java.lang.reflect.Proxy Maven / Gradle / Ivy

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This work corresponds to the API signatures of JSR 219: Foundation Profile 1.1. In the event of a discrepency between this work and the JSR 219 specification, which is available at http://www.jcp.org/en/jsr/detail?id=219, the latter takes precedence. */ package java.lang.reflect; import java.lang.ref.*; import java.util.*; /** * Proxy provides static methods for creating dynamic proxy * classes and instances, and it is also the superclass of all * dynamic proxy classes created by those methods. * *

To create a proxy for some interface Foo: *

 *     InvocationHandler handler = new MyInvocationHandler(...);
 *     Class proxyClass = Proxy.getProxyClass(
 *         Foo.class.getClassLoader(), new Class[] { Foo.class });
 *     Foo f = (Foo) proxyClass.
 *         getConstructor(new Class[] { InvocationHandler.class }).
 *         newInstance(new Object[] { handler });
 * 

 *     Foo f = (Foo) Proxy.newProxyInstance(Foo.class.getClassLoader(),
 *                                          new Class[] { Foo.class },
 *                                          handler);
 * 

A dynamic proxy class (simply referred to as a proxy * class below) is a class that implements a list of interfaces * specified at runtime when the class is created, with behavior as * described below. * * A proxy interface is such an interface that is implemented * by a proxy class. * * A proxy instance is an instance of a proxy class. * * Each proxy instance has an associated invocation handler * object, which implements the interface {@link InvocationHandler}. * A method invocation on a proxy instance through one of its proxy * interfaces will be dispatched to the {@link InvocationHandler#invoke * invoke} method of the instance's invocation handler, passing the proxy * instance, a java.lang.reflect.Method object identifying * the method that was invoked, and an array of type Object * containing the arguments. The invocation handler processes the * encoded method invocation as appropriate and the result that it * returns will be returned as the result of the method invocation on * the proxy instance. * *

A proxy class has the following properties: * *

*
• Proxy classes are public, final, and not abstract. * *
• The unqualified name of a proxy class is unspecified. The space * of class names that begin with the string "$Proxy" * should be, however, reserved for proxy classes. * *
• A proxy class extends java.lang.reflect.Proxy. * *
• A proxy class implements exactly the interfaces specified at its * creation, in the same order. * *
• If a proxy class implements a non-public interface, then it will * be defined in the same package as that interface. Otherwise, the * package of a proxy class is also unspecified. Note that package * sealing will not prevent a proxy class from being successfully defined * in a particular package at runtime, and neither will classes already * defined by the same class loader and the same package with particular * signers. * *
• Since a proxy class implements all of the interfaces specified at * its creation, invoking getInterfaces on its * Class object will return an array containing the same * list of interfaces (in the order specified at its creation), invoking * getMethods on its Class object will return * an array of Method objects that include all of the * methods in those interfaces, and invoking getMethod will * find methods in the proxy interfaces as would be expected. * *
• The {@link Proxy#isProxyClass Proxy.isProxyClass} method will * return true if it is passed a proxy class-- a class returned by * Proxy.getProxyClass or the class of an object returned by * Proxy.newProxyInstance-- and false otherwise. * *
• The java.security.ProtectionDomain of a proxy class * is the same as that of system classes loaded by the bootstrap class * loader, such as java.lang.Object, because the code for a * proxy class is generated by trusted system code. This protection * domain will typically be granted * java.security.AllPermission. * *
• Each proxy class has one public constructor that takes one argument, * an implementation of the interface {@link InvocationHandler}, to set * the invocation handler for a proxy instance. Rather than having to use * the reflection API to access the public constructor, a proxy instance * can be also be created by calling the {@link Proxy#newProxyInstance * Proxy.newInstance} method, which combines the actions of calling * {@link Proxy#getProxyClass Proxy.getProxyClass} with invoking the * constructor with an invocation handler. *

A proxy instance has the following properties: * *

*
• Given a proxy instance proxy and one of the * interfaces implemented by its proxy class Foo, the * following expression will return true: *

   *     proxy instanceof Foo
   * 

  * and the following cast operation will succeed (rather than throwing * a ClassCastException): *

   *     (Foo) proxy
   * 

  * *
• Each proxy instance has an associated invocation handler, the one * that was passed to its constructor. The static * {@link Proxy#getInvocationHandler Proxy.getInvocationHandler} method * will return the invocation handler associated with the proxy instance * passed as its argument. * *
• An interface method invocation on a proxy instance will be * encoded and dispatched to the invocation handler's {@link * InvocationHandler#invoke invoke} method as described in the * documentation for that method. * *
• An invocation of the hashCode, * equals, or toString methods declared in * java.lang.Object on a proxy instance will be encoded and * dispatched to the invocation handler's invoke method in * the same manner as interface method invocations are encoded and * dispatched, as described above. The declaring class of the * Method object passed to invoke will be * java.lang.Object. Other public methods of a proxy * instance inherited from java.lang.Object are not * overridden by a proxy class, so invocations of those methods behave * like they do for instances of java.lang.Object. *

