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Java动态代理分析及理解
代理设计模式
定义：为其他对象提供一种代理以控制对这个对象的访问。
动态代理使用
java动态代理机制以巧妙的方式实现了代理模式的设计理念。
代理模式示例代码
public interface Subject
{
public void doSomething();
}
public class RealSubject implements Subject
{
public void doSomething()
{
System.out.println( "call doSomething()" );
}
}
public class ProxyHandler implements InvocationHandler
{
private Object proxied;
public ProxyHandler( Object proxied )
{
this.proxied = proxied;
}
public Object invoke( Object proxy, Method method, Object[] args ) throws Throwable
{
//在转调具体目标对象之前，可以执行一些功能处理
//转调具体目标对象的方法
return method.invoke( proxied, args);
//在转调具体目标对象之后，可以执行一些功能处理
}
}
</div>
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import sun.misc.ProxyGenerator;
import java.io.*;
public class DynamicProxy
{
public static void main( String args[] )
{
RealSubject real = new RealSubject();
Subject proxySubject = (Subject)Proxy.newProxyInstance(Subject.class.getClassLoader(),
new Class[]{Subject.class},
new ProxyHandler(real));
proxySubject.doSomething();
//write proxySubject class binary data to file
createProxyClassFile();
}
public static void createProxyClassFile()
{
String name = "ProxySubject";
byte[] data = ProxyGenerator.generateProxyClass( name, new Class[] { Subject.class } );
try
{
FileOutputStream out = new FileOutputStream( name + ".class" );
out.write( data );
out.close();
}
catch( Exception e )
{
e.printStackTrace();
}
}
}
</div>
动态代理内部实现
首先来看看类Proxy的代码实现 Proxy的主要静态变量
// 映射表：用于维护类装载器对象到其对应的代理类缓存 private static Map loaderToCache = new WeakHashMap(); // 标记：用于标记一个动态代理类正在被创建中 private static Object pendingGenerationMarker = new Object(); // 同步表：记录已经被创建的动态代理类类型，主要被方法 isProxyClass 进行相关的判断 private static Map proxyClasses = Collections.synchronizedMap(new WeakHashMap()); // 关联的调用处理器引用 protected InvocationHandler h;</div>
Proxy的构造方法
// 由于 Proxy 内部从不直接调用构造函数，所以 private 类型意味着禁止任何调用
private Proxy() {}
// 由于 Proxy 内部从不直接调用构造函数，所以 protected 意味着只有子类可以调用
protected Proxy(InvocationHandler h) {this.h = h;}
</div>
Proxy静态方法newProxyInstance
public static Object newProxyInstance(ClassLoader loader, Class<?>[]interfaces,InvocationHandler h) throws IllegalArgumentException {
// 检查 h 不为空，否则抛异常
if (h == null) {
throw new NullPointerException();
}
// 获得与指定类装载器和一组接口相关的代理类类型对象
Class cl = getProxyClass(loader, interfaces);
// 通过反射获取构造函数对象并生成代理类实例
try {
Constructor cons = cl.getConstructor(constructorParams);
return (Object) cons.newInstance(new Object[] { h });
} catch (NoSuchMethodException e) { throw new InternalError(e.toString());
} catch (IllegalAccessException e) { throw new InternalError(e.toString());
} catch (InstantiationException e) { throw new InternalError(e.toString());
} catch (InvocationTargetException e) { throw new InternalError(e.toString());
}
}
</div>
类Proxy的getProxyClass方法调用ProxyGenerator的 generateProxyClass方法产生ProxySubject.class的二进制数据：
public static byte[] generateProxyClass(final String name, Class[] interfaces)</div>
我们可以import sun.misc.ProxyGenerator，调用 generateProxyClass方法产生binary data，然后写入文件，最后通过反编译工具来查看内部实现原理。 反编译后的ProxySubject.java Proxy静态方法newProxyInstance
import java.lang.reflect.*;
public final class ProxySubject extends Proxy
implements Subject
{
private static Method m1;
private static Method m0;
private static Method m3;
private static Method m2;
public ProxySubject(InvocationHandler invocationhandler)
{
super(invocationhandler);
}
public final boolean equals(Object obj)
{
try
{
return ((Boolean)super.h.invoke(this, m1, new Object[] {
obj
})).booleanValue();
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final int hashCode()
{
try
{
return ((Integer)super.h.invoke(this, m0, null)).intValue();
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final void doSomething()
{
try
{
super.h.invoke(this, m3, null);
return;
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final String toString()
{
try
{
return (String)super.h.invoke(this, m2, null);
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
static
{
try
{
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] {
Class.forName("java.lang.Object")
});
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
m3 = Class.forName("Subject").getMethod("doSomething", new Class[0]);
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
}
catch(NoSuchMethodException nosuchmethodexception)
{
throw new NoSuchMethodError(nosuchmethodexception.getMessage());
}
catch(ClassNotFoundException classnotfoundexception)
{
throw new NoClassDefFoundError(classnotfoundexception.getMessage());
}
}
}
</div>
ProxyGenerator内部是如何生成class二进制数据，可以参考源代码。
private byte[] generateClassFile() {
/*
* Record that proxy methods are needed for the hashCode, equals,
* and toString methods of java.lang.Object. This is done before
* the methods from the proxy interfaces so that the methods from
* java.lang.Object take precedence over duplicate methods in the
* proxy interfaces.
*/
addProxyMethod(hashCodeMethod, Object.class);
addProxyMethod(equalsMethod, Object.class);
addProxyMethod(toStringMethod, Object.class);
/*
* Now record all of the methods from the proxy interfaces, giving
* earlier interfaces precedence over later ones with duplicate
* methods.
*/
for (int i = 0; i < interfaces.length; i++) {
Method[] methods = interfaces[i].getMethods();
for (int j = 0; j < methods.length; j++) {
addProxyMethod(methods[j], interfaces[i]);
}
}
/*
* For each