【Java】深刻理解ThreadLocal

時間 2019-11-10

原文原文鏈接

1、前言java

要理解ThreadLocal，首先必須理解線程安全。線程能夠看作是一個具備必定獨立功能的處理過程，它是比進程更細度的單位。當程序以單線程運行的時候，咱們不須要考慮線程安全。然而當一個進程中包含多個線程的時候，就須要考慮線程安全問題，由於此時線程可能會同時操做同一個資源，當兩個或者兩個以上線程同時操做一個資源的時候，就會形成衝突、不一致等問題，即線程不安全。數組

解決線程安全問題，本質上就是解決資源共享問題，通常有如下手段：安全

1）可重入（不依賴環境）；2）互斥（同一時間段只容許一個線程使用）；3）原子操做；4）Thread-Local多線程

2、Thread-Local併發

Thread-Local是一個很簡單的思想：若是一個資源會引發線程競爭，那就爲每個線程配備一個資源。這就是ThreadLocal須要作的事情。less

3、ThreadLocal的用法dom

在理解ThreadLocal以前，首先看一下它的用法：ide

 1 public class ThreadLocalTest {
 2     public static ThreadLocal<Integer> intLocal = new ThreadLocal<Integer>() {
 3         @Override
 4         protected Integer initialValue() {
 5             // TODO Auto-generated method stub
 6             return 0;
 7         }
 8 
 9         @Override
10         public Integer get() {
11             // TODO Auto-generated method stub
12             set(super.get() + 1);
13             return super.get();
14         }
15 
16         @Override
17         public void set(Integer value) {
18             // TODO Auto-generated method stub
19             super.set(value);
20         }
21 
22         @Override
23         public void remove() {
24             // TODO Auto-generated method stub
25             super.remove();
26         }
27     };
28 
29     public static void main(String[] args) {
30         // TODO Auto-generated method stub
31         for(int index = 0; index < 3; index ++)
32             new MyThread(index).start();
33     }
34 }
35 
36 class MyThread extends Thread{
37     int id;
38     
39     public MyThread(int id){
40         this.id = id;
41     }
42 
43     @Override
44     public void run() {
45         // TODO Auto-generated method stub
46         for(int index = 0; index < 3; index ++){
47             System.out.println("Thread-" + id + " : " + ThreadLocalTest.intLocal.get());
48             try {
49                 Thread.sleep((int)(100 * Math.random()));
50             } catch (InterruptedException e) {
51                 // TODO Auto-generated catch block
52                 e.printStackTrace();
53             }
54         }
55     }
56 }

View Code

其打印結果以下：學習

1 Thread-1 : 1
2 Thread-0 : 1
3 Thread-2 : 1
4 Thread-0 : 2
5 Thread-2 : 2
6 Thread-1 : 2
7 Thread-2 : 3
8 Thread-0 : 3
9 Thread-1 : 3

View Code

這是一個很典型的問題，在學習多線程以及同步的時候，幾乎全部的書本都會使用相似的一個例子：銀行存錢取錢問題。咱們知道當多線程併發操做一個int值的加減操做的時候，最後的數值會產生很大的不肯定性，得不到最終正確的結果。ui

而從例子中咱們能夠看到：每個線程對int值的操做都是獨立的，咱們使用的只是同一個靜態的intLocal類型！經過使用TreadLocal，咱們能夠爲每個線程提供獨立的資源副本，從而完成對資源的「共享」操做。

ThreadLocal類中可重載的方法只有四個：

1）set()：設置值，也就是說，咱們選擇將某個值設置爲ThreadLocal類型的；

2）get()：將設置進去的值取出來；

3）remove()：咱們不想將某個值設置爲ThreadLocal了，移除掉；

4）initialValue()：若是get的時候尚未設置值，就使用這個方法進行初始化；

使用過程簡單明瞭，通常重載initialValue()提供一個初始值就能夠了，其他方法不須要重載。

4、ThreadLocal的實現

看源碼是最直接也是最有效的學習方式，不但能夠掌握其原理，也能夠學習Java源碼精巧的實現方式。

ThreadLocal的實現代碼略長，咱們選取重要的方法做爲切入點：

 1     public T get() {
 2         Thread t = Thread.currentThread();
 3         ThreadLocalMap map = getMap(t);
 4         if (map != null) {
 5             ThreadLocalMap.Entry e = map.getEntry(this);
 6             if (e != null)
 7                 return (T)e.value;
 8         }
 9         return setInitialValue();
10     }

