一文读懂ava中的Volatile关键字使用

public class VolatileWithoutUsage {
 private int count = 0;

 public void incrementCount() {
 count++;
 }
 public int getCount() {
 return count;
 }
}

@Test
 public void testWithoutVolatile() throws InterruptedException {
 ExecutorService service= Executors.newFixedThreadPool(3);
 VolatileWithoutUsage volatileWithoutUsage=new VolatileWithoutUsage();

 IntStream.range(0,1000).forEach(count ->service.submit(volatileWithoutUsage::incrementCount) );
 service.shutdown();
 service.awaitTermination(1000, TimeUnit.MILLISECONDS);
 assertEquals(1000,volatileWithoutUsage.getCount() );
 }

public class VolatileFalseUsage {
 private volatile int count = 0;

 public void incrementCount() {
 count++;
 }
 public int getCount() {
 return count;
 }

}

@Test
 public void testWithVolatileFalseUsage() throws InterruptedException {
 ExecutorService service= Executors.newFixedThreadPool(3);
 VolatileFalseUsage volatileFalseUsage=new VolatileFalseUsage();

 IntStream.range(0,1000).forEach(count ->service.submit(volatileFalseUsage::incrementCount) );
 service.shutdown();
 service.awaitTermination(5000, TimeUnit.MILLISECONDS);
 assertEquals(1000,volatileFalseUsage.getCount() );
 }

public class VolatileTrueUsage {

private volatile int count = 0;

public void setCount(int number) {
 count=number;
}
public int getCount() {
 return count;
}
}

@Test
public void testWithVolatileTrueUsage() throws InterruptedException {
 VolatileTrueUsage volatileTrueUsage=new VolatileTrueUsage();
 Thread threadA = new Thread(()->volatileTrueUsage.setCount(10));
 threadA.start();
 Thread.sleep(100);

 Thread reader = new Thread(() -> {
 int valueReadByThread = volatileTrueUsage.getCount();
 assertEquals(10, valueReadByThread);
 });
 reader.start();
}
## Happens-Before 

public class VolatileHappenBeforeUsage {

int a = 0;
volatile boolean flag = false;

public void writer() {
 a = 1;  // 1 线程A修改共享变量
 flag = true; // 2 线程A写volatile变量
}
}