles

Values of the long and double types each take two consecutive words on the operand stack and in the local variables.

Whenever a long or double is moved into a local variable, the subsequent local variable is marked as containing the second half of a long or double. This special value indicates that all references to the long or double must be through the index of the lower-numbered local variable.

Whenever any value is moved to a local variable, the preceding local variable is examined to see if it contains the first word of a long or a double. If so, that preceding local variable is changed to indicate that it now contains an unusable value. Since half of the long or double has been overwritten, the other half must no longer be used.

Dealing with 64-bit quantities on the operand stack is simpler; the verifier treats them as single units on the stack. For example, the verification code for the dadd opcode (add two double values) checks that the top two items on the stack are both of type double. When calculating operand stack length, values of type long and double have length two.

Untyped instructions that manipulate the operand stack must treat values of type double and long as atomic. For example, the verifier reports a failure if the top value on the stack is a double and it encounters an instruction such as pop or dup. The instructions pop2 or dup2 must be used instead.

4.9.4 Instance Initialization Methods and Newly Created Objects

Creating a new class instance is a multistep process. The Java statement


    ...
    new myClass(i, j, k);

    ...


can be implemented by the following:


    ...
    new 	#1		// Allocate uninitialized space for myClass

    dup			// Duplicate object on the operand stack

    iload_1			// Push i

    iload_2			// Push j

    iload_3			// Push k

    invokespecial myClass.<init>	         // Initialize object

    ...


This instruction sequence leaves the newly created and initialized object on top of the operand stack. (More examples of compiling Java code to the instruction set of the Java Virtual Machine are given in Chapter 7, "Compiling for the Java Virtual Machine.")

The instance initialization method <init> for class myClass sees the new uninitialized object as its this argument in local variable 0. It must either invoke an alternative instance initialization method for class myClass or invoke the initialization method of a superclass on the this object before it is allowed to do anything else with this.

When doing dataflow analysis on instance methods, the verifier initializes local variable 0 to contain an object of the current class, or, for instance initialization methods, local variable 0 contains a special type indicating an uninitialized object. After an appropriate initialization method is invoked (from the current class or the current superclass) on this object, all occurrences of this special type on the verifier's model of the operand stack and in the local variables are replaced by the current class type. The verifier rejects code that uses the new object before it has been initialized or that initializes the object twice. In addition, it ensures that every normal return of the method has either invoked an initialization method in the class of this method or in the direct superclass.

Similarly, a special type is created and pushed on the verifier's model of the operand stack as the result of the Java Virtual Machine instruction new. The special type indicates the instruction by which the class instance was created and the type of the uninitialized class instance created. When an initialization method is invoked on that class instance, all occurrences of the special type are replaced by the intended type of the class instance. This change in type may propagate to subsequent instructions as the dataflow analysis proceeds.

The instruction number needs to be stored as part of the special type, as there may be multiple not-yet-initialized instances of a class in existence on the operand stack at one time. For example, the Java Virtual Machine instruction sequence that implements

new InputStream(new Foo(), new InputStream("foo"))

may have two uninitialized instances of InputStream on the operand stack at once. When an initialization method is invoked on a class instance, only those occurrences of the special type on the operand stack or in the registers that are the same object as the class instance are replaced.

A valid instruction sequence must not have an uninitialized object on the operand stack or in a local variable during a backwards branch, or in a local variable in code protected by an exception handler or a finally clause. Otherwise, a devious piece of code might fool the verifier into thinking it had initialized a class instance when it had, in fact, initialized a class instance created in a previous pass through the loop.

4.9.5 Exception Handlers

Java Virtual Machine code produced from Sun's Java compiler always generates exception handlers such that:

These restrictions are not enforced by the class file verifier since they do not pose a threat to the integrity of the Java Virtual Machine. As long as every nonexceptional path to the exception handler causes there to be a single object on the operand stack, and as long as all other criteria of the verifier are met, the verifier will pass the code.

4.9.6 Exceptions and finally

Given the fragment of Java code


    ...
    try {

        startFaucet();

        waterLawn();

    } finally {

        stopFaucet();

    }

    ...


the Java language guarantees that stopFaucet is invoked (the faucet is turned off) whether we finish watering the lawn or whether an exception occurs while starting the faucet or watering the lawn. That is, the finally clause is guaranteed to be executed whether its try clause completes normally, or completes abruptly by throwing an exception.

To implement the try-finally construct, the Java compiler uses the exception-handling facilities together with two special instructions jsr ("jump to subroutine") and ret ("return from subroutine"). The finally clause is compiled as a subroutine within the Java Virtual Machine code for its method, much like the code for an exception handler. When a jsr instruction that invokes the subroutine is executed, it pushes its return address, the address of the instruction after the jsr that is being executed, onto the operand stack as a value of type returnAddress. The code for the subroutine stores the return address in a local variable. At the end of the subroutine, a ret instruction fetches the return address from the local variable and transfers control to the instruction at the return address.

Control can be transferred to the finally clause (the finally subroutine can be invoked) in several different ways. If the try clause completes normally, the finally subroutine is invoked via a jsr instruction before evaluating the next Java expression. A break or continue inside the try clause that transfers control outside the try clause executes a jsr to the code for the finally clause first. If the try clause executes a return, the compiled code does the following:

  1. Saves the return value (if any) in a local variable.
  2. Executes a jsr to the code for the finally clause.
  3. Upon return from the finally clause, returns the value saved in the local variable.
The compiler sets up a special exception handler which catches any exception thrown by the try clause. If an exception is thrown in the try clause, this exception handler does the following:

  1. Saves the exception in a local variable.
  2. Executes a jsr to the finally clause.
  3. Upon return from the finally clause, rethrows the exception.
For more information about the implementation of Java's try-finally construct, see Section 7.13, "Compiling finally."

The code for the finally clause presents a special problem to the verifier. Usually, if a particular instruction can be reached via multiple paths and a particular local variable contains incompatible values through those multiple paths, then the local variable becomes unusable. However, a finally clause might be called from several different places, yielding several different circumstances:

The code for the finally clause itself might pass verification, but after updating all the successors of the ret instruction, the verifier would note that the local variable that the exception handler expects to hold an exception, or that the return code expects to hold a return value, now contains an indeterminate value.

Verifying code that contains a finally clause is complicated. The basic idea is the following:


4.10 Limitations of the Java Virtual Machine and class File Format

The following limitations in the Java Virtual Machine are imposed by this version of the Java Virtual Machine specification:


1 In retrospect, making eight-byte constants take two constant pool entries was a poor choice.

2 The fact that end_pc is exclusive is an historical mistake in the Java Virtual Machine: if the Java Virtual Machine code for a method is exactly 65535 bytes long and ends with an instruction that is one byte long, then that instruction cannot be protected by an exception handler. A compiler writer can work around this bug by limiting the maximum size of the generated Java Virtual Machine code for any method, instance initialization method, or static initializer (the size of any code array) to 65534 bytes.

3 The javac compiler in Sun's JDK 1.0.2 release can in fact generate LineNumberTable attributes which are not in line number order and which are not one-to-one with source lines. This is unfortunate, as we would prefer to specify a one-to-one, ordered mapping of LineNumberTable attributes to source lines, but must yield to backward compatibility.


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Java Virtual Machine Specification

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