第一個方法是get()方法。這邊出現了Thread和Map，聯想到Thread的做用，大體應該能夠猜到ThreadLocal的實現：維護一個Map，以Thread做爲鍵，以變量做爲值，每一次要使用變量的時候，就以當前的Thread做爲鍵去取值，若是沒有，就初始化一個值返回，若是有，就直接返回。

事實上，ThreadLocal的實現思路的確大體如此。可是咱們要作的事情其實更多：

A.若是要設置更多的值怎麼辦？也就是說，咱們有多種資源須要共享怎麼辦？

B.爲每個線程共享一個資源，如何回收？

咱們言歸正傳，從源碼中獲取答案。

get()方法的第3行中出現了一個ThreadLocalMap實例，它是從getMap()方法獲取的，其方法以下：

1 ThreadLocalMap getMap(Thread t) {
2         return t.threadLocals;
3     }

t表示當前的線程，從Thread的源碼中能夠看到，的確是有一個ThreadLocalMap實例，其聲明和註解以下：

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

這個屬性是專門爲ThreadLocal類而存在的，而它的實現也存在於ThreadLocal中，是ThreadLocal的一個靜態內部類。其類註釋以下：

 1 /**
 2      * ThreadLocalMap is a customized hash map suitable only for
 3      * maintaining thread local values. No operations are exported
 4      * outside of the ThreadLocal class. The class is package private to
 5      * allow declaration of fields in class Thread.  To help deal with
 6      * very large and long-lived usages, the hash table entries use
 7      * WeakReferences for keys. However, since reference queues are not
 8      * used, stale entries are guaranteed to be removed only when
 9      * the table starts running out of space.
10      */

　　它是一個定製的HashMap（天然具備HashMap的相關特性，好比自動擴增容量等）。

　　前面提到A,B兩個問題，從源碼來看，A問題的解決方案就是，爲每個Thread維護一個HashMap，在這裏就是維護一個ThreadLocalMap屬性，這個屬性的鍵是ThreadLocal，值就是資源副本，詳細描述以下：

　　每個Thread都有一個ThreadLocalMap屬性，這個屬性是相似於HashMap的，它以ThreadLocal爲鍵，以屬於該線程的資源副本爲值。咱們能夠這樣看待ThreadLocal：ThreadLocal是爲一組線程維護資源副本的對象，經過它，能夠爲每個線程建立資源副本，也能夠正確得到屬於某一線程的資源副本。

　　每個ThreadLocal只能維護一個共享資源，一旦聲明ThreadLocal實例，線程在調用的其get()方法獲取資源副本的時候，就能夠自動設置綁定到該線程自己。

　　好，如今轉了一小圈回到get方法()。get()方法的第二、3行很明顯是獲取屬於當前線程的ThreadLocalMap，若是這個map不爲空，咱們就以當前的ThreadLocal爲鍵，去獲取相應的Entry，Entry是ThreadLocalMap的靜態內部類，其定義以下：

1 static class Entry extends WeakReference<ThreadLocal> {
2             /** The value associated with this ThreadLocal. */
3             Object value;
4 
5             Entry(ThreadLocal k, Object v) {
6                 super(k);
7                 value = v;
8             }
9         }

　　它繼承與弱引用，因此在get()方法裏面如第7行同樣調用e.value方法就能夠獲取實際的資源副本值。可是若是有一個爲空，說明屬於該線程的資源副本還不存在，則須要去建立資源副本，從代碼中能夠看到是調用setInitialValue()方法，其定義以下：

 1 /**
 2      * Variant of set() to establish initialValue. Used instead
 3      * of set() in case user has overridden the set() method.
 4      *
 5      * @return the initial value
 6      */
 7     private T setInitialValue() {
 8         T value = initialValue();
 9         Thread t = Thread.currentThread();
10         ThreadLocalMap map = getMap(t);
11         if (map != null)
12             map.set(this, value);
13         else
14             createMap(t, value);
15         return value;
16     }

