Java 设计模式：代理模式

代理模式三种角色

角色	描述
接口	规定具体方法
委托类	实现接口，完成具体的业务逻辑
代理类	实现接口，在方法里面调用委托类的方法，自己不实现核心业务， 在调用委托类的方法前后可以执行其他操作，也就是增强

代理模式的一个重要的特征就是委托类和代理类实现了相同的接口，所以对于调用者来说除了增强之外委托类和代理类没有什么不同，这也是和适配器模式的重要区别。

静态代理

静态代理是在编译时期就将接口，委托类，代理类全部编码，也就是在运行之前所有的 class 文件已经存在。

接口

public interface Service {

    int increaseOne(int num);

    String toUpperCase(String str);
}

接口实现类（委托类）

public class ServiceImpl implements Service {

    @Override
    public int increaseOne(int num) {
        return ++num;
    }

    @Override
    public String toUpperCase(String str) {
        return str.toUpperCase();
    }
}

代理类

public class ServiceProxy implements Service{

    private Service service;

    public ServiceProxy(Service service) {
        this.service = service;
    }

    @Override
    public int increaseOne(int num) {
        System.out.println("将要执行increaseOne");
        int i = service.increaseOne(num);
        System.out.println("increaseOne执行完毕");
        return i;
    }

    @Override
    public String toUpperCase(String str) {
        return service.toUpperCase(str);
    }
}

测试

public class MainTest {

    public static void main(String[] args) {

        Service service = new ServiceImpl();
        ServiceProxy serviceProxy = new ServiceProxy(service);
        /**
         * 在执行真正 increaseOne方法之前或者之后可以执行一系列操作
         * 这里用sout模拟
         */
        System.out.println(serviceProxy.increaseOne(5));

        /**
         * 调用真正的 toUpperCase方法
         */
        System.out.println(serviceProxy.toUpperCase("hello world"));
    }
}

输出

将要执行increaseOne
increaseOne执行完毕
6
HELLO WORLD

JDK动态代理

JDK动态代理简化了静态代理的繁琐操作，不需要人工硬编码代理类。

实现 InvocationHandler 接口

public class DynamicProxyHandler implements InvocationHandler {

    private Object target;

    public DynamicProxyHandler(Object target) {
        this.target = target;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Object result;
        if ("increaseOne".equals(method.getName())) {
            System.out.println("将要执行increaseOne");
            result = method.invoke(target, args);
            System.out.println("increaseOne执行完毕");
        } else {
            result = method.invoke(target, args);
        }

        return result;
    }

测试

public class MainTest {

    public static void main(String[] args) {

        Service service = new ServiceImpl();

        DynamicProxyHandler dynamicProxyHandler = new DynamicProxyHandler(service);

        Service serviceProxy = (Service) Proxy.newProxyInstance(
                Thread.currentThread().getContextClassLoader(),
                service.getClass().getInterfaces(), dynamicProxyHandler);

        System.out.println(serviceProxy.increaseOne(5));
        System.out.println(serviceProxy.toUpperCase("hello world"));
    }
}

输出

将要执行increaseOne
increaseOne执行完毕
6
HELLO WORLD

静态代理的实现方式

代理类实现接口，实例化代理类，代理类的成员变量引用委托类对象，代理类调用方法，方法内部委托类对象调用方法。

JDK动态代理的实现方式

与静态代理的过程是一样的，但是代理类编码是由 JVM 完成，我们只需要关注方法的实现细节。

实现InvocationHandler接口

先看看InvocationHandler接口：

public interface InvocationHandler {
  public Object invoke(Object proxy, Method method, Object[] args)
      throws Throwable;
}

只有invoke方法，三个参数：

类型	参数	说明
Object	proxy	自动生成的代理对象，后面详细说
Method	method	调用的方法
Object[]	args	调用的方法的参数

method.invoke(target, args)使用反射让委托对象调用方法，target就是委托对象

public class DynamicProxyHandler implements InvocationHandler {

  private Object target;
  
  public DynamicProxyHandler(Object target) {
      this.target = target;
  }
  
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      Object result;
      result = method.invoke(target, args);
      return result;
  }
}

