线程系列3---ThreadLocal类研究

2013-12-23 17:44:44

Java为线程安全提供了一些工具类，如ThreadLocal类，它代表一个线程局部变量，通过把数据放在ThreadLocal中就可以让每个线程创建一个该变量的副本，从而避免并发访问的线程安全问题。

线程局部变量的功能其实很简单，就是为每一个使用该变量的线程提供一个副本，使每一个线程都可以独立的访问属于自己的副本，而不会和其他线程的副本产生冲突，就好像每一个线程都完全拥有该变量一样。

ThreadLocal类并不能替代同步机制，两者面向的问题领域不同。

同步机制是为了同步多个线程对相同资源的并发访问，是多个线程之间进行通信的有效方式；

ThreadLocal是为了隔离多个线程的数据共享，从根本上避免多个线程对共享资源的竞争，也就不需要对多个线程进行同步了。

自己做了一个Demo， 如下：

ThreadLocalTest.java

 package thread;

 public class ThreadLocalTest {

     public ThreadLocal<String> local = new ThreadLocal<String>();

     private String name;

     public ThreadLocalTest() {
     }

     public String getName() {
         // return name;
         return local.get();
     }

     public void setName(String name) {
         // this.name = name;
         local.set(name);
     }

     public static void main(String[] args) {
         ThreadLocalTest one = new ThreadLocalTest();
         ThreadOne t1 = new ThreadOne(one);
         ThreadTwo t2 = new ThreadTwo(one);
         t1.start();
         t2.start();
     }
 }

ThreadOne.java

 package thread;

 public class ThreadOne extends Thread {
     ThreadLocalTest one;

     public ThreadOne(ThreadLocalTest o) {
         one = o;
     }

     @Override
     public void run() {
         for (int i = 0; i < 10; i++) {
             StringBuilder ss = new StringBuilder("      one_");
             ss.append(i);
             one.setName(ss.toString());
             try {
                 Thread.sleep(280);
             } catch (InterruptedException e) {
                 e.printStackTrace();
             }
             System.out.println("线程一：" + Thread.currentThread().getName()
                     + one.getName());
         }
     }
 }

ThreadTwo.java

 package thread;

 public class ThreadTwo extends Thread {
     ThreadLocalTest one;

     public ThreadTwo(ThreadLocalTest o) {
         one = o;
     }

     @Override
     public void run() {
         for (int i = 0; i < 10; i++) {
             StringBuilder ss = new StringBuilder("------------two_");
             ss.append(i);
             one.setName(ss.toString());
             try {
                 Thread.sleep(100);
             } catch (InterruptedException e) {
                 e.printStackTrace();
             }
             System.out.println("线程二：" + Thread.currentThread().getName()
                     + one.getName());
         }
     }
 }

先看不是用ThreadLocal的log（使用ThreadLocalTest.java中注释掉的部分），如下：

 线程二：Thread-1------------two_0
 线程二：Thread-1------------two_1
 线程一：Thread-0------------two_2
 线程二：Thread-1      one_1
 线程二：Thread-1------------two_3
 线程二：Thread-1------------two_4
 线程一：Thread-0------------two_5
 线程二：Thread-1      one_2
 线程二：Thread-1------------two_6
 线程二：Thread-1------------two_7
 线程一：Thread-0------------two_8
 线程二：Thread-1      one_3
 线程二：Thread-1------------two_9
 线程一：Thread-0------------two_9
 线程一：Thread-0      one_4
 线程一：Thread-0      one_5
 线程一：Thread-0      one_6
 线程一：Thread-0      one_7
 线程一：Thread-0      one_8
 线程一：Thread-0      one_9

