java子线程中获取父线程的threadLocal中的值

package com.example.springbootstudy.test.threadLocal;

public class BaseTest {

    public static final InheritableThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>();

    public static final ThreadLocal<String> threadLocal = new ThreadLocal<>();

    public static void main(String[] args) {
        inheritableThreadLocal.set("Inheritable hello");
        threadLocal.set("hello");

        new Thread(() -> {
            System.out.println(String.format("子线程可继承值：%s", inheritableThreadLocal.get()));
            System.out.println(String.format("子线程值：%s", threadLocal.get()));

            new Thread(() -> {
                System.out.println(String.format("孙子线程可继承值：%s", inheritableThreadLocal.get()));
                System.out.println(String.format("孙子线程值：%s", threadLocal.get()));
            }).start();
        }).start();

        System.out.println(String.format("主线程-inheritableThreadLocal：%s", inheritableThreadLocal.get()));
        System.out.println(String.format("主线程-threadLocal：%s", threadLocal.get()));
    }

}

运行结果如下所示：

子线程可继承值：Inheritable hello
主线程-inheritableThreadLocal：Inheritable hello
子线程值：null
主线程-threadLocal：hello
孙子线程可继承值：Inheritable hello
孙子线程值：null

InheritableThreadLocal 是如何做到的呢？
我们先看下类结构

public class Thread{
     ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
}

public class InheritableThreadLocal<T> extends ThreadLocal<T> {
    ThreadLocalMap getMap(Thread t) {
       return t.inheritableThreadLocals;
    }
    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }
}
    

这里重写了ThreadLocal的操作ThreadLocalMap的方法，其实就是把从Thread类操作threadLocals变量，切换到inheritableThreadLocals变量上。
这不就是跟ThreadLocal一样吗？只是换了个变量名！ 但是实际上Thread类初始化时还有一些骚操作，请看：

public class Thread{
    public Thread(){
        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
    }
}

//ThreadLocal 类里的方法
   static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
        return new ThreadLocalMap(parentMap);
    }

这里判断父线程的inheritableThreadLocals有值， 就拷贝到当前子线程的inheritableThreadLocals ， 这样我们就能在子线程获取到父线程的值了。
这里要注意的是，并不是复制的指针，而是复制了副本。你在子线程重新set值后， 并不会影响父线程的值。