InheritableThreadLocal变量无法传递

背景知识

我们都知道InheritableThreadLocal类型的变量可以在父子线程之间传递。具体实现为：每个线程都维护了threadLocals和inheritableThreadLocals成员变量，线程在初始化时会根据父线程的inheritableThreadLocals创建自身的inheritableThreadLocals。具体的创建由ThreadLocal类负责实现。

线程的threadLocals和inheritableThreadLocals成员变量都是ThreadLocal.ThreadLocalMap类型，这个类在功能上类似于HashMap，是ThreadLocal或者InheritableThreadLocal类型变量的集合。
Thread.java

/* 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;

init是线程的初始化方法，注意这里省略了无关的一些代码
Thread.java

/**
 * 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) {
    ……
    Thread parent = currentThread();
    ……
    if (inheritThreadLocals && parent.inheritableThreadLocals != null)
        this.inheritableThreadLocals =
            ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
    ……
}

问题描述

笔者最近在使用InheritableThreadLocal时无意中碰到了一个问题。先来看我自定义的ThreadLocal类型，这个类的默认初始值为当前时间戳。

MyThreadLocal.java

import java.util.Date;

public class MyThreadLocal extends InheritableThreadLocal {

    @Override
    protected Long initialValue() {
        return new Date().getTime();
    }
}

在这里我创建了一个子线程myThread，然后打印当前线程的ThreadLocal变量的值，过2秒后启动子线程，由子线程打印这个ThreadLocal变量的值。
MyThread.java

public class MyThread extends Thread {

    public static ThreadLocal tl = new MyThreadLocal();

    @Override
    public void run() {
        System.out.println(tl.get());
    }

    public static void main(String args[]) {
        MyThread myThread = new MyThread();
        System.out.println(tl.get());

        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        myThread.start();
    }
}

由于MyThreadLocal继承了InheritableThreadLocal，因此我期望两者可以打印出相同的结果。然而无论运行多少遍这个程序，实际结果总是相差2秒以上（非常接近2秒）。由此可见这个ThreadLocal变量并没有从父线程传递到子线程。

问题分析

笔者试着通过分析源代码来查找原因。可以看到线程中的threadLocals变量默认为null，而且Thread类中并没有任何对其赋值的地方，事实上通过注释也可以发现threadLocals和inheritableThreadLocals都是由ThreadLocal类维护的

ThreadLocal类的createMap方法为线程初始化threadLocals变量
ThreadLocal.java

/**
 * 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) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

InheritableThreadLocal类的createMap方法为线程初始化inheritableThreadLocals变量
InheritableThreadLocal.java

/**
 * 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);
}

而调用createMap方法的地方为ThreadLocal的set方法和setInitialValue方法，后者被get方法调用。ThreadLocal本身的构造函数并没有做任何事情，即使我们通过重写initialValue方法设置默认值。也就是说，如果我们创建了ThreadLocal类型的对象后没有调用set方法设置初始值或者调用get方法设置默认的初始值，那么这个对象不会被添加到线程的threadLocals或者inheritableThreadLocals中。此时创建的子线程也不能获取到父线程的InheritableThreadLocal类型的变量。由此导致了InheritableThreadLocal变量无法在父子线程之间传递。
当然解决方法也很简单，在子线程创建之前调用set方法设置变量的值或者调用get方法获取默认值即可。

结论

InheritableThreadLocal类型的变量要在子线程创建之前设置初始值，否则不能在父子线程之间共享。