ThreadLocal的源码解析

java.lang.ThreadLocal

存数据

1、获取当前线程持有的threadLocalMap对象
2、如果map对象不为空，则set值，key为threadLocal对象本身，value为set的值
3、如果map对象为null，则创建map对象并set值

    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

map对象为当前线程维护的ThreadLocalMap对象

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

如果是第一次set值，则new一个新的ThreadLocalMap对象并赋值，然后交给当前线程维护

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

Thread类中维护了一个ThreadLocalMap对象

• java.lang.Thread

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

• java.lang.ThreadLocal.ThreadLocalMap

    static class ThreadLocalMap {
        static class Entry extends WeakReference> {
            /** 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;
    }

       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;
            // 对key的hash与table.length取模，获取entry在数组中存储的位置
            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();
        }

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

取数据

1、获取当前线程维护的threadLocalMap
2、从map中获取Entry，key值为threadLocal本身
3、返回entry中的value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();
    }