【Java】深入理解ThreadLocal

一、前言

要理解ThreadLocal，首先必须理解线程安全。线程可以看做是一个具有一定独立功能的处理过程，它是比进程更细度的单位。当程序以单线程运行的时候，我们不需要考虑线程安全。然而当一个进程中包含多个线程的时候，就需要考虑线程安全问题，因为此时线程可能会同时操作同一个资源，当两个或者两个以上线程同时操作一个资源的时候，就会造成冲突、不一致等问题，即线程不安全。

解决线程安全问题，本质上就是解决资源共享问题，一般有以下手段：

1）可重入（不依赖环境）；2）互斥（同一时间段只允许一个线程使用）；3）原子操作；4）Thread-Local

二、Thread-Local

Thread-Local是一个很简单的思想：如果一个资源会引起线程竞争，那就为每一个线程配备一个资源。这就是ThreadLocal需要做的事情。

三、ThreadLocal的用法

在理解ThreadLocal之前，首先看一下它的用法：

 public class ThreadLocalTest {
     public static ThreadLocal<Integer> intLocal = new ThreadLocal<Integer>() {
         @Override
         protected Integer initialValue() {
             // TODO Auto-generated method stub
             return 0;
         }

         @Override
         public Integer get() {
             // TODO Auto-generated method stub
             set(super.get() + 1);
             return super.get();
         }

         @Override
         public void set(Integer value) {
             // TODO Auto-generated method stub
             super.set(value);
         }

         @Override
         public void remove() {
             // TODO Auto-generated method stub
             super.remove();
         }
     };

     public static void main(String[] args) {
         // TODO Auto-generated method stub
         for(int index = 0; index < 3; index ++)
             new MyThread(index).start();
     }
 }

 class MyThread extends Thread{
     int id;

     public MyThread(int id){
         this.id = id;
     }

     @Override
     public void run() {
         // TODO Auto-generated method stub
         for(int index = 0; index < 3; index ++){
             System.out.println("Thread-" + id + " : " + ThreadLocalTest.intLocal.get());
             try {
                 Thread.sleep((int)(100 * Math.random()));
             } catch (InterruptedException e) {
                 // TODO Auto-generated catch block
                 e.printStackTrace();
             }
         }
     }
 }

其打印结果如下：

 Thread-1 : 1
 Thread-0 : 1
 Thread-2 : 1
 Thread-0 : 2
 Thread-2 : 2
 Thread-1 : 2
 Thread-2 : 3
 Thread-0 : 3
 Thread-1 : 3

这是一个很典型的问题，在学习多线程以及同步的时候，几乎所有的书本都会使用类似的一个例子：银行存钱取钱问题。我们知道当多线程并发操作一个int值的加减操作的时候，最后的数值会产生很大的不确定性，得不到最终正确的结果。

而从例子中我们可以看到：每一个线程对int值的操作都是独立的，我们使用的只是同一个静态的intLocal类型！通过使用TreadLocal，我们可以为每一个线程提供独立的资源副本，从而完成对资源的“共享”操作。

ThreadLocal类中可重载的方法只有四个：

1）set()：设置值，也就是说，我们选择将某个值设置为ThreadLocal类型的；

2）get()：将设置进去的值取出来；

3）remove()：我们不想将某个值设置为ThreadLocal了，移除掉；

4）initialValue()：如果get的时候还没有设置值，就使用这个方法进行初始化；

使用过程简单明了，一般重载initialValue()提供一个初始值就可以了，其余方法不需要重载。

四、ThreadLocal的实现

看源码是最直接也是最有效的学习方式，不但可以掌握其原理，也可以学习Java源码精巧的实现方式。

ThreadLocal的实现代码略长，我们选取重要的方法作为切入点：

     public T get() {
         Thread t = Thread.currentThread();
         ThreadLocalMap map = getMap(t);
         if (map != null) {
             ThreadLocalMap.Entry e = map.getEntry(this);
             if (e != null)
                 return (T)e.value;
         }
         return setInitialValue();
     }

第一个方法是get()方法。这边出现了Thread和Map，联想到Thread的作用，大致应该可以猜到ThreadLocal的实现：维护一个Map，以Thread作为键，以变量作为值，每一次要使用变量的时候，就以当前的Thread作为键去取值，如果没有，就初始化一个值返回，如果有，就直接返回。

