为什么 ThreadLocal 的 initialValue 不增加我的变量?

Why initialValue of ThreadLocal doesn't increment my variables?

我正在学习多线程;我有以下 ThreadID class:

public class ThreadID {

    private static volatile int nextID=0;

    private static class ThreadLocalID extends ThreadLocal<Integer>{
        protected synchronized Integer initialValue(){
            return nextID ++;
        }
    }

    private static ThreadLocalID threadID =new ThreadLocalID();

    public static int get(){
        return threadID.get();
    }

    public static void set (int index){
        threadID.set(index);
    }
}

和以下 Thread class:

class MyThread1 extends Thread {
    int x;
    public ThreadID tID;
    public int myid;

    public MyThread1(String name) {
        tID = new ThreadID();
        myid = tID.get();
    }

    public void run() {
        System.out.println("la thread =" + tID.get() + " myid= " + myid);
        try {
            this.sleep(10);
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println("la thread =" + tID.get() + "   apres le sommeil ");
    }
}

和我的 main Class:

public static void main(String[] args) {
    MyThread1 TH[] = new MyThread1[10];
    for (int i = 0; i < 10; i++)
        TH[i] = new MyThread1("nom" + i);
    try {
        for (int i = 0; i < 10; i++) TH[i].start();
        for (int i = 0; i < 10; i++) TH[i].join();
    } catch (InterruptedException e) {
    }
}

问题:

我想要的是给每个Thread一个ID;我发现当我在线程构造函数中初始化 id 时,值总是 0 (通常 initialValue 应该递增 nextID

la thread =1 myid= 0
la thread =3 myid= 0
la thread =2 myid= 0

但是当我在 Run 函数中初始化 id 时,它起作用了!

la thread =1 myid= 1
la thread =3 myid= 3
la thread =2 myid= 2

谁能解释为什么会这样?

What I want is to give each Thread an ID I found that when I init the id in the thread constructor the value is always 0 (normally the initialValue should increment the nextID)

所以在 MyThread1 class 构造函数中你做:

public MyThread1(String name) {
    tID = new ThreadID();
    myid = tID.get();
}

在这种情况下,实际调用tID.get();的线程是main线程,,正在调用的线程这些构造函数,来自主要 class:

MyThread1 TH[] = new MyThread1[10];
for (int i = 0; i < 10; i++)
    TH[i] = new MyThread1("nom" + i); 

第一次调用 tID.get() 将生成一个新的 ID 作为 0,因为这是任何线程第一次调用 tID.get()。来自同一线程(主线程)的下一次调用不会生成新的ID,而是始终return相同的ID,在在这种情况下,return 0 用于 main 线程。

but when I init the id inside the Run function it works!

run 方法内部:

public void run() {
    System.out.println("la thread =" + tID.get() + " myid= " + myid);
    try {
        this.sleep(10);
    } catch (Exception e) {
        e.printStackTrace();
    }
    System.out.println("la thread =" + tID.get() + "   apres le sommeil ");
}

tID.get() 将被不同的线程调用,这就是您获得新 ID 的原因。一个新 ID per 线程第一次调用 tID.get().