为什么局部变量不允许使用 volatile 关键字？

Question

考虑片段：

如果在主线程中，我在方法 -

中有这个

volatile CountDownLatch latch = new CountDownLatch(3);

new Thread(new ProcessThread("Worker1",latch, 20000)).start();//20 secs
new Thread(new ProcessThread("Worker2",latch, 60000)).start();//60 secs
new Thread(new ProcessThread("Worker3",latch, 40000)).start();//40 secs

我看到 volatile 显示为非法修饰符。并且只允许 final。 并最终保证初始化安全。

public static class ProcessThread implements Runnable {
  final CountDownLatch latch;
  final long workDuration;
  final String name;

  public ProcessThread(String name, CountDownLatch latch, long duration){
      this.name= name;
      this.latch = latch;
      this.workDuration = duration;
  }
}

下面的对象即 new CountDownLatch(3) 已正确构造，但我还想确保分配给上述对象的引用 latch 保证对其下面的代码可见。

final CountDownLatch latch = new CountDownLatch(3);

以上代码是否保证初始化，以便 latch 对下面的代码完全可见，即

new Thread(new ProcessThread("Worker1",latch, 20000)).start();

Answer 1

局部变量存在于栈中；当然，当您两次调用 same 方法时，它们的局部变量 all 在各自的堆栈上。

volatile 仅在多个线程写入 same 内存位置时才有意义（在堆上）。

这对于方法体内的局部变量完全没有意义！

Answer 2

And final guarantees initialization safety.

不适用于局部变量：它只会阻止您重新分配该变量。

final CountDownLatch latch = new CountDownLatch(3);
Does the above code will guarantee initialization so that latch is perfectly visible to the code below i.e

没有。正是这段代码保证了它：

public static class ProcessThread implements Runnable {

    final CountDownLatch latch;

    // Plus the assignment in the constructor.

}

一旦构造函数完成（通常），

final 字段保证可见。来自 JLS Sec 17.5:

An object is considered to be completely initialized when its constructor finishes. A thread that can only see a reference to an object after that object has been completely initialized is guaranteed to see the correctly initialized values for that object's final fields.

Answer 3

您在本地执行的操作不会有其他线程的可见性或干扰问题，因此声明局部变量 volatile 没有意义。

为什么局部变量不允许使用 volatile 关键字？

Why is volatile keyword not allowed for local variables?

java

multithreading

final

volatile