Java线程:新特征-原子量

作者: ldsea 分类: 程序生活 发布时间: 2011-08-19 15:44
Java线程:新特征-原子量
 
所谓的原子量即操作变量的操作是“原子的”,该操作不可再分,因此是线程安全的。
 
为何要使用原子变量呢,原因是多个线程对单个变量操作也会引起一些问题。在Java5之前,可以通过volatile、synchronized关键字来解决并发访问的安全问题,但这样太麻烦。
Java5之后,专门提供了用来进行单变量多线程并发安全访问的工具包java.util.concurrent.atomic,其中的类也很简单。
 

下面给出一个反面例子(切勿模仿):
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicLong;

/**
* Java线程:新特征-原子量
*
* @author leizhimin 2009-11-6 9:53:11
*/

public class Test {
        public static void main(String[] args) {
                ExecutorService pool = Executors.newFixedThreadPool(2);
                Runnable t1 = new MyRunnable("张三", 2000);
                Runnable t2 = new MyRunnable("李四", 3600);
                Runnable t3 = new MyRunnable("王五", 2700);
                Runnable t4 = new MyRunnable("老张", 600);
                Runnable t5 = new MyRunnable("老牛", 1300);
                Runnable t6 = new MyRunnable("胖子", 800);
                //执行各个线程
                pool.execute(t1);
                pool.execute(t2);
                pool.execute(t3);
                pool.execute(t4);
                pool.execute(t5);
                pool.execute(t6);
                //关闭线程池
                pool.shutdown();
        }
}

class MyRunnable implements Runnable {
        private static AtomicLong aLong = new AtomicLong(10000);        //原子量,每个线程都可以自由操作
        private String name;                //操作人
        private int x;                            //操作数额

        MyRunnable(String name, int x) {
                this.name = name;
                this.x = x;
        }

        public void run() {
                System.out.println(name + "执行了" + x + ",当前余额:" + aLong.addAndGet(x));
        }
}

 
运行结果:
李四执行了3600,当前余额:13600
王五执行了2700,当前余额:16300
老张执行了600,当前余额:16900
老牛执行了1300,当前余额:18200
胖子执行了800,当前余额:19000
张三执行了2000,当前余额:21000

Process finished with exit code 0

 
张三执行了2000,当前余额:12000
王五执行了2700,当前余额:18300
老张执行了600,当前余额:18900
老牛执行了1300,当前余额:20200
胖子执行了800,当前余额:21000
李四执行了3600,当前余额:15600

Process finished with exit code 0

 
张三执行了2000,当前余额:12000
李四执行了3600,当前余额:15600
老张执行了600,当前余额:18900
老牛执行了1300,当前余额:20200
胖子执行了800,当前余额:21000
王五执行了2700,当前余额:18300

Process finished with exit code 0

 
从运行结果可以看出,虽然使用了原子量,但是程序并发访问还是有问题,那究竟问题出在哪里了?
 
这里要注意的一点是,原子量虽然可以保证单个变量在某一个操作过程的安全,但无法保证你整个代码块,或者整个程序的安全性。因此,通常还应该使用锁等同步机制来控制整个程序的安全性。
 
下面是对这个错误修正:
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.atomic.AtomicLong;

/**
* Java线程:新特征-原子量
*
* @author leizhimin 2009-11-6 9:53:11
*/

public class Test {
        public static void main(String[] args) {
                ExecutorService pool = Executors.newFixedThreadPool(2);
                Lock lock = new ReentrantLock(false);
                Runnable t1 = new MyRunnable("张三", 2000,lock);
                Runnable t2 = new MyRunnable("李四", 3600,lock);
                Runnable t3 = new MyRunnable("王五", 2700,lock);
                Runnable t4 = new MyRunnable("老张", 600,lock);
                Runnable t5 = new MyRunnable("老牛", 1300,lock);
                Runnable t6 = new MyRunnable("胖子", 800,lock);
                //执行各个线程
                pool.execute(t1);
                pool.execute(t2);
                pool.execute(t3);
                pool.execute(t4);
                pool.execute(t5);
                pool.execute(t6);
                //关闭线程池
                pool.shutdown();
        }
}

class MyRunnable implements Runnable {
        private static AtomicLong aLong = new AtomicLong(10000);        //原子量,每个线程都可以自由操作
        private String name;                //操作人
        private int x;                            //操作数额
        private Lock lock;

        MyRunnable(String name, int x,Lock lock) {
                this.name = name;
                this.x = x;
                this.lock = lock;
        }

        public void run() {
                lock.lock();
                System.out.println(name + "执行了" + x + ",当前余额:" + aLong.addAndGet(x));
                lock.unlock();
        }
}

 
执行结果:
张三执行了2000,当前余额:12000
王五执行了2700,当前余额:14700
老张执行了600,当前余额:15300
老牛执行了1300,当前余额:16600
胖子执行了800,当前余额:17400
李四执行了3600,当前余额:21000

Process finished with exit code 0

 
这里使用了一个对象锁,来控制对并发代码的访问。不管运行多少次,执行次序如何,最终余额均为21000,这个结果是正确的。
 
有关原子量的用法很简单,关键是对原子量的认识,原子仅仅是保证变量操作的原子性,但整个程序还需要考虑线程安全的。
 

本文出自 “熔 岩” 博客,请务必保留此出处http://lavasoft.blog.51cto.com/62575/222541

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