事实上，ThreadLocal的实现思路的确大致如此。但是我们要做的事情其实更多：

A.如果要设置更多的值怎么办？也就是说，我们有多种资源需要共享怎么办？

     B.为每一个线程共享一个资源，如何回收？

我们言归正传，从源码中获取答案。

get()方法的第3行中出现了一个ThreadLocalMap实例，它是从getMap()方法获取的，其方法如下：

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

t表示当前的线程，从Thread的源码中可以看到，的确是有一个ThreadLocalMap实例，其声明和注解如下：

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

这个属性是专门为ThreadLocal类而存在的，而它的实现也存在于ThreadLocal中，是ThreadLocal的一个静态内部类。其类注释如下：

 /**
      * 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.
      */

　　它是一个定制的HashMap（自然具有HashMap的相关特性，比如自动扩增容量等）。

　　前面提到A,B两个问题，从源码来看，A问题的解决方案就是，为每一个Thread维护一个HashMap，在这里就是维护一个ThreadLocalMap属性，这个属性的键是ThreadLocal，值就是资源副本，详细描述如下：

　　每一个Thread都有一个ThreadLocalMap属性，这个属性是类似于HashMap的，它以ThreadLocal为键，以属于该线程的资源副本为值。我们可以这样看待ThreadLocal：ThreadLocal是为一组线程维护资源副本的对象，通过它，可以为每一个线程创建资源副本，也可以正确获得属于某一线程的资源副本。

　　每一个ThreadLocal只能维护一个共享资源，一旦声明ThreadLocal实例，线程在调用的其get()方法获取资源副本的时候，就可以自动设置绑定到该线程本身。

　　好，现在转了一小圈回到get方法()。get()方法的第2、3行很明显是获取属于当前线程的ThreadLocalMap，如果这个map不为空，我们就以当前的ThreadLocal为键，去获取相应的Entry，Entry是ThreadLocalMap的静态内部类，其定义如下：

 static class Entry extends WeakReference<ThreadLocal> {
             /** The value associated with this ThreadLocal. */
             Object value;

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

　　它继承与弱引用，所以在get()方法里面如第7行一样调用e.value方法就可以获取实际的资源副本值。但是如果有一个为空，说明属于该线程的资源副本还不存在，则需要去创建资源副本，从代码中可以看到是调用setInitialValue()方法，其定义如下：

 /**
      * 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;
     }

　　第8行调用initialValue()方法初始化一个值，还记得在一开始的例子中，我们重载这个方法产生一个初始化的值么？

　　接下来是判断线程的ThreadLocalMap是否为空，不为空就直接这是值（键为this，值为value），为空则创建一个Map，调用方法为createMap()，其定义如下：

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

　　简单明了，而ThreadLocalMap的这个构造方法的实现如下：

/**
         * 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);
        }

　　实例化table数组用于存储键值对，然后通过映射将键值对存储进入相应的位置。

　　至于set方法，看完get()后应该很简单了，自己都可以实现：

 /**
      * 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);
     }

　　现在还剩一个问题B，会造成内存泄露吗？ThreadLocal的类注释里面有一段话：

/* 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).
*/

　　每一个线程对资源副本都有一个隐式引用：只要线程还在运行，只要ThreadLocal还是可以获取的。当一个线程运行结束销毁时，所有的资源副本都是可以被垃圾回收的。这段注释表明，ThreadLocal的使用是不会造成内训泄露的。

　　但是我们来仔细分析一下，我想画一张Thread、ThreadLocal和ThreadLocalMap的依赖关系图，但是在绘制过程中，我发现：根本就没有依赖！

　　非常惊讶，我在这里把ThreadLocal完整的源码贴一遍，读者可以自行审视。

 package java.lang;
 import java.lang.ref.*;
 import java.util.concurrent.atomic.AtomicInteger;

 /**
  * 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).
  *
  * <p>For example, the class below generates unique identifiers local to each
  * thread.
  * A thread's id is assigned the first time it invokes <tt>ThreadId.get()</tt>
  * and remains unchanged on subsequent calls.
  * <pre>
  * 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&lt;Integer> threadId =
  *         new ThreadLocal&lt;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();
  *     }
  * }
  * </pre>
  * <p>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).
  *
  * @author  Josh Bloch and Doug Lea
  * @since   1.2
  */
 public class ThreadLocal<T> {
     /**
      * 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();

     /**
      * 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;

