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ThreadPoolExecutor - Java Thread Pool Example

Published on August 3, 2022
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By Pankaj
Developer and author at DigitalOcean.
ThreadPoolExecutor - Java Thread Pool Example

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Java thread pool manages the pool of worker threads. It contains a queue that keeps tasks waiting to get executed. We can use ThreadPoolExecutor to create thread pool in Java. Java thread pool manages the collection of Runnable threads. The worker threads execute Runnable threads from the queue. java.util.concurrent.Executors provide factory and support methods for java.util.concurrent.Executor interface to create the thread pool in java. Executors is a utility class that also provides useful methods to work with ExecutorService, ScheduledExecutorService, ThreadFactory, and Callable classes through various factory methods. Let’s write a simple program to explain it’s working. First, we need to have a Runnable class, named WorkerThread.java

package com.journaldev.threadpool;

public class WorkerThread implements Runnable {
  
    private String command;
    
    public WorkerThread(String s){
        this.command=s;
    }

    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName()+" Start. Command = "+command);
        processCommand();
        System.out.println(Thread.currentThread().getName()+" End.");
    }

    private void processCommand() {
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    @Override
    public String toString(){
        return this.command;
    }
}

ExecutorService Example

Here is the test program class SimpleThreadPool.java, where we are creating fixed thread pool from Executors framework.

package com.journaldev.threadpool;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class SimpleThreadPool {

    public static void main(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(5);
        for (int i = 0; i < 10; i++) {
            Runnable worker = new WorkerThread("" + i);
            executor.execute(worker);
          }
        executor.shutdown();
        while (!executor.isTerminated()) {
        }
        System.out.println("Finished all threads");
    }
}

In the above program, we are creating a fixed-size thread pool of 5 worker threads. Then we are submitting 10 jobs to this pool, since the pool size is 5, it will start working on 5 jobs and other jobs will be in wait state, as soon as one of the job is finished, another job from the wait queue will be picked up by worker thread and get’s executed. Here is the output of the above program.

pool-1-thread-2 Start. Command = 1
pool-1-thread-4 Start. Command = 3
pool-1-thread-1 Start. Command = 0
pool-1-thread-3 Start. Command = 2
pool-1-thread-5 Start. Command = 4
pool-1-thread-4 End.
pool-1-thread-5 End.
pool-1-thread-1 End.
pool-1-thread-3 End.
pool-1-thread-3 Start. Command = 8
pool-1-thread-2 End.
pool-1-thread-2 Start. Command = 9
pool-1-thread-1 Start. Command = 7
pool-1-thread-5 Start. Command = 6
pool-1-thread-4 Start. Command = 5
pool-1-thread-2 End.
pool-1-thread-4 End.
pool-1-thread-3 End.
pool-1-thread-5 End.
pool-1-thread-1 End.
Finished all threads

The output confirms that there are five threads in the pool named from “pool-1-thread-1” to “pool-1-thread-5” and they are responsible to execute the submitted tasks to the pool.

ThreadPoolExecutor Example

Executors class provide simple implementation of ExecutorService using ThreadPoolExecutor but ThreadPoolExecutor provides much more feature than that. We can specify the number of threads that will be alive when we create ThreadPoolExecutor instance and we can limit the size of thread pool and create our own RejectedExecutionHandler implementation to handle the jobs that can’t fit in the worker queue. Here is our custom implementation of RejectedExecutionHandler interface.

package com.journaldev.threadpool;

import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;

public class RejectedExecutionHandlerImpl implements RejectedExecutionHandler {

    @Override
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
        System.out.println(r.toString() + " is rejected");
    }

}

ThreadPoolExecutor provides several methods using which we can find out the current state of the executor, pool size, active thread count and task count. So I have a monitor thread that will print the executor information at a certain time interval.

package com.journaldev.threadpool;

import java.util.concurrent.ThreadPoolExecutor;

public class MyMonitorThread implements Runnable
{
    private ThreadPoolExecutor executor;
    private int seconds;
    private boolean run=true;

    public MyMonitorThread(ThreadPoolExecutor executor, int delay)
    {
        this.executor = executor;
        this.seconds=delay;
    }
    public void shutdown(){
        this.run=false;
    }
    @Override
    public void run()
    {
        while(run){
                System.out.println(
                    String.format("[monitor] [%d/%d] Active: %d, Completed: %d, Task: %d, isShutdown: %s, isTerminated: %s",
                        this.executor.getPoolSize(),
                        this.executor.getCorePoolSize(),
                        this.executor.getActiveCount(),
                        this.executor.getCompletedTaskCount(),
                        this.executor.getTaskCount(),
                        this.executor.isShutdown(),
                        this.executor.isTerminated()));
                try {
                    Thread.sleep(seconds*1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
        }
            
    }
}

Here is the thread pool implementation example using ThreadPoolExecutor.

package com.journaldev.threadpool;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class WorkerPool {

    public static void main(String args[]) throws InterruptedException{
        //RejectedExecutionHandler implementation
        RejectedExecutionHandlerImpl rejectionHandler = new RejectedExecutionHandlerImpl();
        //Get the ThreadFactory implementation to use
        ThreadFactory threadFactory = Executors.defaultThreadFactory();
        //creating the ThreadPoolExecutor
        ThreadPoolExecutor executorPool = new ThreadPoolExecutor(2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(2), threadFactory, rejectionHandler);
        //start the monitoring thread
        MyMonitorThread monitor = new MyMonitorThread(executorPool, 3);
        Thread monitorThread = new Thread(monitor);
        monitorThread.start();
        //submit work to the thread pool
        for(int i=0; i<10; i++){
            executorPool.execute(new WorkerThread("cmd"+i));
        }
        
