Java基础之线程原子量

Posted 2020-08-06 潇湘剑雨

    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

package unit_fifteen;
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
* 
*/ 
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));
        } 
}

上面是个反例，代码的结果是多变的

注意：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

package unit_fifteen;
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
* 
*/ 
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(); 
        } 
}