Java8 ThreadLocal 源码解析

前言

ThreadLocal ，像是一个神秘的黑衣人，令人望而生畏。唯有下定决心，一探究竟，方能解开他神秘的面纱、在Android中，Handler，EventBus，ConnectionPool 等等，都曾出现它的身影

是什么东西？

看到Thread，就想到应该是与线程有关吧，其次，Local是说本地，那组合起来就是线程私有，就是说每个线程都有备份，各备份不是同一个对象，一般来说，他的用途就是让各个线程拥有不用的对象，它的对象都是 new 出来的，哪有人可能会问，既然是 new 出来的，那为什么还要用 ThreadLocal，直接 new 一个对象不就行了，你如果不仔细了解 ThreadLocal，就很难解答这样的问题

前世今生

是一个泛型类

public class ThreadLocal<T> {}

源码解析

属性

nextHashCode 是获取 hashcode 值，HASH_INCREMENT 初始值为0x61c88647，nextHashCode()函数每次调用增加增量获取下一个值，大家可以好好看看Atomic的实现，以及底层Unsafe的语义

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);
}

函数

初始值函数

protected T initialValue() {
    return null;
}

set ，将值拷贝到当前线程，使用ThreadLocalMap，这是一个内部类，要了解ThreadLocal，这个类是不得不了解的

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;
}

一探ThreadLocalMap，前方高能，他是一个静态内部类，类似于WeakHashMap，只不过他没有WeakHashMap那么强大，没有Map那么多的接口，不过作为一个内部类，只需要为ThreadLocal服务就可以了，这一条可以在EffectiveJava里找到，废话不多说，看一下他的Entry，继承自WeakReference，关于WeakReference，又可以花大篇幅来介绍了，不过现在暂时不说了，你只要记住当发生GC的时候，WeakReference的对象都会被回收。同样内部也是用Entry数组，默认大小为16，set 函数需要注意的是如何处理哈希碰撞的问题，这里扯一下，处理哈希碰撞的如HashMap使用了拉链法，而这里使用了简单的开发地址法，具体来说就是如果发生了碰撞，就位置就+1，一直到有位置。

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 函数，同样也是使用key的hashcode 与 len-1的
        // 与值
        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();
        }

最重要的 get 函数，我们需要记住的是，每个线程有一个ThreadLocalMap，每个Map可以放很多个值，也就是不用的ThreadLocal对象。如果Get 不到数据的话，就会调用InitialValue函数进行赋值，所以有时候我们会重写这个函数，以保证get函数都能获取到正确的值

/**
 * 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) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();
}

由于使用了WeakReference，可能会存在值丢失，那ThreadLocalMap又是怎么处理这种情况呢？首先自然是处理hash碰撞的问题，通过nextIndex来遍历，其中如果出现某个key获取不到的话，就会执行
expungeStaleEntry(i) 删除旧的entry，具体的做法就是删除这个entry，以及之后出现为空的所有entry

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;
        }

        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;
        }

小结

一个 ThreadLocal 竟包含了如此多的知识，当你熟读各种源码的时候，各种设计模式，各种细节，作者实现的非常有借鉴意义，理解了这样的源码，在今后的项目中使用类似的，便可以知其然知其所以然，做到各种框架的深度定制，甚至实现自己的框架！

欢迎讨论～