彻底理解Java动态代理

青旅半醒 2022-05-26 09:08 193阅读 0赞

# 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
      }    
    }

import java.lang.reflect.InvocationHandler;   
    import java.lang.reflect.Method;   
    import java.lang.reflect.Proxy;   
    import sun.misc.ProxyGenerator;   
    import java.io.*;   
    /** * Java学习交流QQ群：589809992 我们一起学Java！ */
    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

// 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.*;   
    /** * Java学习交流QQ群：589809992 我们一起学Java！ */
    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。