【Java】深入理解ThreadLocal

一、前言

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

解决线程安全问题,本质上就是解决资源共享问题,一般有以下手段:

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

二、Thread-Local

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

三、ThreadLocal的用法

在理解ThreadLocal之前,首先看一下它的用法:

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

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

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

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

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

class MyThread extends Thread {
    int id;

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

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

其打印结果如下:

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

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

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

ThreadLocal类中可重载的方法只有四个:

  1. set():设置值,也就是说,我们选择将某个值设置为ThreadLocal类型的;
  2. get():将设置进去的值取出来;
  3. remove():我们不想将某个值设置为ThreadLocal了,移除掉;
  4. initialValue():如果get的时候还没有设置值,就使用这个方法进行初始化;

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

四、ThreadLocal的实现

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

ThreadLocal的实现代码略长,我们选取重要的方法作为切入点:

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

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

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

A.如果要设置更多的值怎么办?也就是说,我们有多种资源需要共享怎么办?
B.为每一个线程共享一个资源,如何回收?

我们言归正传,从源码中获取答案。

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

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

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

/*ThreadLocal values pertaining to this thread. This map is maintained
 * by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
 这个属性是专门为ThreadLocal类而存在的,而它的实现也存在于ThreadLocal中,是ThreadLocal的一个静态内部类。其类注释如下:
/**
 * ThreadLocalMap is a customized hash map suitable only for
 * maintaining thread local values. No operations are exported
 * outside of the ThreadLocal class. The class is package private to
 * allow declaration of fields in class Thread.  To help deal with
 * very large and long-lived usages, the hash table entries use
 * WeakReferences for keys. However, since reference queues are not
 * used, stale entries are guaranteed to be removed only when
 * the table starts running out of space.
 */

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

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

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

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

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

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

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

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

/**
 * Variant of set() to establish initialValue. Used instead
 * of set() in case user has overridden the set() method.
 *
 * @return the initial value
 */
private T setInitialValue() {
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
    return value;
}

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

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

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

简单明了,而ThreadLocalMap的这个构造方法的实现如下:

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

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

至于set方法,看完get()后应该很简单了,自己都可以实现:

/**
 * Sets the current thread's copy of this thread-local variable
 * to the specified value.  Most subclasses will have no need to
 * override this method, relying solely on the {@link #initialValue}
 * method to set the values of thread-locals.
 *
 * @param value the value to be stored in the current thread's copy of
 *        this thread-local.
 */
public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}

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

/* Each thread holds an implicit reference to its copy of a thread-local
 * variable as long as the thread is alive and the <tt>ThreadLocal</tt>
 * instance is accessible; after a thread goes away, all of its copies of
 * thread-local instances are subject to garbage collection (unless other
 * references to these copies exist).
*/

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

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

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

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

/**
 * This class provides thread-local variables.  These variables differ from
 * their normal counterparts in that each thread that accesses one (via its
 * <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized
 * copy of the variable.  <tt>ThreadLocal</tt> instances are typically private
 * static fields in classes that wish to associate state with a thread (e.g.,
 * a user ID or Transaction ID).
 *
 * <p>For example, the class below generates unique identifiers local to each
 * thread.
 * A thread's id is assigned the first time it invokes <tt>ThreadId.get()</tt>
 * and remains unchanged on subsequent calls.
 * <pre>
 * import java.util.concurrent.atomic.AtomicInteger;
 *
 * public class ThreadId {
 *     // Atomic integer containing the next thread ID to be assigned
 *     private static final AtomicInteger nextId = new AtomicInteger(0);
 *
 *     // Thread local variable containing each thread's ID
 *     private static final ThreadLocal&lt;Integer> threadId =
 *         new ThreadLocal&lt;Integer>() {
 *             &#64;Override protected Integer initialValue() {
 *                 return nextId.getAndIncrement();
 *         }
 *     };
 *
 *     // Returns the current thread's unique ID, assigning it if necessary
 *     public static int get() {
 *         return threadId.get();
 *     }
 * }
 * </pre>
 * <p>Each thread holds an implicit reference to its copy of a thread-local
 * variable as long as the thread is alive and the <tt>ThreadLocal</tt>
 * instance is accessible; after a thread goes away, all of its copies of
 * thread-local instances are subject to garbage collection (unless other
 * references to these copies exist).
 *
 * @author  Josh Bloch and Doug Lea
 * @since   1.2
 */
public class ThreadLocal<T> {
    /**
     * ThreadLocals rely on per-thread linear-probe hash maps attached
     * to each thread (Thread.threadLocals and
     * inheritableThreadLocals).  The ThreadLocal objects act as keys,
     * searched via threadLocalHashCode.  This is a custom hash code
     * (useful only within ThreadLocalMaps) that eliminates collisions
     * in the common case where consecutively constructed ThreadLocals
     * are used by the same threads, while remaining well-behaved in
     * less common cases.
     */
    private final int threadLocalHashCode = nextHashCode();

