Java 动态代理作用是什么？

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();   
    }   
  }   
}  

// 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;

// 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;} 

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()); 
    } 
}

public static byte[] generateProxyClass(final String name, Class[] interfaces)

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();