最近做了一个日志调用链路跟踪的项目，涉及到操作标识在线程和子线程，线程池以及远程调用之间的传递问题。最终采用了阿里开源的TransmittableThreadLocal插件(https://github.com/alibaba/transmittable-thread-local)完美解决。在分析源码以及中途修复bug的过程中，被ThreadLocal搞得晕头转向。好在静下心来细细啃了一下午,终于能理解各种ThreadLocal相关问题了。这里准备用博客记录下来。
要想了解ThreadLocal的底层原理首先就要了解弱引用。本篇不会详细介绍是强引用，啥是弱引用、软引用以及虚幻引用，有兴趣的同学可以自己百度。这里直接给出弱引用的简单代码说明：
Object obj = new Object(); WeakReference<Object> wf = new WeakReference<Object>(obj); // 对堆内存中对象建立一个弱引用 obj = null; // 去掉堆中对象的强引用 System.gc(); System.out.println(wf.get()); // 输出null可以看到弱引用的作用就在于当堆内存中对象不存在强引用的时候，在下一次gc的时候可能会回收掉堆内存占用。
了解了弱引用之后，其实就能够很好地理解ThreadLocal(题外话：其实这个ThreadLocal我摸索了好久才弄得比较透彻)。直接上代码：
首先我们需要注意Thread类中的两个属性：
public class Thread implements Runnable { // .... /* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */ // ThreadLocal实际值的存储所在 ThreadLocal.ThreadLocalMap threadLocals = null; /* * InheritableThreadLocal values pertaining to this thread. This map is * maintained by the InheritableThreadLocal class. */ // 后面要将的InheritableThreadLocal的实际值存储所在 ThreadLocal.ThreadLocalMap inheritableThreadLocals = null; }这两个属性特别关键：
他是每个线程所特有的
两个属性的类型是ThreadLocal的内部静态类
他们是ThreadLocal的神奇魔法之关键~
接下来我们来看看ThreadLocal的关键方法：
/** * 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); } /** * 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) { // set时候的关键，实际上是创建一个当前ThreadLocal的弱引用为key的Map t.threadLocals = new ThreadLocalMap(this, firstValue); } /** * 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(); } /** * Get the map associated with a ThreadLocal. Overridden in * InheritableThreadLocal. * 最为关键的方法： 可以看出getMap实际上就是得到传入线程的threadLocals属性的值 * @param t the current thread * @return the map */ ThreadLocalMap getMap(Thread t) { return t.threadLocals; } /** * Removes the current thread's value for this thread-local * variable. If this thread-local variable is subsequently * {@linkplain #get read} by the current thread, its value will be * reinitialized by invoking its {@link #initialValue} method, * unless its value is {@linkplain #set set} by the current thread * in the interim. This may result in multiple invocations of the * {@code initialValue} method in the current thread. * * @since 1.5 */ public void remove() { ThreadLocalMap m = getMap(Thread.currentThread()); if (m != null) m.remove(this); }不难看出，所有方法都是围绕着一个ThreadLocalMap来操作的，那么这个ThreadLocalMap究竟是啥，我们进一步来分析：
/** * ThreadLocalMap is a customized hash map suitable only for * maintaining thread local values. No operations are exported * outside of the ThreadLocal class. The class is package private to * allow declaration of fields in class Thread. To help deal with * very large and long-lived usages, the hash table entries use * WeakReferences for keys. However, since reference queues are not * used, stale entries are guaranteed to be removed only when * the table starts running out of space. */ static class ThreadLocalMap { /** * 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. */ // 实际上他的存储也是利用Entry结构来进行的，只不过这个Entry的key值是弱音用对象，实际上可以将ThreadLocalMap看做WeakHashMap static class Entry extends WeakReference<ThreadLocal<?>> { /** The value associated with this ThreadLocal. */ Object value; Entry(ThreadLocal<?> k, Object v) { super(k); value = v; } } }
至此，我们已经知道了ThreadLocal是如何实现的了，具体来说是下面几个关键点：
ThreadLocal本身并不存储值，而是作为ThreadLocalMap的key用来查找对象所存储的值的
用来存储值的ThreadLocalMap是每个线程都有的非静态属性，当前线程实例该属性的值对其他线程实例是不可见的，这也就实现了线程隔离
ThreadLocal的get方法实际上是先获取当前线程的ThreadLocalMap属性值，然后再通过ThreadLocal作为key获取实际上存储在Map中的值