Volatile in Java does not guarantee thread safety (example explained)

Volatile in Java does not guarantee thread safety (example explained)

Reprinted 2017-09-04 Think-007 I want to comment

The following small series to bring you a piece of Java in the volatile can not guarantee thread safety (the example explained). Small series feel very good, now share to everyone, also for everyone to make a reference. Follow the small part together to look at it today hit the code to see if the Java volatile keyword can guarantee thread safety, after practice, volatile is not guaranteed to be thread-safe, it just guarantees the visibility of the data, no more cache, each thread is read from main memory data, Instead of reading the data from the cache, attach the code as follows, and when the synchronized is removed, the result of each thread is messy, plus the result is correct.

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/** *  * 类简要描述 *  * <p> * 类详细描述 * </p> *  * @author think *  */public class VolatileThread implements Runnable { private volatile int a = 0; @Override public void run() {  // TODO Auto-generated method stub//  synchronized (this) {   a = a + 1;   System.out.println(Thread.currentThread().getName() + ":----" + a);   try {    Thread.sleep(100);    a = a + 2;   } catch (InterruptedException e) {    // TODO Auto-generated catch block    e.printStackTrace();   }   System.out.println(Thread.currentThread().getName() + ":----" + a);//  } }}

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/** *  * 类简要描述 *  * <p> * 类详细描述 * </p> *  * @author think *  */ public class VolatileMain { public static void main(String[] args) {  VolatileThread s = new VolatileThread();  Thread t1 = new Thread(s);  Thread t2 = new Thread(s);  Thread t3 = new Thread(s);  Thread t4 = new Thread(s);  t1.start();  t2.start();  t3.start();  t4.start();    /*  同步的结果  Thread-2:----1  Thread-2:----3  Thread-0:----4  Thread-0:----6  Thread-3:----7  Thread-3:----9  Thread-1:----10  Thread-1:----12*/  /*    去掉同步的结果  Thread-0:----1  Thread-1:----2  Thread-2:----3  Thread-3:----4  Thread-0:----8  Thread-3:----10  Thread-1:----10  Thread-2:----12*/   }}

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Volatile in Java does not guarantee thread safety (example explained)