Java线程变量ThreadLocal源码分析

//这里用到了泛型，ThreadLocal中只保存Looper对象。
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static void prepare(boolean quitAllowed) {
    if (sThreadLocal.get() != null) { //保证Looper只被创建一次。
        throw new RuntimeException("Only one Looper may be created per thread");
    }
    sThreadLocal.set(new Looper(quitAllowed));
}

  public void set(T value) {
        //拿到当前线程
        Thread t = Thread.currentThread();
        //得到一个map
        ThreadLocalMap map = getMap(t);
        if (map != null){
        // 这个map是以当前对象为key的，这个this就是 ThreadLocal的实例 sThreadLocal
            map.set(this, value);
        }else{
            createMap(t, value);
        }
    }

//getMap 是从Thread中拿到了一个threadLocals变量，是ThreadLocal.ThreadLocalMap 的实例。
//保存的数据也是存在了这个map中，这也就是为什么ThreadLocal是和线程绑定的，对其他线程来说是隔离的原因所在。
ThreadLocalMap getMap(Thread t) {
      return t.threadLocals;
}

private void set(ThreadLocal<?> key, Object value) {
      Entry[] tab = table;
      int len = tab.length;
      // 计算出key在集合中的索引，index
      int i = key.threadLocalHashCode & (len-1);
       //开始遍历整个数组，
       //取出索引为i的Entry，如果不为空，取出下一个，进行遍历
        for (Entry e = tab[i];
               e != null;
               e = tab[i = nextIndex(i, len)]) {
              ThreadLocal<?> k = e.get();
              //如果取出的k和传进来的key一致，则把新的值存起来。
              if (k == key) {
                  e.value = value;
                  return;
              }
              //直到取出最有一个，k==null则进行存储。
              if (k == null) {
                  replaceStaleEntry(key, value, i);
                  return;
              }
          }
          //如果索引i的位置，没有Entry，则把传进来的key和value保存在这个位置。
          tab[i] = new Entry(key, value);
          int sz =   size;
          //如果大于阈值了，则进行扩容
          if (!cleanSomeSlots(i, sz) && sz >= threshold)
              rehash();
}

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

 ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
    //创建一个大小为16的数组
    table = new Entry[INITIAL_CAPACITY];
    //计算得到的i是数组的角标。可以参考hashMap源码分析
    int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
    //赋值，保存数据
    table[i] = new Entry(firstKey, firstValue);
    size = 1;
    //扩容的阈值
    setThreshold(INITIAL_CAPACITY);
}

public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    //如果map不为空，则从map中取值
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    //如果map为空
    return setInitialValue();
}

//如果map不为空
 private Entry getEntry(ThreadLocal<?> key) {
    //拿到key对应的索引
    int i = key.threadLocalHashCode & (table.length - 1);
    //从数组中拿到Entry
    Entry e = table[i];
    if (e != null && e.get() == key){如果key一样直接返回
        return e;
    }else{//如果不一致则开始遍历
         return getEntryAfterMiss(key, i, e);
    }
}

private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
		Entry[] tab = table;
		int len = tab.length;
		while (e != null) {
			ThreadLocal<?> k = e.get();
			if (k == key)
				return e;
			if (k == null)
				expungeStaleEntry(i);
			else
				i = nextIndex(i, len);
			e = tab[i];
		}
		return null;
}

//如果map为空，则调用这个方法，initialValue由用户去实现。
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;
}

//下面是Choreographer中的例子：
private static final ThreadLocal<Choreographer> sThreadInstance =
        new ThreadLocal<Choreographer>() {
    @Override
    protected Choreographer initialValue() {
        Looper looper = Looper.myLooper();
        if (looper == null) {
            throw new IllegalStateException("The current thread must have a looper!");
        }
        Choreographer choreographer = new Choreographer(looper, VSYNC_SOURCE_APP);
        if (looper == Looper.getMainLooper()) {
            mMainInstance = choreographer;
        }
        return choreographer;
    }
};