ThreadLocal是什么

1、概念

ThreadLocal是一个线程内部数据存储类，每一个使用该变量的线程都获得该变量的一个副本，每个副本之间相互独立，使得每一个线程可以任意的修改和访问自己的变量副本，而不会对其他线程产生任何影响，即不同线程使用同一个ThreadLocal变量，但是他们通过TheadLocal获取到的值不同。

2、原理

ThreadLocal是一个泛型类，下面看一下set和get方法

    public class ThreadLocal<T>;

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

    ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {  // key指Thread对象引用
        table = new Entry[INITIAL_CAPACITY];
        int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
        table[i] = new Entry(firstKey, firstValue);
        size = 1;
        setThreshold(INITIAL_CAPACITY);
    }

Thread内部有一个ThreadLocalMap，用来存储线程的ThreadLocal数据，由ThreadLocal来维护，ThreadLocal内部有一个初始容量为16的Entry数组

    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;  // Thread.class成员变量

    Entry(ThreadLocal<?> k, Object v) {
        reference = k;  // k是ThreadLocal
        value = v;
    }

下面看一下map如何set的

        /**
         * 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();
        }

可以看出存储规则：由key计算在table中的位置，更新或者重新创建Entry

下面看一下get方法：首先通过reference（ThreadLocal）计算ThreadLocal在table中的位置并返回该位置处的Entry，Entry.value即是所需要的值

    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();
    }

总结：每一个Thread内部有一个ThreadLocalMap，其实是一个默认大小为16的Entry数组，Entry中保存了reference（该ThreadLocal对象）和对应的值value，ThreadLocal的get和set方法所操作的都是当前线程的Entry数组table