生成代理对象

通过Proxy类的静态方法newProxyInstance

public static Object newProxyInstance(ClassLoader loader,
                                        Class<?>[] interfaces,
                                        InvocationHandler h)

newProxyInstance需要三个参数：

类型	参数	说明
ClassLoader	loadery	类加载器
Class<?>	interfaces	代理类代理的接口，相当于示例中的Service接口
InvocationHandler	h	回调对象，代理方法的具体实现细节在这里完成

具体过程

1. 调用Proxy.newProxyInstance(ClassLoader, Class<?>[] , InvocationHandler)生成代理类

 public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
        Objects.requireNonNull(h);

        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.
         */
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }

            final Constructor<?> cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            if (!Modifier.isPublic(cl.getModifiers())) {
                AccessController.doPrivileged(new PrivilegedAction<Void>() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            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);
        }
    }

重要的有俩步：

• Class<?> cl = getProxyClass0(loader, intfs)根据类加载器和接口参数会生成代理类的字节码，
• return cons.newInstance(new Object[]{h})返回生成代理类的实例

来看看生成的代理类（在内存中生成 .class 文件，需要先输出保存，再反编译）：

public final class $ServiceProxy extends Proxy implements Service {
    private static Method m1;
    private static Method m2;
    private static Method m3;
    private static Method m4;
    private static Method m0;

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

    public final boolean equals(Object var1) throws  {
        try {
            return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
        } 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 String toUpperCase(String var1) throws  {
        try {
            return (String)super.h.invoke(this, m3, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final int increaseOne(int var1) throws  {
        try {
            return (Integer)super.h.invoke(this, m4, 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);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m3 = Class.forName("com.Service").getMethod("toUpperCase", Class.forName("java.lang.String"));
            m4 = Class.forName("com.Service").getMethod("increaseOne", Integer.TYPE);
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

生成的代理类实现l了Service接口，这和静态代理的代码类似，但是这里还继承了Proxy 类，构造函数接受InvocationHandler实现类对象，然后调用Proxy的构造函数，把对象传给Proxy的InvocationHandler类型成员变量，示例中获取代理对象后有个强制转换类型，之前一直有疑问为什么不会报错，直到反编译代理类后看到实现了Service接口才明白。

2. 代理类对象调用接口方法

继续看代理类，例如调用increaseOne方法：

public final int increaseOne(int var1) throws  {
    try {
        return (Integer)super.h.invoke(this, m4, new Object[]{var1});
    } catch (RuntimeException | Error var3) {
        throw var3;
    } catch (Throwable var4) {
        throw new UndeclaredThrowableException(var4);
    }
}

其实最终调用的就是我们传入的InvocationHandler对象的invoke方法，this和new Object[]{var1}可以理解，但是m4什么时候生成的呢？

代理类有个static代码块：

static {
    try {
        m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
        m2 = Class.forName("java.lang.Object").getMethod("toString");
        m3 = Class.forName("com.Service").getMethod("toUpperCase", Class.forName("java.lang.String"));
        m4 = Class.forName("com.Service").getMethod("increaseOne", Integer.TYPE);
        m0 = Class.forName("java.lang.Object").getMethod("hashCode");
    } catch (NoSuchMethodException var2) {
        throw new NoSuchMethodError(var2.getMessage());
    } catch (ClassNotFoundException var3) {
        throw new NoClassDefFoundError(var3.getMessage());
    }
}

可以看到m3和m4是Service接口方法的引用，其他的三个方法引用是Object类的equals，toString以及hashCode方法，而且这三个方法里面也调用了invoke方法，所以不能在invoke方法里面输出代理对象，这相当于循环调用toString()。

CGLIB动态代理

CGLIB 代理的是类，不同于JDK只能代理接口，这是因为 Java 不支持多继承，JDK 动态代理类已经继承了 Proxy类。

委托类

public class DelegateClass {

    public int increaseOne(int num) {
        return ++num;
    }

    public String toUpperCase(String str) {
        return str.toUpperCase();
    }
}

代理类

public class CGLIBProxy implements MethodInterceptor {

  private Object target;

  public CGLIBProxy(Object target) {
      this.target = target;
  }

  @Override
  public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
      Object result;
      if ("increaseOne".equals(method.getName())) {
          System.out.println("将要执行increaseOne");
          result = method.invoke(target, args);
          System.out.println("increaseOne执行完毕");
      } else {
          result = method.invoke(target, args);
      }
      return result;
  }
}

测试类

public class MainTest {

    public static void main(String[] args) {

        DelegateClass delegateClass = new DelegateClass();
        MethodInterceptor methodInterceptor = new CGLIBProxy(delegateClass);

