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 > () {
 *             &#64;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？