　　第8行調用initialValue()方法初始化一個值，還記得在一開始的例子中，咱們重載這個方法產生一個初始化的值麼？

　　接下來是判斷線程的ThreadLocalMap是否爲空，不爲空就直接這是值（鍵爲this，值爲value），爲空則建立一個Map，調用方法爲createMap()，其定義以下：

1 void createMap(Thread t, T firstValue) {
2         t.threadLocals = new ThreadLocalMap(this, firstValue);
3     }

　　簡單明瞭，而ThreadLocalMap的這個構造方法的實現以下：

/**
         * Construct a new map initially containing (firstKey, firstValue).
         * ThreadLocalMaps are constructed lazily, so we only create
         * one when we have at least one entry to put in it.
         */
        ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY];
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }

　　實例化table數組用於存儲鍵值對，而後經過映射將鍵值對存儲進入相應的位置。

　　至於set方法，看完get()後應該很簡單了，本身均可以實現：

 1 /**
 2      * Sets the current thread's copy of this thread-local variable
 3      * to the specified value.  Most subclasses will have no need to
 4      * override this method, relying solely on the {@link #initialValue}
 5      * method to set the values of thread-locals.
 6      *
 7      * @param value the value to be stored in the current thread's copy of
 8      *        this thread-local.
 9      */
10     public void set(T value) {
11         Thread t = Thread.currentThread();
12         ThreadLocalMap map = getMap(t);
13         if (map != null)
14             map.set(this, value);
15         else
16             createMap(t, value);
17     }

　　如今還剩一個問題B，會形成內存泄露嗎？ThreadLocal的類註釋裏面有一段話：

/* Each thread holds an implicit reference to its copy of a thread-local
 * variable as long as the thread is alive and the <tt>ThreadLocal</tt>
 * 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).
*/

　　每個線程對資源副本都有一個隱式引用：只要線程還在運行，只要ThreadLocal仍是能夠獲取的。當一個線程運行結束銷燬時，全部的資源副本都是能夠被垃圾回收的。這段註釋代表，ThreadLocal的使用是不會形成內訓泄露的。

