ThreadLocal源码分析

1、概述

ThreadLocal，可以理解为线程的局部变量，作用就是为每一个使用该变量的线程都提供一个变量值的副本，每一个线程都可以独立地改变自己的副本，而不会和其它线程的副本冲突。

ThreadLocal是如何做到为每一个线程维护变量的副本的呢？

每个线程中都有一个ThreadLocalMap（Thread.threadLocals），用于存储每一个线程的变量的副本。

ThreadLocalMap使用数组Entry[] table保存ThreadLocal-->Object键值对象，数组保存位置：int i = key.nextHashCode() & (table.length - 1);。

ThreadLocal和Synchonized区别：

都用于解决多线程并发访问。

Synchronized用于线程间的数据共享（使变量或代码块在某一时该只能被一个线程访问），是一种以延长访问时间来换取线程安全性的策略；

而ThreadLocal则用于线程间的数据隔离（为每一个线程都提供了变量的副本），是一种以空间来换取线程安全性的策略。

2、ThreadLocalMap

用于存储每一个线程的变量的副本

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

        /**
         * 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);
        }
/**
         * 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;
                }
            }
        }

        ......　
    }

3、ThreadLocal

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 {@code initialValue} 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 {@code null}; if the
     * programmer desires thread-local variables to have an initial
     * value other than {@code null}, {@code ThreadLocal} must be
     * subclassed, and this method overridden.  Typically, an
     * anonymous inner class will be used.
     *
     * @return the initial value for this thread-local
     */
　　　//返回此线程局部变量的当前线程的初始值
　　 protected T initialValue() {
        return null;
    }

    /**
     * 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
     */
　　 //返回此线程局部变量的当前线程副本中的值
    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = t.threadLocals;
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }


    /**
     * 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.
     */
　　//将此线程局部变量的当前线程副本中的值设置为指定值
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = t.threadLocals;
        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
     * {@code initialValue} method in the current thread.
     *
     * @since 1.5
     */
　　　//移除此线程局部变量的值
     public void remove() {
         ThreadLocalMap m = Thread.currentThread().threadLocals;
         if (m != null)
             m.remove(this);
     }

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

　　......
}

为什么取HASH_INCREMENT = 0x61c88647?

（可以阅读Why 0x61c88647?）

由来：

This number represents the golden ratio (sqrt(5)-1) times two to the power of 31. The result is then a golden number, either 2654435769 or -1640531527. You can see the calculation here:

long l1 = (long) ((1L << 31) * (Math.sqrt(5) - 1));  //Math.sqrt(5) - 1 = 1.2360679774997898
System.out.println("as 32 bit unsigned: " + l1);  //2654435769
        
int i1 = (int) l1;
System.out.println("as 32 bit signed:   " + i1);  //-1640531527 = -0x61c88647

与fibonacci hashing(斐波那契散列法)以及黄金分割有关，特殊的哈希码0x61c88647大大降低碰撞的几率，能让哈希码能均匀的分布在2的N次方的数组里。

key.threadLocalHashCode & (len-1)，ThreadLocalMap 中 Entry[] table 的大小必须是2的N次方呀(len = 2^N)，那 len-1 的二进制表示就是低位连续的N个1， 那 key.threadLocalHashCode & (len-1) 的值就是 threadLocalHashCode 的低N位。

测试：

private static AtomicInteger nextHashCode = new AtomicInteger();

    private static final int HASH_INCREMENT = 0x61c88647;

    private static int nextHashCode() {
        return nextHashCode.getAndAdd(HASH_INCREMENT);
    }

    public static void main(String[] args) {
        for (int j = 0; j < 5; j++) {
            int size = 2 << j;
            // hash = 0;
            int[] indexArray = new int[size];
            for (int i = 0; i < size; i++) {
                indexArray[i] = nextHashCode() & (size - 1);
            }
            System.out.println("indexs = "+ Arrays.toString(indexArray));
        }
    }

结果：

indexs = [0, 1]
indexs = [2, 1, 0, 3]
indexs = [2, 1, 0, 7, 6, 5, 4, 3]
indexs = [2, 9, 0, 7, 14, 5, 12, 3, 10, 1, 8, 15, 6, 13, 4, 11]
indexs = [18, 25, 0, 7, 14, 21, 28, 3, 10, 17, 24, 31, 6, 13, 20, 27, 2, 9, 16, 23, 30, 5, 12, 19, 26, 1, 8, 15, 22, 29, 4, 11]

没有看到重复的索引值，要哈希表的大小是2的N次方,那么基本上可以保证每次计算出的index值都不会重复。

为什么HashCode不直接用自增的方式(HASH_INCREMENT=1)?

我的理解是，随着不用的 ThreadLocal 变量被回收掉，这种自增的方式的性能会越来越差，因为临近的 slot 为空的可能性很小。而 ThreadLocal 实际所采用的方式，其下标是在跳跃分布，这样即使出现冲突，在临近找到空 slot 的可能性更大一些，性能也会更好。

4、例子

Android Looper的实现：

public final class Looper {
　　
　　 ......
    
    // sThreadLocal.get() will return null unless you've called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

    private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

    /**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static Looper myLooper() {
        return sThreadLocal.get();
    }

　　 ......

}

 

ThreadId:

维护每个线程的id

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<Integer> threadId = new ThreadLocal<Integer>(){
            @Override
            protected Integer initialValue () {
                return nextId.getAndIncrement();
            }
        };

        // Returns the current thread's unique ID, assigning it if necessary
        public static int get() {
            return threadId.get();
        }
    }