多线程与AtomicInteger

Question

我把这段代码摘自原子学教程，它说:-

“通过使用AtomicInteger代替Integer，我们可以在线程安全庄园中并发地增加数字，而不需要同步对变量的访问。方法incrementAndGet()是一个原子操作，因此我们可以从多个线程安全地调用该方法。”

它说这将返回正确的结果，1000但是两个实例都没有达到1000，它们通常要少得多。

test1 result = 532
test2 result = 128

出什么事了？

public class AtomicsTest {

    public static void main(String... args){

        AtomicsTest test = new AtomicsTest();        
        test.test1();        
        test.test2();                             
    }

    public void test1() {
        AtomicInteger atomicInt = new AtomicInteger(0);

        ExecutorService executor = Executors.newSingleThreadExecutor();                
        IntStream.range(0,1000).forEach(i->executor.submit( atomicInt::incrementAndGet ));
        System.out.println("test1 result = "+ atomicInt.get());
        executor.shutdown();
    }

    public void test2() {

        AtomicInteger atomicInt = new AtomicInteger(0);

        ExecutorService executor = Executors.newFixedThreadPool(2);            
        IntStream.range(0,1000).forEach(i->executor.submit( atomicInt::incrementAndGet ));
        System.out.println("test2 result = " + atomicInt.get());        
        executor.shutdown();           
    }    
}

Answer 1

您的问题是，提交线程后立即打印值，尽管并非所有线程都已完成。

public void test1() {
    AtomicInteger atomicInt = new AtomicInteger(0);

    ExecutorService executor = Executors.newSingleThreadExecutor();                
    IntStream.range(0,1000).forEach(i->executor.submit( atomicInt::incrementAndGet ));

    // All threads submitted (not necessarily finished)

    System.out.println("test1 result = "+ atomicInt.get());

    executor.shutdown();

    // Threads are still not necessarily done
}

如果您在打印值之前显式地等待executor服务完成，您应该看到预期的结果：

public void test1() {
    try {
        AtomicInteger atomicInt = new AtomicInteger(0);

        ExecutorService executor = Executors.newSingleThreadExecutor();
        IntStream.range(0,1000).forEach(i->executor.submit( atomicInt::incrementAndGet ));

        executor.shutdown();
        // Wait a maximum of ~a million billion years
        executor.awaitTermination(Long.MAX_VALUE, TimeUnit.HOURS);

        System.out.println("test1 result = "+ atomicInt.get());
    }
    catch (InterruptedException ex)
    {
        // Oh no!
    }
}