     /**
      * Returns the next hash code.
      */
     private static int nextHashCode() {
         return nextHashCode.getAndAdd(HASH_INCREMENT);
     }

     /**
      * Returns the current thread's "initial value" for this
      * thread-local variable.  This method will be invoked the first
      * time a thread accesses the variable with the {@link #get}
      * method, unless the thread previously invoked the {@link #set}
      * method, in which case the <tt>initialValue</tt> method will not
      * be invoked for the thread.  Normally, this method is invoked at
      * most once per thread, but it may be invoked again in case of
      * subsequent invocations of {@link #remove} followed by {@link #get}.
      *
      * <p>This implementation simply returns <tt>null</tt>; if the
      * programmer desires thread-local variables to have an initial
      * value other than <tt>null</tt>, <tt>ThreadLocal</tt> must be
      * subclassed, and this method overridden.  Typically, an
      * anonymous inner class will be used.
      *
      * @return the initial value for this thread-local
      */
     protected T initialValue() {
         return null;
     }

     /**
      * Creates a thread local variable.
      */
     public ThreadLocal() {
     }

     /**
      * 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)
                 return (T)e.value;
         }
         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;
     }

     /**
      * 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);
     }

     /**
      * Removes the current thread's value for this thread-local
      * variable.  If this thread-local variable is subsequently
      * {@linkplain #get read} by the current thread, its value will be
      * reinitialized by invoking its {@link #initialValue} method,
      * unless its value is {@linkplain #set set} by the current thread
      * in the interim.  This may result in multiple invocations of the
      * <tt>initialValue</tt> method in the current thread.
      *
      * @since 1.5
      */
      public void remove() {
          ThreadLocalMap m = getMap(Thread.currentThread());
          if (m != null)
              m.remove(this);
      }

     /**
      * 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);
     }

     /**
      * Factory method to create map of inherited thread locals.
      * Designed to be called only from Thread constructor.
      *
      * @param  parentMap the map associated with parent thread
      * @return a map containing the parent's inheritable bindings
      */
     static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
         return new ThreadLocalMap(parentMap);
     }

     /**
      * Method childValue is visibly defined in subclass
      * InheritableThreadLocal, but is internally defined here for the
      * sake of providing createInheritedMap factory method without
      * needing to subclass the map class in InheritableThreadLocal.
      * This technique is preferable to the alternative of embedding
      * instanceof tests in methods.
      */
     T childValue(T parentValue) {
         throw new UnsupportedOperationException();
     }

     /**
      * 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 {

         /**
          * 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

         /**
          * Set the resize threshold to maintain at worst a 2/3 load factor.
          */
         private void setThreshold(int len) {
             threshold = len * 2 / 3;
         }

         /**
          * Increment i modulo len.
          */
         private static int nextIndex(int i, int len) {
             return ((i + 1 < len) ? i + 1 : 0);
         }

         /**
          * Decrement i modulo len.
          */
         private static int prevIndex(int i, int len) {
             return ((i - 1 >= 0) ? i - 1 : len - 1);
         }

         /**
          * 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);
         }

         /**
          * Construct a new map including all Inheritable ThreadLocals
          * from given parent map. Called only by createInheritedMap.
          *
          * @param parentMap the map associated with parent thread.
          */
         private ThreadLocalMap(ThreadLocalMap parentMap) {
             Entry[] parentTable = parentMap.table;
             int len = parentTable.length;
             setThreshold(len);
             table = new Entry[len];

             for (int j = 0; j < len; j++) {
                 Entry e = parentTable[j];
                 if (e != null) {
                     ThreadLocal key = e.get();
                     if (key != null) {
                         Object value = key.childValue(e.value);
                         Entry c = new Entry(key, value);
                         int h = key.threadLocalHashCode & (len - 1);
                         while (table[h] != null)
                             h = nextIndex(h, len);
                         table[h] = c;
                         size++;
                     }
                 }
             }
         }

         /**
          * 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) {
             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);
         }

         /**
          * Version of getEntry method for use when key is not found in
          * its direct hash slot.
          *
          * @param  key the thread local object
          * @param  i the table index for key's hash code
          * @param  e the entry at table[i]
          * @return the entry associated with key, or null if no such
          */
         private Entry getEntryAfterMiss(ThreadLocal key, int i, Entry e) {
             Entry[] tab = table;
             int len = tab.length;

             while (e != null) {
                 ThreadLocal k = e.get();
                 if (k == key)
                     return e;
                 if (k == null)
                     expungeStaleEntry(i);
                 else
                     i = nextIndex(i, len);
                 e = tab[i];
             }
             return null;
         }