    /**
     * The next hash code to be given out. Updated atomically. Starts at
     * zero.
     */
    private static AtomicInteger nextHashCode =
        new AtomicInteger();

    /**
     * The difference between successively generated hash codes - turns
     * implicit sequential thread-local IDs into near-optimally spread
     * multiplicative hash values for power-of-two-sized tables.
     */
    private static final int HASH_INCREMENT = 0x61c88647;

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

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

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

    /**
     * Returns the value in the current thread's copy of this
     * thread-local variable.  If the variable has no value for the
     * current thread, it is first initialized to the value returned
     * by an invocation of the {@link #initialValue} method.
     *
     * @return the current thread's value of this thread-local
     */
    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();
    }

    /**
     * Variant of set() to establish initialValue. Used instead
     * of set() in case user has overridden the set() method.
     *
     * @return the initial value
     */
    private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

    /**
     * Sets the current thread's copy of this thread-local variable
     * to the specified value.  Most subclasses will have no need to
     * override this method, relying solely on the {@link #initialValue}
     * method to set the values of thread-locals.
     *
     * @param value the value to be stored in the current thread's copy of
     *        this thread-local.
     */
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

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

    /**
     * Get the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param  t the current thread
     * @return the map
     */
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

    /**
     * Create the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param t the current thread
     * @param firstValue value for the initial entry of the map
     * @param map the map to store.
     */
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

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

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

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

        /**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal> {
            /** The value associated with this ThreadLocal. */
            Object value;

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

        /**
         * The initial capacity -- MUST be a power of two.
         */
        private static final int INITIAL_CAPACITY = 16;

        /**
         * The table, resized as necessary.
         * table.length MUST always be a power of two.
         */
        private Entry[] table;

        /**
         * The number of entries in the table.
         */
        private int size = 0;

        /**
         * The next size value at which to resize.
         */
        private int threshold; // Default to 0

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

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

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

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

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

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

        /**
         * Get the entry associated with key.  This method
         * itself handles only the fast path: a direct hit of existing
         * key. It otherwise relays to getEntryAfterMiss.  This is
         * designed to maximize performance for direct hits, in part
         * by making this method readily inlinable.
         *
         * @param  key the thread local object
         * @return the entry associated with key, or null if no such
         */
        private Entry getEntry(ThreadLocal key) {
            int i = key.threadLocalHashCode & (table.length - 1);
            Entry e = table[i];
            if (e != null && e.get() == key)
                return e;
            else
                return getEntryAfterMiss(key, i, e);
        }

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

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

        /**
         * Set the value associated with key.
         *
         * @param key the thread local object
         * @param value the value to be set
         */
        private void set(ThreadLocal key, Object value) {

            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.

            Entry[] tab = table;
            int len = tab.length;
            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal k = e.get();

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

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

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

        /**
         * Remove the entry for key.
         */
        private void remove(ThreadLocal key) {
            Entry[] tab = table;
            int len = tab.length;
            int i = key.threadLocalHashCode & (len-1);
            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                if (e.get() == key) {
                    e.clear();
                    expungeStaleEntry(i);
                    return;
                }
            }
        }

        /**
         * Replace a stale entry encountered during a set operation
         * with an entry for the specified key.  The value passed in
         * the value parameter is stored in the entry, whether or not
         * an entry already exists for the specified key.
         *
         * As a side effect, this method expunges all stale entries in the
         * "run" containing the stale entry.  (A run is a sequence of entries
         * between two null slots.)
         *
         * @param  key the key
         * @param  value the value to be associated with key
         * @param  staleSlot index of the first stale entry encountered while
         *         searching for key.
         */
        private void replaceStaleEntry(ThreadLocal key, Object value,
                                       int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;
            Entry e;