　　可是咱們來仔細分析一下，我想畫一張Thread、ThreadLocal和ThreadLocalMap的依賴關係圖，可是在繪製過程當中，我發現：根本就沒有依賴！

　　很是驚訝，我在這裏把ThreadLocal完整的源碼貼一遍，讀者能夠自行審視。

  1 package java.lang;
  2 import java.lang.ref.*;
  3 import java.util.concurrent.atomic.AtomicInteger;
  4 
  5 /**
  6  * This class provides thread-local variables.  These variables differ from
  7  * their normal counterparts in that each thread that accesses one (via its
  8  * <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized
  9  * copy of the variable.  <tt>ThreadLocal</tt> instances are typically private
 10  * static fields in classes that wish to associate state with a thread (e.g.,
 11  * a user ID or Transaction ID).
 12  *
 13  * <p>For example, the class below generates unique identifiers local to each
 14  * thread.
 15  * A thread's id is assigned the first time it invokes <tt>ThreadId.get()</tt>
 16  * and remains unchanged on subsequent calls.
 17  * <pre>
 18  * import java.util.concurrent.atomic.AtomicInteger;
 19  *
 20  * public class ThreadId {
 21  *     // Atomic integer containing the next thread ID to be assigned
 22  *     private static final AtomicInteger nextId = new AtomicInteger(0);
 23  *
 24  *     // Thread local variable containing each thread's ID
 25  *     private static final ThreadLocal&lt;Integer> threadId =
 26  *         new ThreadLocal&lt;Integer>() {
 27  *             &#64;Override protected Integer initialValue() {
 28  *                 return nextId.getAndIncrement();
 29  *         }
 30  *     };
 31  *
 32  *     // Returns the current thread's unique ID, assigning it if necessary
 33  *     public static int get() {
 34  *         return threadId.get();
 35  *     }
 36  * }
 37  * </pre>
 38  * <p>Each thread holds an implicit reference to its copy of a thread-local
 39  * variable as long as the thread is alive and the <tt>ThreadLocal</tt>
 40  * instance is accessible; after a thread goes away, all of its copies of
 41  * thread-local instances are subject to garbage collection (unless other
 42  * references to these copies exist).
 43  *
 44  * @author  Josh Bloch and Doug Lea
 45  * @since   1.2
 46  */
 47 public class ThreadLocal<T> {
 48     /**
 49      * ThreadLocals rely on per-thread linear-probe hash maps attached
 50      * to each thread (Thread.threadLocals and
 51      * inheritableThreadLocals).  The ThreadLocal objects act as keys,
 52      * searched via threadLocalHashCode.  This is a custom hash code
 53      * (useful only within ThreadLocalMaps) that eliminates collisions
 54      * in the common case where consecutively constructed ThreadLocals
 55      * are used by the same threads, while remaining well-behaved in
 56      * less common cases.
 57      */
 58     private final int threadLocalHashCode = nextHashCode();
 59 
 60     /**
 61      * The next hash code to be given out. Updated atomically. Starts at
 62      * zero.
 63      */
 64     private static AtomicInteger nextHashCode =
 65         new AtomicInteger();
 66 
 67     /**
 68      * The difference between successively generated hash codes - turns
 69      * implicit sequential thread-local IDs into near-optimally spread
 70      * multiplicative hash values for power-of-two-sized tables.
 71      */
 72     private static final int HASH_INCREMENT = 0x61c88647;
 73 
 74     /**
 75      * Returns the next hash code.
 76      */
 77     private static int nextHashCode() {
 78         return nextHashCode.getAndAdd(HASH_INCREMENT);
 79     }
 80 
 81     /**
 82      * Returns the current thread's "initial value" for this
 83      * thread-local variable.  This method will be invoked the first
 84      * time a thread accesses the variable with the {@link #get}
 85      * method, unless the thread previously invoked the {@link #set}
 86      * method, in which case the <tt>initialValue</tt> method will not
 87      * be invoked for the thread.  Normally, this method is invoked at
 88      * most once per thread, but it may be invoked again in case of
 89      * subsequent invocations of {@link #remove} followed by {@link #get}.
 90      *
 91      * <p>This implementation simply returns <tt>null</tt>; if the
 92      * programmer desires thread-local variables to have an initial
 93      * value other than <tt>null</tt>, <tt>ThreadLocal</tt> must be
 94      * subclassed, and this method overridden.  Typically, an
 95      * anonymous inner class will be used.
 96      *
 97      * @return the initial value for this thread-local
 98      */
 99     protected T initialValue() {
100         return null;
101     }
102 
103     /**
104      * Creates a thread local variable.
105      */
106     public ThreadLocal() {
107     }
108 
109     /**
110      * Returns the value in the current thread's copy of this
111      * thread-local variable.  If the variable has no value for the
112      * current thread, it is first initialized to the value returned
113      * by an invocation of the {@link #initialValue} method.
114      *
115      * @return the current thread's value of this thread-local
116      */
117     public T get() {
118         Thread t = Thread.currentThread();
119         ThreadLocalMap map = getMap(t);
120         if (map != null) {
121             ThreadLocalMap.Entry e = map.getEntry(this);
122             if (e != null)
123                 return (T)e.value;
124         }
125         return setInitialValue();
126     }
127 
128     /**
129      * Variant of set() to establish initialValue. Used instead
130      * of set() in case user has overridden the set() method.
131      *
132      * @return the initial value
133      */
134     private T setInitialValue() {
135         T value = initialValue();
136         Thread t = Thread.currentThread();
137         ThreadLocalMap map = getMap(t);
138         if (map != null)
139             map.set(this, value);
140         else
141             createMap(t, value);
142         return value;
143     }
144 
145     /**
146      * Sets the current thread's copy of this thread-local variable
147      * to the specified value.  Most subclasses will have no need to
148      * override this method, relying solely on the {@link #initialValue}
149      * method to set the values of thread-locals.
150      *
151      * @param value the value to be stored in the current thread's copy of
152      *        this thread-local.
153      */
154     public void set(T value) {
155         Thread t = Thread.currentThread();
156         ThreadLocalMap map = getMap(t);
157         if (map != null)
158             map.set(this, value);
159         else
160             createMap(t, value);
161     }
162 
163     /**
