ThreadLocal

ThreadLocal example on Oracle documentation does not work, I tried to report it but the steps involved are so cumbersome, I have chosen to post the solution here.

Here is the code that works as intended (ignore the wrong indentation).

import java.util.concurrent.atomic.AtomicInteger;

public class Threads implements Runnable {

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

    private static final ThreadLocal < Integer > threadIdHolder = 

        new ThreadLocal < Integer > ();

    public int getCurrentThreadId() {

        return threadIdHolder.get();

    }

    public Threads() {

    threadIdHolder.set(numThreads.getAndIncrement());

    }

@Override

public void run() {

System.out.println(getCurrentThreadId());

}

private static Integer getNumberOfThreads() {

return numThreads.get();

}

public static void main(String[] args) {

for (int i=0; i<4 i="" p="">

(new Thread(new Threads())).run();

}

System.out.println("Number of threads is: "+getNumberOfThreads());

}

}

The trouble with the code posted in Oracle documentation (http://docs.oracle.com/javase/6/docs/api/java/lang/ThreadLocal.html) is that it tries to use the initialValue() method to increment the numThreads variable. However it is called only once because it is static. It can be fixed by making it non static. Alternately keep it static and assign the value to threadIdHolder in the constructor like in this code.

Here is the output:

0

1

2

3

Number of threads is: 4