Java thread variable ThreadLocal source code analysis

1.ThreadLocal thread variables are bound to the current thread and only save the variables of the current thread. They are isolated from other threads and cannot access the data inside.

2. ThreadLocal is used in Looper, and a Looper is created and saved in ThreadLocal.

//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));
}

Look at how the sThreadLocal.set() method saves data.

First get the current thread, then get the ThreadLocalMap member variable of the thread, and then save it to this map.

The key is the created ThreadLocal object, and the value is the value passed in.

  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;
}

1) Save the data if the map is not empty. Take the first branch of the if judgment above. This storage method is similar to HashMap

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();
}

2) Save data if the map is empty. Then create a ThreadLocalMap and assign it to the current thread t.

 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);
}

3. Let’s look at how ThreadLocal gets its value.

Also get the current thread t first, and then get its member variable ThreadLocalMap through t.

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();
}

1) If the map is not empty, get the value from the map.

//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;
}

2) If the map obtained is empty when getting, you need to initialize it at this time

//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;
    }
};

ThreadLocalMap is used to store ThreadLocal data. This ThreadLocalMap object is obtained from the current thread through

and is obtained from the current thread. The design of ThreadLocalMap realizes the isolation of data between different threads and only saves the data of the corresponding thread.

The ThreadLocal object itself is used as a key to store data in ThreadLocalMap. map.set(this, value);

If you want to store more data in ThreadLocalMap, you need to create multiple objects.

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