ThreadLocal in java programming

* The java.lang.ThreadLocal [2] allows thread-specific variables to be added in an ad-hoc fasion to just about any code.
* The ThreadLocal class maintains a table associating data (Object references) with Thread instances. ThreadLocal supports set and get methods to access data held by the current Thread. [1]
* The following example is from [3] . When running the program notice that the ThreadLocal variable in column one remains the same throughout the program's run.

import java.util.Random;

public class ThreadLocal1
{

// Define/create thread local variable
static ThreadLocal threadLocal = new ThreadLocal();

// Create class variable
static volatile int counter = 0;

// For random number generation
static Random random = new Random();

// Displays thread local variable, counter,
// and thread name
private static void displayValues()
{
System.out.println(threadLocal.get() + "\t" + counter + "\t"
+ Thread.currentThread().getName());
}

public static void main(String args[])
{

// Each thread increments counter
// Displays variable info
// And sleeps for the random amount of time
// Before displaying info again
Runnable runner = new Runnable()
{
public void run()
{
synchronized (ThreadLocal1.class)
{
counter++;
}
threadLocal.set(new Integer(random.nextInt(1000)));
displayValues();
try
{
Thread.sleep(((Integer) threadLocal.get()).intValue());
displayValues();
} catch (InterruptedException e)
{
e.printStackTrace();
}
}
};

// Increment counter, access thread local from
// a different thread, and display the values
synchronized (ThreadLocal1.class)
{
counter++;
}
threadLocal.set(new Integer(random.nextInt(1000)));
displayValues();

// Here's where the other threads
// are actually created
for (int i = 0; i < 5; i++)
{
Thread t = new Thread(runner);
t.start();
}
}
}


(from http://programmingexamples.wikidot.com/threadlocal)