How to make a Scheduledexecutorservice launch a new task while the previous one has not yet finished?

Question

I am implementing a Scheduler to run some threads on my system at x time intervals. The big problem is that if thread 1 has not yet finished running, 2 does not start, even though its time has come.

In the example below I "forced" this error, because I was suspicious that this could be happening.

Thread

package backgroundProcesses;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.TimerTask;

public class MinhaThread implements Runnable {

    @Override
    public void run() {
        Calendar cal = Calendar.getInstance();
        SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss");
        System.out.println("FOI, COMEÇOOU A THREAD: " +  sdf.format(cal.getTime()));
        int i = 0;
        while(i < 1000000000) {
            int a = 1;
        }

        System.out.println("CHEGOU AO FIM");
    }

}

Executioner

package backgroundProcesses;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener
public class ExecutorThreads implements ServletContextListener {

     private ScheduledExecutorService scheduler;


     @Override
    public void contextInitialized(ServletContextEvent arg0) {
        // TODO Auto-generated method stub
        scheduler = Executors.newScheduledThreadPool(4);
        scheduler.scheduleAtFixedRate(new MinhaThread(), 0, 10, TimeUnit.SECONDS); 
    }

    @Override
    public void contextDestroyed(ServletContextEvent arg0) {
        // TODO Auto-generated method stub
        scheduler.shutdownNow();
    }



}

I’m running the application on a simple Tomcat, I don’t use Tomcatee.

Desired Result:

When starting the program, the first thread will be created. 10 seconds later, although the first thread is still in the loop, the second thread is also created and starts to rotate.

Answer 1

It is not possible and this is highlighted in documentation.

If any Execution of this task takes longer than its period, then subsequent executions may start late, but will not concurrently execute.

If the execution of the task takes longer than your period, subsequent executions may start later, but will not be performed simultaneously.

A possible approach⁽¹⁾ is the Scheduler use another Executor to perform the "time consuming/long task".

@WebListener
public class Executor implements ServletContextListener {

    private ScheduledExecutorService scheduler;
    private ExecutorService executorService;

    @Override
    public void contextInitialized(ServletContextEvent arg0) {
        // TODO Auto-generated method stub

        executorService = Executors.newCachedThreadPool();
        scheduler = Executors.newScheduledThreadPool(4);

        scheduler.scheduleWithFixedDelay(new Runnable() {
            @Override
            public void run() {
                executorService.submit(new MinhaThread());
            }
        }, 0, 10, TimeUnit.SECONDS);

    @Override
    public void contextDestroyed(ServletContextEvent arg0) {
        // TODO Auto-generated method stub
        scheduler.shutdownNow();
    }    

}

When the Scheduler fires, the task MinhaThread is executed using a Cachedthreadpool "type" Executor.

newCachedThreadPool() creates an Executor that uses a thread pool. New threads are created if necessary or are used if available.

Note: The class name MinhaThread should be MinhaTarefa or MyTask.

_{⁽¹⁾More or less as suggested by Carlosheuberger at your comment.}

Answer 2

The solution proposed by @Carlos Heuberger really worked:

Main thread

package backgroundProcesses;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.TimerTask;

public class MinhaThread implements Runnable {

    @Override
    public void run() {
        Calendar cal = Calendar.getInstance();
        SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss");
        System.out.println("FOI, COMEÇOOU A THREAD: " +  sdf.format(cal.getTime()));
        int i = 0;
        while(i < 1000000000) {
            int a = 1;
        }

        System.out.println("CHEGOU AO FIM");
    }

}

Secondary Thread

package backgroundProcesses;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class OutraThread implements Runnable {

    @Override
    public void run() {
        // TODO Auto-generated method stub
        System.out.println("ENTROU NA THREAD SECUNDARIA");
        new Thread(new MinhaThread()).start();

    }

}

Executioner

package backgroundProcesses;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;



@WebListener
public class Executor implements ServletContextListener {

     private ScheduledExecutorService scheduler;


     @Override
    public void contextInitialized(ServletContextEvent arg0) {
        // TODO Auto-generated method stub
        scheduler = Executors.newScheduledThreadPool(4);
        scheduler.scheduleAtFixedRate(new OutraThread(), 0, 10, TimeUnit.SECONDS); //roda a qualidade a cada 5 minutos.
    }

    @Override
    public void contextDestroyed(ServletContextEvent arg0) {
        // TODO Auto-generated method stub
        scheduler.shutdownNow();
    }



}

Console:

ENTROU NA THREAD SECUNDARIA
FOI, COMEÇOOU A THREAD: 11:54:33
ENTROU NA THREAD SECUNDARIA
FOI, COMEÇOOU A THREAD: 11:54:43
ENTROU NA THREAD SECUNDARIA
FOI, COMEÇOOU A THREAD: 11:54:53
ENTROU NA THREAD SECUNDARIA
FOI, COMEÇOOU A THREAD: 11:55:03
ENTROU NA THREAD SECUNDARIA
FOI, COMEÇOOU A THREAD: 11:55:13
ENTROU NA THREAD SECUNDARIA
FOI, COMEÇOOU A THREAD: 11:55:23

I’ll leave the question open to see if any other solution will come up, but for now I will implement this one.