可以上面红色部分，可以发现就是多线程导致资源混乱的情况。

下面是使用了ThreadLocal后的log：

 线程二：Thread-1------------two_0
 线程二：Thread-1------------two_1
 线程一：Thread-0      one_0
 线程二：Thread-1------------two_2
 线程二：Thread-1------------two_3
 线程二：Thread-1------------two_4
 线程一：Thread-0      one_1
 线程二：Thread-1------------two_5
 线程二：Thread-1------------two_6
 线程二：Thread-1------------two_7
 线程一：Thread-0      one_2
 线程二：Thread-1------------two_8
 线程二：Thread-1------------two_9
 线程一：Thread-0      one_3
 线程一：Thread-0      one_4
 线程一：Thread-0      one_5
 线程一：Thread-0      one_6
 线程一：Thread-0      one_7
 线程一：Thread-0      one_8
 线程一：Thread-0      one_9

一切正常。

但是我发先了一个问题，那就是ThreadLocal类在Android和Java中的实现不太一样，把它们的源码都贴出来，有兴趣的同学可以好好研究一下：

Java SE-1.6

 /*
  * %W% %E%
  *
  * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
  */

 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>UniqueThreadIdGenerator.getCurrentThreadId()</tt> and remains unchanged on subsequent calls.
  * <pre>
  * import java.util.concurrent.atomic.AtomicInteger;
  *
  * public class UniqueThreadIdGenerator {
  *
  *     private static final AtomicInteger uniqueId = new AtomicInteger(0);
  *
  *     private static final ThreadLocal &lt; Integer > uniqueNum =
  *         new ThreadLocal &lt; Integer > () {
  *             @Override protected Integer initialValue() {
  *                 return uniqueId.getAndIncrement();
  *         }
  *     };
  *
  *     public static int getCurrentThreadId() {
  *         return uniqueId.get();
  *     }
  * } // UniqueThreadIdGenerator
  * </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
  * @version %I%, %G%
  * @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);
             }
         }
     }
 }

Android 4.4

 /*
  * Copyright (C) 2008 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 package java.lang;

 import java.lang.ref.Reference;
 import java.lang.ref.WeakReference;
 import java.util.concurrent.atomic.AtomicInteger;

 /**
  * Implements a thread-local storage, that is, a variable for which each thread
  * has its own value. All threads share the same {@code ThreadLocal} object,
  * but each sees a different value when accessing it, and changes made by one
  * thread do not affect the other threads. The implementation supports
  * {@code null} values.
  *
  * @see java.lang.Thread
  * @author Bob Lee
  */
 public class ThreadLocal<T> {

     /* Thanks to Josh Bloch and Doug Lea for code reviews and impl advice. */

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

     /**
      * Returns the value of this variable for the current thread. If an entry
      * doesn't yet exist for this variable on this thread, this method will
      * create an entry, populating the value with the result of
      * {@link #initialValue()}.
      *
      * @return the current value of the variable for the calling thread.
      */
     @SuppressWarnings("unchecked")
     public T get() {
         // Optimized for the fast path.
         Thread currentThread = Thread.currentThread();
         Values values = values(currentThread);
         if (values != null) {
             Object[] table = values.table;
             int index = hash & values.mask;
             if (this.reference == table[index]) {
                 return (T) table[index + 1];
             }
         } else {
             values = initializeValues(currentThread);
         }

         return (T) values.getAfterMiss(this);
     }

     /**
      * Provides the initial value of this variable for the current thread.
      * The default implementation returns {@code null}.
      *
      * @return the initial value of the variable.
      */
     protected T initialValue() {
         return null;
     }

     /**
      * Sets the value of this variable for the current thread. If set to
      * {@code null}, the value will be set to null and the underlying entry will
      * still be present.
      *
      * @param value the new value of the variable for the caller thread.
      */
     public void set(T value) {
         Thread currentThread = Thread.currentThread();
         Values values = values(currentThread);
         if (values == null) {
             values = initializeValues(currentThread);
         }
         values.put(this, value);
     }

     /**
      * Removes the entry for this variable in the current thread. If this call
      * is followed by a {@link #get()} before a {@link #set},
      * {@code #get()} will call {@link #initialValue()} and create a new
      * entry with the resulting value.
      *
      * @since 1.5
      */
     public void remove() {
         Thread currentThread = Thread.currentThread();
         Values values = values(currentThread);
         if (values != null) {
             values.remove(this);
         }
     }

