具体场景
- 为了使代理类和被代理类对第三方有相同的函数，代理类和被代理类一般实现一个公共的interface，该interface定义如下
public interface Calculator { public Integer add(Integer num1, Integer num2); public Integer minus(Integer num1, Integer num2); }
- 被代理类定义如下
public class CalculatorImpl implements Calculator { @Override public Integer add(Integer num1, Integer num2) { int ret = num1 + num2; System.out.println("in calculatorImpl, res: " + ret); return ret; } @Override public Integer minus(Integer num1, Integer num2) { int ret = num1 - num2; System.out.println("int calculatorImpl, res: " + ret); return ret; } }
- 代理需求：在add函数和minus函数调用前后分别输出before invocation和after invocation字样
静态代理解决方案
- 代码如下：简单直接，无需赘言，如果calculator里边不仅有add和minus，还有divide，product,log,sin…呢，呵呵哒
public class StaticCalculatorProxy implements Calculator { Calculator obj; public StaticCalculatorProxy(Calculator obj) { this.obj = obj; } @Override public Integer add(Integer num1, Integer num2) { System.out.println("in StaticCalculatorProxy, before invocation"); Integer ret = obj.add(num1, num2); System.out.println("in StaticCalculatorProxy, after invocation"); return ret; } @Override public Integer minus(Integer num1, Integer num2) { System.out.println("in StaticCalculatorProxy, before invocation"); Integer ret = obj.minus(num1, num2); System.out.println("in StaticCalculatorProxy, after invocation"); return ret; } }
动态代理解决方案
- 首先编写实现InvocationHandler接口的类，用于请求转发，实现如下
public class CalculatorHandler implements InvocationHandler { private Object obj; //被代理类 public CalculatorHandler(Object obj) { this.obj = obj; } @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.println("in calculatorhandler, before invocation"); Object ret = method.invoke(obj, args); //执行被代理类方法 System.out.println("in calculationhandler, after invocation"); return ret; } }
- 生成动态代理
CalculatorImpl calculatorImpl = new CalculatorImpl();//被代理类 CalculatorHandler calculatorHandler = new CalculatorHandler(calculatorImpl); Calculator calculator = (Calculator) Proxy.newProxyInstance(calculatorImpl.getClass().getClassLoader(), calculatorImpl.getClass().getInterfaces(), calculatorHandler); System.out.println(calculator.add(1,2)); System.out.println(calculator.minus(1, 2));
无论calculator中包含多少函数，动态代理只需实现一次，实际工程中，System.out.println(“in calculatorhandler, before invocation”)可能是加缓存，打日志等操作
动态代理工作原理
- 为了搞清楚动态代理如何工作，首先看看生成的动态代理的代码是什么，借助[1]中ProxyUtil代码
public class ProxyUtils { /** * Save proxy class to path * * @param path path to save proxy class * @param proxyClassName name of proxy class * @param interfaces interfaces of proxy class * @return */ public static boolean saveProxyClass(String path, String proxyClassName, Class[] interfaces) { if (proxyClassName == null || path == null) { return false; } // get byte of proxy class byte[] classFile = ProxyGenerator.generateProxyClass(proxyClassName, interfaces); FileOutputStream out = null; try { out = new FileOutputStream(path); out.write(classFile); out.flush(); return true; } catch (Exception e) { e.printStackTrace(); } finally { try { out.close(); } catch (IOException e) { e.printStackTrace(); } } return false; } }
- 得到了生成的动态代理代码如下：
public final class $Proxy0 extends Proxy implements Calculator { public $Proxy0(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 String toString() { try { return (String)super.h.invoke(this, m2, null); } catch(Error _ex) { } catch(Throwable throwable) { throw new UndeclaredThrowableException(throwable); } } public final Integer minus(Integer integer, Integer integer1) { try { return (Integer)super.h.invoke(this, m4, new Object[] { integer, integer1 }); } catch(Error _ex) { } catch(Throwable throwable) { throw new UndeclaredThrowableException(throwable); } } public final Integer add(Integer integer, Integer integer1) { try { return (Integer)super.h.invoke(this, m3, new Object[] { integer, integer1 }); } 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); } } private static Method m1; private static Method m2; private static Method m4; private static Method m3; private static Method m0; 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]); m4 = Class.forName("com.langrx.mq.Calculator").getMethod("minus", new Class[] { Class.forName("java.lang.Integer"), Class.forName("java.lang.Integer") }); m3 = Class.forName("com.langrx.mq.Calculator").getMethod("add", new Class[] { Class.forName("java.lang.Integer"), Class.forName("java.lang.Integer") }); m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]); } catch(NoSuchMethodException nosuchmethodexception) { throw new NoSuchMethodError(nosuchmethodexception.getMessage()); } catch(ClassNotFoundException classnotfoundexception) { throw new NoClassDefFoundError(classnotfoundexception.getMessage()); } } }
- 有点长，按照初始化顺序慢慢来分析，首先分析静态代码块：
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]); m4 = Class.forName("com.langrx.mq.Calculator").getMethod("minus", new Class[] { Class.forName("java.lang.Integer"), Class.forName("java.lang.Integer") }); m3 = Class.forName("com.langrx.mq.Calculator").getMethod("add", new Class[] { Class.forName("java.lang.Integer"), Class.forName("java.lang.Integer") }); m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);得到公共interface中的add函数和minus函数对应的Method方法，同事也得到了equals，toString，hashCode三个函数的Method，所以调用代理类的equals，toString，hashCode也是要执行被代理类的方法的，知道这点很有必要
- 构造函数
public $Proxy0(InvocationHandler invocationhandler) { super(invocationhandler); }
初始化了内部的InvocationHandler变量，也就是下文的super.h
- 以add为例看一下请求的转发
public final Integer add(Integer integer, Integer integer1) { try { return (Integer)super.h.invoke(this, m3, new Object[] { integer, integer1 }); } catch(Error _ex) { } catch(Throwable throwable) { throw new UndeclaredThrowableException(throwable); } }
super.h.invoke就是invocationhandler.invoke就是传入的CalculatorHandler中实现的
@Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.println("in calculatorhandler, before invocation"); Object ret = method.invoke(obj, args); //执行被代理类方法 System.out.println("in calculationhandler, after invocation"); return ret; }
最终执行的就是CalculatorHandler对应的invoke函数
总结
Calculator calculator = (Calculator) Proxy.newProxyInstance(calculatorImpl.getClass().getClassLoader(), calculatorImpl.getClass().getInterfaces(), calculatorHandler);
生成动态代理的过程步骤如下[2]：
// InvocationHandlerImpl 实现了 InvocationHandler 接口，并能实现方法调用从代理类到委托类的分派转发 // 其内部通常包含指向委托类实例的引用，用于真正执行分派转发过来的方法调用 InvocationHandler handler = new InvocationHandlerImpl(..); // 通过 Proxy 为包括 Interface 接口在内的一组接口动态创建代理类的类对象 Class clazz = Proxy.getProxyClass(classLoader, new Class[] { Interface.class, ... }); // 通过反射从生成的类对象获得构造函数对象 Constructor constructor = clazz.getConstructor(new Class[] { InvocationHandler.class }); // 通过构造函数对象创建动态代理类实例 Interface Proxy = (Interface)constructor.newInstance(new Object[] { handler });
Proxy.newProxyInstance帮我们做了2，3，4步，直接返回给我们一个动态代理对象，代理对象最终执行InvocationHandler中invoke函数。顺便强推文章[2]