semovy

 TheadLocal包含了一組get(),set(Object)方法，可以獲得當前線程的放進去的對象。

    ThreadLocal 不是用來解決共享對象的多線程訪問問題的，一般情況下，通過ThreadLocal.set() 到線程中的對象是該線程自己使用的對象，其他線程是不需要訪問的，也訪問不到的。各個線程中訪問的是不同的對象。

    另外，說ThreadLocal使得各線程能夠保持各自獨立的一個對象，這個對象的創建并不是通過ThreadLocal.set()來實現的，set()并不會做什么對象的拷貝，而是每個線程之前已經創建好的對象。通 過ThreadLocal.set()將這個新創建的對象的引用以當前線程為key，保存TheadLocal的一個map中，執行 ThreadLocal.get()時，各線程從map中取出以當前線程為key的對象，因此取出來的是各自自己線程中的對象。

 

下面來看看ThreadLocal的實現原理（jdk1.5源碼）

1. public class ThreadLocal<T> {  
2.     /**
3.       * ThreadLocals rely on per-thread hash maps attached to each thread
4.       * (Thread.threadLocals and inheritableThreadLocals).   The ThreadLocal
5.       * objects act as keys, searched via threadLocalHashCode.   This is a
6.       * custom hash code (useful only within ThreadLocalMaps) that eliminates
7.       * collisions in the common case where consecutively constructed
8.       * ThreadLocals are used by the same threads, while remaining well-behaved
9.       * in less common cases.
10.       */  
11.     private final int threadLocalHashCode = nextHashCode();  
12.   
13.     /**
14.       * The next hash code to be given out. Accessed only by like-named method.
15.       */  
16.     private static int nextHashCode = 0;  
17.   
18.     /**
19.       * The difference between successively generated hash codes - turns
20.       * implicit sequential thread-local IDs into near-optimally spread
21.       * multiplicative hash values for power-of-two-sized tables.
22.       */  
23.     private static final int HASH_INCREMENT = 0x61c88647;  
24.   
25.     /**
26.       * Compute the next hash code. The static synchronization used here
27.       * should not be a performance bottleneck. When ThreadLocals are
28.       * generated in different threads at a fast enough rate to regularly
29.       * contend on this lock, memory contention is by far a more serious
30.       * problem than lock contention.
31.       */  
32.     private static synchronized int nextHashCode() {  
33.         int h = nextHashCode;  
34.          nextHashCode = h + HASH_INCREMENT;  
35.         return h;  
36.      }  
37.   
38.     /**
39.       * Creates a thread local variable.
40.       */  
41.     public ThreadLocal() {  
42.      }  
43.   
44.     /**
45.       * Returns the value in the current thread's copy of this thread-local
46.       * variable.   Creates and initializes the copy if this is the first time
47.       * the thread has called this method.
48.       *
49.       * @return the current thread's value of this thread-local
50.       */  
51.     public T get() {  
52.          Thread t = Thread.currentThread();  
53.          ThreadLocalMap map = getMap(t);  
54.         if (map != null)  
55.             return (T)map.get(this);  
56.   
57.         // Maps are constructed lazily.   if the map for this thread  
58.         // doesn't exist, create it, with this ThreadLocal and its  
59.         // initial value as its only entry.  
60.          T value = initialValue();  
61.          createMap(t, value);  
62.         return value;  
63.      }  
64.   
65.     /**
66.       * Sets the current thread's copy of this thread-local variable
67.       * to the specified value.   Many applications will have no need for
68.       * this functionality, relying solely on the {@link #initialValue}
69.       * method to set the values of thread-locals.
70.       *
71.       * @param value the value to be stored in the current threads' copy of
72.       *         this thread-local.
73.       */  
74.     public void set(T value) {  
75.          Thread t = Thread.currentThread();  
76.          ThreadLocalMap map = getMap(t);  
77.         if (map != null)  
78.              map.set(this, value);  
79.         else  
80.              createMap(t, value);  
81.      }  
82.   
83.     /**
84.       * Get the map associated with a ThreadLocal. Overridden in
85.       * InheritableThreadLocal.
86.       *
87.       * @param   t the current thread
88.       * @return the map
89.       */  
90.      ThreadLocalMap getMap(Thread t) {  
91.         return t.threadLocals;  
92.      }  
93.   
94.     /**
95.       * Create the map associated with a ThreadLocal. Overridden in
96.       * InheritableThreadLocal.
97.       *
98.       * @param t the current thread
99.       * @param firstValue value for the initial entry of the map
100.       * @param map the map to store.
101.       */  
102.     void createMap(Thread t, T firstValue) {  
103.          t.threadLocals = new ThreadLocalMap(this, firstValue);  
104.      }  
105.   
106.      .......  
107.   
108.     /**
109.       * ThreadLocalMap is a customized hash map suitable only for
110.       * maintaining thread local values. No operations are exported
111.       * outside of the ThreadLocal class. The class is package private to
112.       * allow declaration of fields in class Thread.   To help deal with
113.       * very large and long-lived usages, the hash table entries use
114.       * WeakReferences for keys. However, since reference queues are not
115.       * used, stale entries are guaranteed to be removed only when
116.       * the table starts running out of space.
117.       */  
118.     static class ThreadLocalMap {  
119.   
120.      ........  
121.   
122.      }
     

以hibernate中典型的ThreadLocal的應用：