彻底理解JAVA动态代理

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。

代理模式的结构如下图所示。

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
  }
} 

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();
    }
  }
}  

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;

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;} 

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());
    }
}

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)

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());
        }
    }
}  

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 set of proxy methods with the same signature,
   * verify that the methods' return types are compatible.
   */
  for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
      checkReturnTypes(sigmethods);
  }
  /* ============================================================
   * Step 2: Assemble FieldInfo and MethodInfo structs for all of
   * fields and methods in the class we are generating.
   */
  try {
      methods.add(generateConstructor());
      for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
    for (ProxyMethod pm : sigmethods) {
        // add static field for method's Method object
        fields.add(new FieldInfo(pm.methodFieldName,
      "Ljava/lang/reflect/Method;",
       ACC_PRIVATE | ACC_STATIC));
        // generate code for proxy method and add it
        methods.add(pm.generateMethod());
    }
      }
      methods.add(generateStaticInitializer());
  } catch (IOException e) {
      throw new InternalError("unexpected I/O Exception");
  }
  /* ============================================================
   * Step 3: Write the final class file.
   */
  /*
   * Make sure that constant pool indexes are reserved for the
   * following items before starting to write the final class file.
   */
  cp.getClass(dotToSlash(className));
  cp.getClass(superclassName);
  for (int i = 0; i < interfaces.length; i++) {
      cp.getClass(dotToSlash(interfaces[i].getName()));
  }
  /*
   * Disallow new constant pool additions beyond this point, since
   * we are about to write the final constant pool table.
   */
  cp.setReadOnly();
  ByteArrayOutputStream bout = new ByteArrayOutputStream();
  DataOutputStream dout = new DataOutputStream(bout);
  try {
      /*
       * Write all the items of the "ClassFile" structure.
       * See JVMS section 4.1.
       */
          // u4 magic;
      dout.writeInt(0xCAFEBABE);
          // u2 minor_version;
      dout.writeShort(CLASSFILE_MINOR_VERSION);
          // u2 major_version;
      dout.writeShort(CLASSFILE_MAJOR_VERSION);
      cp.write(dout);   // (write constant pool)
          // u2 access_flags;
      dout.writeShort(ACC_PUBLIC | ACC_FINAL | ACC_SUPER);
          // u2 this_class;
      dout.writeShort(cp.getClass(dotToSlash(className)));
          // u2 super_class;
      dout.writeShort(cp.getClass(superclassName));
          // u2 interfaces_count;
      dout.writeShort(interfaces.length);
          // u2 interfaces[interfaces_count];
      for (int i = 0; i < interfaces.length; i++) {
    dout.writeShort(cp.getClass(
        dotToSlash(interfaces[i].getName())));
      }
          // u2 fields_count;
      dout.writeShort(fields.size());
          // field_info fields[fields_count];
      for (FieldInfo f : fields) {
    f.write(dout);
      }
          // u2 methods_count;
      dout.writeShort(methods.size());
          // method_info methods[methods_count];
      for (MethodInfo m : methods) {
    m.write(dout);
      }
             // u2 attributes_count;
      dout.writeShort(0); // (no ClassFile attributes for proxy classes)
  } catch (IOException e) {
      throw new InternalError("unexpected I/O Exception");
  }
  return bout.toByteArray(); 

总结

一个典型的动态代理创建对象过程可分为以下四个步骤：
1、通过实现InvocationHandler接口创建自己的调用处理器 IvocationHandler handler = new InvocationHandlerImpl(...);
2、通过为Proxy类指定ClassLoader对象和一组interface创建动态代理类
Class clazz = Proxy.getProxyClass(classLoader,new Class[]{...});
3、通过反射机制获取动态代理类的构造函数，其参数类型是调用处理器接口类型
Constructor constructor = clazz.getConstructor(new Class[]{InvocationHandler.class});
4、通过构造函数创建代理类实例，此时需将调用处理器对象作为参数被传入
Interface Proxy = (Interface)constructor.newInstance(new Object[] (handler));
为了简化对象创建过程，Proxy类中的newInstance方法封装了2~4，只需两步即可完成代理对象的创建。
生成的ProxySubject继承Proxy类实现Subject接口，实现的Subject的方法实际调用处理器的invoke方法，而invoke方法利用反射调用的是被代理对象的的方法（Object result=method.invoke(proxied,args)）

美中不足

诚然，Proxy已经设计得非常优美，但是还是有一点点小小的遗憾之处，那就是它始终无法摆脱仅支持interface代理的桎梏，因为它的设计注定了这个遗憾。回想一下那些动态生成的代理类的继承关系图，它们已经注定有一个共同的父类叫Proxy。Java的继承机制注定了这些动态代理类们无法实现对class的动态代理，原因是多继承在Java中本质上就行不通。有很多条理由，人们可以否定对 class代理的必要性，但是同样有一些理由，相信支持class动态代理会更美好。接口和类的划分，本就不是很明显，只是到了Java中才变得如此的细化。如果只从方法的声明及是否被定义来考量，有一种两者的混合体，它的名字叫抽象类。实现对抽象类的动态代理，相信也有其内在的价值。此外，还有一些历史遗留的类，它们将因为没有实现任何接口而从此与动态代理永世无缘。如此种种，不得不说是一个小小的遗憾。但是，不完美并不等于不伟大，伟大是一种本质，Java动态代理就是佐例。

参考资料

1、JDK动态代理实现原理

2、Java动态代理机制分析及扩展