        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(DelegateClass.class);
        enhancer.setCallback(methodInterceptor);
        DelegateClass cglibProxy = (DelegateClass) enhancer.create();

        System.out.println(cglibProxy.increaseOne(5));
        System.out.println(cglibProxy.toUpperCase("hello world"));
    }
}

自动生成的代理类

public class DelegateClass$$EnhancerByCGLIB$$50d8bc59 extends DelegateClass implements Factory {
    private boolean CGLIB$BOUND;
    public static Object CGLIB$FACTORY_DATA;
    private static final ThreadLocal CGLIB$THREAD_CALLBACKS;
    private static final Callback[] CGLIB$STATIC_CALLBACKS;
    private MethodInterceptor CGLIB$CALLBACK_0;
    private static Object CGLIB$CALLBACK_FILTER;
    private static final Method CGLIB$toUpperCase$0$Method;
    private static final MethodProxy CGLIB$toUpperCase$0$Proxy;
    private static final Object[] CGLIB$emptyArgs;
    private static final Method CGLIB$increaseOne$1$Method;
    private static final MethodProxy CGLIB$increaseOne$1$Proxy;
    private static final Method CGLIB$equals$2$Method;
    private static final MethodProxy CGLIB$equals$2$Proxy;
    private static final Method CGLIB$toString$3$Method;
    private static final MethodProxy CGLIB$toString$3$Proxy;
    private static final Method CGLIB$hashCode$4$Method;
    private static final MethodProxy CGLIB$hashCode$4$Proxy;
    private static final Method CGLIB$clone$5$Method;
    private static final MethodProxy CGLIB$clone$5$Proxy;

    static void CGLIB$STATICHOOK1() {
        CGLIB$THREAD_CALLBACKS = new ThreadLocal();
        CGLIB$emptyArgs = new Object[0];
        Class var0 = Class.forName("com.cglibproxy.DelegateClass$$EnhancerByCGLIB$$50d8bc59");
        Class var1;
        Method[] var10000 = ReflectUtils.findMethods(new String[]{"equals", "(Ljava/lang/Object;)Z", "toString", "()Ljava/lang/String;", "hashCode", "()I", "clone", "()Ljava/lang/Object;"}, (var1 = Class.forName("java.lang.Object")).getDeclaredMethods());
        CGLIB$equals$2$Method = var10000[0];
        CGLIB$equals$2$Proxy = MethodProxy.create(var1, var0, "(Ljava/lang/Object;)Z", "equals", "CGLIB$equals$2");
        CGLIB$toString$3$Method = var10000[1];
        CGLIB$toString$3$Proxy = MethodProxy.create(var1, var0, "()Ljava/lang/String;", "toString", "CGLIB$toString$3");
        CGLIB$hashCode$4$Method = var10000[2];
        CGLIB$hashCode$4$Proxy = MethodProxy.create(var1, var0, "()I", "hashCode", "CGLIB$hashCode$4");
        CGLIB$clone$5$Method = var10000[3];
        CGLIB$clone$5$Proxy = MethodProxy.create(var1, var0, "()Ljava/lang/Object;", "clone", "CGLIB$clone$5");
        var10000 = ReflectUtils.findMethods(new String[]{"toUpperCase", "(Ljava/lang/String;)Ljava/lang/String;", "increaseOne", "(I)I"}, (var1 = Class.forName("com.cglibproxy.DelegateClass")).getDeclaredMethods());
        CGLIB$toUpperCase$0$Method = var10000[0];
        CGLIB$toUpperCase$0$Proxy = MethodProxy.create(var1, var0, "(Ljava/lang/String;)Ljava/lang/String;", "toUpperCase", "CGLIB$toUpperCase$0");
        CGLIB$increaseOne$1$Method = var10000[1];
        CGLIB$increaseOne$1$Proxy = MethodProxy.create(var1, var0, "(I)I", "increaseOne", "CGLIB$increaseOne$1");
    }

    final String CGLIB$toUpperCase$0(String var1) {
        return super.toUpperCase(var1);
    }

    public final String toUpperCase(String var1) {
        MethodInterceptor var10000 = this.CGLIB$CALLBACK_0;
        if (this.CGLIB$CALLBACK_0 == null) {
            CGLIB$BIND_CALLBACKS(this);
            var10000 = this.CGLIB$CALLBACK_0;
        }

