JDK动态代理源码分析

先抛出一个问题，JDK的动态代理为什么不支持对实现类的代理，只支持接口的代理？？？

首先来看一下如何使用JDK动态代理。JDK提供了Java.lang.reflect.Proxy类来实现动态代理的，可通过它的newProxyInstance来获得代理实现类。同时对于代理的接口的实际处理，是一个java.lang.reflect.InvocationHandler，它提供了一个invoke方法供实现者提供相应的代理逻辑的实现。

下面实现一个jdk动态代理的例子：

1.被代理的接口，编写一个接口HelloTest

package com.proxy.test2;
public interface HelloTest {
	void say(String name);
}

2.HelloTestImpl 实现接口HelloTest

package com.proxy.test2;
public class HelloTestImpl implements HelloTest {
	@Override
	public void say(String name) {
		System.out.println("Hello:"+name);
	}
}

3.JDK的动态代码需要实现InvocationHandler

package com.proxy.test2;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class CustomInvocationHandler implements InvocationHandler {
    private Object target;  
    public CustomInvocationHandler(Object target) {
        this.target = target;
    }  
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("Before invocation");
        Object retVal = method.invoke(target, args);
        System.out.println("After invocation");
        return retVal;
    }
}

4.编写一个测试类ProxyTest

package com.proxy.test2;
import java.lang.reflect.Proxy;
public class ProxyTest {
	public static void main(String[] args) {
		//设置为true,会在工程根目录生成$Proxy0.class代理类（com.sun.proxy.$Proxy0.class）
		System.getProperties().put(
				"sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
		String saveGeneratedFiles = System.getProperty("sun.misc.ProxyGenerator.saveGeneratedFiles");
		System.out.println(saveGeneratedFiles);
		HelloTest helloWord = new HelloTestImpl();
		CustomInvocationHandler customInvocationHandler = new CustomInvocationHandler(
				helloWord);
		//通过Proxy.newProxyInstance生成代理对象
		HelloTest proxy = (HelloTest) Proxy.newProxyInstance(
				HelloTest.class.getClassLoader(),
				helloWord.getClass().getInterfaces(), customInvocationHandler);
		//调用say方法
		proxy.say("test");
	}
}

5.运行测试类

true
Before invocation
Hello:test
After invocation

以上5步编写了一个JDK动态代理的例子，到底是如果代理的呢？？

首先看ProxyTest 类中

HelloTest proxy = (HelloTest) Proxy.newProxyInstance(
HelloTest.class.getClassLoader(),
helloWord.getClass().getInterfaces(), customInvocationHandler);

查看Proxy.newProxyInstance源码：（JDK版本为jdk1.7.0_80）

@CallerSensitive
    public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
        if (h == null) {
            throw new NullPointerException();
        }
        final Class<?>[] intfs = interfaces.clone();
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }
        /*
         * Look up or generate the designated proxy class.
         */
         //动态生成class的地方，重点是看这里面的方法
        Class<?> cl = getProxyClass0(loader, intfs);
        /*
         * Invoke its constructor with the designated invocation handler.
         */
         //获取代理类的实例
        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());
        }
    }

查看getProxyClass0方法

   /**
     * Generate a proxy class.  Must call the checkProxyAccess method
     * to perform permission checks before calling this.
     */
    private static Class<?> getProxyClass0(ClassLoader loader,
                                           Class<?>... interfaces) {
        if (interfaces.length > 65535) {//
            throw new IllegalArgumentException("interface limit exceeded");
        }
        // If the proxy class defined by the given loader implementing
        // the given interfaces exists, this will simply return the cached copy;
        // otherwise, it will create the proxy class via the ProxyClassFactory
        // JDK对代理进行了缓存，如果已经存在相应的代理类，则直接返回，否则才会通过ProxyClassFactory来创建代理
        return proxyClassCache.get(loader, interfaces);
    }

点击proxyClassCache

   /**
     * a cache of proxy classes
     */
    private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

