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无法在继承的Thread类中将值设置为Inheritable ThreadLocal字段

inheritable-thread-local thread-local multithreading java

1

bepopov · 技术社区 · 7 月前

不知何故,当我在创建继承Thread的类的对象时尝试为Inheritable ThreadLocal字段设置值时,我得到了奇怪的行为:当执行到达run()方法中的打印行时,我的对象没有我在调用构造函数时设置的值。同时,当我尝试运行一个继承我的类的线程时,它会看到我在创建previos时设置的值。

我尝试运行以下代码:

public class InheritableThreadLocalExample {

  public static void main(String[] args) {
    ThreadOne threadOne = new ThreadOne("user-one", "thread-one");
    threadOne.start();
    ThreadTwo threadTwo = new ThreadTwo("user-two", "thread-two");
    threadTwo.start();
  }

  public static class ThreadOne extends Thread {

    protected static final InheritableThreadLocal<String> USERNAME = new InheritableThreadLocal<>();

    public ThreadOne(String username, String threadName) {
      super(threadName);
      USERNAME.set(username);
    }

    @Override
    public void run() {
      System.out.println(Thread.currentThread().getName() + ". Username: " + USERNAME.get());
    }
  }

  public static class ThreadTwo extends ThreadOne {
    public ThreadTwo(String username, String threadName) {
      super(username, threadName);
    }
  }

}

输出为:

thread-two. Username: user-one
thread-one. Username: null

为什么会这样?

1 回复 | 直到 7 月前

1

3

Holger 7 月前

建设者 ThreadOne 由主线程执行,因此,语句 USERNAME.set(username); 在这个构造函数中设置主线程的值。但是主线程的可继承线程局部变量的实际值被复制到构造函数中 Thread ,即前面的 super(threadName); ,已经。

因此,第一个线程继承了第一个线程之前的状态 USERNAME.set(用户名); 调用,而第二个线程继承调用后的状态;继承关系无关紧要,正如我们可以证明的那样:

public class InheritableThreadLocalExample {
  public static void main(String[] args) {
    ThreadOne threadOne = new ThreadOne("user-one", "thread-one");
    printValue();
    threadOne.start();
    Thread threadTwo = new Thread(
                       InheritableThreadLocalExample::printValue, "thread-two");
     // things happening after Thread's constructor are irrelevant:
    ThreadOne.USERNAME.set("user-two");
    threadTwo.start();
  }

  static void printValue() {
    System.out.println(Thread.currentThread().getName()
                     + ". Username: " + ThreadOne.USERNAME.get());
  }

  public static class ThreadOne extends Thread {
    protected static final InheritableThreadLocal<String> USERNAME
                                  = new InheritableThreadLocal<>();

    public ThreadOne(String username, String threadName) {
      super(threadName);
      USERNAME.set(username);
    }

    @Override
    public void run() {
      printValue();
    }
  }
}

main. Username: user-one
thread-one. Username: null
thread-two. Username: user-one

我们可以很容易地在主线程中设置预期的值,而不需要子类:

public class InheritableThreadLocalExample {
  static final InheritableThreadLocal<String> USERNAME
                                            = new InheritableThreadLocal<>();

  public static void main(String[] args) {
    USERNAME.set("user-one");
    Thread threadOne = new Thread(
        InheritableThreadLocalExample::printValue, "thread-one");
    USERNAME.set("user-two");
    Thread threadTwo = new Thread(
        InheritableThreadLocalExample::printValue, "thread-two");

    // demonstrate that each thread has its own value
    // and main doesn't need a value:
    USERNAME.remove();

    threadOne.start();
    threadTwo.start();
  }

  static void printValue() {
    System.out.println(Thread.currentThread().getName()
                     + ". Username: " + USERNAME.get());
  }
}

thread-one. Username: user-one
thread-two. Username: user-two