学步园

本文译自博文 Understanding Java Dynamic Proxy 

假定，我们需要一个计算器，就把这个接口叫做Calculator吧。

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

有了接口，我们还需要一个真正的计算器实现，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;
    }
}

这个计算器的代码很直观。

OK，新需求来了，虽然做加法和做减法的核心业务不用需要改变，但是我们希望在进行加减之前能够打印出日志。同时，目前已经有其他代码在利用Calculator接口和CalculatorImpl类了，我们也特么不想影响这些已有的代码。咋办呢？来吧，试试动态代理类

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);		
	}
}

运行，输出为

[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

现在来简单解释一下。呵呵，道理请见前一篇博文。