java - 线程同步问题(更新变量 sum)

Question

这个问题已经有答案了:

ReentrantLock doesn't work out (2 个回答)

已关闭 4 年前。

当我想使用同步关键字或锁进行更新时，当 sum 变量为 int 时它会起作用，但当它是 Integer 对象时则不起作用。

代码如下 -

public class TestSynchronized {
  private  Integer sum = new Integer(0);

  public static void main(String[] args) {
    TestSynchronized test = new TestSynchronized();
    System.out.println("The sum is :" + test.sum);
  }

  public TestSynchronized() {
    ExecutorService executor = Executors.newFixedThreadPool(1000);

    for (int i = 0; i <=2000; i++) {
      executor.execute(new SumTask());
    }

    executor.shutdown();

    while(!executor.isTerminated()) {
    }
  }

  class SumTask implements Runnable {
    Lock lock = new ReentrantLock();

    public void run() {
    lock.lock();

      int value = sum.intValue() + 1;
      sum = new Integer(value);

        lock.unlock(); // Release the lock
      }

  }
}

Answer 1

问题是您为每个 SumTask 对象都有单独的锁。这些对象应该共享锁。

在TestSynchronized()方法中创建一个lock对象。这应该由所有new SumTask(lock)对象共享。

所以你的 SumTask 类看起来像:

class SumTask implements Runnable {
    Lock lock;

    public SumTask(Lock commonLock) {
        this.lock = commonLock;
    }

    public void run() {
        lock.lock();

        int value = sum.intValue() + 1;
        sum = new Integer(value);

        lock.unlock(); // Release the lock
    }

}

并且不要忘记在 TestSynchronized() 方法中创建一个 commonLock 对象:

  Lock lock = new ReentrantLock();

  executor.execute(new SumTask(commonLock));