使用ThreadLocal为每个线程分配ID(Assign ID to each thread using ThreadLocal)

 在下面的程序中，我想为每个线程分配不同的id，但是在输出中，每个线程都有不一致的id，如输出中所示。 但是，如果我取消注释system.out语句，每个线程都被分配了唯一的ID，不知道原因。 
 
class ThreadLocalDemo {
public static void main(String[] args) throws InterruptedException, 
ExecutionException {
    CustomerThread custThread1 = new CustomerThread("Sampath");
    CustomerThread custThread2 = new CustomerThread("Harish");
    CustomerThread custThread3 = new CustomerThread("Harsha");
    CustomerThread custThread4 = new CustomerThread("Gowtham");
    custThread1.start();
    custThread2.start();
    custThread3.start();
    custThread4.start();
    }
}

class CustomerThread extends Thread {
static Integer custId = 0;
private  static ThreadLocal<Integer> tl = new ThreadLocal<Integer>() {
    @Override
    protected Integer initialValue() {
        //System.out.println("will work");
        return ++custId;
    }
};

CustomerThread(String name) {
    super(name);
}

public void run() {
    System.out.println(Thread.currentThread().getName() + " executing with id: " + tl.get());
}
}
 
 输出是： 
 
Sampath executing with id: 1
Harish executing with id: 
Harsha executing with id: 2
Gowtham executing with id: 1
 
 预期的输出是具有唯一ID的线程： 
 
Sampath executing with id: 1
Harish executing with id: 2
Harsha executing with id: 3
Gowtham executing with id: 4              

In the below program i want to assign distinct id to each thread, but in the output each thread is having inconsistent id's as shown in output. However if I uncomment the system.out statement each thread is assigned unique ID's, not sure about the reason.
 
class ThreadLocalDemo {
public static void main(String[] args) throws InterruptedException, 
ExecutionException {
    CustomerThread custThread1 = new CustomerThread("Sampath");
    CustomerThread custThread2 = new CustomerThread("Harish");
    CustomerThread custThread3 = new CustomerThread("Harsha");
    CustomerThread custThread4 = new CustomerThread("Gowtham");
    custThread1.start();
    custThread2.start();
    custThread3.start();
    custThread4.start();
    }
}

class CustomerThread extends Thread {
static Integer custId = 0;
private  static ThreadLocal<Integer> tl = new ThreadLocal<Integer>() {
    @Override
    protected Integer initialValue() {
        //System.out.println("will work");
        return ++custId;
    }
};

CustomerThread(String name) {
    super(name);
}

public void run() {
    System.out.println(Thread.currentThread().getName() + " executing with id: " + tl.get());
}
}
 
Output is: 
 
Sampath executing with id: 1
Harish executing with id: 
Harsha executing with id: 2
Gowtham executing with id: 1
 
Expected output is threads with unique id's:
 
Sampath executing with id: 1
Harish executing with id: 2
Harsha executing with id: 3
Gowtham executing with id: 4              

原文：https://stackoverflow.com/questions/44338430