        return var10000 != null ? (String)var10000.intercept(this, CGLIB$toUpperCase$0$Method, new Object[]{var1}, CGLIB$toUpperCase$0$Proxy) : super.toUpperCase(var1);
    }

    final int CGLIB$increaseOne$1(int var1) {
        return super.increaseOne(var1);
    }

    public final int increaseOne(int var1) {
        MethodInterceptor var10000 = this.CGLIB$CALLBACK_0;
        if (this.CGLIB$CALLBACK_0 == null) {
            CGLIB$BIND_CALLBACKS(this);
            var10000 = this.CGLIB$CALLBACK_0;
        }

        if (var10000 != null) {
            Object var2 = var10000.intercept(this, CGLIB$increaseOne$1$Method, new Object[]{new Integer(var1)}, CGLIB$increaseOne$1$Proxy);
            return var2 == null ? 0 : ((Number)var2).intValue();
        } else {
            return super.increaseOne(var1);
        }
    }

    final boolean CGLIB$equals$2(Object var1) {
        return super.equals(var1);
    }

    public final boolean equals(Object var1) {
        MethodInterceptor var10000 = this.CGLIB$CALLBACK_0;
        if (this.CGLIB$CALLBACK_0 == null) {
            CGLIB$BIND_CALLBACKS(this);
            var10000 = this.CGLIB$CALLBACK_0;
        }

        if (var10000 != null) {
            Object var2 = var10000.intercept(this, CGLIB$equals$2$Method, new Object[]{var1}, CGLIB$equals$2$Proxy);
            return var2 == null ? false : (Boolean)var2;
        } else {
            return super.equals(var1);
        }
    }

    final String CGLIB$toString$3() {
        return super.toString();
    }

    public final String toString() {
        MethodInterceptor var10000 = this.CGLIB$CALLBACK_0;
        if (this.CGLIB$CALLBACK_0 == null) {
            CGLIB$BIND_CALLBACKS(this);
            var10000 = this.CGLIB$CALLBACK_0;
        }

        return var10000 != null ? (String)var10000.intercept(this, CGLIB$toString$3$Method, CGLIB$emptyArgs, CGLIB$toString$3$Proxy) : super.toString();
    }

    final int CGLIB$hashCode$4() {
        return super.hashCode();
    }

    public final int hashCode() {
        MethodInterceptor var10000 = this.CGLIB$CALLBACK_0;
        if (this.CGLIB$CALLBACK_0 == null) {
            CGLIB$BIND_CALLBACKS(this);
            var10000 = this.CGLIB$CALLBACK_0;
        }

        if (var10000 != null) {
            Object var1 = var10000.intercept(this, CGLIB$hashCode$4$Method, CGLIB$emptyArgs, CGLIB$hashCode$4$Proxy);
            return var1 == null ? 0 : ((Number)var1).intValue();
        } else {
            return super.hashCode();
        }
    }

    final Object CGLIB$clone$5() throws CloneNotSupportedException {
        return super.clone();
    }

    protected final Object clone() throws CloneNotSupportedException {
        MethodInterceptor var10000 = this.CGLIB$CALLBACK_0;
        if (this.CGLIB$CALLBACK_0 == null) {
            CGLIB$BIND_CALLBACKS(this);
            var10000 = this.CGLIB$CALLBACK_0;
        }

        return var10000 != null ? var10000.intercept(this, CGLIB$clone$5$Method, CGLIB$emptyArgs, CGLIB$clone$5$Proxy) : super.clone();
    }

    public static MethodProxy CGLIB$findMethodProxy(Signature var0) {
        String var10000 = var0.toString();
        switch(var10000.hashCode()) {
        case -1961280283:
            if (var10000.equals("increaseOne(I)I")) {
                return CGLIB$increaseOne$1$Proxy;
            }
            break;
        case -508378822:
            if (var10000.equals("clone()Ljava/lang/Object;")) {
                return CGLIB$clone$5$Proxy;
            }
            break;
        case 893465080:
            if (var10000.equals("toUpperCase(Ljava/lang/String;)Ljava/lang/String;")) {
                return CGLIB$toUpperCase$0$Proxy;
            }
            break;
        case 1826985398:
            if (var10000.equals("equals(Ljava/lang/Object;)Z")) {
                return CGLIB$equals$2$Proxy;
            }
            break;
        case 1913648695:
            if (var10000.equals("toString()Ljava/lang/String;")) {
                return CGLIB$toString$3$Proxy;
            }
            break;
        case 1984935277:
            if (var10000.equals("hashCode()I")) {
                return CGLIB$hashCode$4$Proxy;
            }
        }

