private static final ThreadLocal threadSession = new ThreadLocal();  
  
public static Session getSession() throws InfrastructureException {  
    Session s = (Session) threadSession.get();  
    try {  
        if (s == null) {  
            s = getSessionFactory().openSession();  
            threadSession.set(s);  
        }  
    } catch (HibernateException ex) {  
        throw new InfrastructureException(ex);  
    }  
    return s;  
}

/**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal k, Object v) {
                super(k);
                value = v;
            }
        }

        /**
         * The initial capacity -- MUST be a power of two.
         */
        private static final int INITIAL_CAPACITY = 16;

        /**
         * The table, resized as necessary.
         * table.length MUST always be a power of two.
         */
        private Entry[] table;

        /**
         * The number of entries in the table.
         */
        private int size = 0;

        /**
         * The next size value at which to resize.
         */
        private int threshold; // Default to 0

   /**
    * The next hash code to be given out. Updated atomically. Starts at
    * zero.
    */
   private static AtomicInteger nextHashCode = 
new AtomicInteger();

/**
 * 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;

/**
     * ThreadLocals rely on per-thread linear-probe 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();

/* ThreadLocal values pertaining to this thread. This map is maintained
 * by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;

/**
    * 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();
       // 1
       ThreadLocalMap map = getMap(t);
       if (map != null) {
           // 2
           ThreadLocalMap.Entry e = map.getEntry(this);
           if (e != null)
               return (T)e.value;
       }
       return setInitialValue();
   }

/**
 * 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;
}

/**
         * Get the entry associated with key.  This method
         * itself handles only the fast path: a direct hit of existing
         * key. It otherwise relays to getEntryAfterMiss.  This is
         * designed to maximize performance for direct hits, in part
         * by making this method readily inlinable.
         *
         * @param  key the thread local object
         * @return the entry associated with key, or null if no such
         */
        private Entry getEntry(ThreadLocal key) {
            // 1
            int i = key.threadLocalHashCode & (table.length - 1);

            Entry e = table[i];
            if (e != null && e.get() == key)
                return e;
            else
                return getEntryAfterMiss(key, i, e);
        }

/**
 * 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);
}

/**
 * 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);
}

/**
         * Set the value associated with key.
         *
         * @param key the thread local object
         * @param value the value to be set
         */
        private void set(ThreadLocal key, Object value) {

            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.

            Entry[] tab = table;
            int len = tab.length;
            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i];
		 e != null;
		 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal k = e.get();
                // 1
                if (k == key) {
                    e.value = value;
                    return;
                }
                // 2
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
            // 3
            tab[i] = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

/**
         * Replace a stale entry encountered during a set operation
         * with an entry for the specified key.  The value passed in
         * the value parameter is stored in the entry, whether or not
         * an entry already exists for the specified key.
         *
         * As a side effect, this method expunges all stale entries in the
         * "run" containing the stale entry.  (A run is a sequence of entries
         * between two null slots.)
         *
         * @param  key the key
         * @param  value the value to be associated with key
         * @param  staleSlot index of the first stale entry encountered while
         *         searching for key.
         */
        private void replaceStaleEntry(ThreadLocal key, Object value,
                                       int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;
            Entry e;

            // Back up to check for prior stale entry in current run.
            // We clean out whole runs at a time to avoid continual
            // incremental rehashing due to garbage collector freeing
            // up refs in bunches (i.e., whenever the collector runs).
            // 1
            int slotToExpunge = staleSlot;
            // 2
            for (int i = prevIndex(staleSlot, len);
		 (e = tab[i]) != null;
                 i = prevIndex(i, len))

                if (e.get() == null)
                    slotToExpunge = i;

            // Find either the key or trailing null slot of run, whichever
            // occurs first
            for (int i = nextIndex(staleSlot, len);
		 (e = tab[i]) != null;
                 i = nextIndex(i, len)) {
                ThreadLocal k = e.get();

                // If we find key, then we need to swap it
                // with the stale entry to maintain hash table order.
                // The newly stale slot, or any other stale slot
                // encountered above it, can then be sent to expungeStaleEntry
                // to remove or rehash all of the other entries in run.
                // 3
                if (k == key) {
                    e.value = value;
                    // 4
                    tab[i] = tab[staleSlot];
                    tab[staleSlot] = e;

                    // Start expunge at preceding stale entry if it exists
                    // 5
                    if (slotToExpunge == staleSlot)
                        slotToExpunge = i;
                    // 6
                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
                    return;
                }

