ThreadLocal提供了一個線程獨立的變量闽铐。在不同線程中調(diào)set,get設(shè)置的值都是和該線程相關(guān)的蝶怔。網(wǎng)上對ThreadLocal的實現(xiàn)眾說紛紜，今天來看下源碼兄墅√咝牵基于JDK1.8。

支持原創(chuàng)隙咸，轉(zhuǎn)載請注明出處沐悦。

類圖

ThreadLocal.png

ThreadLocal.set方法

    public void set(T value) {
        Thread t = Thread.currentThread();   //獲取當(dāng)前線程對象
        ThreadLocalMap map = getMap(t);   //獲取ThreadLocalMap對象
        if (map != null)
            map.set(this, value);               //調(diào)用ThreadLocalMap的set方法
        else
            createMap(t, value);
    }

    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;              //返回Thread的ThreadLocalMap成員變量
    }

    ThreadLocal.ThreadLocalMap threadLocals = null;  //Thread的ThreadLocalMap成員

該方法獲取當(dāng)前線程的Thread對象中的trheadLocal成員變量，調(diào)用它的set方法五督。

ThreadLocalMap.set方法

        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);   //通過hash值藏否，計算下標(biāo)

            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) { //不斷獲取下一個下標(biāo)直到某個槽沒有元素
                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);             //將該元素加入這個槽
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

我們看下Entry這個類：

        static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

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

這個類繼承自WeakReference，所以只要ThreadLocal對象存在充包，那么設(shè)置在里面的值就不會被回收副签。我們看下get方法。

ThreadLocal.get方法

    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();
    }

和set方法類似基矮。我們看下map.getEntry方法：

       private Entry getEntry(ThreadLocal<?> key) {
            int i = key.threadLocalHashCode & (table.length - 1); //計算下標(biāo)
            Entry e = table[i]; 
            if (e != null && e.get() == key)
                return e;
            else
                return getEntryAfterMiss(key, i, e);
        }

和set方法類似淆储，一看就明白了。

總結(jié)

我們最后回顧下類圖:

ThreadLocal.png

支持原創(chuàng)家浇，轉(zhuǎn)載請注明出處本砰。
github：https://github.com/gatsbydhn