Spec-Zone .ru
спецификации, руководства, описания, API
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There are two reflective methods for creating instances of classes:
java.lang.reflect.Constructor.newInstance()
and
Class.newInstance()
Class.newInstance()
Constructor.newInstance()
Class.newInstance()
InvocationTargetException
Class.newInstance()
Constructor.newInstance()
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
Console
character set is stored in an private field and is not necessarily the same as the Java virtual machine default character set returned by
java.nio.charset.Charset.defaultCharset()
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()
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()
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
HashMap.class
since the parameter to any get*Constructor()
method requires a class only for type purposes. Due 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