探秘ThreadLocal

一　类结构

主要是set(T), get(), remove()方法

二  TheadLocal是什么时候创建的

　　threadLocal的初始化， lazy creating， 用到的时候(get 或者 set)再初始化

　　　　　　

void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);// this 指代的是theadlocal
}

private T setInitialValue() {
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
    return 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();
}

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

三 TheadLocal 是如何保存的

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

初始化时，threadlocal作为第一个key，

由ThreadLocal中的 createMap方法和set方法 new ThreadLocalMap(this, firstValue) 结合 

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

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();//threadLocal的实际引用地址。

        if (k == key) {
            e.value = value; //第N次set，将线程变量值实际存入Entry，ThreadLocalMap中维护由Entry组成的数组
            return;
        }

        if (k == null) {
            replaceStaleEntry(key, value, i);//第一次set
            return;
        }
    }

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

上文的代码解读：由于Entry继承自WeakReference，所以e.get()指代的是TheadLocal的实际引用地址。

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

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

以上代码就是变量值是如何存入的，下面来看怎么取出线程变量值

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();
}

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

总结，线程变量threadLocal是通过自定义实现的hash存于TheadLocalMap中的Entry数组的。

四 TheadLocal是如何销毁的

public void remove() {
    ThreadLocalMap m = getMap(Thread.currentThread());
    if (m != null)
        m.remove(this);
}

结束语

为什么要用WeakReference, 当threadlocal最后一次被get后， 就不会再有引用指向threadlocal了，从而ThreadLocalMap中的Entry所存的变量值就可以被GC回收了。

在使用完线程变量后， 一定要及时清理，remove，尤其是在使用线程池的情况下。否则会造成内存溢出。