Java线程打印多余行问题：最后线程重复输出唤醒信息

问题现象 #

运行一个包含线程的Java程序，逻辑是让线程随机休眠一段时间后打印唤醒信息。程序执行到末尾时，最后一个线程的“is NOW AWAKE”信息被重复打印6次，其余线程均正常打印一次休眠和唤醒信息。

相关代码 #

sampleThread.java #

import java.util.Random;

public class sampleThread extends Thread{
    sampleThread thread;
    Random rand = new Random();

    public void run() {
        thread = new sampleThread();
        int randSleep = rand.nextInt(1000);

        System.out.println(thread.getName() + " is sleeping for " + randSleep + " milliseconds");

        try {
            Thread.sleep(randSleep);
            System.out.println(thread.getName() + " is NOW AWAKE");
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }
}

driver.java #

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class driver {
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        List<Future<?>> futArray = new ArrayList<>();
        ExecutorService es = Executors.newFixedThreadPool(6);

        sampleThread temp = new sampleThread();
        for (int i=0; i<120; i++) {
            Future<?> future = es.submit(temp);
            futArray.add(future);
        }
    }
}

异常输出 #

Thread-117 is sleeping for 547 milliseconds
Thread-117 is NOW AWAKE
Thread-118 is sleeping for 442 milliseconds
Thread-118 is NOW AWAKE
Thread-119 is sleeping for 182 milliseconds
Thread-119 is NOW AWAKE
Thread-120 is sleeping for 487 milliseconds
Thread-120 is NOW AWAKE
Thread-120 is NOW AWAKE
Thread-120 is NOW AWAKE
Thread-120 is NOW AWAKE
Thread-120 is NOW AWAKE
Thread-120 is NOW AWAKE

原因分析 #

核心问题在于线程安全与变量作用域：

1. 代码仅创建了一个sampleThread实例temp，并将其重复提交给线程池120次。线程池的6个工作线程会并发调用这个实例的run方法。
2. sampleThread类中的thread是实例级成员变量，属于共享资源。当多个工作线程同时执行run方法时，会不断覆盖该变量的引用，最终所有线程的thread都会指向最后一次创建的sampleThread实例（即Thread-120）。
3. 当各个线程休眠结束后，打印的都是共享变量thread的名称，因此出现了Thread-120的唤醒信息被重复打印6次（对应线程池的6个工作线程）的现象。

解决方案 #

将sampleThread类中的成员变量thread移至run方法内部，改为局部变量。这样每个线程执行run方法时，都会拥有独立的thread变量，不会相互干扰。

修改后的sampleThread.java代码：

import java.util.Random;

public class sampleThread extends Thread{
    Random rand = new Random();

    public void run() {
        // 将thread改为run方法内的局部变量
        sampleThread thread = new sampleThread();
        int randSleep = rand.nextInt(1000);

        System.out.println(thread.getName() + " is sleeping for " + randSleep + " milliseconds");

        try {
            Thread.sleep(randSleep);
            System.out.println(thread.getName() + " is NOW AWAKE");
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }
}

验证结果 #

修改后，每个线程的休眠和唤醒信息都会仅打印一次，不再出现重复输出的异常情况。

内容的提问来源于stack exchange，提问作者suttertub