JD side: how can a child thread get the value of the parent thread ThreadLocal? I got...

source ：blog.csdn.net/weixin_44912855

Recently, a netizen in the wechat group shared the process of his Jingdong side , Let me share with you one of the interview questions .

On the east side of Beijing 」 How does a child thread get the parent thread ThreadLocal Value

How does a child thread get the parent thread ThreadLocal Value

Want the child thread to get the parent thread ThreadLocal The value in , Its subclasses are required InheritableThreadLocal Realization .

The test code is as follows ：

public static void main(String[] args) throws InterruptedException {    Thread parentParent = new Thread(() -> {        ThreadLocal<Integer> threadLocal = new ThreadLocal<>();        threadLocal.set(1);        InheritableThreadLocal<Integer> inheritableThreadLocal = new InheritableThreadLocal<>();        inheritableThreadLocal.set(2);        new Thread(() -> {            System.out.println("threadLocal=" + threadLocal.get());            System.out.println("inheritableThreadLocal=" + inheritableThreadLocal.get());        }).start();    }, " Parent thread ");    parentParent.start();}

The operation results are as follows ：

The child thread gets the from the parent thread ThreadLocal The value in

The principle is as follows ：

First we need to know Thread Class maintains two ThreadLocalMap

To follow up new Thread() Method

Its construction method calls init Method , init Method to inheritThreadLocals The value is set to true

Continue to follow up .

When inheritThreadLocals The value of is true And its parent thread inheritableThreadLocals Not for null when , Put its parent thread inheritableThreadLocals Assigned to the current thread inheritableThreadLocals

This is how the child thread can get the parent thread ThreadLocal The key to value .

Continue to follow up have a look InheritableThreadLocal Of get() Method

get() There's nothing to look at , Namely ThreadLocal Of get() Method .

Be careful ：InheritableThreadLocal Yes ThreadLocal Of getMap() Method

ThreadLocalMap getMap(Thread t) { // Get the thread's own variables threadLocals, And bind to the member variables of the current calling thread. threadLocals On     return t.threadLocals;}void createMap(Thread t, T firstValue) {    t.threadLocals = new ThreadLocalMap(this, firstValue);    // Create for ThreadLocalMap Of table Attribute assignment , And will firstValue At the top of the array .}

createMap Method not only creates threadLocals, The local variable value to be added is also added to the threadLocals in .

InheritableThreadLocal Class inherited ThreadLocal class , Rewrite the childValue、getMap、createMap Method .

among createMap When the method is called , Created is inheritableThreadLocal instead of threadLocals.

Empathy ,getMap Method to call the current caller thread get Method is not returned threadLocals It is inheritableThreadLocal.

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