Java Dynamic Proxy

本文譯自博文 Understanding Java Dynamic Proxy 

假定，我們需要一個計算機，就把這個介面叫做Calculator吧。

interface Calculator {    public int add(int a, int b);    public int sub(int a, int b);}

有了介面，我們還需要一個真正的計算機實現，就把它叫做CalculatorImpl吧

public class CalculatorImpl implements Calculator {    public int add(int a,int b) {        int c = a + b;        System.out.println("[TARGET METHOD CALL (add)] Result is " + c);        return c;    }        @Override    public int sub(int a, int b) {    int c = a - b;    System.out.println("[TARGET METHOD CALL (sub)] Result is " + c);    return c;    }}

這個計算機的代碼很直觀。

OK，新需求來了，雖然做加法和做減法的核心業務不用需要改變，但是我們希望在進行加減之前能夠列印出日誌。同時，目前已經有其他代碼在利用Calculator介面和CalculatorImpl類了，我們也特麼不想影響這些已有的代碼。咋辦呢？來吧，試試動態代理類

import java.lang.reflect.InvocationHandler;import java.lang.reflect.Method;import java.util.Arrays; public class LoggingValidationHandler implements InvocationHandler{    private Object target;     public LoggingValidationHandler(Object target) {        this.target = target;    }     public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {        System.out.println("[BEFORE METHOD CALL] The method " + method.getName() + "() begins with "+ Arrays.toString(args));          Object result = method.invoke(target, args);         System.out.println("[AFTER METHOD CALL ] The method " + method.getName() + "() ends with " + result.toString());         return result;    }}

import java.lang.reflect.Proxy;public class Clinet4 {public static void main(String[] args) {Calculator plainCalculator = new CalculatorImpl();Calculator proxiedCalculator = (Calculator) Proxy.newProxyInstance(plainCalculator.getClass().getClassLoader(),plainCalculator.getClass().getInterfaces(),new LoggingValidationHandler(plainCalculator));proxiedCalculator.add(10, 6);System.out.println("--------------------------------------------------------------------");proxiedCalculator.sub(10, 6);}}

運行，輸出為

[BEFORE METHOD CALL] The method add() begins with [10, 6][TARGET METHOD CALL (add)] Result is 16[AFTER METHOD CALL ] The method add() ends with 16--------------------------------------------------------------------[BEFORE METHOD CALL] The method sub() begins with [10, 6][TARGET METHOD CALL (sub)] Result is 4[AFTER METHOD CALL ] The method sub() ends with 4

現在來簡單解釋一下。呵呵，道理請見前一篇博文。