JAVA动态代理（proxy）实现和源码剖析

曾经终败给现在 2022-09-30 06:08 268阅读 0赞

## JDK动态代理实例 ##

（1）动态代理首先提供一个调度处理器接口（Invocationhandler），该接口实例封装了我们要代理的对象实例的数据。

public class TranceHander implements InvocationHandler{   
            private Object tObject;  
            public TranceHander(Object t){  
                tObject=t;  
            }  
            @Override  
            public Object invoke(Object proxy, Method method, Object[] args)  
                    throws Throwable {  
                // TODO Auto-generated method stub 
                System.out.println("被代理对象："+tObject);  
                System.out.println("方法："+method.getName());  
                return method.invoke(tObject,args);//方法反射 
            }  
        }

（2）使用Proxy类的newProxyInstance方法创建代理对象或getProxyClass方法获得代理类Class。

测试mian方法中，写了一个代理类，从1-1000的整数数组中使用二分查找随机查找一个整数，实现对象比较和toString接口代理

public class ProxyTest {   
    
            /** * @param args */  
            public static void main(String[] args) {  
                // TODO Auto-generated method stub 
                Object[] elements=new Object[1000];  
                for(int i=0;i<elements.length;i++){  
                    Integer value=i+1;//被代理对象 
                    InvocationHandler handler=new TranceHander(value);//构建调度处理器 
                    Object proxy=Proxy.newProxyInstance(null, new Class[]{Comparable.class}, handler);  
                    elements[i]=proxy;  
                }  
                Integer key=new Random().nextInt(1000)+1;  
                int res=Arrays.binarySearch(elements, key);  
                if(res>=0){  
                    System.out.println(elements[res].toString());  
                }  
                System.out.println("查找Key："+key);  
            }  
        }

## JDK源码实现原理分析 ##

（1）代理对象生成：

Object proxy=Proxy.newProxyInstance(null, new Class[]{ Comparable.class}, handler);

（2）查看Proxy类的newProxyInstance方法  
从生成对象方法中，我们看到三个关键的地方：

Class<?> cl = getProxyClass0(loader, interfaces);//得到代理类
    final Constructor<?> cons = cl.getConstructor(constructorParams);   
    newInstance(cons, ih);//将InvocationHandler h传入代理对象中

public static Object newProxyInstance(ClassLoader loader,  
                                              Class<?>[] interfaces,  
                                              InvocationHandler h)  
            throws IllegalArgumentException  
        {  
            if (h == null) {  
                throw new NullPointerException();  
            }  
    
            final SecurityManager sm = System.getSecurityManager();  
            if (sm != null) {  
                checkProxyAccess(Reflection.getCallerClass(), loader, interfaces);  
            }  
    
            /* * Look up or generate the designated proxy class. */  
            Class<?> cl = getProxyClass0(loader, interfaces);  
    
            /* * Invoke its constructor with the designated invocation handler. */  
            try {  
                final Constructor<?> cons = cl.getConstructor(constructorParams);  
                final InvocationHandler ih = h;  
                if (sm != null && ProxyAccessHelper.needsNewInstanceCheck(cl)) {  
                    // create proxy instance with doPrivilege as the proxy class may 
                    // implement non-public interfaces that requires a special permission 
                    return AccessController.doPrivileged(new PrivilegedAction<Object>() {  
                        public Object run() {  
                            return newInstance(cons, ih);  
                        }  
                    });  
                } else {  
                    return newInstance(cons, ih);  
                }  
            } catch (NoSuchMethodException e) {  
                throw new InternalError(e.toString());  
            }  
        }

（3）生成Proxy类，查看getProxyClass0方法，

这个方法有点长，抽取关键的地方：  
1.先抽取被代理类实现的接口Interface，检测是否符合要求

/* collect interface names to use as key for proxy class cache */  
        String[] interfaceNames = new String[interfaces.length];  
    
        // for detecting duplicates 
        Set<Class<?>> interfaceSet = new HashSet<>();  
    
        for (int i = 0; i < interfaces.length; i++) {  
            /* 
             * Verify that the class loader resolves the name of this 
             * interface to the same Class object. 
             */  
            String interfaceName = interfaces[i].getName();  
            Class<?> interfaceClass = null;  
            try {  
                interfaceClass = Class.forName(interfaceName, false, loader);  
            } catch (ClassNotFoundException e) {  
            }  
            if (interfaceClass != interfaces[i]) {  
                throw new IllegalArgumentException(  
                    interfaces[i] + " is not visible from class loader");  
            }  
    
            /* 
             * Verify that the Class object actually represents an 
             * interface. 
             */  
            if (!interfaceClass.isInterface()) {  
                throw new IllegalArgumentException(  
                    interfaceClass.getName() + " is not an interface");  
            }  
    
            /* 
             * Verify that this interface is not a duplicate. 
             */  
            if (interfaceSet.contains(interfaceClass)) {  
                throw new IllegalArgumentException(  
                    "repeated interface: " + interfaceClass.getName());  
            }  
            interfaceSet.add(interfaceClass);  
    
            interfaceNames[i] = interfaceName;  
        }

查看代理对象缓存中是否有我们要创建的代理类，如果有，直接获取；没有则创建  
如果有则： return proxyClassCache.get(loader, interfaces);  
没有则创建，先定义代理类的包名，如果所有的接口都是公共的，则默认将创建的类放在String PROXY\_PACKAGE = “com.sun.proxy”;中，若果存在  
非public接口则所有的非public接口必须在同一包下，代理类也放在此包下，否则系统抛异常。