164      * Removes the current thread's value for this thread-local
165      * variable.  If this thread-local variable is subsequently
166      * {@linkplain #get read} by the current thread, its value will be
167      * reinitialized by invoking its {@link #initialValue} method,
168      * unless its value is {@linkplain #set set} by the current thread
169      * in the interim.  This may result in multiple invocations of the
170      * <tt>initialValue</tt> method in the current thread.
171      *
172      * @since 1.5
173      */
174      public void remove() {
175          ThreadLocalMap m = getMap(Thread.currentThread());
176          if (m != null)
177              m.remove(this);
178      }
179 
180     /**
181      * Get the map associated with a ThreadLocal. Overridden in
182      * InheritableThreadLocal.
183      *
184      * @param  t the current thread
185      * @return the map
186      */
187     ThreadLocalMap getMap(Thread t) {
188         return t.threadLocals;
189     }
190 
191     /**
192      * Create the map associated with a ThreadLocal. Overridden in
193      * InheritableThreadLocal.
194      *
195      * @param t the current thread
196      * @param firstValue value for the initial entry of the map
197      * @param map the map to store.
198      */
199     void createMap(Thread t, T firstValue) {
200         t.threadLocals = new ThreadLocalMap(this, firstValue);
201     }
202 
203     /**
204      * Factory method to create map of inherited thread locals.
205      * Designed to be called only from Thread constructor.
206      *
207      * @param  parentMap the map associated with parent thread
208      * @return a map containing the parent's inheritable bindings
209      */
210     static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
211         return new ThreadLocalMap(parentMap);
212     }
213 
214     /**
215      * Method childValue is visibly defined in subclass
216      * InheritableThreadLocal, but is internally defined here for the
217      * sake of providing createInheritedMap factory method without
218      * needing to subclass the map class in InheritableThreadLocal.
219      * This technique is preferable to the alternative of embedding
220      * instanceof tests in methods.
221      */
222     T childValue(T parentValue) {
223         throw new UnsupportedOperationException();
224     }
225 
226     /**
227      * ThreadLocalMap is a customized hash map suitable only for
228      * maintaining thread local values. No operations are exported
229      * outside of the ThreadLocal class. The class is package private to
230      * allow declaration of fields in class Thread.  To help deal with
231      * very large and long-lived usages, the hash table entries use
232      * WeakReferences for keys. However, since reference queues are not
233      * used, stale entries are guaranteed to be removed only when
234      * the table starts running out of space.
235      */
236     static class ThreadLocalMap {
237 
238         /**
239          * The entries in this hash map extend WeakReference, using
240          * its main ref field as the key (which is always a
241          * ThreadLocal object).  Note that null keys (i.e. entry.get()
242          * == null) mean that the key is no longer referenced, so the
243          * entry can be expunged from table.  Such entries are referred to
244          * as "stale entries" in the code that follows.
245          */
246         static class Entry extends WeakReference<ThreadLocal> {
247             /** The value associated with this ThreadLocal. */
248             Object value;
249 
250             Entry(ThreadLocal k, Object v) {
251                 super(k);
252                 value = v;
253             }
254         }
255 
256         /**
257          * The initial capacity -- MUST be a power of two.
258          */
259         private static final int INITIAL_CAPACITY = 16;
260 
261         /**
262          * The table, resized as necessary.
263          * table.length MUST always be a power of two.
264          */
265         private Entry[] table;
266 
267         /**
268          * The number of entries in the table.
269          */
270         private int size = 0;
271 
272         /**
273          * The next size value at which to resize.
274          */
275         private int threshold; // Default to 0
276 
277         /**
278          * Set the resize threshold to maintain at worst a 2/3 load factor.
279          */
280         private void setThreshold(int len) {
281             threshold = len * 2 / 3;
282         }
283 
284         /**
285          * Increment i modulo len.
286          */
287         private static int nextIndex(int i, int len) {
288             return ((i + 1 < len) ? i + 1 : 0);
289         }
290 
291         /**
292          * Decrement i modulo len.
293          */
294         private static int prevIndex(int i, int len) {
295             return ((i - 1 >= 0) ? i - 1 : len - 1);
296         }
297 
298         /**
299          * Construct a new map initially containing (firstKey, firstValue).
300          * ThreadLocalMaps are constructed lazily, so we only create
301          * one when we have at least one entry to put in it.
302          */
303         ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
304             table = new Entry[INITIAL_CAPACITY];
305             int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
306             table[i] = new Entry(firstKey, firstValue);
307             size = 1;
308             setThreshold(INITIAL_CAPACITY);
309         }
310 
311         /**
312          * Construct a new map including all Inheritable ThreadLocals
313          * from given parent map. Called only by createInheritedMap.
314          *
315          * @param parentMap the map associated with parent thread.
316          */
317         private ThreadLocalMap(ThreadLocalMap parentMap) {
318             Entry[] parentTable = parentMap.table;
319             int len = parentTable.length;
320             setThreshold(len);
321             table = new Entry[len];
322 
323             for (int j = 0; j < len; j++) {
324                 Entry e = parentTable[j];
325                 if (e != null) {
326                     ThreadLocal key = e.get();
327                     if (key != null) {
328                         Object value = key.childValue(e.value);
329                         Entry c = new Entry(key, value);
330                         int h = key.threadLocalHashCode & (len - 1);
331                         while (table[h] != null)
332                             h = nextIndex(h, len);
