The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
Here are a few common problems encountered by developers with explanations for why the occur and how to resolve them.
The
example will generate an
FieldTrouble
IllegalArgumentException
.
Field.setInt()
is invoked to set a field that is of the reference type Integer
with a value of primitive type. In the non-reflection equivalent Integer val = 42
, the compiler would convert (or box) the primitive type 42
to a reference type as new Integer(42)
so that its type checking will accept the statement. When using reflection, type checking only occurs at runtime so there is no opportunity to box the value.
import java.lang.reflect.Field; public class FieldTrouble { public Integer val; public static void main(String... args) { FieldTrouble ft = new FieldTrouble(); try { Class<?> c = ft.getClass(); Field f = c.getDeclaredField("val"); f.setInt(ft, 42); // IllegalArgumentException // production code should handle these exceptions more gracefully } catch (NoSuchFieldException x) { x.printStackTrace(); } catch (IllegalAccessException x) { x.printStackTrace(); } } }
$ java FieldTrouble Exception in thread "main" java.lang.IllegalArgumentException: Can not set java.lang.Object field FieldTrouble.val to (long)42 at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException (UnsafeFieldAccessorImpl.java:146) at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException (UnsafeFieldAccessorImpl.java:174) at sun.reflect.UnsafeObjectFieldAccessorImpl.setLong (UnsafeObjectFieldAccessorImpl.java:102) at java.lang.reflect.Field.setLong(Field.java:831) at FieldTrouble.main(FieldTrouble.java:11)
To eliminate this exception, the problematic line should be replaced by the following invocation of
Field.set(Object obj, Object value)
:
f.set(ft, new Integer(43));
Class.isAssignableFrom()
. The example is expected to fail because isAssignableFrom()
will return false
in this test which can be used programmatically to verify whether a particular conversion is possible:
Integer.class.isAssignableFrom(int.class) == false
Similarly, automatic conversion from primitive to reference type is also impossible in reflection.
int.class.isAssignableFrom(Integer.class) == false
The astute reader may notice that if the
example shown earlier is used to get information on a non-public field, it will fail:FieldSpy
$ java FieldSpy java.lang.String count java.lang.NoSuchFieldException: count at java.lang.Class.getField(Class.java:1519) at FieldSpy.main(FieldSpy.java:12)
Class.getField()
and
Class.getFields()
methods return the public member field(s) of the class, enum, or interface represented by the Class
object. To retrieve all fields declared (but not inherited) in the Class
, use the
Class.getDeclaredFields()
method.
An
IllegalAccessException
may be thrown if an attempt is made to get or set the value of a private
or otherwise inaccessible field or to set the value of a final
field (regardless of its access modifiers).
The
example illustrates the type of stack trace which results from attempting to set a final field.FieldTroubleToo
import java.lang.reflect.Field; public class FieldTroubleToo { public final boolean b = true; public static void main(String... args) { FieldTroubleToo ft = new FieldTroubleToo(); try { Class<?> c = ft.getClass(); Field f = c.getDeclaredField("b"); // f.setAccessible(true); // solution f.setBoolean(ft, Boolean.FALSE); // IllegalAccessException // production code should handle these exceptions more gracefully } catch (NoSuchFieldException x) { x.printStackTrace(); } catch (IllegalArgumentException x) { x.printStackTrace(); } catch (IllegalAccessException x) { x.printStackTrace(); } } }
$ java FieldTroubleToo java.lang.IllegalAccessException: Can not set final boolean field FieldTroubleToo.b to (boolean)false at sun.reflect.UnsafeFieldAccessorImpl. throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:55) at sun.reflect.UnsafeFieldAccessorImpl. throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:63) at sun.reflect.UnsafeQualifiedBooleanFieldAccessorImpl.setBoolean (UnsafeQualifiedBooleanFieldAccessorImpl.java:78) at java.lang.reflect.Field.setBoolean(Field.java:686) at FieldTroubleToo.main(FieldTroubleToo.java:12)
final
fields from being set after initialization of the class. However, Field
is declared to extend
AccessibleObject
which provides the ability to suppress this check.AccessibleObject.setAccessible()
succeeds, then subsequent operations on this field value will not fail do to this problem. This may have unexpected side-effects; for example, sometimes the original value will continue to be used by some sections of the application even though the value has been modified.
AccessibleObject.setAccessible()
will only succeed if the operation is allowed by the security context.