子线程如何获取父线程的ThreadLocal变量

           之前面试的时候，当谈到ThreadLocal时，被人问到，在子线程中如何获取父线程的ThreadLocal里面存储的变量。今天看到了一个类InheritableThreadLocal才明白原来是这样来做的。

           先来看一下InheritableThreadLocal的源码。

           

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);
    }
}

可以看到，它继承自ThreadLocal,既然有了ThreadLocal,为什么还要弄这么个类出来呢，原因就是通过这个类可以帮我们在子线程中获取到父线程的ThreadLocal变量。

下面来演示对比一下。

 

public class TestThreadLocal {

    private static ThreadLocal T = new ThreadLocal();

    public static void main(String[] args) {
        T.set("23abc");
        System.out.println("main" + T.get());
        new Thread(() -> System.out.println("sub" + T.get())).start();
    }
}

输出

main23abc
subnull

然后我们将ThreadLocal换成InheritableThreadLocal

然后再次输出

main23abc
sub23abc

可以看到，子线程已经拿到了父线程的ThreadLocal里面的值了。