Java中的ThreadLocal源码解析（上）

这一篇之所以讲ThreadLocal，是因为之前在读Handler,Looper的源码过程（见http://maosidiaoxian.iteye.com/blog/1927735）中，看到了这个类，引起了我的兴趣。而后来发现JAVA1.6中的TheadLocal类，和我在android源码看到的这个ThreadLocal类代码是不一样的。所以这篇先讲一下Java的ThreadLocal。
Java中ThreadLocal在Java1.2就已经提出了，后来重构过，所以我在想android中的这个类的实现是不是重构过之前的版本优化过的，由于未找到早前版本该类的代码，所以只能进行猜测。

ThreadLocal用于提供线程局部变量，即通过这里的get()和set()方法访问某个变量的线程都有自己的局部变量。但在这里需要注意的是，它是通过各个线程自己创建对象然后调用这里的set方法设置进去，而不是由ThreadLocal自己来创建变量的副本的。就像上面链接的那一篇文章里面提到的android的Looper类所用的（虽然Android里ThreadLocal实现代码不一样，但用法是一样的），代码如下：

    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));
    }

可以看到，线程中的局部变量，是在线程中创建，然后调用ThreadLocal.set()方法设置进去的。

下面详细读一下ThreadLocal这个类，看看它是怎么实现的（jdk1.6的源码）。


从上图可以看到ThreadLocal有三个变（常）量，代码如下：

    /**
     * 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;

其中HASH_INCREMENT 是一个常量，它表示连续的两个ThreadLocal实例的threadLocalHashCode值的增量。至于为什么用0x61c88647常量，我也没明白，只百度出一堆跟加密算法有关的内容。nextHashCode变量是AtomicInteger类型，AtomicInteger是一个提供原子操作的Integer类，简单而言就是不用Synchronized关键字也可以进行线程安全的加减操作。

当创建一个ThreadLocal对象时，会进行以下操作：

    public ThreadLocal() {
    }

    private final int threadLocalHashCode = nextHashCode();

我们再来看nextHashCode()这个方法，代码如下：

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

它是返回一个hash code，同时对nextHashCode加上增量HASH_INCREMENT。也就是在初始化ThreadLocal的实例过程中，做的仅仅是将一个哈希值赋给实例的threadLocalHashCode，并生成下一个哈希值。而前面可以看到threadLocalHashCode是final的，在个类中，它用来区分不同的ThreadLocal对象。

在前面的图当中，我们可以看到在ThreadLocal里还有一个ThreadLocalMap的内部类，从类名来看应该是被设计来保存线程局部变量的Map，但是在ThreadLocal类或它的实例当中，并没有其他变量，那么通过ThreadLocal.set()放进去的值又是怎么保存的呢？
我们继续看ThreadLocal的set()和其他方法：

    /**
     * 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);
    }

    /**
     * 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);
    }

从上面的代码可以看到，set()方法是调用ThreadLocalMap.set()方法保存到ThreadLocalMap实例当中，但是ThreadLocalMap实例却不是保存在Thread中，它通过getMap()方法去取，当取不到的时候就去创建，但是创建之后却是保存在Thread实例中。可以看到Thread.java的代码有如下声明：

    ThreadLocal.ThreadLocalMap threadLocals = null;

这种设计很巧妙，线程中有各自的map，而把ThreadLocal实例作为key，这样除了当线程销毁时相关的线程局部变量被销毁之外，还让性能提升了很多。

看完set()方法，再来看get()方法，它的代码如下：

    /**
     * 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;
    }

    /**
     * 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;
    }

可以看到在get()方法中，是通过当前线程获取map，再从map当中取得对象。如果取不到（如map为null或取到的值为null），则调用setInitialValue()方法对其设置初始值。setInitialValue()方法会调用initialValue()方法得到一个初始值（默认为null），然后当Thread中的map不为空时，把初始值设置进去，否则为它创建一个ThreadLocalMap对象（但不会调用map的set方法保存这个初始值），最后返回这个初始值。

最后看remove(）方法，它提供了移除此线程局部变量在当前进程的值。代码如下：

    /**
     * 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);
     }

关于ThreadLocal就读到这里，有时间再写下它的内部类ThreadLocalMap。
参考文章：
lujh99. 正确理解ThreadLocal. http://www.iteye.com/topic/103804