     /**
      * Creates Values instance for this thread and variable type.
      */
     Values initializeValues(Thread current) {
         return current.localValues = new Values();
     }

     /**
      * Gets Values instance for this thread and variable type.
      */
     Values values(Thread current) {
         return current.localValues;
     }

     /** Weak reference to this thread local instance. */
     private final Reference<ThreadLocal<T>> reference
             = new WeakReference<ThreadLocal<T>>(this);

     /** Hash counter. */
     private static AtomicInteger hashCounter = new AtomicInteger(0);

     /**
      * Internal hash. We deliberately don't bother with #hashCode().
      * Hashes must be even. This ensures that the result of
      * (hash & (table.length - 1)) points to a key and not a value.
      *
      * We increment by Doug Lea's Magic Number(TM) (*2 since keys are in
      * every other bucket) to help prevent clustering.
      */
     private final int hash = hashCounter.getAndAdd(0x61c88647 * 2);

     /**
      * Per-thread map of ThreadLocal instances to values.
      */
     static class Values {

         /**
          * Size must always be a power of 2.
          */
         private static final int INITIAL_SIZE = 16;

         /**
          * Placeholder for deleted entries.
          */
         private static final Object TOMBSTONE = new Object();

         /**
          * Map entries. Contains alternating keys (ThreadLocal) and values.
          * The length is always a power of 2.
          */
         private Object[] table;

         /** Used to turn hashes into indices. */
         private int mask;

         /** Number of live entries. */
         private int size;

         /** Number of tombstones. */
         private int tombstones;

         /** Maximum number of live entries and tombstones. */
         private int maximumLoad;

         /** Points to the next cell to clean up. */
         private int clean;

         /**
          * Constructs a new, empty instance.
          */
         Values() {
             initializeTable(INITIAL_SIZE);
             this.size = 0;
             this.tombstones = 0;
         }

         /**
          * Used for InheritableThreadLocals.
          */
         Values(Values fromParent) {
             this.table = fromParent.table.clone();
             this.mask = fromParent.mask;
             this.size = fromParent.size;
             this.tombstones = fromParent.tombstones;
             this.maximumLoad = fromParent.maximumLoad;
             this.clean = fromParent.clean;
             inheritValues(fromParent);
         }

         /**
          * Inherits values from a parent thread.
          */
         @SuppressWarnings({"unchecked"})
         private void inheritValues(Values fromParent) {
             // Transfer values from parent to child thread.
             Object[] table = this.table;
             for (int i = table.length - 2; i >= 0; i -= 2) {
                 Object k = table[i];

                 if (k == null || k == TOMBSTONE) {
                     // Skip this entry.
                     continue;
                 }

                 // The table can only contain null, tombstones and references.
                 Reference<InheritableThreadLocal<?>> reference
                         = (Reference<InheritableThreadLocal<?>>) k;
                 // Raw type enables us to pass in an Object below.
                 InheritableThreadLocal key = reference.get();
                 if (key != null) {
                     // Replace value with filtered value.
                     // We should just let exceptions bubble out and tank
                     // the thread creation
                     table[i + 1] = key.childValue(fromParent.table[i + 1]);
                 } else {
                     // The key was reclaimed.
                     table[i] = TOMBSTONE;
                     table[i + 1] = null;
                     fromParent.table[i] = TOMBSTONE;
                     fromParent.table[i + 1] = null;

                     tombstones++;
                     fromParent.tombstones++;

                     size--;
                     fromParent.size--;
                 }
             }
         }

         /**
          * Creates a new, empty table with the given capacity.
          */
         private void initializeTable(int capacity) {
             this.table = new Object[capacity * 2];
             this.mask = table.length - 1;
             this.clean = 0;
             this.maximumLoad = capacity * 2 / 3; // 2/3
         }

         /**
          * Cleans up after garbage-collected thread locals.
          */
         private void cleanUp() {
             if (rehash()) {
                 // If we rehashed, we needn't clean up (clean up happens as
                 // a side effect).
                 return;
             }

             if (size == 0) {
                 // No live entries == nothing to clean.
                 return;
             }

