关于java线程的问题

[moonaa]

package thread_test;

class Accum1 {
	private static Accum1 a = new Accum1();
	private int counter = 0;

	private Accum1() {
	};

	public static Accum1 getAccum1() {
		return a;
	}

	public void updateCounter(int add) {
		counter += add;
	}

	public int getCount() {
		return counter;
	}
}

class ThreadOne implements Runnable {

	Accum1 a = Accum1.getAccum1();

	@Override
	public void run() {
		// TODO Auto-generated method stub
		for (int i = 0; i < 98; i++) {
			a.updateCounter(1000);
			try {
				Thread.sleep(50);
			} catch (InterruptedException ex) {
			}
		}
		System.out.println("one " + a.getCount());
	}

}

class ThreadTwo implements Runnable {

	Accum1 a = Accum1.getAccum1();

	@Override
	public void run() {
		// TODO Auto-generated method stub
		for (int i = 0; i < 99; i++) {
			a.updateCounter(1);
			try {
				Thread.sleep(50);
			} catch (InterruptedException ex) {
			}
		}
		System.out.println("two " + a.getCount());
	}

}

public class Accum {
	public static void main(String[] args) {
		ThreadOne t1=new ThreadOne();
		ThreadTwo t2=new ThreadTwo();
		Thread one=new Thread(t1);
		Thread two=new Thread(t2);
		one.start();
		two.start();
	}
}

上面是书上的代码，书上说是不会出现脏数据的情况，但是我自己敲出了试了下，为什么会有脏数据的现象，应该怎么改，代码才能对

[武小凡]

要加线程锁。具体百度吧。

[wangxizzz]

首先我觉得楼主的代码是不存在线程安全性问题的，也就是说不会发生数据脏读的现象。

[zhangpf的代码人生]

增加一个Object变量，给update方法加一把锁，就能防止脏数据了 class Accum1 { private static Accum1 a = new Accum1(); private int counter = 0; private Object obj = new Object();//对象 private Accum1() { }; public static Accum1 getAccum1() { return a; } public void updateCounter(int add) { synchronized (obj) {//加锁 counter += add; } } public int getCount() { return counter; } }

[maradona1984]

引用 2 楼 qq_36387730 的回复:

首先我觉得楼主的代码是不存在线程安全性问题的，也就是说不会发生数据脏读的现象。

会有的

[maradona1984]

private int counter = 0; 换成 private AtomicInteger counter = new AtomicInteger(); updateCounter改一下就行了

[txwh_2169]

两个线程同时对一个对象操作，会有线程安全问题，加锁