This class provides thread-local variables. 
These variables differ from their normal counterparts in that each thread that accesses
 one (via its get or set method) has its own, independently initialized copy of the variable. ThreadLocal instances are typically private static fields in classes that
 wish to associate state with a thread (e.g., a user ID or Transaction ID).
For example, the class below generates unique identifiers local to each thread.
 A thread's id is assigned the first time it invokes ThreadId.get() and 
remains unchanged on subsequent calls.
  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();
    }
  }
Each thread holds an implicit reference to its copy of a thread-local 
variable as long as the thread is alive and the ThreadLocal instance is 
accessible; after a thread goes away, all of its copies of thread-local
 instances are subject to garbage collection (unless other references to 
these copies exist).

  /**
   * Sets the current thread's copy of this thread-local variable
   * to the specified value. Most subclasses will have no need to
   * override this method, relying solely on the {@link #initialValue}
   * method to set the values of thread-locals.
   * @param value the value to be stored in the current thread's copy of
   *    this thread-local.
   **/
  public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
      map.set(this, value);
    else
      createMap(t, value);
  }
  /**
   * Get the map associated with a ThreadLocal. Overridden in
   * InheritableThreadLocal.
   * @param t the current thread
   * @return the map
   **/
  ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
  }
  /**
   * Create the map associated with a ThreadLocal. Overridden in
   * InheritableThreadLocal.
   * @param t the current thread
   * @param firstValue value for the initial entry of the map
   **/
  void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
  }

  /**
   * Returns the value in the current thread's copy of this
   * thread-local variable. If the variable has no value for the
   * current thread, it is first initialized to the value returned
   * by an invocation of the {@link #initialValue} method.
   * @return the current thread's value of this thread-local
   **/
  public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
      ThreadLocalMap.Entry e = map.getEntry(this);
      if (e != null) {
        @SuppressWarnings("unchecked")
        T result = (T)e.value;
        return result;
      }
    }
    return setInitialValue();
  }
  /**
   * Variant of set() to establish initialValue. Used instead
   * of set() in case user has overridden the set() method.
   * @return the initial value
   **/
  private T setInitialValue() {
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
      map.set(this, value);
    else
      createMap(t, value);
    return value;
  }
  protected T initialValue() {
    return null;
  }