java中Atomic类之AtomicBoolean

在java.util.concurrent.atomic包下,有AtomicBoolean , AtomicInteger, AtomicLong, AtomicReference等类,它们的基本特性就是在多线程环境下,执行这些类实例包含的方法时,具有排他性,即当某个线程进入方法,执行其中的指令时,不会被其他线程打断,而别的线程就像自旋锁一样,一直等到该方法执行完成,才由JVM从等待队列中选择一个另一个线程进入。

举例说明

以AtomicBoolean为例,单线程执行普通的方法(如下),不会出现线程问题:

package com.secbro.test.atomic;

/**
 * @author zhuzhisheng
 * @Description
 * @date on 2016/5/26.
 */
public class NormalBoolean implements Runnable{

    public static boolean exits = false;

    private String name;

    public NormalBoolean(String name){
        this.name = name;
    }


    @Override
    public void run() {
        if(!exits){
            exits = true;
            System.out.println(name + ",step 1");
            System.out.println(name + ",step 2");
            System.out.println(name + ",step 3");
            exits = false;
        } else {
            System.out.println(name + ",step else");
        }
    }

    public static void main(String[] args) {
        new NormalBoolean("张三").run();
    }
}

然而,当多线程执行时,就会出现在执行判断之后的命令时,会有其他线程插入,变更exits的值。如下段代码:

package com.secbro.test.atomic;

/**
 * @author zhuzhisheng
 * @Description
 * @date on 2016/5/26.
 */
public class NormalBoolean2 implements Runnable{


    public static boolean exits = false;

    private String name;

    public NormalBoolean2(String name) {
        this.name = name;
    }

    @Override
    public void run() {

        if(!exits){
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            exits = true;
            System.out.println(name + ",step 1");

            System.out.println(name + ",step 2");

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(name + ",step 3");
            exits = false;
        } else {
            System.out.println(name + ",step else");
        }

    }
}

同时执行两线程,打印结果为:

张三,step 1
李四,step 1
张三,step 2
李四,step 2
张三,step 3
李四,step 3

现在,使用AtomicBoolean就可以确保多线程的情况下安全的运行,只有一个线程进行业务处理。

package com.secbro.test.atomic;

import java.util.concurrent.atomic.AtomicBoolean;

/**
 * @author zhuzhisheng
 * @Description
 * @date on 2016/5/26.
 */
public class TestAtomicBoolean implements Runnable{

    public static AtomicBoolean exits = new AtomicBoolean(false);

    private String name;

    public TestAtomicBoolean(String name) {
        this.name = name;
    }

    @Override
    public void run() {

        if(exits.compareAndSet(false,true)){
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(name + ",step 1");

            System.out.println(name + ",step 2");

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(name + ",step 3");
            exits.set(false);
        } else {
            System.out.println(name + ",step else");
        }

    }
}

当两个线程执行此类时,打印结果如下:

张三,step else
李四,step 1
李四,step 2
李四,step 3

测试类:

package com.secbro.test.atomic;

/**
 * @author zhuzhisheng
 * @Description
 * @date on 2016/5/26.
 */
public class TestBoolean {
    public static void main(String[] args) {

        Thread thread1 = new Thread(new NormalBoolean2("李四"));
        Thread thread2 = new Thread(new NormalBoolean2("张三"));

        thread1.start();
        thread2.start();

        //-------------------------------------------------------

        Thread thread3 = new Thread(new TestAtomicBoolean("李四"));
        Thread thread4 = new Thread(new TestAtomicBoolean("张三"));

        thread3.start();
        thread4.start();
    }
}

参考文章:http://thomaschen2011.iteye.com/blog/1468085

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