ThreadLocal琢磨(或者:太阳的javadoc错了吗？)

Question

我一直在阅读ThreadLocal,试图了解它是如何工作的以及我们为什么需要它.

到目前为止,我能够学到的是以下内容:

1. ThreadLocal类允许在线程级别保存一个对象实例
2. 通过覆盖initialValue()创建实例
3. 实例实际上存储在每个线程的HashMap中
4. 可以在此处找到常识用法示例

一切似乎都很好,直到我尝试从javadoc运行示例,代码提供如下:

 import java.util.concurrent.atomic.AtomicInteger;

 public class UniqueThreadIdGenerator {

     private static final AtomicInteger uniqueId = new AtomicInteger(0);

     private static final ThreadLocal < Integer > uniqueNum = 
         new ThreadLocal < Integer > () {
             @Override protected Integer initialValue() {
                 return uniqueId.getAndIncrement();
         }
     };

     public static int getCurrentThreadId() {
         return uniqueId.get();
     }
 } // UniqueThreadIdGenerator

如果我正确理解了这段代码,调用getCurrentThreadId()应该返回正确的自动递增的线程号,唉它为我返回0.总是0,不考虑我已经启动了多少线程.

为了让这对我有用,我不得不改变getCurrentThreadId()来阅读

     public static int getCurrentThreadId() {
         return uniqueId.get();
     } 

在这种情况下,我得到正确的价值观.

我的代码在下面提供,我缺少什么？(这不是javadoc实际上是错的,对吧？)

package org.vekslers;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

public class UniqueThreadIdGenerator extends Thread {

    private static final AtomicInteger uniqueId = new AtomicInteger(0);

    private static final ThreadLocal <Integer> uniqueNum = 
        new ThreadLocal <Integer> () {
            @Override protected Integer initialValue() {
                return uniqueId.getAndIncrement();
        }
    };

    public static int getCurrentThreadId() {
        return uniqueNum.get();
    }




    //////////////////////////////////////////////////
    // Testing code...
    //////////////////////////////////////////////////
    private static volatile boolean halt = false;

    public UniqueThreadIdGenerator(String threadName) {
        super(threadName);
    }

    @Override
    public void run() {
        System.out.println(Thread.currentThread() + " PREHALT " + getCurrentThreadId());
        while(!halt)
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
            }
        System.out.println(Thread.currentThread() + " POSTHALT " + getCurrentThreadId());
    }

    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new UniqueThreadIdGenerator("t1");
        Thread t2 = new UniqueThreadIdGenerator("t2");
        Thread t3 = new UniqueThreadIdGenerator("t3");
        Thread t4 = new UniqueThreadIdGenerator("t4");

        t3.start();
        t1.start();
        t2.start();
        t4.start();

        TimeUnit.SECONDS.sleep(10);
        halt = true;
    }
} // UniqueThreadIdGenerator

输出:

Thread[t3,5,main] PREHALT 0
Thread[t1,5,main] PREHALT 1
Thread[t2,5,main] PREHALT 2
Thread[t4,5,main] PREHALT 3
Thread[t4,5,main] POSTHALT 3
Thread[t2,5,main] POSTHALT 2
Thread[t1,5,main] POSTHALT 1
Thread[t3,5,main] POSTHALT 0

ps代码评论OT或点到欢迎评论.

Answer 1

javadoc是错误的.
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6475885

Java 7的javadoc包括

 import java.util.concurrent.atomic.AtomicInteger;

 public class ThreadId {
     // Atomic integer containing the next thread ID to be assigned
     private static final AtomicInteger nextId = new AtomicInteger(0);

     // Thread local variable containing each thread's ID
     private static final ThreadLocal<Integer> threadId =
         new ThreadLocal<Integer>() {
             @Override protected Integer initialValue() {
                 return nextId.getAndIncrement();
         }
     };

     // Returns the current thread's unique ID, assigning it if necessary
     public static int get() {
         return threadId.get();
     }
 }