java - Bruce Eckel 《Java 思维》第 4 版，ThreadLocal 变量持有者

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已关闭。这个问题是 not reproducible or was caused by typos 。目前不接受答案。

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这个问题是由拼写错误或无法再重现的问题引起的。虽然类似的问题可能是 on-topic在这里，这个问题的解决方式不太可能帮助 future 的读者。

已关闭 8 年前。

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书中的两个片段(并发章节):

 class Accessor implements Runnable {
    private final int id;

    public Accessor(int idn) {
      id = idn;
    }

    public void run() {
      while(!Thread.currentThread().isInterrupted()) {
        ThreadLocalVariableHolder.increment();
      System.out.println(this);
      Thread.yield();
      }
   }

   public String toString() {
   return "#" + id + ": " +
     ThreadLocalVariableHolder.get();
   }
}

public class ThreadLocalVariableHolder {

private static ThreadLocal<Integer> value =
  new ThreadLocal<Integer>(); /*{
  private Random rand = new Random(47);
  protected synchronized Integer initialValue() {
    return rand.nextInt(10000);
  }
};*/

public static void increment() {
  value.set(value.get() + 1);
}

public static int get() {
  return value.get();
}

public static void main(String[] args) throws Exception {
  ExecutorService exec = Executors.newCachedThreadPool();

  for(int i = 0; i < 5; i++)
    exec.execute(new Accessor(i));

  TimeUnit.SECONDS.sleep(3); // Run for a while
  exec.shutdownNow();
  // All Accessors will quit
}
}

在ThreadLocalVariableHolder类中是内部类ThreadLocal，其方法initialValue和属性rand未显式使用。如果我评论类(class)的全部内容(就像我在这里所做的那样)，它仍然会产生类似的结果。我想，这段代码已经过时了。你知道为什么这段代码在内部类中，其目的是什么吗？

Answer 1

我怀疑，如果我运行你的代码，它会在 increment() -> value.get() 处抛出 NPE

这是有效的，因为 ThreadLocal 中的默认实现将返回 null

protected T initialValue() {
        return null;
    }

来自JavaDoc

Returns the current thread's "initial value" for this thread-local variable. This method will be invoked the first time a thread accesses the variable with the get method, unless the thread previously invoked the set method, in which case the initialValue method will not be invoked for the thread. Normally, this method is invoked at most once per thread, but it may be invoked again in case of subsequent invocations of remove followed by get.

This implementation simply returns null; if the programmer desires thread-local variables to have an initial value other than null, ThreadLocal must be subclassed, and this method overridden. Typically, an anonymous inner class will be used.