Java线程学习笔记（五）资源共享问题

IncreaseClient 中持有一个base，每次调用起increase方法后，返回原先的值，

increase方法自加两次，在check方法中判断返回值和最新值是否相差2.在多线程的环境下就会出错，因为++base不是原子性操作

package com.woxiaoe.study.thread;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * 自加类
 * @author 小e
 *
 * 2010-4-24 下午08:39:19
 */
public class IncreaseClient {
	private int base = 0;

	public IncreaseClient() {
	}
	public int increase(){
		int pre = base;
		++ base;
		Thread.currentThread().yield();
		++ base;
		return pre;
	}
	public boolean check(){
		int pre = increase();
		System.out.println("pre:" + pre + "\t" + "increase:" + base);
		return pre - base == -2?true:false;
	}
	
	public static void main(String[] args) {
		IncreaseClient ic = new IncreaseClient();
		ExecutorService exec = Executors.newCachedThreadPool();
		for(int i = 0; i < 10; i++){
			exec.execute(new IncreaseChecker(ic));
		}
	}
	

}

 

package com.woxiaoe.study.thread;
/**
 * 验证多线程下自加的准确性
 * @author 小e
 *
 * 2010-4-24 下午08:38:14
 */
public class IncreaseChecker extends Thread {
	private IncreaseClient ic;
	
	public IncreaseChecker(IncreaseClient ic) {
		this.ic = ic;
	}



	@Override
	public void run() {
		while(ic.check()){
		}
		System.out.println(Thread.currentThread() + "自增出错");
		System.exit(0);
	}
	
	

}

 解决资源共享的最简单方法极为给会产生冲突的方法加把锁，本例中只需将chenk()方法做如下修改

public synchronized boolean check(){
		int pre = increase();
		System.out.println("pre:" + pre + "\t" + "increase:" + base);
		return increase() - base == -2?true:false;
	}

 另一种方法是显示的给代码加把锁：

public boolean check(){
		lock.lock();
		try{
		int pre = increase();
		System.out.println("pre:" + pre + "\t" + "increase:" + base);
		return pre - base == -2?true:false;
		}finally{
			lock.unlock();
		}
	}

 

大体上当使用synchronized关键字时，需要写的代码量更少，并且用户出现的可能性也会降低，因此通常只在解决特殊问题时，才会显示的Lock对象。Lock对象提供了更细粒度的操作。