            // Back up to check for prior stale entry in current run.
            // We clean out whole runs at a time to avoid continual
            // incremental rehashing due to garbage collector freeing
            // up refs in bunches (i.e., whenever the collector runs).
            int slotToExpunge = staleSlot;
            for (int i = prevIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = prevIndex(i, len))
                if (e.get() == null)
                    slotToExpunge = i;

            // Find either the key or trailing null slot of run, whichever
            // occurs first
            for (int i = nextIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = nextIndex(i, len)) {
                ThreadLocal k = e.get();

                // If we find key, then we need to swap it
                // with the stale entry to maintain hash table order.
                // The newly stale slot, or any other stale slot
                // encountered above it, can then be sent to expungeStaleEntry
                // to remove or rehash all of the other entries in run.
                if (k == key) {
                    e.value = value;

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

                    // Start expunge at preceding stale entry if it exists
                    if (slotToExpunge == staleSlot)
                        slotToExpunge = i;
                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
                    return;
                }

                // If we didn't find stale entry on backward scan, the
                // first stale entry seen while scanning for key is the
                // first still present in the run.
                if (k == null && slotToExpunge == staleSlot)
                    slotToExpunge = i;
            }

            // If key not found, put new entry in stale slot
            tab[staleSlot].value = null;
            tab[staleSlot] = new Entry(key, value);

            // If there are any other stale entries in run, expunge them
            if (slotToExpunge != staleSlot)
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
        }

        /**
         * Expunge a stale entry by rehashing any possibly colliding entries
         * lying between staleSlot and the next null slot.  This also expunges
         * any other stale entries encountered before the trailing null.  See
         * Knuth, Section 6.4
         *
         * @param staleSlot index of slot known to have null key
         * @return the index of the next null slot after staleSlot
         * (all between staleSlot and this slot will have been checked
         * for expunging).
         */
        private int expungeStaleEntry(int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;

            // expunge entry at staleSlot
            tab[staleSlot].value = null;
            tab[staleSlot] = null;
            size--;

            // Rehash until we encounter null
            Entry e;
            int i;
            for (i = nextIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = nextIndex(i, len)) {
                ThreadLocal k = e.get();
                if (k == null) {
                    e.value = null;
                    tab[i] = null;
                    size--;
                } else {
                    int h = k.threadLocalHashCode & (len - 1);
                    if (h != i) {
                        tab[i] = null;

                        // Unlike Knuth 6.4 Algorithm R, we must scan until
                        // null because multiple entries could have been stale.
                        while (tab[h] != null)
                            h = nextIndex(h, len);
                        tab[h] = e;
                    }
                }
            }
            return i;
        }

        /**
         * Heuristically scan some cells looking for stale entries.
         * This is invoked when either a new element is added, or
         * another stale one has been expunged. It performs a
         * logarithmic number of scans, as a balance between no
         * scanning (fast but retains garbage) and a number of scans
         * proportional to number of elements, that would find all
         * garbage but would cause some insertions to take O(n) time.
         *
         * @param i a position known NOT to hold a stale entry. The
         * scan starts at the element after i.
         *
         * @param n scan control: <tt>log2(n)</tt> cells are scanned,
         * unless a stale entry is found, in which case
         * <tt>log2(table.length)-1</tt> additional cells are scanned.
         * When called from insertions, this parameter is the number
         * of elements, but when from replaceStaleEntry, it is the
         * table length. (Note: all this could be changed to be either
         * more or less aggressive by weighting n instead of just
         * using straight log n. But this version is simple, fast, and
         * seems to work well.)
         *
         * @return true if any stale entries have been removed.
         */
        private boolean cleanSomeSlots(int i, int n) {
            boolean removed = false;
            Entry[] tab = table;
            int len = tab.length;
            do {
                i = nextIndex(i, len);
                Entry e = tab[i];
                if (e != null && e.get() == null) {
                    n = len;
                    removed = true;
                    i = expungeStaleEntry(i);
                }
            } while ( (n >>>= 1) != 0);
            return removed;
        }

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

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

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

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

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

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

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

五、ThreadLocal作用

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

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

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

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