线程变量，每一个线程中都维护了一个ThreadLocalMap对象，用于存放该线程中的ThreadLocal->Object类型的数据。

我们在ThreadLocal中设置的值都是直接和线程相关联的，不存在线程安全问题。

但是每个线程中都维护一个map会极大消耗内存，所以这是一个以空间换时间的策略

源代码解析

构造函数什么都没有做，所以看set()

ThreadLocal.class
/**
 * Creates a thread local variable.
 */
public ThreadLocal() {
}

在set方法中初始化

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) {
        // 获取当前线程
        Thread t = Thread.currentThread();
        // 从当前线程中获取ThreadLocalMap对象
        ThreadLocalMap map = getMap(t);
       
        if (map != null)   // 若已经初始化map，即不是第一次
            // 将当前 ThreadLocal对象为key，任意对象为值存入map
            map.set(this, value);
        else // 如果没有初始化map，即第一次
            //实例化ThreadLocalMap，将value添加到map
            createMap(t, value);
    }



    /**
     * 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;
    }

    /**
     * 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) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

//get()

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);  //获取当前线程中的map
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);  //根据ThreadLocal获取entry对象
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();
    }

    /**
     * Variant of set() to establish initialValue. Used instead
     * of set() in case user has overridden the set() method.
     *
     * @return the initial value
     */
    private T setInitialValue() {
        T value = initialValue();  //null
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);  //获取当前线程中的ThreadLocalMap对象
        if (map != null)
            map.set(this, value);  //将null添加到map
        else
            createMap(t, value);  //初始化map，将null添加进map
        return value;
    }

   protected T initialValue() {
        return null;
    }

// 线程之中存在一个 ThreadLocalMap 的对象

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

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