摘要：下來我們來看中的方法。從中可以看到真正保存的是在中，接著看看的源碼。數組的初始長度為，最多可保存一旦超過就進行擴容增加一倍。而內部利用數組來保存和值的，數組的索引就是的哈希值數組的長度。

在Android-27中查看源碼：

在Looper源碼中，我們看到通過ThreadLocal的set方法來保存Looper,通過get方法來取出Looper。下來我們來看ThreadLocal中的set/get方法。

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

從中可以看到真正保存的是在Thread.threadLocals中，接著看看ThreadLocalMap的源碼。

在ThreadLcoalMap中：

    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);
        } 
        
    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();

                if (k == key) {
                    e.value = value;
                    return;
                }

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

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

在ThreadLcoalMap中創建了數組，利用數組來保存Entry，Entry中保存了ThreadLocal和值。數組的初始長度為16，最多可保存length/2一旦超過就進行擴容（增加一倍）。

所以ThreaLocal的作用是用來實現線程內部的數據共享，其實現原理是利用線程內部的ThreaLocalMap來保存數據。而ThreadLocalMap內部利用數組來保存ThreadLocal和值的，數組的索引就是ThreadLocal的哈希值&(數組的長度-1)。