333                         table[h] = c;
334                         size++;
335                     }
336                 }
337             }
338         }
339 
340         /**
341          * Get the entry associated with key.  This method
342          * itself handles only the fast path: a direct hit of existing
343          * key. It otherwise relays to getEntryAfterMiss.  This is
344          * designed to maximize performance for direct hits, in part
345          * by making this method readily inlinable.
346          *
347          * @param  key the thread local object
348          * @return the entry associated with key, or null if no such
349          */
350         private Entry getEntry(ThreadLocal key) {
351             int i = key.threadLocalHashCode & (table.length - 1);
352             Entry e = table[i];
353             if (e != null && e.get() == key)
354                 return e;
355             else
356                 return getEntryAfterMiss(key, i, e);
357         }
358 
359         /**
360          * Version of getEntry method for use when key is not found in
361          * its direct hash slot.
362          *
363          * @param  key the thread local object
364          * @param  i the table index for key's hash code
365          * @param  e the entry at table[i]
366          * @return the entry associated with key, or null if no such
367          */
368         private Entry getEntryAfterMiss(ThreadLocal key, int i, Entry e) {
369             Entry[] tab = table;
370             int len = tab.length;
371 
372             while (e != null) {
373                 ThreadLocal k = e.get();
374                 if (k == key)
375                     return e;
376                 if (k == null)
377                     expungeStaleEntry(i);
378                 else
379                     i = nextIndex(i, len);
380                 e = tab[i];
381             }
382             return null;
383         }
384 
385         /**
386          * Set the value associated with key.
387          *
388          * @param key the thread local object
389          * @param value the value to be set
390          */
391         private void set(ThreadLocal key, Object value) {
392 
393             // We don't use a fast path as with get() because it is at
394             // least as common to use set() to create new entries as
395             // it is to replace existing ones, in which case, a fast
396             // path would fail more often than not.
397 
398             Entry[] tab = table;
399             int len = tab.length;
400             int i = key.threadLocalHashCode & (len-1);
401 
402             for (Entry e = tab[i];
403                  e != null;
404                  e = tab[i = nextIndex(i, len)]) {
405                 ThreadLocal k = e.get();
406 
407                 if (k == key) {
408                     e.value = value;
409                     return;
410                 }
411 
412                 if (k == null) {
413                     replaceStaleEntry(key, value, i);
414                     return;
415                 }
416             }
417 
418             tab[i] = new Entry(key, value);
419             int sz = ++size;
420             if (!cleanSomeSlots(i, sz) && sz >= threshold)
421                 rehash();
422         }
423 
424         /**
425          * Remove the entry for key.
426          */
427         private void remove(ThreadLocal key) {
428             Entry[] tab = table;
429             int len = tab.length;
430             int i = key.threadLocalHashCode & (len-1);
431             for (Entry e = tab[i];
432                  e != null;
433                  e = tab[i = nextIndex(i, len)]) {
434                 if (e.get() == key) {
435                     e.clear();
436                     expungeStaleEntry(i);
437                     return;
438                 }
439             }
440         }
441 
442         /**
443          * Replace a stale entry encountered during a set operation
444          * with an entry for the specified key.  The value passed in
445          * the value parameter is stored in the entry, whether or not
446          * an entry already exists for the specified key.
447          *
448          * As a side effect, this method expunges all stale entries in the
449          * "run" containing the stale entry.  (A run is a sequence of entries
450          * between two null slots.)
451          *
452          * @param  key the key
453          * @param  value the value to be associated with key
454          * @param  staleSlot index of the first stale entry encountered while
455          *         searching for key.
456          */
457         private void replaceStaleEntry(ThreadLocal key, Object value,
458                                        int staleSlot) {
459             Entry[] tab = table;
460             int len = tab.length;
461             Entry e;
462 
463             // Back up to check for prior stale entry in current run.
464             // We clean out whole runs at a time to avoid continual
465             // incremental rehashing due to garbage collector freeing
466             // up refs in bunches (i.e., whenever the collector runs).
467             int slotToExpunge = staleSlot;
468             for (int i = prevIndex(staleSlot, len);
469                  (e = tab[i]) != null;
470                  i = prevIndex(i, len))
471                 if (e.get() == null)
472                     slotToExpunge = i;
473 
474             // Find either the key or trailing null slot of run, whichever
475             // occurs first
476             for (int i = nextIndex(staleSlot, len);
477                  (e = tab[i]) != null;
478                  i = nextIndex(i, len)) {
479                 ThreadLocal k = e.get();
480 
481                 // If we find key, then we need to swap it
482                 // with the stale entry to maintain hash table order.
483                 // The newly stale slot, or any other stale slot
484                 // encountered above it, can then be sent to expungeStaleEntry
485                 // to remove or rehash all of the other entries in run.
486                 if (k == key) {
487                     e.value = value;
488 
489                     tab[i] = tab[staleSlot];
490                     tab[staleSlot] = e;
491 
492                     // Start expunge at preceding stale entry if it exists
493                     if (slotToExpunge == staleSlot)
494                         slotToExpunge = i;
495                     cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
496                     return;
