Question

I’ve been reading about ThreadLocal, trying to understand how it works and why we need it.

So far what I’ve been able to learn is the following:

1. ThreadLocal class allows to hold 1 instance of an object at the thread level
2. The instance is created by overriding initialValue()
3. The instance is actually stored in the each thread’s HashMap
4. A common sense usage example can be found here

All seemed fine, until I tried to run the example from the javadoc, the code is provided as following:

 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

If I understand this code correctly, calling getCurrentThreadId() should return the correct auto incremented thread number, alas it returns 0 for me. ALWAYS 0, without consideration of how many threads I have started.

To get this working for me I had to change getCurrentThreadId() to read

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

In which case I am getting correct values.

My code is provided below, what am I missing? (It’s not that the javadoc is actually wrong, right??)

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

Output:

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

p.s. Code comments OT or to the point are welcome in comments.

Answer 1

The javadocs are wrong.
https://bugs.java.com/bugdatabase/view_bug?bug_id=6475885

Java 7’s javadoc includes

 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();
     }
 }