             // Clean log(table.length) entries picking up where we left off
             // last time.
             int index = clean;
             Object[] table = this.table;
             for (int counter = table.length; counter > 0; counter >>= 1,
                     index = next(index)) {
                 Object k = table[index];

                 if (k == TOMBSTONE || k == null) {
                     continue; // on to next entry
                 }

                 // The table can only contain null, tombstones and references.
                 @SuppressWarnings("unchecked")
                 Reference<ThreadLocal<?>> reference
                         = (Reference<ThreadLocal<?>>) k;
                 if (reference.get() == null) {
                     // This thread local was reclaimed by the garbage collector.
                     table[index] = TOMBSTONE;
                     table[index + 1] = null;
                     tombstones++;
                     size--;
                 }
             }

             // Point cursor to next index.
             clean = index;
         }

         /**
          * Rehashes the table, expanding or contracting it as necessary.
          * Gets rid of tombstones. Returns true if a rehash occurred.
          * We must rehash every time we fill a null slot; we depend on the
          * presence of null slots to end searches (otherwise, we'll infinitely
          * loop).
          */
         private boolean rehash() {
             if (tombstones + size < maximumLoad) {
                 return false;
             }

             int capacity = table.length >> 1;

             // Default to the same capacity. This will create a table of the
             // same size and move over the live entries, analogous to a
             // garbage collection. This should only happen if you churn a
             // bunch of thread local garbage (removing and reinserting
             // the same thread locals over and over will overwrite tombstones
             // and not fill up the table).
             int newCapacity = capacity;

             if (size > (capacity >> 1)) {
                 // More than 1/2 filled w/ live entries.
                 // Double size.
                 newCapacity = capacity * 2;
             }

             Object[] oldTable = this.table;

             // Allocate new table.
             initializeTable(newCapacity);

             // We won't have any tombstones after this.
             this.tombstones = 0;

             // If we have no live entries, we can quit here.
             if (size == 0) {
                 return true;
             }

             // Move over entries.
             for (int i = oldTable.length - 2; i >= 0; i -= 2) {
                 Object k = oldTable[i];
                 if (k == null || k == TOMBSTONE) {
                     // Skip this entry.
                     continue;
                 }

                 // The table can only contain null, tombstones and references.
                 @SuppressWarnings("unchecked")
                 Reference<ThreadLocal<?>> reference
                         = (Reference<ThreadLocal<?>>) k;
                 ThreadLocal<?> key = reference.get();
                 if (key != null) {
                     // Entry is still live. Move it over.
                     add(key, oldTable[i + 1]);
                 } else {
                     // The key was reclaimed.
                     size--;
                 }
             }

             return true;
         }

         /**
          * Adds an entry during rehashing. Compared to put(), this method
          * doesn't have to clean up, check for existing entries, account for
          * tombstones, etc.
          */
         void add(ThreadLocal<?> key, Object value) {
             for (int index = key.hash & mask;; index = next(index)) {
                 Object k = table[index];
                 if (k == null) {
                     table[index] = key.reference;
                     table[index + 1] = value;
                     return;
                 }
             }
         }

         /**
          * Sets entry for given ThreadLocal to given value, creating an
          * entry if necessary.
          */
         void put(ThreadLocal<?> key, Object value) {
             cleanUp();

             // Keep track of first tombstone. That's where we want to go back
             // and add an entry if necessary.
             int firstTombstone = -1;

             for (int index = key.hash & mask;; index = next(index)) {
                 Object k = table[index];

                 if (k == key.reference) {
                     // Replace existing entry.
                     table[index + 1] = value;
                     return;
                 }

                 if (k == null) {
                     if (firstTombstone == -1) {
                         // Fill in null slot.
                         table[index] = key.reference;
                         table[index + 1] = value;
                         size++;
                         return;
                     }

                     // Go back and replace first tombstone.
                     table[firstTombstone] = key.reference;
                     table[firstTombstone + 1] = value;
                     tombstones--;
                     size++;
                     return;
                 }

