ThreadLocal MDC

因为MDC底层是用ThreadLocal实现的，所以这里补充一些和ThreadLocal相关的知识点。

1.ThreadLocal的三个层次

关于ThreadLocal有三个层次，可以按照这三个层次去理解就不会乱。

三个层次
 * 第一层是Thread空间，通过Thread.currentThread()获得。
 * 第二层是Thread中的两个ThreadLocalMap，threadLocals和inheritableThreadLocals，访问thread对应的两个ThreadLocalMap成员变量获得。
 * 第三层是每个ThreadLocalMap中key——ThreadLocal和value——ThreadLocal的set方法set的值，在get方法中用ThreadLocal的this作为ThreadLocalMap的key获取value。
 * 无论什么操作都要按照这三个层次依次进行才不会乱

2.ThreadLocalMap

保存了当前Thread中存放的ThreadLocal和ThreadLocal对应的值的键值对。

当前线程的所有ThreadLocal变量组成了这个map的keyset，对应的值组成了这个map的valueset。

3.ThreadLocal的操作

第一步都是先用Thread.currentThread()获得当前线程，然后getMap获取线程中的ThreadLocalMap。

像getMap这些操作方法都是包可见性的，包外部无法操作。以ThreadLocal的get方法举例，

/**
     * 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();
    }

/**
     * 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();
    }

4.InheritableThreadLocal

Thread类中有两个ThreadLocalMap，一个是threadLocals，一个是inheritableThreadLocals。

threadLocals保存的是当前线程中的ThreadLocal变量们，inheritableThreadLocals保存的是当前线程父线程中的变量们。

InheritableThreadLocal类覆写了getMap和createMap这两个方法，

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

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

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

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

可以看出在初始化或者getMap的时候，获取到的都是inheritableThreadLocals引用，操作的也是inheritableThreadLocals这个ThreadLocalMap。

5.threadLocals和inheritableThreadLocals的初始化

/**
     * Initializes a Thread.
     *
     * @param g the Thread group
     * @param target the object whose run() method gets called
     * @param name the name of the new Thread
     * @param stackSize the desired stack size for the new thread, or
     *        zero to indicate that this parameter is to be ignored.
     */
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
        ...
        Thread parent = currentThread();
        ...
        if (parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        ...
    }

/**
     * Initializes a Thread.
     *
     * @param g the Thread group
     * @param target the object whose run() method gets called
     * @param name the name of the new Thread
     * @param stackSize the desired stack size for the new thread, or
     *        zero to indicate that this parameter is to be ignored.
     */
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
        ...
        Thread parent = currentThread();
        ...
        if (parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        ...
    }

在new一个Thread的时候会调用Thread的init方法，该方法中如果parent线程的inheritableThreadLocals不是null的话，就会用createInheritedMap方法，用parent的inheritableThreadLocals中的元素构造一个新的ThreadLocalMap。

注意：该操作只在线程初始化的时候进行，所以在该线程初始化之后，parent线程对parent线程自己的inheritableThreadLocals变量的操作不会影响到当前线程的inheritableThreadLocals了，因为已经不是同一个map了。

MDC就是利用这个InheritableThreadLocal把父线程的context带到子线程中，把上下文传递到子线程中通过日志输出，把一次完整的请求串联起来。

6.parent线程

Thread parent = currentThread();

Thread parent = currentThread();

在Thread的init方法中，是通过获得当前线程作为parent线程，也就是说，在哪个线程中new的这个Thread并start的，执行该操作的线程就是new的新Thread的parent线程。

但是init方法只在线程初始化的时候执行一次，所以如果用的线程池来使线程重用的话，就不会再调用这个init方法了，这会带来一些问题，后面会具体说。