Spring Boot shared thread

Question

I am developing my Spring boot application wich gets two requests: /start and /stop. I need to create one shared thread for all clients requests.

When the first request "/start" will be received from client, app will create one thread shared by local variable T1.

When the second request "/stop" will be received, app will set boolean variable of thread "stopped" to stop it and the thread should stop.

Is next code provides safe for this shared thread? Should i use the local variable for thread object or need to do it by another way?

package com.direct.webflow;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

@EnableAutoConfiguration
@Controller
public class WebApp {

    ThreadDemo T1;

    @RequestMapping("/start")
    @ResponseBody
    String start() {

        synchronized(this){
        if (T1 == null || T1.stopped) {
            T1= new ThreadDemo( "Thread-1");
            T1.start();
        } else {
            return "Already started!";
        }
        }
        return "Thread started!";

    }

    @RequestMapping("/stop")
    @ResponseBody
    String end() {
        if (T1 == null) {
            System.out.println("Not started!");
            return "Not started!";
        } else if (!T1.stopped) {
            T1.stopped=true;
            System.out.println("Trying to stop!");
            return "Stopped!";
        } else {
            return "Already stopped!";
        }
    }

    public static void main(String[] args) throws Exception {
        SpringApplication.run(WebApp.class, args);
    }
}


package com.direct.webflow;

public class ThreadDemo extends Thread {
       private Thread t;
       private String threadName;
       public volatile boolean stopped=false;
       ThreadDemo(String name){
           threadName = name;
           System.out.println("Creating " +  threadName );
       }

       public void run() {
          int i=0;
          System.out.println("Running " +  threadName );
          while (!stopped) {
            System.out.println("Thread: " +this.isInterrupted()+ threadName + ", " + i++);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
                System.out.println("Thread: STOP!");
                break;
            }
         }
        System.out.println("Thread " +  threadName + " exiting.");
       }

       public void start ()
       {
          stopped=false; 
          System.out.println("Starting " +  threadName );
          if (t == null)
          {
             t = new Thread (this, threadName);
             t.start ();
          }
       }

    }

Answer 1

This is very close. You need to add the synchronized(this) block in your controller end() method. Otherwise you may have a race condition if /stop and /start are being called simultaneously.

Since Spring controllers are singletons you are OK to use a member variable like you have done here.