Class Proxy

Proxy provides static methods for creating objects that act like instances of interfaces but allow for customized method invocation. To create a proxy instance for some interface Foo: InvocationHandler handler = new MyInvocationHandler(...); Foo f = (Foo) Proxy.newProxyInstance(Foo.class.getClassLoader(), new Class<?>[] { Foo.class }, handler);

A proxy class is a class created at runtime that implements a specified list of interfaces, known as proxy interfaces. A proxy instance is an instance of a proxy class. Each proxy instance has an associated invocation handler object, which implements the interface InvocationHandler. A method invocation on a proxy instance through one of its proxy interfaces will be dispatched to the 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:

• 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.
• The package and module in which a proxy class is defined is specified below.
• A proxy class is final and non-abstract.
• A proxy class extends java.lang.reflect.Proxy.
• A proxy class implements exactly the interfaces specified at its creation, in the same order. 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 ProtectionDomain of a proxy class is the same as that of system classes loaded by the bootstrap class loader, such as java.lang.Object.
• The Proxy.isProxyClass method can be used to determine if a given class is a proxy class.

A proxy instance has the following properties:

• Given a proxy instance proxy and one of the interfaces, Foo, implemented by its proxy class, 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 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 invoke method as described in the documentation for that method.
• A proxy interface may define a default method or inherit a default method from its superinterface directly or indirectly. An invocation handler can invoke a default method of a proxy interface by calling InvocationHandler::invokeDefault.
• 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.

Package and Module Membership of Proxy Class

The package and module to which a proxy class belongs are chosen such that the accessibility of the proxy class is in line with the accessibility of the proxy interfaces. Specifically, the package and the module membership of a proxy class defined via the getProxyClass(ClassLoader, Class[]) or newProxyInstance(ClassLoader, Class[], InvocationHandler) methods is specified as follows:

1. If all the proxy interfaces are in exported or open packages:
   1. if all the proxy interfaces are public, then the proxy class is public in an unconditionally exported but non-open package. The name of the package and the module are unspecified.
   2. if at least one of all the proxy interfaces is non-public, then the proxy class is non-public in the package and module of the non-public interfaces. All the non-public interfaces must be in the same package and module; otherwise, proxying them is not possible.
2. If at least one proxy interface is in a package that is non-exported and non-open:
   1. if all the proxy interfaces are public, then the proxy class is public in a non-exported, non-open package of dynamic module. The names of the package and the module are unspecified.
   2. if at least one of all the proxy interfaces is non-public, then the proxy class is non-public in the package and module of the non-public interfaces. All the non-public interfaces must be in the same package and module; otherwise, proxying them is not possible.

Note that if proxy interfaces with a mix of accessibilities -- for example, an exported public interface and a non-exported non-public interface -- are proxied by the same instance, then the proxy class's accessibility is governed by the least accessible proxy interface.

Note that it is possible for arbitrary code to obtain access to a proxy class in an open package with setAccessible, whereas a proxy class in a non-open package is never accessible to code outside the module of the proxy class.

Throughout this specification, a "non-exported package" refers to a package that is not exported to all modules, and a "non-open package" refers to a package that is not open to all modules. Specifically, these terms refer to a package that either is not exported/open by its containing module or is exported/open in a qualified fashion by its containing module.

Dynamic Modules

A dynamic module is a named module generated at runtime. A proxy class defined in a dynamic module is encapsulated and not accessible to any module. Calling Constructor.newInstance(Object...) on a proxy class in a dynamic module will throw IllegalAccessException; Proxy.newProxyInstance method should be used instead.

A dynamic module can read the modules of all of the superinterfaces of a proxy class and the modules of the classes and interfaces referenced by all public method signatures of a proxy class. If a superinterface or a referenced class or interface, say T, is in a non-exported package, the module of T is updated to export the package of T to the dynamic module.

Methods Duplicated in Multiple Proxy Interfaces

When two or more proxy interfaces 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 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.