                // If we didn't find stale entry on backward scan, the
                // first stale entry seen while scanning for key is the
                // first still present in the run.
                if (k == null && slotToExpunge == staleSlot)
                    slotToExpunge = i;
            }

            // If key not found, put new entry in stale slot
            // 7
            tab[staleSlot].value = null;   
            tab[staleSlot] = new Entry(key, value);

            // If there are any other stale entries in run, expunge them
            if (slotToExpunge != staleSlot)
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
        }

/**
        * Expunge a stale entry by rehashing any possibly colliding entries
        * lying between staleSlot and the next null slot.  This also expunges
        * any other stale entries encountered before the trailing null.  See
        * Knuth, Section 6.4
        *
        * @param staleSlot index of slot known to have null key
        * @return the index of the next null slot after staleSlot
        * (all between staleSlot and this slot will have been checked
        * for expunging).
        */
       private int expungeStaleEntry(int staleSlot) {
           Entry[] tab = table;
           int len = tab.length;

           // expunge entry at staleSlot
           tab[staleSlot].value = null;   
           tab[staleSlot] = null;
           size--;

           // Rehash until we encounter null
           // 1
           Entry e;
           int i;
           for (i = nextIndex(staleSlot, len);
	 (e = tab[i]) != null;
                i = nextIndex(i, len)) {
               ThreadLocal k = e.get();
               // 2
               if (k == null) {
                   e.value = null;
                   tab[i] = null;
                   size--;
               } else {
                   // 3
                   int h = k.threadLocalHashCode & (len - 1);
                   // 4
                   if (h != i) {
                       tab[i] = null;

                       // Unlike Knuth 6.4 Algorithm R, we must scan until
                       // null because multiple entries could have been stale.
                       while (tab[h] != null)
                           h = nextIndex(h, len);
                       tab[h] = e;
                   }
               }
           }
           return i;
       }

/**
         * Heuristically scan some cells looking for stale entries.
         * This is invoked when either a new element is added, or
         * another stale one has been expunged. It performs a
         * logarithmic number of scans, as a balance between no
         * scanning (fast but retains garbage) and a number of scans
         * proportional to number of elements, that would find all
         * garbage but would cause some insertions to take O(n) time.
         *
         * @param i a position known NOT to hold a stale entry. The
         * scan starts at the element after i.
         *
         * @param n scan control: <tt>log2(n)</tt> cells are scanned,
         * unless a stale entry is found, in which case
         * <tt>log2(table.length)-1</tt> additional cells are scanned.
         * When called from insertions, this parameter is the number
         * of elements, but when from replaceStaleEntry, it is the
         * table length. (Note: all this could be changed to be either
         * more or less aggressive by weighting n instead of just
         * using straight log n. But this version is simple, fast, and
         * seems to work well.)
         *
         * @return true if any stale entries have been removed.
         */
        private boolean cleanSomeSlots(int i, int n) {
            boolean removed = false;
            Entry[] tab = table;
            int len = tab.length;
            do {
                i = nextIndex(i, len);
                Entry e = tab[i];
                // 1
                if (e != null && e.get() == null) {
                    n = len;
                    removed = true;
                    i = expungeStaleEntry(i);
                }
            } while ( (n >>>= 1) != 0);
            return removed;
        }

package threadLocal;

public class T {

	protected static ThreadLocal<String> threadLocal = new ThreadLocal<String>();

	public static void main(String[] args) {
		System.out.println("threadLocal  = " + threadLocal);
		threadLocal.set("1000");
		 
		System.out.println("main = " + threadLocal.get());
		threadLocal.set("4000");
 		System.out.println("main = " + threadLocal.get());
	}

}

package threadLocal;

public class T {

	public static ThreadLocal<String> threadLocal = new ThreadLocal<String>();

	public static void main(String[] args) {
		Thread t1 = new Thread(new Runnable() {
			@Override
			public void run() {
				System.out.println("执行线程1.....");
				threadLocal.set("1001");

				try {
					Thread.sleep(5000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}

				System.out.println("t1 = " + T.threadLocal.get());
			}
		});
		System.out.println("thread1  = " + t1);
		t1.start();

		Thread t2 = new Thread(new Runnable() {
			@Override
			public void run() {
				System.out.println("执行线程2.....");
				threadLocal.set("1002");

				try {
					Thread.sleep(10000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}

				System.out.println("t2 = " + T.threadLocal.get());
			}
		});
		System.out.println("thread2  = " + t2);
		t2.start();
		  
	}

}