点击ProxyClassFactory

   /**
     * A factory function that generates, defines and returns the proxy class given
     * the ClassLoader and array of interfaces.
     */
    private static final class ProxyClassFactory
        implements BiFunction<ClassLoader, Class<?>[], Class<?>>
        // 所有代理类名字的前缀
        private static final String proxyClassNamePrefix = "$Proxy";
        ///用于生成唯一代理类名称的下一个数字
        private static final AtomicLong nextUniqueNumber = new AtomicLong();
        @Override
        public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
            Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
            for (Class<?> intf : interfaces) {
                /*
                 * Verify that the class loader resolves the name of this
                 * interface to the same Class object.
                 */
                Class<?> interfaceClass = null;
                try {
                    interfaceClass = Class.forName(intf.getName(), false, loader);
                } catch (ClassNotFoundException e) {
                }
                if (interfaceClass != intf) {
                    throw new IllegalArgumentException(
                        intf + " 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.put(interfaceClass, Boolean.TRUE) != null) {
                    throw new IllegalArgumentException(
                        "repeated interface: " + interfaceClass.getName());
                }
            }
            String proxyPkg = null;     // package to define proxy class in
            /*
             * Record the package of a non-public proxy interface so that the
             * proxy class will be defined in the same package.  Verify that
             * all non-public proxy interfaces are in the same package.
             */
            for (Class<?> intf : interfaces) {
                int flags = intf.getModifiers();
                if (!Modifier.isPublic(flags)) {
                    String name = intf.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) {
                // 如果没有公开的代理接口，使用com.sun.proxy包
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }
            /*
             * Choose a name for the proxy class to generate.
             */
            long num = nextUniqueNumber.getAndIncrement();
        /默认情况下，代理类的完全限定名为：com.sun.proxy.$Proxy0，com.sun.proxy.$Proxy1……依次递增
            String proxyName = proxyPkg + proxyClassNamePrefix + num;
            /*
             * Generate the specified proxy class.
             */
             /真正的生成代理类的字节码的地方/
           byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces);
            try {
                // 根据二进制字节码返回相应的Class实例
                return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);
            } catch (ClassFormatError e) {
                /*
                 * A ClassFormatError here means that (barring bugs in the
                 * proxy class generation code) there was some other
                 * invalid aspect of the arguments supplied to the proxy
                 * class creation (such as virtual machine limitations
                 * exceeded).
                 */
                throw new IllegalArgumentException(e.toString());
            }
        }
    }

ProxyGenerator是sun.misc包中的类，它没有开源，但是可以反编译来一探究竟：

public static byte[] generateProxyClass(final String var0, Class[] var1) {
    ProxyGenerator var2 = new ProxyGenerator(var0, var1);
    final byte[] var3 = var2.generateClassFile();
    // 这里根据参数配置，决定是否把生成的字节码（.class文件）保存到本地磁盘，我们可以通过把相应的class文件保存到本地，再反编译来看看具体的实现，这样更直观
    if(saveGeneratedFiles) {
        AccessController.doPrivileged(new PrivilegedAction() {
            public Void run() {
                try {
                    FileOutputStream var1 = new FileOutputStream(ProxyGenerator.dotToSlash(var0) + ".class");
                    var1.write(var3);
                    var1.close();
                    return null;
                } catch (IOException var2) {
                    throw new InternalError("I/O exception saving generated file: " + var2);
                }
            }
        });
    }
    return var3;
}

在前面的测试类ProxyTest中，我例子中

//设置为true,会在工程根目录生成$Proxy0.class代理类（com.sun.proxy.$Proxy0.class）
System.getProperties().put(
"sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

反编译$Proxy0.class

package com.sun.proxy;
import com.proxy.test2.HelloTest;
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 HelloTest
{
  private static Method m1;
  private static Method m0;
  private static Method m3;
  private static Method m2;
  public $Proxy0(InvocationHandler paramInvocationHandler)
    throws
  {
    super(paramInvocationHandler);
  }
  public final boolean equals(Object paramObject)
    throws
  {
    try
    {
      return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
    }
    throw new UndeclaredThrowableException(localThrowable);
  }
  public final int hashCode()
    throws
  {
    try
    {
      return ((Integer)this.h.invoke(this, m0, null)).intValue();
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
    }
    throw new UndeclaredThrowableException(localThrowable);
  }
  public final void say(String paramString)
    throws
  {
    try
    {
      this.h.invoke(this, m3, new Object[] { paramString });
      return;
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
    }
    throw new UndeclaredThrowableException(localThrowable);
  }
  public final String toString()
    throws
  {
    try
    {
      return (String)this.h.invoke(this, m2, null);
    }
    catch (RuntimeException localRuntimeException)
    {
      throw localRuntimeException;
    }
    catch (Throwable localThrowable)
    {
    }
    throw new UndeclaredThrowableException(localThrowable);
  }
  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("com.proxy.test2.HelloTest").getMethod("say", new Class[] { Class.forName("java.lang.String") });
      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());
  }
}

可以看到，动态生成的代理类有如下特性：

1. 继承了Proxy类，实现了代理的接口，由于java不能多继承，这里已经继承了Proxy类了，不能再继承其他的类，所以JDK的动态代理不支持对实现类的代理，只支持接口的代理。
2. 提供了一个使用InvocationHandler作为参数的构造方法。
3. 生成静态代码块来初始化接口中方法的Method对象，以及Object类的equals、hashCode、toString方法。
4. 重写了Object类的equals、hashCode、toString，它们都只是简单的调用了InvocationHandler的invoke方法，即可以对其进行特殊的操作，也就是说JDK的动态代理还可以代理上述三个方法。
5. 代理类实现代理接口的say方法中，只是简单的调用了InvocationHandler的invoke方法，我们可以在invoke方法中进行一些特殊操作，甚至不调用实现的方法，直接返回。