ThreadLocal的继承性该如何实现

开发过程中发现,同一个ThreadLocal变量在父线程中被设置值后,在子线程中是获取不到的,经过查询学习,发现有InheritableThreadLocal这么一个神奇的类,提供了一个特性: 让子线程可以访问父线程中设置的本地变量,我们来看下源码:

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);
    }

如上代码所示,InheritableThreadLocal继承了ThreadLocal,并重写了3个方法.
createMap被重写,当第一次调用set方法的时候,创建的是当前线程的inheritableThreadLocals 变量的实例而不再是ThreadLocals.
get方法也是同样.

我们看下childValue(T parentValue)何时被执行,以及如何让子线程可以访问父线程的本地变量. 打开Thread类的默认构造函数,代码如下:

    /**
     * Initializes a Thread with the current AccessControlContext.
     * @see #init(ThreadGroup,Runnable,String,long,AccessControlContext,boolean)
     */
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
        init(g, target, name, stackSize, null, true);
    }

    /**
     * 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.
     * @param acc the AccessControlContext to inherit, or
     *            AccessController.getContext() if null
     * @param inheritThreadLocals if {@code true}, inherit initial values for
     *            inheritable thread-locals from the constructing thread
     */
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
        if (name == null) {
            throw new NullPointerException("name cannot be null");
        }

        this.name = name;

        Thread parent = currentThread();
        SecurityManager security = System.getSecurityManager();
        if (g == null) {
            /* Determine if it's an applet or not */

            /* If there is a security manager, ask the security manager
               what to do. */
            if (security != null) {
                g = security.getThreadGroup();
            }

            /* If the security doesn't have a strong opinion of the matter
               use the parent thread group. */
            if (g == null) {
                g = parent.getThreadGroup();
            }
        }

        /* checkAccess regardless of whether or not threadgroup is
           explicitly passed in. */
        g.checkAccess();

        /*
         * Do we have the required permissions?
         */
        if (security != null) {
            if (isCCLOverridden(getClass())) {
                security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
            }
        }

        g.addUnstarted();

        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);
        // (1)如果父线程的inheritableThreadLocals 不为null
        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
        // (2)设置子线程中的inheritableThreadLocals 变量
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

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

如上代码在创建线程时,会调用init方法
Thread parent = currentThread();获取了当前线程(这里指main函数所在线程,也就是父线程),然后(1)判断了父线程的属性输不是null,如果不是null则会执行代码(2)

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

可以看到, 在createInheritedMap内部使用父线程的inheritableThreadLocals变量作为构造函数创建了一个新的ThreadLocalMap变量,然后赋值给了子线程的inheritableThreadLocals变量,下面我们看下在ThreadLocalMap构造函数内部做了些什么:

 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