497                 }
498 
499                 // If we didn't find stale entry on backward scan, the
500                 // first stale entry seen while scanning for key is the
501                 // first still present in the run.
502                 if (k == null && slotToExpunge == staleSlot)
503                     slotToExpunge = i;
504             }
505 
506             // If key not found, put new entry in stale slot
507             tab[staleSlot].value = null;
508             tab[staleSlot] = new Entry(key, value);
509 
510             // If there are any other stale entries in run, expunge them
511             if (slotToExpunge != staleSlot)
512                 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
513         }
514 
515         /**
516          * Expunge a stale entry by rehashing any possibly colliding entries
517          * lying between staleSlot and the next null slot.  This also expunges
518          * any other stale entries encountered before the trailing null.  See
519          * Knuth, Section 6.4
520          *
521          * @param staleSlot index of slot known to have null key
522          * @return the index of the next null slot after staleSlot
523          * (all between staleSlot and this slot will have been checked
524          * for expunging).
525          */
526         private int expungeStaleEntry(int staleSlot) {
527             Entry[] tab = table;
528             int len = tab.length;
529 
530             // expunge entry at staleSlot
531             tab[staleSlot].value = null;
532             tab[staleSlot] = null;
533             size--;
534 
535             // Rehash until we encounter null
536             Entry e;
537             int i;
538             for (i = nextIndex(staleSlot, len);
539                  (e = tab[i]) != null;
540                  i = nextIndex(i, len)) {
541                 ThreadLocal k = e.get();
542                 if (k == null) {
543                     e.value = null;
544                     tab[i] = null;
545                     size--;
546                 } else {
547                     int h = k.threadLocalHashCode & (len - 1);
548                     if (h != i) {
549                         tab[i] = null;
550 
551                         // Unlike Knuth 6.4 Algorithm R, we must scan until
552                         // null because multiple entries could have been stale.
553                         while (tab[h] != null)
554                             h = nextIndex(h, len);
555                         tab[h] = e;
556                     }
557                 }
558             }
559             return i;
560         }
561 
562         /**
563          * Heuristically scan some cells looking for stale entries.
564          * This is invoked when either a new element is added, or
565          * another stale one has been expunged. It performs a
566          * logarithmic number of scans, as a balance between no
567          * scanning (fast but retains garbage) and a number of scans
568          * proportional to number of elements, that would find all
569          * garbage but would cause some insertions to take O(n) time.
570          *
571          * @param i a position known NOT to hold a stale entry. The
572          * scan starts at the element after i.
573          *
574          * @param n scan control: <tt>log2(n)</tt> cells are scanned,
575          * unless a stale entry is found, in which case
576          * <tt>log2(table.length)-1</tt> additional cells are scanned.
577          * When called from insertions, this parameter is the number
578          * of elements, but when from replaceStaleEntry, it is the
579          * table length. (Note: all this could be changed to be either
580          * more or less aggressive by weighting n instead of just
581          * using straight log n. But this version is simple, fast, and
582          * seems to work well.)
583          *
584          * @return true if any stale entries have been removed.
585          */
586         private boolean cleanSomeSlots(int i, int n) {
587             boolean removed = false;
588             Entry[] tab = table;
589             int len = tab.length;
590             do {
591                 i = nextIndex(i, len);
592                 Entry e = tab[i];
593                 if (e != null && e.get() == null) {
594                     n = len;
595                     removed = true;
596                     i = expungeStaleEntry(i);
597                 }
598             } while ( (n >>>= 1) != 0);
599             return removed;
600         }
601 
602         /**
603          * Re-pack and/or re-size the table. First scan the entire
604          * table removing stale entries. If this doesn't sufficiently
605          * shrink the size of the table, double the table size.
606          */
607         private void rehash() {
608             expungeStaleEntries();
609 
610             // Use lower threshold for doubling to avoid hysteresis
611             if (size >= threshold - threshold / 4)
612                 resize();
613         }
614 
615         /**
616          * Double the capacity of the table.
617          */
618         private void resize() {
619             Entry[] oldTab = table;
620             int oldLen = oldTab.length;
621             int newLen = oldLen * 2;
622             Entry[] newTab = new Entry[newLen];
623             int count = 0;
624 
625             for (int j = 0; j < oldLen; ++j) {
626                 Entry e = oldTab[j];
627                 if (e != null) {
628                     ThreadLocal k = e.get();
629                     if (k == null) {
630                         e.value = null; // Help the GC
631                     } else {
632                         int h = k.threadLocalHashCode & (newLen - 1);
633                         while (newTab[h] != null)
634                             h = nextIndex(h, newLen);
635                         newTab[h] = e;
636                         count++;
637                     }
638                 }
639             }
640 
641             setThreshold(newLen);
642             size = count;
643             table = newTab;
644         }
645 
646         /**
647          * Expunge all stale entries in the table.
648          */
649         private void expungeStaleEntries() {
650             Entry[] tab = table;
651             int len = tab.length;
652             for (int j = 0; j < len; j++) {
653                 Entry e = tab[j];
654                 if (e != null && e.get() == null)
655                     expungeStaleEntry(j);
656             }
657         }
658     }
659 }

