JAVA动态代理

1、简单的demo

public interface HelloService {
    /**
     * 目标方法
     */
    public abstract void add(String str);
}

public class HelloServiceImpl implements HelloService {
    /*
     * @see dynamic.proxy.UserService#add()
     */
    public void add(String str) {
        System.out.println("==>"+str);
    }
}

//代理类
public class HelloServiceProxy implements InvocationHandler {
    // 目标对象
    private Object target;

    /**
     * 执行目标对象的方法
     */
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

        // 在目标对象的方法执行之前简单的打印一下
        System.out.println("------------------before------------------");

        // 执行目标对象的方法
        Object result = method.invoke(target, args);

        // 在目标对象的方法执行之后简单的打印一下
        System.out.println("-------------------after------------------");

        return result;
    }

    /**
     * 获取目标对象的代理对象
     * @param target 目标对象
     * @return 代理对象
     */
    public Object getProxy(Object target) {
        this.target = target;
        return Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(),
                target.getClass().getInterfaces(), this);
    }
}

测试代码

public class Test{
    public static void main(String[] args) {
        HelloService service = new HelloServiceImpl();

        HelloServiceProxy proxy = new HelloServiceProxy();
        //绑定代理对象。
        service = (HelloService) proxy.getProxy(service);
        //这里service经过绑定，就会进入invoke方法里面了。
        service.add("hello");
    }
}

执行结果:
——————before——————
==>hello
——————-after——————

2、代码分析

//Proxy.java
//通过反射 获取代理类HelloServiceImpl的实例
public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
        Objects.requireNonNull(h);
        //克隆一份被代理目标类所实现所有接口的 Class对象数组  
        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.
         */
         //拿到代理类的Class 主要的方法
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
            //拿到代理类的Constructor对象，以便用于构造代理对象 
            final Constructor<?> cons = cl.getConstructor(constructorParams);
            //h为InvocationHandler的实现类HelloServiceProxy
            final InvocationHandler ih = h;
            if (!Modifier.isPublic(cl.getModifiers())) {
                AccessController.doPrivileged(new PrivilegedAction<Void>() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            ////返回构造的代理类对象,这里留个意,传入的是 实现了InvocationHandler接口的对象 
            return cons.newInstance(new Object[]{h});
        } catch (IllegalAccessException|InstantiationException e) {
            throw new InternalError(e.toString(), e);
        } catch (InvocationTargetException e) {
            Throwable t = e.getCause();
            if (t instanceof RuntimeException) {
                throw (RuntimeException) t;
            } else {
                throw new InternalError(t.toString(), t);
            }
        } catch (NoSuchMethodException e) {
            throw new InternalError(e.toString(), e);
        }
    }

主要是跟getProxyClass0的代码，

//主要通过这两个方法获取HelloServiceImpl的Class对象
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);

return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);

详见：http://blog.youkuaiyun.com/scplove/article/details/52451899

3、实际的调用过程

通过ProxyGeneratorUtils生成HelloServiceImpl的class文件并保持在磁盘上

public class ProxyGeneratorUtils {
    /**
     * 把代理类的字节码写到硬盘上
     */
    public static void writeProxyClassToHardDisk(String path) {

        // 获取代理类的字节码  方法的第二个参数为被代理类所实现的全部接口
        byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy11", HelloServiceImpl.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();
            }
        }
    }
}

用idea可以查看HelloServiceImpl生成的class文件

public final class $Proxy11 extends Proxy implements HelloService {
    private static Method m1;
    private static Method m2;
    private static Method m3;
    private static Method m0;

    public $Proxy11(InvocationHandler var1) throws  {
        super(var1);
    }

    public final boolean equals(Object var1) throws  {
        try {
            return ((Boolean)super.h.invoke(this, m1, new Object[]{var1})).booleanValue();
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final String toString() throws  {
        try {
            return (String)super.h.invoke(this, m2, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final void add(String var1) throws  {
        try {
            //h为HelloServiceProxy的实例 直接调用HelloServiceProxy的invoke方法。
            //在HelloServiceProxy的invoke中，通过反射调用HelloServiceImpl的add方法
            super.h.invoke(this, m3, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final int hashCode() throws  {
        try {
            return ((Integer)super.h.invoke(this, m0, (Object[])null)).intValue();
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[]{Class.forName("java.lang.Object")});
            m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
            //通过反射获取add的Method实例
            m3 = Class.forName("com.company.HelloService").getMethod("add", new Class[]{Class.forName("java.lang.String")});
            m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

总结：
主要步骤为
1、通过ProxyGenerator.generateProxyClass 获取HelloServiceImpl的class对象并使用反射实例化，还会获得add的Method对象
2、调用HelloServiceImpl通过反射得到的实例的add方法，在add方法中会调用HelloServiceProxy的invoke方法。最终通过反射调用HelloServiceImpl的add方法