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<Long> {
@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<Long> 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。