        Thread.sleep(30000);
        //shut down the pool
        executorPool.shutdown();
        //shut down the monitor thread
        Thread.sleep(5000);
        monitor.shutdown();
        
    }
}

Notice that while initializing the ThreadPoolExecutor, we are keeping initial pool size as 2, maximum pool size to 4 and work queue size as 2. So if there are 4 running tasks and more tasks are submitted, the work queue will hold only 2 of them and the rest of them will be handled by RejectedExecutionHandlerImpl. Here is the output of the above program that confirms the above statement.

pool-1-thread-1 Start. Command = cmd0
pool-1-thread-4 Start. Command = cmd5
cmd6 is rejected
pool-1-thread-3 Start. Command = cmd4
pool-1-thread-2 Start. Command = cmd1
cmd7 is rejected
cmd8 is rejected
cmd9 is rejected
[monitor] [0/2] Active: 4, Completed: 0, Task: 6, isShutdown: false, isTerminated: false
[monitor] [4/2] Active: 4, Completed: 0, Task: 6, isShutdown: false, isTerminated: false
pool-1-thread-4 End.
pool-1-thread-1 End.
pool-1-thread-2 End.
pool-1-thread-3 End.
pool-1-thread-1 Start. Command = cmd3
pool-1-thread-4 Start. Command = cmd2
[monitor] [4/2] Active: 2, Completed: 4, Task: 6, isShutdown: false, isTerminated: false
[monitor] [4/2] Active: 2, Completed: 4, Task: 6, isShutdown: false, isTerminated: false
pool-1-thread-1 End.
pool-1-thread-4 End.
[monitor] [4/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [0/2] Active: 0, Completed: 6, Task: 6, isShutdown: true, isTerminated: true
[monitor] [0/2] Active: 0, Completed: 6, Task: 6, isShutdown: true, isTerminated: true

Notice the change in active, completed and total completed task count of the executor. We can invoke shutdown() method to finish execution of all the submitted tasks and terminate the thread pool. If you want to schedule a task to run with delay or periodically then you can use ScheduledThreadPoolExecutor class. Read more about them at Java Schedule Thread Pool Executor.

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Pankaj

author

Developer and author at DigitalOcean.

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JournalDev
DigitalOcean Employee
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September 10, 2021

Hi Pankaj , Thanks a lot for this great tutorial, My lots of concept cleared about executorpoolservice. It would be great if my one doubt clear. I am trying to implement this threadpoolexecutor , As we know Use threadpoolExecutor we creating thead pool, My question how this thread worker is internally connected with http rest call, bcz there also we are creating tomcat thread, Suppose we have 50 tomcat thread , and 10 worker thread, and 10k request, how each request sync with there own response if we calling async call.

- Ravi kumar

    JournalDev
    DigitalOcean Employee
    DigitalOcean Employee badge
    April 28, 2020

    Is it work in same way for JAVA 8, Or need to change code

    - Amol

      JournalDev
      DigitalOcean Employee
      DigitalOcean Employee badge
      December 24, 2019

      Thanks, my most of the basic doubts got clarified.

      - maha

        JournalDev
        DigitalOcean Employee
        DigitalOcean Employee badge
        October 18, 2019

        Hi, I have created ExecutorService with 10 worker threads, and there are N number of tasks, wherein each task connects to DB and does some processing. My problem is, I am unable to understand why thread names keep changing. For example : pool-9-thread-1, pool-9-thread-2,…pool-9-thread-10 after few executions the thread name changes to pool-10-thread-1,pool-10-thread-2,…pool-10-thread-10 . Can you please explain, me why is it happening?

        - Rakesh

          JournalDev
          DigitalOcean Employee
          DigitalOcean Employee badge
          February 27, 2019

          ”Java thread pool manages the collection of Runnable threads and worker threads execute Runnable from the queue“ I think this should be —> Java thread pool manages the collection of Runnable threads and worker threads executING Runnable from the queue Please answer me . Am I right?

          - linjiejun

            JournalDev
            DigitalOcean Employee
            DigitalOcean Employee badge
            December 17, 2018

            can you help me wihts this? Implement a thread pool executor, similar to the one present in Java platform: java.util.concurrent.ThreadPoolExecutor WITHOUT using any class from java.util.concurrent package. The component will accept for execution tasks of type java.lang.Runnable. The input parameters are: - int corePoolSize - int maximumPoolSize - int keepAliveTime (resolution = 1 second) - int queueSize

            - Florin

              JournalDev
              DigitalOcean Employee
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              October 12, 2018

              I am not able to see Executors is implementing Executor interface

              - Akshay

                JournalDev
                DigitalOcean Employee
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                April 25, 2018

                thanks a lot, it’s really do me a favor

                - Christophe

                  JournalDev
                  DigitalOcean Employee
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                  April 13, 2018

                  Nice example. i have some requirement. I have to read number of files and push the data to the table. I think executor service can used here to do it in a optimum way. Can you please help me how to do it using executor service

                  - Ashish Mishra

                    JournalDev
                    DigitalOcean Employee
                    DigitalOcean Employee badge
                    February 12, 2018

                    Very helpful. Thanking you

                    - Prakash

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