Java并发：InheritableThreadLocal详解

概述

最新看项目代码时, 发现有地方用到了InheritableThreadLocal，之前只用过ThreadLocal，于是就查了点资料，看了下源码，稍微学习一下。

InheritableThreadLocal是ThreadLocal的子类，可以先了解下ThreadLocal：Java并发：ThreadLocal详解

1.定义

InheritableThreadLocal继承了ThreadLocal，此类扩展了ThreadLocal以提供从父线程到子线程的值的继承：当创建子线程时，子线程接收父线程具有的所有可继承线程局部变量的初始值。 通常子线程的值与父线程的值是一致的。 但是，通过重写此类中的childValue方法，可以将子线程的值作为父线程的任意函数。

2.源码

public class InheritableThreadLocal extends ThreadLocal {
    /**
     * Computes the child's initial value for this inheritable thread-local
     * variable as a function of the parent's value at the time the child
     * thread is created.  This method is called from within the parent
     * thread before the child is started.
     * 

     * This method merely returns its input argument, and should be overridden
     * if a different behavior is desired.
     *
     * @param parentValue the parent thread's value
     * @return the child thread's initial value
     */
    protected T childValue(T parentValue) {
        return parentValue;
    }

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

InheritableThreadLocal的源码很短，只有3个很短的方法，我们主要关注getMap()方法和creatMap()方法，这两个方法都是重写的，跟ThreadLocal中的差别在于把ThreadLocal中的threadLocals换成了inheritableThreadLocals，这两个变量都是ThreadLocalMap类型，并且都是Thread类的属性。

    /* 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;

3.InheritableThreadLocal继承父线程的值

上文的定义说道了InheritableThreadLocal会继承父线程的值，这是InheritableThreadLocal被创造出来的意义，具体是怎么实现的？

让我们从子线程被创建出来开始看起

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

    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
        init(g, target, name, stackSize, null);
    }

    private void init(ThreadGroup g, Runnable target, String name,
            long stackSize, AccessControlContext acc) {
		// ... 省略掉一部分代码
		if (parent.inheritableThreadLocals != null)
		  this.inheritableThreadLocals =
		      ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
		// ... 省略掉一部分代码
	}

    static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
        return new ThreadLocalMap(parentMap);
    }

        /**
         * Construct a new map including all Inheritable ThreadLocals
         * from given parent map. Called only by createInheritedMap.
         *
         * @param parentMap the map associated with parent thread.
         */
        private ThreadLocalMap(ThreadLocalMap parentMap) {
            Entry[] parentTable = parentMap.table;
            int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];// 创建跟父线程相同大小的table

            for (int j = 0; j < len; j++) {// 遍历父线程的inheritableThreadLocals, 在上面第3个代码块作为参数传下来
                Entry e = parentTable[j];
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    ThreadLocal