ThreadLocal

public class ThreadLocal<T> {  
    /** 
     * ThreadLocals rely on per-thread hash maps attached to each thread 
     * (Thread.threadLocals and inheritableThreadLocals).  The ThreadLocal 
     * objects act as keys, searched via threadLocalHashCode.  This is a 
     * custom hash code (useful only within ThreadLocalMaps) that eliminates 
     * collisions in the common case where consecutively constructed 
     * ThreadLocals are used by the same threads, while remaining well-behaved 
     * in less common cases. 
     */  
    private final int threadLocalHashCode = nextHashCode();  
  
    /** 
     * The next hash code to be given out. Accessed only by like-named method. 
     */  
    private static int nextHashCode = 0;  
  
    /** 
     * The difference between successively generated hash codes - turns 
     * implicit sequential thread-local IDs into near-optimally spread 
     * multiplicative hash values for power-of-two-sized tables. 
     */  
    private static final int HASH_INCREMENT = 0x61c88647;  
  
    /** 
     * Compute the next hash code. The static synchronization used here 
     * should not be a performance bottleneck. When ThreadLocals are 
     * generated in different threads at a fast enough rate to regularly 
     * contend on this lock, memory contention is by far a more serious 
     * problem than lock contention. 
     */  
    private static synchronized int nextHashCode() {  
        int h = nextHashCode;  
        nextHashCode = h + HASH_INCREMENT;  
        return h;  
    }  
  
    /** 
     * Creates a thread local variable. 
     */  
    public ThreadLocal() {  
    }  
  
    /** 
     * Returns the value in the current thread‘s copy of this thread-local 
     * variable.  Creates and initializes the copy if this is the first time 
     * the thread has called this 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)  
            return (T)map.get(this);  
  
        // Maps are constructed lazily.  if the map for this thread  
        // doesn‘t exist, create it, with this ThreadLocal and its  
        // initial value as its only entry.  
        T value = initialValue();  
        createMap(t, value);  
        return value;  
    }  
  
    /** 
     * Sets the current thread‘s copy of this thread-local variable 
     * to the specified value.  Many applications will have no need for 
     * this functionality, relying solely on the {@link #initialValue} 
     * method to set the values of thread-locals. 
     * 
     * @param value the value to be stored in the current threads‘ 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 
     * @param map the map to store. 
     */  
    void createMap(Thread t, T firstValue) {  
        t.threadLocals = new ThreadLocalMap(this, firstValue);  
    }  
  
    .......  
  
    /** 
     * ThreadLocalMap is a customized hash map suitable only for 
     * maintaining thread local values. No operations are exported 
     * outside of the ThreadLocal class. The class is package private to 
     * allow declaration of fields in class Thread.  To help deal with 
     * very large and long-lived usages, the hash table entries use 
     * WeakReferences for keys. However, since reference queues are not 
     * used, stale entries are guaranteed to be removed only when 
     * the table starts running out of space. 
     */  
    static class ThreadLocalMap {  
  
    ........  
  
    }  
  }