代码之家 › 专栏 › 技术社区 › Maxim Veksler

ThreadLocal思考(或者:sun的javadoc错了吗?)

jdk1.6 sun javadoc java

5

Maxim Veksler · 技术社区 · 15 年前

我一直在阅读关于ThreadLocal的文章,试图了解它是如何工作的以及为什么我们需要它。

到目前为止,我学到的是:

ThreadLocal类允许在线程级别保存一个对象的实例
实例是通过重写initialValue()创建的
each thread's HashMap
常识用法示例 can be found here

一切似乎都很好,直到我试着从 javadoc ,代码如下:

 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,那么调用getCurrentThreadId()应该会返回正确的自动递增线程数,唉,它会为我返回0。总是0,不考虑我已经启动了多少线程。

为了让它为我工作,我必须将getCurrentThreadId()改为

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

在这种情况下,我得到了正确的值。

下面提供了我的代码,我遗漏了什么(不是javadoc真的错了,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

1 回复 | 直到 8 年前

1

9

Stephen Denne 15 年前

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

Java 7's javadoc

 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();
     }
 }