Dynamic Proxy Classes

Contents

Introduction
Dynamic Proxy API
Serialization
Examples

Introduction

A dynamic proxy class is a class that implements a list of interfaces specified at runtime such that a method invocation through one of the interfaces on an instance of the class will be encoded and dispatched to another object through a uniform interface. Thus, a dynamic proxy class can be used to create a type-safe proxy object for a list of interfaces without requiring pre-generation of the proxy class, such as with compile-time tools. Method invocations on an instance of a dynamic proxy class are dispatched to a single method in the instance's invocation handler, and they are encoded with a java.lang.reflect.Method object identifying the method that was invoked and an array of type Object containing the arguments.

Dynamic proxy classes are useful to an application or library that needs to provide type-safe reflective dispatch of invocations on objects that present interface APIs. For example, an application can use a dynamic proxy class to create an object that implements multiple arbitrary event listener interfaces-- interfaces that extend java.util.EventListener-- to process a variety of events of different types in a uniform fashion, such as by logging all such events to a file.

Dynamic Proxy Class API

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.

A proxy interface is such an interface that is implemented by a proxy class.

A proxy instance is an instance of a proxy class.

Creating a Proxy Class

Proxy classes, as well as instances of them, are created using the static methods of the class java.lang.reflect.Proxy.

The Proxy.getProxyClass method 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 in 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 in the class loader, then the existing proxy class will be returned; otherwise, a proxy class for those interfaces will be generated dynamically and defined in the class loader.

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

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. Proxy classes are distinguished by the order of their proxy interfaces in order to provide deterministic method invocation encoding in cases where two or more of the proxy interfaces share a method with the same name and parameter signature; this reasoning is described in more detail in the section below titled Methods Duplicated in Multiple Proxy Interfaces.

So that a new proxy class does not need to be generated each time Proxy.getProxyClass is invoked with the same class loader and list of interfaces, the implementation of the dynamic proxy class API should keep a cache of generated proxy classes, keyed by their corresponding loaders and interface list. The implementation should be careful not to refer to the class loaders, interfaces, and proxy classes in such a way as to prevent class loaders, and all of their classes, from being garbage collected when appropriate.

Proxy Class Properties

A proxy class has the following properties:

Creating a Proxy Instance

Each proxy class has one public constructor that takes one argument, an implementation of the interface InvocationHandler.

Each proxy instance has an associated invocation handler object, the one that was passed to its constructor. Rather than having to use the reflection API to access the public constructor, a proxy instance can be also be created by calling the Proxy.newProxyInstance method, which combines the actions of calling Proxy.getProxyClass with invoking the constructor with an invocation handler. Proxy.newProxyInstance throws IllegalArgumentException for the same reasons that Proxy.getProxyClass does.

Proxy Instance Properties

A proxy instance has the following properties:

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 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.

Serialization

Since java.lang.reflect.Proxy implements java.io.Serializable, proxy instances can be serialized, as described in this section. If a proxy instance contains an invocation handler that is not assignable to java.io.Serializable, however, then a java.io.NotSerializableException will be thrown if such an instance is written to a java.io.ObjectOutputStream. Note that for proxy classes, implementing java.io.Externalizable has the same effect with respect to serialization as implementing java.io.Serializable: the writeExternal and readExternal methods of the Externalizable interface will never be invoked on a proxy instance (or an invocation handler) as part of its serialization process. As with all Class objects, the Class object for a proxy class is always serializable.

A proxy class has no serializable fields and a serialVersionUID of 0L. In other words, when the Class object for a proxy class is passed to the static lookup method of java.io.ObjectStreamClass, the returned ObjectStreamClass instance will have the following properties:

The stream protocol for Object Serialization supports a type code named TC_PROXYCLASSDESC, which is a terminal symbol in the grammar for the stream format; its type and value are defined by the following constant field in the java.io.ObjectStreamConstants interface:

    final static byte TC_PROXYCLASSDESC = (byte)0x7D;

The grammar also includes the following two rules, the first being an alternate expansion of the original newClassDesc rule:

newClassDesc:
        TC_PROXYCLASSDESC newHandle proxyClassDescInfo

proxyClassDescInfo:
        (int)<count> proxyInterfaceName[count] classAnnotation superClassDesc

proxyInterfaceName:
        (utf)

When an ObjectOutputStream serializes the class descriptor for a class that is a proxy class, as determined by passing its Class object to the Proxy.isProxyClass method, it uses the TC_PROXYCLASSDESC type code instead of TC_CLASSDESC, following the rules above. In the expansion of proxyClassDescInfo, the sequence of proxyInterfaceName items are the names of all of the interfaces implemented by the proxy class, in the order that they are returned by invoking the getInterfaces method on the Class object. The classAnnotation and superClassDesc items have the same meaning as they do in the classDescInfo rule. For a proxy class, superClassDesc is the class descriptor for its superclass, java.lang.reflect.Proxy; including this descriptor allows for the evolution of the serialized representation of the class Proxy for proxy instances.

For non-proxy classes, ObjectOutputStream calls its protected annotateClass method to allow subclasses to write custom data to the stream for a particular class. For proxy classes, instead of annotateClass, the following method in java.io.ObjectOutputStream is called with the Class object for the proxy class:

    protected void annotateProxyClass(Class cl) throws IOException;

The default implementation of annotateProxyClass in ObjectOutputStream does nothing.

When an ObjectInputStream encounters the type code TC_PROXYCLASSDESC, it deserializes the class descriptor for a proxy class from the stream, formatted as described above. Instead of calling its resolveClass method to resolve the Class object for the class descriptor, the following method in java.io.ObjectInputStream is called:

    protected Class resolveProxyClass(String[] interfaces)
        throws IOException, ClassNotFoundException;

The list of interface names that were deserialized in the proxy class descriptor are passed as the interfaces argument to resolveProxyClass.