        return null;
    }

    public DelegateClass$$EnhancerByCGLIB$$50d8bc59() {
        CGLIB$BIND_CALLBACKS(this);
    }

    public static void CGLIB$SET_THREAD_CALLBACKS(Callback[] var0) {
        CGLIB$THREAD_CALLBACKS.set(var0);
    }

    public static void CGLIB$SET_STATIC_CALLBACKS(Callback[] var0) {
        CGLIB$STATIC_CALLBACKS = var0;
    }

    private static final void CGLIB$BIND_CALLBACKS(Object var0) {
        DelegateClass$$EnhancerByCGLIB$$50d8bc59 var1 = (DelegateClass$$EnhancerByCGLIB$$50d8bc59)var0;
        if (!var1.CGLIB$BOUND) {
            var1.CGLIB$BOUND = true;
            Object var10000 = CGLIB$THREAD_CALLBACKS.get();
            if (var10000 == null) {
                var10000 = CGLIB$STATIC_CALLBACKS;
                if (CGLIB$STATIC_CALLBACKS == null) {
                    return;
                }
            }

            var1.CGLIB$CALLBACK_0 = (MethodInterceptor)((Callback[])var10000)[0];
        }

    }

    public Object newInstance(Callback[] var1) {
        CGLIB$SET_THREAD_CALLBACKS(var1);
        DelegateClass$$EnhancerByCGLIB$$50d8bc59 var10000 = new DelegateClass$$EnhancerByCGLIB$$50d8bc59();
        CGLIB$SET_THREAD_CALLBACKS((Callback[])null);
        return var10000;
    }

    public Object newInstance(Callback var1) {
        CGLIB$SET_THREAD_CALLBACKS(new Callback[]{var1});
        DelegateClass$$EnhancerByCGLIB$$50d8bc59 var10000 = new DelegateClass$$EnhancerByCGLIB$$50d8bc59();
        CGLIB$SET_THREAD_CALLBACKS((Callback[])null);
        return var10000;
    }

    public Object newInstance(Class[] var1, Object[] var2, Callback[] var3) {
        CGLIB$SET_THREAD_CALLBACKS(var3);
        DelegateClass$$EnhancerByCGLIB$$50d8bc59 var10000 = new DelegateClass$$EnhancerByCGLIB$$50d8bc59;
        switch(var1.length) {
        case 0:
            var10000.<init>();
            CGLIB$SET_THREAD_CALLBACKS((Callback[])null);
            return var10000;
        default:
            throw new IllegalArgumentException("Constructor not found");
        }
    }

    public Callback getCallback(int var1) {
        CGLIB$BIND_CALLBACKS(this);
        MethodInterceptor var10000;
        switch(var1) {
        case 0:
            var10000 = this.CGLIB$CALLBACK_0;
            break;
        default:
            var10000 = null;
        }

        return var10000;
    }

    public void setCallback(int var1, Callback var2) {
        switch(var1) {
        case 0:
            this.CGLIB$CALLBACK_0 = (MethodInterceptor)var2;
        default:
        }
    }

    public Callback[] getCallbacks() {
        CGLIB$BIND_CALLBACKS(this);
        return new Callback[]{this.CGLIB$CALLBACK_0};
    }

    public void setCallbacks(Callback[] var1) {
        this.CGLIB$CALLBACK_0 = (MethodInterceptor)var1[0];
    }

    static {
        CGLIB$STATICHOOK1();
    }
}

类继承了委托类DelegateClass，测试类代码中一行代码

DelegateClass cglibProxy = (DelegateClass) enhancer.create();

cglibProxy引用的就是DelegateClass$$EnhancerByCGLIB$$50d8bc59的实例，属于子类强制转父类，除了代理increaseOne和toUpperCase方法外，还有toString，equal，hashCode，clone方法也被代理，代理类的方法会判断是否有回调对象，也就是MethodInterceptor的实例，有的话调用回调对象的intercept方法，否则使用super.increaseOne(var1)调用委托类的方法，没有增强处理。