ThreadLocal的一些总结

ThreadLocal.class
/**
     * 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();
        //根据线程局部变量ThreadLocal实例来获取与之对应的ThreadLocalMap实例
        ThreadLocalMap map = getMap(t);
        if (map != null)
            //如果ThreadLocalMap 不为null，那么就以ThreadLocal为key,以value为值，进行绑定
            map.set(this, value);
        else
            /**
              *如果根据当前的线程局部变量没有获取到与之绑定的ThreadLocal.ThreadLocalMap实例，
              *那么就创建一个 ThreadLocal.ThreadLocalMap实例 
              *且以当前的线程为 key，传入的绑定值 为 value
              *
            */
            createMap(t, value);
    }



ThreadLocal.class

// 在ThreadLocal.java的 224行
/**
     * 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
     * @param map the map to store.
     */
    void createMap(Thread t, T firstValue) {
        //这一段代码就把当前线程与 ThreadLocal(下面一行代码里的this就是 ThreadLocal)以及要绑定的值 关联了起来 
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }


Thread.class的180行

/* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;
/*
*通过 Thread类的180行的这行代码我们可以知道，每个线程实例都有一个
* 类型为 ThreadLocal.ThreadLocalMap 的threadLocals属性，
* 这个属性的一个很重要的作用就是 接收 在ThreadLocal.java的224行的
* 方法 createMap(Thread t,T firstValue)里 new的TheadLocalMap变量
*/
ThrealLocal.ThreadLocalMap.class

    /**
         * 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.
         *  这里是 ThrealLocal.ThreadLocalMap 构造器，
         */
        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);
        }





ThreadLocal.class
    /**
     * 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)
                return (T)e.value;
        }
        return setInitialValue();
    }