View Code

　　ThreadLocal對Thread的引用所有經過局部變量完成，而沒有一個全局變量。而實際的資源副本則存儲在Thread的自身的屬性ThreadLocalMap中，這說明，其實ThreadLocal只是關聯一個Thread和其資源副本的橋樑，而且實際上Thread和資源副本的生命週期是緊密相連的，的的確確如ThreadLocal所說，在線程被回收的時候，其資源副本也會被回收，雖然ThreadLocal是靜態的，可是它既不引用Thread，也不引用ThreadLocalMap。真是精巧的設計！

5、ThreadLocal做用

其實ThreadLocal所實現的功能前面已經描述的很清楚了，可是從網上的討論來看，你們對ThreadLocal所要達成的目的意見卻不是很統一，好比博客《完全理解ThreadLocal》，具體能夠看一下類的註釋：

/**
 * This class provides thread-local variables.  These variables differ from
 * their normal counterparts in that each thread that accesses one (via its
 * <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized
 * copy of the variable.  <tt>ThreadLocal</tt> instances are typically private
 * static fields in classes that wish to associate state with a thread (e.g.,
 * a user ID or Transaction ID).
 */

從這個註解來看，和TLS（能夠參見《Thread Local Storage》）大體是同樣的，其實不用去深究其具體目的。我的以爲，ThreadLocal的功能就是爲每個線程提供一個資源副本，當你有這樣的需求的時候，就可使用ThreadLocal去解決問題。