ThreadLocal是个很重要的多线程类，里面数据结构的设计很有意思，很巧妙。但是我们平时使用它的时候常常容易对它的使用感到迷惑，因为它跟其它的API很不一样，使用很不一样，设计也很不一样。

但是不用担心，这篇文章将从源码出发，一步步深入剖析ThreadLocal内部构造，理清楚它的来龙去脉。

example

还是从一个使用用例出发：

public class ThreadLocalExample {
    // 声明一个 ThreadLocal 变量
    private static ThreadLocal<Integer> threadLocal = new ThreadLocal<>();

    public static void main(String[] args) {
        // 在主线程中设置变量值
        threadLocal.set(10);

        // 创建并启动一个新线程
        Thread thread = new Thread(() -> {
            // 在新线程中获取变量值
            System.out.println("ThreadLocal value in new thread: " + threadLocal.get());
        });
        thread.start();

        // 在主线程中获取变量值
        System.out.println("ThreadLocal value in main thread: " + threadLocal.get());
    }
}

打印结果：

ThreadLocal value in main thread: 10
ThreadLocal value in new thread: null

可以看出同一个threadLocal 对象，在不同的线程里面调用get方法，获取的是不一样的结果！也就是说，threadLocal 对象存储了不同线程的私有变量。

现在可能我们还是觉得云里雾里，那现在我们就从源码出发，来一步步进行分析。

从threadLocal.set(10);方法进去：

set(T value)

    /**
     * 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);
    }

创建map或者设置key&value。

createMap(t, value);

先来看看假如map==null它怎么做：

    /**
     * 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);
    }

这里把new出来的ThreadLocalMap赋值给了t.threadLocals，t是个线程。

        /**
         * 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);
        }

这里是ThreadLocalMap的构造方法，可以看出ThreadLocal作为key，传进来的参数作为value。

set(ThreadLocal<?> key, Object value)

现在回退一下，看看map.set(this, value);做了什么：

        /**
         * Set the value associated with key.
         *
         * @param key the thread local object
         * @param value the value to be set
         */
        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();
        }

这里有必要知道的是该方法位于ThreadLocalMap类里面：

看下Entry 的实现代码：

        /**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }

Entry 是个弱引用的子类。设计为弱引用，说明它跟内存泄漏有关。这里先不深入探讨。

到这里我们可以得知set方法执行的时候以ThreadLocal对象作为key，以value入参作为value，传到了ThreadLocal的 Entry[] 里面，设置的时候根据threadLocal对象的hash值来确定其在ThreadLocalMap中的位置。

ThreadLocalMap和Thread的关系

还记得前面的这行代码吗？

 t.threadLocals = new ThreadLocalMap(this, firstValue);

它createMap(t, value);里面，来看看ThreadLocalMap和Thread的关系：

java/lang/Thread.java

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

也就是说ThreadLocalMap 其实是属于线程的成员变量。

全貌

其实到这里，我们已经有能力知道整体的数据结构的设计了，下面我们通过前面给出的example代码，通过画图的方式把它们之间关系全貌绘制出来：

ThreadLocalMap里面是Entry数组，那么其它Entry元素怎么用呢？
这是个好问题，我们迭代下前面的example：

class ThreadLocalExample {
  // 声明一个 ThreadLocal 变量
  private static ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
  private static ThreadLocal<String> threadLocal2 = new ThreadLocal<>();
  public static void main(String[] args) {
    // 在主线程中设置变量值
    threadLocal.set(10);
    threadLocal2.set("hello");

    // 创建并启动一个新线程
    Thread thread = new Thread(() -> {
      // 在新线程中获取变量值
      System.out.println("ThreadLocal value in new thread: " + threadLocal.get());
      System.out.println("ThreadLocal2 value in new thread: " + threadLocal2.get());
    });
    thread.start();

    // 在主线程中获取变量值
    System.out.println("ThreadLocal value in main thread: " + threadLocal.get());
    System.out.println("ThreadLocal2 value in main thread: " + threadLocal2.get());
  }
}

运行结果：

ThreadLocal value in main thread: 10
ThreadLocal value in new thread: null
ThreadLocal2 value in main thread: hello
ThreadLocal2 value in new thread: null

再来一个图：

一图胜千言，到这里我们应该对ThreadLocal这个线程类的整体有个清晰的把握了。

enjoy it。