Examples of java.lang.reflect.Proxy$Key2

• java.lang.reflect.Proxy
  {@code Proxy} provides static methods for creating dynamic proxyclasses and instances, and it is also the superclass of all dynamic proxy classes created by those methods.

  To create a proxy for some interface {@code Foo}:

   InvocationHandler handler = new MyInvocationHandler(...); Class<?> proxyClass = Proxy.getProxyClass(Foo.class.getClassLoader(), Foo.class); Foo f = (Foo) proxyClass.getConstructor(InvocationHandler.class). newInstance(handler); 

  or more simply:

   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 proxyinstance, a {@code java.lang.reflect.Method} object identifyingthe method that was invoked, and an array of type {@code 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 if all proxy interfaces are public.
  • Proxy classes are non-public, final, and not abstract if any of the proxy interfaces is non-public.
  • The unqualified name of a proxy class is unspecified. The space of class names that begin with the string {@code "$Proxy"}should be, however, reserved for proxy classes.
  • A proxy class extends {@code 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 {@code getInterfaces} on its{@code Class} object will return an array containing the samelist of interfaces (in the order specified at its creation), invoking {@code getMethods} on its {@code Class} object will returnan array of {@code Method} objects that include all of themethods in those interfaces, and invoking {@code getMethod} willfind methods in the proxy interfaces as would be expected.
  • The {@link Proxy#isProxyClass Proxy.isProxyClass} method willreturn true if it is passed a proxy class-- a class returned by {@code Proxy.getProxyClass} or the class of an object returned by{@code Proxy.newProxyInstance}-- and false otherwise.
  • The {@code java.security.ProtectionDomain} of a proxy classis the same as that of system classes loaded by the bootstrap class loader, such as {@code java.lang.Object}, because the code for a proxy class is generated by trusted system code. This protection domain will typically be granted {@code 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.newProxyInstance} method, which combines the actions of calling{@link Proxy#getProxyClass Proxy.getProxyClass} with invoking theconstructor with an invocation handler.

  A proxy instance has the following properties:

  • Given a proxy instance {@code proxy} and one of theinterfaces implemented by its proxy class {@code Foo}, the following expression will return true:

     {@code proxy instanceof Foo}

    and the following cast operation will succeed (rather than throwing a {@code ClassCastException}):

     {@code (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} methodwill 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 thedocumentation for that method.
  • An invocation of the {@code hashCode}, {@code equals}, or {@code toString} methods declared in{@code java.lang.Object} on a proxy instance will be encoded anddispatched to the invocation handler's {@code invoke} method inthe same manner as interface method invocations are encoded and dispatched, as described above. The declaring class of the {@code Method} object passed to {@code invoke} will be{@code java.lang.Object}. Other public methods of a proxy instance inherited from {@code java.lang.Object} are notoverridden by a proxy class, so invocations of those methods behave like they do for instances of {@code 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 {@code Method} object passedto 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 {@code Method} object for the method in the foremost interfacethat 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 {@code invoke} method, regardless of thereference type through which the method invocation occurred.

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

  Note also that when a duplicate method is dispatched to an invocation handler, the {@code invoke} method may only throwchecked exception types that are assignable to one of the exception types in the {@code throws} clause of the method in all ofthe proxy interfaces that it can be invoked through. If the {@code invoke} method throws a checked exception that is notassignable to any of the exception types declared by the method in one of the proxy interfaces that it can be invoked through, then an unchecked {@code UndeclaredThrowableException} will be thrown bythe invocation on the proxy instance. This restriction means that not all of the exception types returned by invoking {@code getExceptionTypes} on the {@code Method} objectpassed to the {@code invoke} method can necessarily be thrownsuccessfully by the {@code invoke} method. @author Peter Jones @see InvocationHandler @since 1.3

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