<span style="white-space:pre">    </span>   for (int i = 0; i < interfaces.length; i++) {  
                       int flags = interfaces[i].getModifiers();  
                       if (!Modifier.isPublic(flags)) {  
                           String name = interfaces[i].getName();  
                           int n = name.lastIndexOf('.');  
                           String pkg = ((n == -1) ? "" : name.substring(0, n + 1));  
                           if (proxyPkg == null) {  
                               proxyPkg = pkg;  
                           } else if (!pkg.equals(proxyPkg)) {  
                               throw new IllegalArgumentException(  
                                   "non-public interfaces from different packages");  
                           }  
                       }  
                   }  
    
                   if (proxyPkg == null) {  
                       // if no non-public proxy interfaces, use com.sun.proxy package 
                       proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";  
                   }

接下来创建类，和将字节码写入类文件中：

byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, interfaces);  
        proxyClass = defineClass0(loader, proxyName,proxyClassFile, 0, proxyClassFile.length);  
        return proxyClass;

(4)创建代理实例，将

newInstance(cons, ih);//将InvocationHandler h传入代理对象中

看完源码，我还是有点困惑，直到我看到生成的代理类文件源码后，豁然开朗。其实主要一步就在于生成类  
的过程,类文件中建立被代理对象接口的invoke调用，话不多说看源码（我直接在测试用例调用的ProxyGenerator.generateProxyClass( proxyName, interfaces)接口生成的）

public static void main(String[] args) {  
            // TODO Auto-generated method stub 
            Integer value=1;  
            InvocationHandler handler=new TranceHander(value);//构建调度处理器 
            Object proxy=Proxy.newProxyInstance(null, Integer.class.getInterfaces(), handler);  
            String path = "D://aaa.class";    
            byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0",Integer.class.getInterfaces());   
               FileOutputStream out = null;    
    
               try {    
                   out = new FileOutputStream(path);    
                   out.write(classFile);    
                   out.flush();    
               } catch (Exception e) {    
                   e.printStackTrace();    
               } finally {    
                   try {    
                       out.close();    
                   } catch (IOException e) {    
                       e.printStackTrace();    
                   }    
               }    
        }

接着看class文件的反编译源码吧，直接用的Method的反射机制实现的代理，equal、hascode、toString方法是所有类都有的，compareTo则是被代理对象的实现接口：

import java.lang.reflect.InvocationHandler;  
        import java.lang.reflect.Method;  
        import java.lang.reflect.Proxy;  
        import java.lang.reflect.UndeclaredThrowableException;  
    
        public final class $Proxy0 extends Proxy implements Comparable {   
          private static Method m3;  
          private static Method m1;  
          private static Method m0;  
          private static Method m2;  
    
          public $Proxy0(InvocationHandler paramInvocationHandler)  
            throws   
          {  
            super(paramInvocationHandler);  
          }  
    
          public final int compareTo(Object paramObject)  
            throws   
          {  
            try  
            {  
              return ((Integer)this.h.invoke(this, m3, new Object[] { paramObject })).intValue();  
            }  
            catch (Error|RuntimeException localError)  
            {  
              throw localError;  
            }  
            catch (Throwable localThrowable)  
            {  
              throw new UndeclaredThrowableException(localThrowable);  
            }  
          }  
    
          public final boolean equals(Object paramObject)  
            throws   
          {  
            try  
            {  
              return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();  
            }  
            catch (Error|RuntimeException localError)  
            {  
              throw localError;  
            }  
            catch (Throwable localThrowable)  
            {  
              throw new UndeclaredThrowableException(localThrowable);  
            }  
          }  
    
          public final int hashCode()  
            throws   
          {  
            try  
            {  
              return ((Integer)this.h.invoke(this, m0, null)).intValue();  
            }  
            catch (Error|RuntimeException localError)  
            {  
              throw localError;  
            }  
            catch (Throwable localThrowable)  
            {  
              throw new UndeclaredThrowableException(localThrowable);  
            }  
          }  
    
          public final String toString()  
            throws   
          {  
            try  
            {  
              return (String)this.h.invoke(this, m2, null);  
            }  
            catch (Error|RuntimeException localError)  
            {  
              throw localError;  
            }  
            catch (Throwable localThrowable)  
            {  
              throw new UndeclaredThrowableException(localThrowable);  
            }  
          }  
    
          static  
          {  
            try  
            {  
              m3 = Class.forName("java.lang.Comparable").getMethod("compareTo", new Class[] { Class.forName("java.lang.Object") });  
              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]);  
              m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);  
              return;  
            }  
            catch (NoSuchMethodException localNoSuchMethodException)  
            {  
              throw new NoSuchMethodError(localNoSuchMethodException.getMessage());  
            }  
            catch (ClassNotFoundException localClassNotFoundException)  
            {  
              throw new NoClassDefFoundError(localClassNotFoundException.getMessage());  
            }  
          }  
        }