ThreadLocal源码阅读

首先，从set方法入手，

    // ThreadLocal
public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);//这里可以看出，从Thread对象获取了一个ThreadLocalMap
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

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

    ThreadLocal.ThreadLocalMap threadLocals = null;

接下来，看如何创建的 这个特殊的Map

首次创建

ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
    table = new Entry[INITIAL_CAPACITY];//初始化空间为16
    int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);//机损索引
    table[i] = new Entry(firstKey, firstValue);//赋值
    size = 1;
    setThreshold(INITIAL_CAPACITY);//计算扩容的临界点
}

注意点：Entry 的 key 采用弱引用，在内存不足时候，会被回收

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

    Entry(ThreadLocal<?> k, Object v) {
        super(k);// k 采用弱引用，在内存不足时候，会被回收
        value = v;
    }
}

到这里，细节不说 应该了解大体的数据结构了

每个线程独有一个 Map，Map里的存储结构为 Entry <ThreadLocal,Object> 数组 

如果已存在，

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)]) {//循环，直到i位置无数据
        ThreadLocal<?> k = e.get();

        if (k == key) {//同一个ThreadLocal 替换 value
            e.value = value;
            return;
        }

        if (k == null) {// k 为空，替换 
            replaceStaleEntry(key, value, i);
            return;
        }
    }

    tab[i] = new Entry(key, value);
    int sz = ++size;
    if (!cleanSomeSlots(i, sz) && sz >= threshold)//清除无效Entry 并扩容
        rehash();
}

扩容

private void resize() {
    Entry[] oldTab = table;
    int oldLen = oldTab.length;
    int newLen = oldLen * 2;// 双倍空间
    Entry[] newTab = new Entry[newLen];
    int count = 0;

    for (int j = 0; j < oldLen; ++j) {
        Entry e = oldTab[j];
        if (e != null) {
            ThreadLocal<?> k = e.get();
            if (k == null) {
                e.value = null; // Help the GC
            } else {
                int h = k.threadLocalHashCode & (newLen - 1);
                while (newTab[h] != null)
                    h = nextIndex(h, newLen);
                newTab[h] = e;
                count++;
            }
        }
    }

    setThreshold(newLen);
    size = count;
    table = newTab;
}

基于逻辑完成，

get就比较简单了

先获取 Map ，然后根据 key 获取 value

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();
}

关于ThreadLocal 内存溢出问题，一般只会出现在使用 线程池的时候，ThreadLocalMap 一直得不到释放，即使 key 因为软引用，但value没有释放造成的，但可以通过执行 remove 方法来主动释放