Methods Duplicated in Multiple Proxy Interfaces

When two or more interfaces of a proxy class contain a method with * the same name and parameter signature, the order of the proxy class's * interfaces becomes significant. When such a duplicate method * is invoked on a proxy instance, the Method object passed * to the invocation handler will not necessarily be the one whose * declaring class is assignable from the reference type of the interface * that the proxy's method was invoked through. This limitation exists * because the corresponding method implementation in the generated proxy * class cannot determine which interface it was invoked through. * Therefore, when a duplicate method is invoked on a proxy instance, * the Method object for the method in the foremost interface * that contains the method (either directly or inherited through a * superinterface) in the proxy class's list of interfaces is passed to * the invocation handler's invoke method, regardless of the * reference type through which the method invocation occurred. * *

If a proxy interface contains a method with the same name and * parameter signature as the hashCode, equals, * or toString methods of java.lang.Object, * when such a method is invoked on a proxy instance, the * Method object passed to the invocation handler will have * java.lang.Object as its declaring class. In other words, * the public, non-final methods of java.lang.Object * logically precede all of the proxy interfaces for the determination of * which Method object to pass to the invocation handler. * *

Note also that when a duplicate method is dispatched to an * invocation handler, the invoke method may only throw * checked exception types that are assignable to one of the exception * types in the throws clause of the method in all of * the proxy interfaces that it can be invoked through. If the * invoke method throws a checked exception that is not * assignable to any of the exception types declared by the method in one * of the the proxy interfaces that it can be invoked through, then an * unchecked UndeclaredThrowableException will be thrown by * the invocation on the proxy instance. This restriction means that not * all of the exception types returned by invoking * getExceptionTypes on the Method object * passed to the invoke method can necessarily be thrown * successfully by the invoke method. * * @author Peter Jones * @version 1.8, 00/02/02 * @see InvocationHandler * @since JDK1.3 */ public class Proxy implements java.io.Serializable { /** * the invocation handler for this proxy instance. * @serial */ protected InvocationHandler h; /** * Constructs a new Proxy instance from a subclass * (typically, a dynamic proxy class) with the specified value * for its invocation handler. * * @param h the invocation handler for this proxy instance */ protected Proxy(InvocationHandler h) { } /** * Returns the java.lang.Class object for a proxy class * given a class loader and an array of interfaces. The proxy class * will be defined by the specified class loader and will implement * all of the supplied interfaces. If a proxy class for the same * permutation of interfaces has already been defined by the class * loader, then the existing proxy class will be returned; otherwise, * a proxy class for those interfaces will be generated dynamically * and defined by the class loader. * *

There are several restrictions on the parameters that may be * passed to Proxy.getProxyClass: * *

*
• All of the Class objects in the * interfaces array must represent interfaces, not * classes or primitive types. * *
• No two elements in the interfaces array may * refer to identical Class objects. * *
• All of the interface types must be visible by name through the * specified class loader. In other words, for class loader * cl and every interface i, the following * expression must be true: *

       *     Class.forName(i.getName(), false, cl) == i
       * 

  * *
• All non-public interfaces must be in the same package; * otherwise, it would not be possible for the proxy class to * implement all of the interfaces, regardless of what package it is * defined in. * *
• No two interfaces may each have a method with the same name * and parameter signature but different return type. * *
• The resulting proxy class must not exceed any limits imposed * on classes by the virtual machine. For example, the VM may limit * the number of interfaces that a class may implement to 65535; in * that case, the size of the interfaces array must not * exceed 65535. *

If any of these restrictions are violated, * Proxy.getProxyClass will throw an * IllegalArgumentException. If the interfaces * array argument or any of its elements are null, a * NullPointerException will be thrown. * *

Note that the order of the specified proxy interfaces is * significant: two requests for a proxy class with the same combination * of interfaces but in a different order will result in two distinct * proxy classes. * * @param loader the class loader to define the proxy class * @param interfaces the list of interfaces for the proxy class * to implement * @return a proxy class that is defined in the specified class loader * and that implements the specified interfaces * @throws IllegalArgumentException if any of the restrictions on the * parameters that may be passed to getProxyClass * are violated * @throws NullPointerException if the interfaces array * argument or any of its elements are null */ public static Class getProxyClass(ClassLoader loader, Class[] interfaces) throws IllegalArgumentException { return null; } /** * Returns an instance of a proxy class for the specified interfaces * that dispatches method invocations to the specified invocation * handler. This method is equivalent to: *

     *     Proxy.getProxyClass(loader, interfaces).
     *         getConstructor(new Class[] { InvocationHandler.class }).
     *         newInstance(new Object[] { handler });
     * 

Proxy.newProxyInstance throws * IllegalArgumentException for the same reasons that * Proxy.getProxyClass does. * * @param loader the class loader to define the proxy class * @param interfaces the list of interfaces for the proxy class * to implement * @param h the invocation handler to dispatch method invocations to * @return a proxy instance with the specified invocation handler of a * proxy class that is defined by the specified class loader * and that implements the specified interfaces * @throws IllegalArgumentException if any of the restrictions on the * parameters that may be passed to getProxyClass * are violated * @throws NullPointerException if the interfaces array * argument or any of its elements are null, or * if the invocation handler, h, is * null */ public static Object newProxyInstance(ClassLoader loader, Class[] interfaces, InvocationHandler h) throws IllegalArgumentException { return null; } /** * Returns true if and only if the specified class was dynamically * generated to be a proxy class using the getProxyClass * method or the newProxyInstance method. * *