子线程如何获取父线程ThreadLocal的值

不卖关子，用InheritableThreadLocal。
关于ThreadLocal可以参考我的另一篇文章
ThreadLocal使用及原理_编程还未的博客-优快云博客

ThreadLocal实战源码在文末。

1.使用

直接上代码。

public class InheritableThreadLocalTest1 {

    public static void main(String[] args) {
        ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
        threadLocal.set(1);
        InheritableThreadLocal<Integer> inheritableThreadLocal = new InheritableThreadLocal<>();
        inheritableThreadLocal.set(2);
        new Thread(() -> {
            System.out.println("threadLocal=" + threadLocal.get());
            //debug到这一行看源码
            System.out.println("inheritableThreadLocal=" + inheritableThreadLocal.get());
        }).start();
    }
}

运行结果

threadLocal=null
inheritableThreadLocal=2

2.原理

Thread类里面有两个ThreadLocal对象，一个是threadLocals（ThreadLocal对象），一个是inheritableThreadLocals（InheritableThreadLocal对象）。

    /* 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;

默认的new Thread()会获取parent线程的inheritableThreadLocals，并赋值。

public Thread(Runnable target) {
        this(null, target, "Thread-" + nextThreadNum(), 0);
}
public Thread(ThreadGroup group, Runnable target, String name,
                  long stackSize) {
       this(group, target, name, stackSize, null, true);
}
/**
*精简后的代码
*/
private Thread(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);
 
    }

InheritableThreadLocal本身也比较简单。主要是getMap()、createMap()的时候操作的是Thread的inheritableThreadLocals

public class InheritableThreadLocal<T> extends ThreadLocal<T> {
 
    public InheritableThreadLocal() {}

    protected T childValue(T parentValue) {
        return parentValue;
    }

    ThreadLocalMap getMap(Thread t) {
       return t.inheritableThreadLocals;
    }

    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }
}

3.实战源码

具体的介绍，已经在我的博文，ThreadLocal使用及原理_编程还未的博客-优快云博客，介绍过了。

ThreadLocal实战，源码地址：highway-resources: threadlocal-current-user

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