主线程可以访问线程的Threadlocal

Question

How main thread is able to access threadlocal of another thread? Although threadlocal of another thread gives another value in main thread than what it gives inside its own run method.

class MyThread13 extends Thread{
    static int id =0;
     ThreadLocal threadLocal = new ThreadLocal(){
       public Integer initialValue(){
            return ++id;
        }
    };
    public MyThread13(String name){
        super(name);
    }
    public void run(){
        System.out.println(Thread.currentThread().getName()+" is executing with id :"+threadLocal.get());

    }

}

public class MultiThreading13ThreadLocalB {
    public static void main(String[] args) {
        MyThread13 myThread13a = new MyThread13("Thread:1");
        MyThread13 myThread13b = new MyThread13("Thread:2");
        myThread13a.start();
        myThread13b.start();
        myThread13c.start();
       // myThread13d.start();
        System.out.println("Accessing threadlocal from main :"+myThread13a.threadLocal.get());

    }
}

when threadlocal of another thread is being accessed from main thread , it should give null. But here it is giving some other value

Answer 1

Your example shows a confusion of ThreadLocal with what are simply member variables of Thread instances. The concepts are similar; both allow you to associate state with a Thread instance, but a ThreadLocal allows you to do it outside of the thread instance itself. As you've noted, a ThreadLocal lives up to its name -- when you call get and set on it, those values are specific to that thread, which is why trying to access another thread's state (as the main thread tries to do in your example) is not going to work.

ThreadLocal s are sometimes helpful, but in day to day programming there is rarely a need for them. One kind of use case would be something like a server process that accepts an incoming request and associates some state with that request; if you don't want to change all of your method signatures to pass that state through, you could put it in a ThreadLocal . Usually there is a single ThreadLocal instance per JVM.