         /**
          * 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();

                 if (k == key) {
                     e.value = value;
                     return;
                 }

                 if (k == null) {
                     replaceStaleEntry(key, value, i);
                     return;
                 }
             }

             tab[i] = new Entry(key, value);
             int sz = ++size;
             if (!cleanSomeSlots(i, sz) && sz >= threshold)
                 rehash();
         }

         /**
          * Remove the entry for key.
          */
         private void remove(ThreadLocal key) {
             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)]) {
                 if (e.get() == key) {
                     e.clear();
                     expungeStaleEntry(i);
                     return;
                 }
             }
         }

         /**
          * 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).
             int slotToExpunge = staleSlot;
             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.
                 if (k == key) {
                     e.value = value;

                     tab[i] = tab[staleSlot];
                     tab[staleSlot] = e;

                     // Start expunge at preceding stale entry if it exists
                     if (slotToExpunge == staleSlot)
                         slotToExpunge = i;
                     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
             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
             Entry e;
             int i;
             for (i = nextIndex(staleSlot, len);
                  (e = tab[i]) != null;
                  i = nextIndex(i, len)) {
                 ThreadLocal k = e.get();
                 if (k == null) {
                     e.value = null;
                     tab[i] = null;
                     size--;
                 } else {
                     int h = k.threadLocalHashCode & (len - 1);
                     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];
                 if (e != null && e.get() == null) {
                     n = len;
                     removed = true;
                     i = expungeStaleEntry(i);
                 }
             } while ( (n >>>= 1) != 0);
             return removed;
         }

         /**
          * Re-pack and/or re-size the table. First scan the entire
          * table removing stale entries. If this doesn't sufficiently
          * shrink the size of the table, double the table size.
          */
         private void rehash() {
             expungeStaleEntries();

             // Use lower threshold for doubling to avoid hysteresis
             if (size >= threshold - threshold / 4)
                 resize();
         }

         /**
          * Double the capacity of the table.
          */
         private void resize() {
             Entry[] oldTab = table;
             int oldLen = oldTab.length;
             int newLen = oldLen * 2;
             Entry[] newTab = new Entry[newLen];
             int count = 0;

             for (int j = 0; j < oldLen; ++j) {
                 Entry e = oldTab[j];
                 if (e != null) {
                     ThreadLocal k = e.get();
                     if (k == null) {
                         e.value = null; // Help the GC
                     } else {
                         int h = k.threadLocalHashCode & (newLen - 1);
                         while (newTab[h] != null)
                             h = nextIndex(h, newLen);
                         newTab[h] = e;
                         count++;
                     }
                 }
             }

             setThreshold(newLen);
             size = count;
             table = newTab;
         }

         /**
          * Expunge all stale entries in the table.
          */
         private void expungeStaleEntries() {
             Entry[] tab = table;
             int len = tab.length;
             for (int j = 0; j < len; j++) {
                 Entry e = tab[j];
                 if (e != null && e.get() == null)
                     expungeStaleEntry(j);
             }
         }
     }
 }

　　ThreadLocal对Thread的引用全部通过局部变量完成，而没有一个全局变量。而实际的资源副本则存储在Thread的自身的属性ThreadLocalMap中，这说明，其实ThreadLocal只是关联一个Thread和其资源副本的桥梁，并且实际上Thread和资源副本的生命周期是紧密相连的，的的确确如ThreadLocal所说，在线程被回收的时候，其资源副本也会被回收，虽然ThreadLocal是静态的，但是它既不引用Thread，也不引用ThreadLocalMap。真是精巧的设计！

五、ThreadLocal作用

其实ThreadLocal所实现的功能前面已经描述的很清楚了，但是从网上的讨论来看，大家对ThreadLocal所要达成的目的意见却不是很统一，比如博客《彻底理解ThreadLocal》，具体可以看一下类的注释：

/**
 * 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).
 */

从这个注解来看，和TLS（可以参见《Thread Local Storage》）大致是一样的，其实不用去深究其具体目的。个人觉得，ThreadLocal的功能就是为每一个线程提供一个资源副本，当你有这样的需求的时候，就可以使用ThreadLocal去解决问题。