如果println函数的参数为常量则不会出现线程并发问题,但是如果参数为表达式形式,则JVM在执行println函数的时候会分为几步来执行,从而造成并发问题。
如下例子所示:
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("老朱", 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 data; //操作数 MyRunnable(String name, int data) { this.name = name; this.data = data; } public void run() { System.out.println(name + "执行了" + data + ",当前余额:" + aLong.addAndGet(data)); } }
执行的结果为:
张三执行了2000,当前余额:12000
李四执行了3600,当前余额:15600
王五执行了2700,当前余额:18300
老牛执行了1300,当前余额:20200
老朱执行了800,当前余额:21000
老张执行了600,当前余额:18900
通过反编译,上面println函数的主要处理过程分为下面三步:
long l = aLong.addAndGet(data); String sb =new StringBuilder(String.valueOf(name))).append("执行了").append(data).append(",当前余额:").append(l); synchronized (this) { print(sb.toString); newLine(); }
所以可以看出上面程序的执行顺序为:、
张三执行完->李四执行完->王五执行完->老张只执行到加操作,被老牛打断->老牛执行完->老朱执行完->老张执行剩下的输出操作
要想有一个正常的输出顺序,应该修改代码为:
import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.atomic.AtomicLong; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; 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("老朱", 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 data; //操作数 private Lock lock; MyRunnable(String name, int data, Lock lock) { this.name = name; this.data = data; this.lock = lock; } public void run() { lock.lock(); System.out.println(name + "执行了" + data + ",当前余额:" + aLong.addAndGet(data)); lock.unlock(); } }
结果为:
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
老朱执行了800,当前余额:17400
李四执行了3600,当前余额:21000
【注意】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。