线程之间数据传递ThreadLocal,InheritableThreadLocal,TransmittableThreadLocal

项目中异步处理方法获取用户对象，发现获取不到，发现用户对象是ThreadLocal进行传递，但是异步属于子线程，ThreadLocal对父子线程传递有问题的，后来将对象的传递值改为了TransmittableThreadLocal进行传递。

public static TransmittableThreadLocal> transmittableThreadLocal = new TransmittableThreadLocal() ;

ThreadLocal 大家都熟知它作为本地线程传递，原理是Thread中存在ThreadLocal.ThreadLocalMap threadLocals 变量的ThreadLocal，而再看ThreadLocal的get set方法，都是获取当前的线程，然后进行对当前线程中的ThreadLocalMap变量进行赋值或者取值，这样便实现了线程之间数据传递。

    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();
    }
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

但是当子线程要获取父进程中的ThreadLocal值，得到的确是null，原因是子线程ThreadLocal调用get方法时的当前线程是子线程，所以取不到父线程的值。

这时InheritableThreadLocal就出现了，进行了父子线程之间的数据传递。具体实现原理如下：

Thread中用threadLocals、inheritableThreadLocals两个ThreadLocal.ThreadLocalMap类型的变量

    ThreadLocal.ThreadLocalMap threadLocals = null;

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

而InheritableThreadLocal中重写了getmap createmap方法，分别赋值、取值于thread中的子线程inheritableThreadLocals 变量的ThreadLocalMap。当InheritableThreadLocal进行get set时，getmap方法获取的就行子线程变量inheritableThreadLocals的值

    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();
    }


ThreadLocalMap getMap(Thread t) {
   return t.inheritableThreadLocals;
}


void createMap(Thread t, T firstValue) {
    t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
}

而父线程是如何把数据传递给子线程的哪？

可以看到Thread创建时，将parent的map赋值copy给了当前线程中的inheritableThreadLocals。而copy过程都是浅copy，key,value都是引用地址的赋值

 public Thread() {
        init(null, null, "Thread-" + nextThreadNum(), 0);
    }

追踪下去init方法中
this.group = g;
        this.daemon = parent.isDaemon();
        this.priority = parent.getPriority();
        if (security == null || isCCLOverridden(parent.getClass()))
            this.contextClassLoader = parent.getContextClassLoader();
        else
            this.contextClassLoader = parent.contextClassLoader;
        this.inheritedAccessControlContext =
                acc != null ? acc : AccessController.getContext();
        this.target = target;
        setPriority(priority);
        if (parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        tid = nextThreadID();


//ThreadLocal.createInheritedMap追踪下去
private ThreadLocalMap(ThreadLocalMap parentMap) {
            Entry[] parentTable = parentMap.table;
            int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];

            for (int j = 0; j < len; j++) {
                Entry e = parentTable[j];
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    ThreadLocal