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.
There are two reflective methods for creating instances of classes:
java.lang.reflect.Constructor.newInstance()
and
Class.newInstance()
. The former is preferred and is thus used in these examples because:
Class.newInstance()
can only invoke the zero-argument constructor, while
Constructor.newInstance()
may invoke any constructor, regardless of the number of parameters.Class.newInstance()
throws any exception thrown by the constructor, regardless of whether it is checked or unchecked.
Constructor.newInstance()
always wraps the thrown exception with an
InvocationTargetException
.Class.newInstance()
requires that the constructor be visible;
Constructor.newInstance()
may invoke private
constructors under certain circumstances.Sometimes it may be desirable to retrieve internal state from an object which is only set after construction. Consider a scenario where it is necessary to obtain the internal character set used by
java.io.Console
. (The Console
character set is stored in a private field and is not necessarily the same as the Java virtual machine default character set returned by
java.nio.charset.Charset.defaultCharset()
). The
example shows how this might be achieved:ConsoleCharset
import java.io.Console; import java.nio.charset.Charset; import java.lang.reflect.Constructor; import java.lang.reflect.Field; import java.lang.reflect.InvocationTargetException; import static java.lang.System.out; public class ConsoleCharset { public static void main(String... args) { Constructor[] ctors = Console.class.getDeclaredConstructors(); Constructor ctor = null; for (int i = 0; i < ctors.length; i++) { ctor = ctors[i]; if (ctor.getGenericParameterTypes().length == 0) break; } try { ctor.setAccessible(true); Console c = (Console)ctor.newInstance(); Field f = c.getClass().getDeclaredField("cs"); f.setAccessible(true); out.format("Console charset : %s%n", f.get(c)); out.format("Charset.defaultCharset(): %s%n", Charset.defaultCharset()); // production code should handle these exceptions more gracefully } catch (InstantiationException x) { x.printStackTrace(); } catch (InvocationTargetException x) { x.printStackTrace(); } catch (IllegalAccessException x) { x.printStackTrace(); } catch (NoSuchFieldException x) { x.printStackTrace(); } } }
Class.newInstance()
will only succeed if the constructor is has zero arguments and is already accessible. Otherwise, it is necessary to use
Constructor.newInstance()
as in the above example.
Example output for a UNIX system:
$ java ConsoleCharset Console charset : ISO-8859-1 Charset.defaultCharset() : ISO-8859-1
Example output for a Windows system:
C:\> java ConsoleCharset Console charset : IBM437 Charset.defaultCharset() : windows-1252
Another common application of
Constructor.newInstance()
is to invoke constructors which take arguments. The
example finds a specific single-argument constructor and invokes it:RestoreAliases
import java.lang.reflect.Constructor; import java.lang.reflect.Field; import java.lang.reflect.InvocationTargetException; import java.util.HashMap; import java.util.Map; import java.util.Set; import static java.lang.System.out; class EmailAliases { private Set<String> aliases; private EmailAliases(HashMap<String, String> h) { aliases = h.keySet(); } public void printKeys() { out.format("Mail keys:%n"); for (String k : aliases) out.format(" %s%n", k); } } public class RestoreAliases { private static Map<String, String> defaultAliases = new HashMap<String, String>(); static { defaultAliases.put("Duke", "duke@i-love-java"); defaultAliases.put("Fang", "fang@evil-jealous-twin"); } public static void main(String... args) { try { Constructor ctor = EmailAliases.class.getDeclaredConstructor(HashMap.class); ctor.setAccessible(true); EmailAliases email = (EmailAliases)ctor.newInstance(defaultAliases); email.printKeys(); // production code should handle these exceptions more gracefully } catch (InstantiationException x) { x.printStackTrace(); } catch (IllegalAccessException x) { x.printStackTrace(); } catch (InvocationTargetException x) { x.printStackTrace(); } catch (NoSuchMethodException x) { x.printStackTrace(); } } }
This example uses
Class.getDeclaredConstructor()
to find the constructor with a single argument of type
java.util.HashMap
. Note that it is sufficient to pass HashMap.class
since the parameter to any get*Constructor()
method requires a class only for type purposes. Due to
type erasure, the following expression evaluates to true
:
HashMap.class == defaultAliases.getClass()
The example then creates a new instance of the class using this constructor with
Constructor.newInstance()
.
$ java RestoreAliases Mail keys: Duke Fang