java - 在构造函数中设置ThreadLocal变量有效吗？

我正在学习Java线程，上了有关本地线程值的课程，因此决定制作一个使用它们的程序。
我在构造函数（线程构造函数）中设置了本地线程值，然后在屏幕上显示该值以检查其是否起作用，但是当我启动线程时，本地值变成了NULL，所以我得到了NullPointerException。
我错过了什么吗，还是只能在start方法中设置本地线程值？

package practice;
import static java.lang.System.out;
import java.util.Scanner;
class Try
{
    Thread a1,a2,a3;
    int x=0;
    synchronized void change(int who){
        out.println("who called"+who);
        out.println("x initial="+x);
        x++;
        out.println("x after="+x);
    }
    class now implements Runnable{
        ThreadLocal<Integer> id=new ThreadLocal<Integer>();
        public void run(){
            for(int i=1;i<=25;i++){
            out.println("im running id="+id.get());
            change(id.get());
            out.println("after call me="+id.get());
            }
        }
        now(int givenid){
            out.println("my givenid is "+givenid);
            id.set(givenid);
            out.println("my id is "+id.get());
        }
    }
    public static void main(String[] args)
    {
        new Try();
    }


    Try(){

    a1=new Thread(new now(1));
    a2=new Thread(new now(2));
    a1.start();
    a2.start();
    }

}

最佳答案

我真的很喜欢Banthar的回答，但是我只想通过添加一些打印来说明您的代码会发生什么...

package threads;

import static java.lang.System.out;

class Main {
    Thread a1, a2, a3;
    int x = 0;

    public Main() {
        out.println("Current thread (in Main's constructor): " + Thread.currentThread().getName());
        a1 = new Thread(new MyRunnable(1));
        a2 = new Thread(new MyRunnable(2));
        a1.start();
        a2.start();
    }

    private synchronized void change(int who) {
        out.println("Current thread (in change() method): " + Thread.currentThread().getName());
        out.println("who called" + who);
        out.println("x initial=" + x);
        x++;
        out.println("x after=" + x);
    }

    private class MyRunnable implements Runnable {
        ThreadLocal<Integer> id = new ThreadLocal<Integer>();

        public MyRunnable(int givenid) {
            out.println("Current thread (in MyRunnable's constructor): " + Thread.currentThread().getName());
            out.println("my givenid is " + givenid);
            id.set(givenid);
            out.println("my id is " + id.get());
        }

        public void run() {
            out.println("Current thread (in run() method): " + Thread.currentThread().getName());
            out.println("im running id=" + id.get());
            change(id.get());
            out.println("after call me=" + id.get());
        }
    }

    public static void main(String[] args) {
        new Main();
    }
}

输出为：

Current thread (in Main's constructor): main
Current thread (in MyRunnable's constructor): main
my givenid is 1
my id is 1
Current thread (in MyRunnable's constructor): main
my givenid is 2
my id is 2
Current thread (in run() method): Thread-0
Current thread (in run() method): Thread-1
im running id=null
im running id=null

抱歉，改名了。

如您在实例化MyRunnable（您的now类）时所看到的，您在主线程中设置了ThreadLocal。这就是为什么您无法通过其他线程访问它的原因。