                 // Remember first tombstone.
                 if (firstTombstone == -1 && k == TOMBSTONE) {
                     firstTombstone = index;
                 }
             }
         }

         /**
          * Gets value for given ThreadLocal after not finding it in the first
          * slot.
          */
         Object getAfterMiss(ThreadLocal<?> key) {
             Object[] table = this.table;
             int index = key.hash & mask;

             // If the first slot is empty, the search is over.
             if (table[index] == null) {
                 Object value = key.initialValue();

                 // If the table is still the same and the slot is still empty...
                 if (this.table == table && table[index] == null) {
                     table[index] = key.reference;
                     table[index + 1] = value;
                     size++;

                     cleanUp();
                     return value;
                 }

                 // The table changed during initialValue().
                 put(key, value);
                 return value;
             }

             // Keep track of first tombstone. That's where we want to go back
             // and add an entry if necessary.
             int firstTombstone = -1;

             // Continue search.
             for (index = next(index);; index = next(index)) {
                 Object reference = table[index];
                 if (reference == key.reference) {
                     return table[index + 1];
                 }

                 // If no entry was found...
                 if (reference == null) {
                     Object value = key.initialValue();

                     // If the table is still the same...
                     if (this.table == table) {
                         // If we passed a tombstone and that slot still
                         // contains a tombstone...
                         if (firstTombstone > -1
                                 && table[firstTombstone] == TOMBSTONE) {
                             table[firstTombstone] = key.reference;
                             table[firstTombstone + 1] = value;
                             tombstones--;
                             size++;

                             // No need to clean up here. We aren't filling
                             // in a null slot.
                             return value;
                         }

                         // If this slot is still empty...
                         if (table[index] == null) {
                             table[index] = key.reference;
                             table[index + 1] = value;
                             size++;

                             cleanUp();
                             return value;
                         }
                     }

                     // The table changed during initialValue().
                     put(key, value);
                     return value;
                 }

                 if (firstTombstone == -1 && reference == TOMBSTONE) {
                     // Keep track of this tombstone so we can overwrite it.
                     firstTombstone = index;
                 }
             }
         }

         /**
          * Removes entry for the given ThreadLocal.
          */
         void remove(ThreadLocal<?> key) {
             cleanUp();

             for (int index = key.hash & mask;; index = next(index)) {
                 Object reference = table[index];

                 if (reference == key.reference) {
                     // Success!
                     table[index] = TOMBSTONE;
                     table[index + 1] = null;
                     tombstones++;
                     size--;
                     return;
                 }

                 if (reference == null) {
                     // No entry found.
                     return;
                 }
             }
         }

         /**
          * Gets the next index. If we're at the end of the table, we wrap back
          * around to 0.
          */
         private int next(int index) {
             return (index + 2) & mask;
         }
     }
 }

简单的看一下Java源码，我们就会发现，你set一个对象到ThreadLocal的时候，其实这个对象是被保存到当前线程的ThreadLocalMap里的，而每一个Thread里都包含一个ThreadLocal.ThreadLocalMap threadLocals = null;代码：

     public void set(T value) {
         Thread t = Thread.currentThread();
         ThreadLocalMap map = getMap(t);
         if (map != null)
             map.set(this, value);
         else
             createMap(t, value);
     }

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

 public
 class Thread implements Runnable {
     /* Make sure registerNatives is the first thing <clinit> does. */
     private static native void registerNatives();
     static {
         registerNatives();
     }

     private char    name[];
     private int         priority;
     private Thread    threadQ;
     private long    eetop;
     ...
     /* ThreadLocal values pertaining to this thread. This map is maintained
      * by the ThreadLocal class. */
     ThreadLocal.ThreadLocalMap threadLocals = null;

重点看这句：

 map.set(this, value);

可能有人要问了，如果只是简单的将value放到Map对象里面，那不可能起到每个线程各自保存一份对象的目的啊，我们继续看map.set(this, value)方法：

         /**
          * 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();
         }

看到了吧，不是简单的值传递，而是重新创建了一个对象并保存起来。然后get的时候当然就是线程自己创建的对象喽，其实实现原理很简单，不是吗？