彻底理解Java动态代理

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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.*;   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。
学习Java的同学注意了！！！ 学习过程中遇到什么问题或者想获取学习资源的话，欢迎加入Java学习交流群，群号码：523047986  我们一起学Java！