ThreadLocal的使用及实现

编程技术  /  houtizong 发布于 3年前   68

 

在java中，如果一个变量需要被多个线程访问，可以使用volatile来声明它为“易变的”。而假如一个变量要被持有它的某个线程独享，在java中，它可以使用java.lang.ThreadLocal类来实现线程本地存储的功能。这样，数据便只在同一个线程内可见和共享，因此，即便不同步也能保证线程之间不出现数据争用。

ThreadLocal使得各线程能够保持各自独立的一个对象，通过ThreadLocal.set()来设置对象的值，保存在每个线程自己都有的一个map对象当中，每个ThreadLocal对象会有一个线程范围内唯一hashcode作为key，ThreadLocal.get()内部通过这个key从map中取值，因此取出来的是各自自己线程中的对象，ThreadLocal实例事实上只是作为map的key来使用的。

一个ThreadLocal的例子：

 

package com.threadlocal.test;/** * @Author: chenkangxian * * @Annotation: * * @Date:2012-4-20 *  */public class test {class ConcurrentCount extends Thread{ThreadLocal<Integer> count ;public ConcurrentCount(ThreadLocal<Integer> count){this.count = count;}@Overridepublic void run() {for(int i = 0; i < 10; i ++){if(count.get() != null){count.set(count.get() + 1);}else{count.set(0);}System.out.println("Thread: " + this.currentThread().getName() + ", count: " + count.get());}return ;}}/** * Author: chenkangxian * * Last Modification Time: 2012-4-20 * * @param args */public static void main(String[] args) {//所有线程均使用该变量，但是却不存在线程安全问题ThreadLocal<Integer> count = new ThreadLocal<Integer>();test test = new test();ConcurrentCount count1 = test.new ConcurrentCount(count);ConcurrentCount count2 = test.new ConcurrentCount(count);ConcurrentCount count3 = test.new ConcurrentCount(count);ConcurrentCount count4 = test.new ConcurrentCount(count);count1.start();count2.start();count3.start();count4.start();}}

 

 

ThreadLocal使用归纳有两点：

1.每个线程中都有一个自己的ThreadLocalMap类对象，可以将线程自己的对象保持到其中，各管各的，线程可以正确的访问到自己的对象。 
2.将一个共用的ThreadLocal实例作为key，将不同对象的引用保存到不同线程的ThreadLocalMap中，然后在线程执行的各处通过这个静态ThreadLocal实例的get()方法取得自己线程保存的那个对象。

 

 

ThreadLocal实现的源代码:

 

/* * @(#)ThreadLocal.java1.42 06/06/23 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN 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 1.42, 06/23/06 * @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);            }        }    }}