The default implementation of resolveProxyClass in ObjectInputStream returns the results of calling Proxy.getProxyClass with the list of Class objects for the interfaces named in the interfaces parameter. The Class object used for each interface name i is the value retuned by calling

        Class.forName(i, false, loader)
where loader is the first non-null class loader up the execution stack, or null if no non-null class loaders are on the stack. This is the same class loader choice made by the default behavior of the resolveClass method. This same value of loader is also the class loader passed to Proxy.getProxyClass. If Proxy.getProxyClass throws an IllegalArgumentException, resolveClass will throw a ClassNotFoundException containing the IllegalArgumentException.

Since a proxy class never has its own serializable fields, the classdata[] in the stream representation of a proxy instance consists wholly of the instance data for its superclass, java.lang.reflect.Proxy. Proxy has one serializable field, h, which contains the invocation handler for the proxy instance.

Examples

Here is a simple example that prints out a message before and after a method invocation on an object that implements an arbitrary list of interfaces:

public interface Foo {
    Object bar(Object obj) throws BazException;
}

public class FooImpl implements Foo {
    Object bar(Object obj) throws BazException {
        // ...
    }
}

public class DebugProxy implements java.lang.reflect.InvocationHandler {

    private Object obj;

    public static Object newInstance(Object obj) {
        return java.lang.reflect.Proxy.newProxyInstance(
            obj.getClass().getClassLoader(),
            obj.getClass().getInterfaces(),
            new DebugProxy(obj));
    }

    private DebugProxy(Object obj) {
        this.obj = obj;
    }

    public Object invoke(Object proxy, Method m, Object[] args)
        throws Throwable
    {
        Object result;
        try {
            System.out.println("before method " + m.getName());
            result = m.invoke(obj, args);
        } catch (InvocationTargetException e) {
            throw e.getTargetException();
        } catch (Exception e) {
            throw new RuntimeException("unexpected invocation exception: " +
                                       e.getMessage());
        } finally {
            System.out.println("after method " + m.getName());
        }
        return result;
    }
}

To construct a DebugProxy for an implementation of the Foo interface and call one of its methods:

    Foo foo = (Foo) DebugProxy.newInstance(new FooImpl());
    foo.bar(null);

Here is an example of a utility invocation handler class that provides default proxy behavior for methods inherited from java.lang.Object and implements delegation of certain proxy method invocations to distinct objects depending on the interface of the invoked method:

import java.lang.reflect.*;

public class Delegator implements InvocationHandler {

    // preloaded Method objects for the methods in java.lang.Object
    private static Method hashCodeMethod;
    private static Method equalsMethod;
    private static Method toStringMethod;
    static {
        try {
            hashCodeMethod = Object.class.getMethod("hashCode", null);
            equalsMethod =
                Object.class.getMethod("equals", new Class[] { Object.class });
            toStringMethod = Object.class.getMethod("toString", null);
        } catch (NoSuchMethodException e) {
            throw new NoSuchMethodError(e.getMessage());
        }
    }

    private Class[] interfaces;
    private Object[] delegates;

    public Delegator(Class[] interfaces, Object[] delegates) {
        this.interfaces = (Class[]) interfaces.clone();
        this.delegates = (Object[]) delegates.clone();
    }

    public Object invoke(Object proxy, Method m, Object[] args)
        throws Throwable
    {
        Class declaringClass = m.getDeclaringClass();

        if (declaringClass == Object.class) {
            if (m.equals(hashCodeMethod)) {
                return proxyHashCode(proxy);
            } else if (m.equals(equalsMethod)) {
                return proxyEquals(proxy, args[0]);
            } else if (m.equals(toStringMethod)) {
                return proxyToString(proxy);
            } else {
                throw new InternalError(
                    "unexpected Object method dispatched: " + m);
            }
        } else {
            for (int i = 0; i < interfaces.length; i++) {
                if (declaringClass.isAssignableFrom(interfaces[i])) {
                    try {
                        return m.invoke(delegates[i], args);
                    } catch (InvocationTargetException e) {
                        throw e.getTargetException();
                    }
                }
            }

            return invokeNotDelegated(proxy, m, args);
        }
    }

    protected Object invokeNotDelegated(Object proxy, Method m,
                                        Object[] args)
        throws Throwable
    {
        throw new InternalError("unexpected method dispatched: " + m);
    }

    protected Integer proxyHashCode(Object proxy) {
        return new Integer(System.identityHashCode(proxy));
    }

    protected Boolean proxyEquals(Object proxy, Object other) {
        return (proxy == other ? Boolean.TRUE : Boolean.FALSE);
    }

    protected String proxyToString(Object proxy) {
        return proxy.getClass().getName() + '@' +
            Integer.toHexString(proxy.hashCode());
    }
}

Subclasses of Delegator can override invokeNotDelegated to implement the behavior of proxy method invocations not to be directly delegated to other objects, and they can override proxyHashCode, proxyEquals, and proxyToString to override the default behavior of the methods the proxy inherits from java.lang.Object.

To construct a Delegator for an implementation of the Foo interface:

    Class[] proxyInterfaces = new Class[] { Foo.class };
    Foo foo = (Foo) Proxy.newProxyInstance(Foo.class.getClassLoader(),
        proxyInterfaces,
        new Delegator(proxyInterfaces, new Object[] { new FooImpl() }));

Note that the implementation of the Delegator class given above is intended to be more illustrative than optimized; for example, instead of caching and comparing the Method objects for the hashCode, equals, and toString methods, it could just match them by their string names, because none of those method names are overloaded in java.lang.Object.


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