ThreadLocal源碼分析

set過程

1、ThreadLocal.set(T value)方法

 /**
     * Sets the current thread's copy of this thread-local variable
     * to the specified value.  Most subclasses will have no need to
     * override this method, relying solely on the {@link #initialValue}
     * method to set the values of thread-locals.
     *
     * @param value the value to be stored in the current thread's copy of
     *        this thread-local.
     */
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

當調用set值方法時，首先從當前線程獲取ThreadLocalMap對象map，判斷map是否爲空，不爲空則直接set值（key爲threadlocal對象）,爲空則調用createMap方法。

2、ThreadLocalMap map = getMap(t)中的getMap(t)源碼。

/**
     * Get the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param  t the current thread
     * @return the map
     */
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

3、t.threadLocals;的源碼

public
class Thread implements Runnable {
     ....
     ....

    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;
}

返回當前線程的ThreadLocalMap對象(默認爲null)，ThreadLocalMap爲ThreadLocal的靜態內部類，由ThreadLocalMap中的enty對象存儲value值。

4、createMap()方法

    /**
     * Create the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param t the current thread
     * @param firstValue value for the initial entry of the map
     */
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

createMap方法時創建一個新的ThreadLocalMap並向這個新的ThreadLocalMap中存儲數據。

5、new ThreadLocalMap(this，firstValue)方法

        /**
         * Construct a new map initially containing (firstKey, firstValue).
         * ThreadLocalMaps are constructed lazily, so we only create
         * one when we have at least one entry to put in it.
         */
        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);
        }

new一個Entry數組（默認大小爲INITIAL_CAPACITY = 16），並將this和value的值初始化到數組中。

get過程

/**
     * Returns the value in the current thread's copy of this
     * thread-local variable.  If the variable has no value for the
     * current thread, it is first initialized to the value returned
     * by an invocation of the {@link #initialValue} method.
     *
     * @return the current thread's value of this thread-local
     */
    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();
    }

如果已經set了值，則返回，如果未set值則返回null。

ps：爲什麼不直接向Thread類中的ThreadLocalMap對象存取數據呢？

threadLocals默認是包級別訪問，所以不能直接從外部訪問該變量，只能同包下的類可以訪問threadlocals變量，而ThreadLocal和Thread恰好在同一個包中。

InheritableThreadLocal的使用