Category Archives: Examples

Simple Thread Programs

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Simple Program to print numbers using threads
NumberPrinter.java

public class NumberPrinter implements Runnable{
    int number;
    public NumberPrinter(int number){
        this.number = number;
    }

    public void run(){
        System.out.println("Printing Number from Thread "+ this.number);
    }
}

Main.java

public class Main {
    public static void main(String[] args) {
        for (int idx=1;idx<=5;idx++){
            Thread objthread = new Thread(new NumberPrinter(idx));
            objthread.start();
        }
    }
}

Output

Printing Number from Thread 5
Printing Number from Thread 1
Printing Number from Thread 4
Printing Number from Thread 3
Printing Number from Thread 2

Simple Program using Executor Service taking Runnable as Argument

ExecutorService is a framework which allows to create thread. Threads can be created from FixedThreadPool, CachedThreadPool and ScheduledThreadPool. submit() method takes runnable or callable object (Functional Interface Type) as argument. The Same code above can be rewritten as below
Main.java

public class Main {
    public static void main(String[] args) throws Exception {
        ExecutorService objExecService = Executors.newFixedThreadPool(2);

        //Lambda Expresssion passed as Argument as Runnable is FI
        objExecService.submit(() -> {
            System.out.println(Thread.currentThread().getName());
        });

        objExecService.shutdown();
    }
}

Output

pool-1-thread-1

Same code with Runnable instance passed as argument to submit

  .
  . 
  //Instance of Runnable passed as argument
  HelloThread1 objHT1 = new HelloThread1();
  objExecService.submit(objHT1);
  .
  .

Output

Hello World from Thread Name (pool-1-thread-1) using Runnable

Same code with Runnable as Anonymous Class passed as argument

ExecutorService exec = Executors.newFixedThreadPool(2);

//Instance of Runnable passed as Anonymous class
exec.execute(new Runnable() {
  public void run() {
    System.out.println("Hello world");
  }
});

exec.shutdown();

Simple Program using Executor Service taking Callable as Argument

public class Main {
    public static void main(String[] args) throws Exception {
        ExecutorService objExecService = Executors.newFixedThreadPool(2);
        Future<String> objFuture = objExecService.submit(new HelloThread2());
        System.out.println(objFuture.get());
        objExecService.shutdown();
    }
}

Output

Hello World from Thread Name (pool-1-thread-1) using Callable

Using Lambda Expression as Submi

.
.
ExecutorService objExecService = Executors.newFixedThreadPool(2);

Future<String> objFuture = objExecService.submit(() -> {
  Thread.sleep(3000);
  return Thread.currentThread().getName();
});

System.out.println(objFuture.get());
.
.

The above could be rewritten in anonymous class as below

ExecutorService objExecService = Executors.newFixedThreadPool(2);

Future<String> objFuture = objExecService.submit(new Callable<String>() {
 @Override
 public String call() throws Exception {
   Thread.sleep(3000);
   return Thread.currentThread().getName();
 }
});

System.out.println(objFuture.get());
objExecService.shutdown();

Program for Creating Thread Pool and executing Task

ThreadPoolExample.java

public class ThreadPoolExample {
    public static void main(String args[]) {
       ExecutorService service = Executors.newFixedThreadPool(10); //create 10 worker threads in Thread Pool
       for (int i =0; i<100; i++){
           service.submit(new Task(i)); //submit that to be done 
       }
       service.shutdown();
    }  
}

Task.java

final class Task implements Runnable {
    private int taskId;  
    public Task(int id){
        this.taskId = id;
    }
  
    @Override
    public void run() {
        System.out.println("Task ID : " + this.taskId +" performed by " 
                           + Thread.currentThread().getName());
    }  
}

Task ID : 0 performed by pool-1-thread-1
Task ID : 3 performed by pool-1-thread-4
Task ID : 2 performed by pool-1-thread-3
Task ID : 1 performed by pool-1-thread-2
Task ID : 5 performed by pool-1-thread-6
Task ID : 4 performed by pool-1-thread-5

Thread Code Examples Basics

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Reading a File extending Thread API

  1. ReadFile.java has a run() method which implements the reading the file code within the try with resources block
  2. In the main method start method is called over the ReadFile class instance
  3. In thread we have coded is asynchrobnous(order of execution cannot be guaranteed) which we can see from the output below

TestThread.java

package com.mugil.test;

import com.mugil.runnables.ReadFile;

public class TestThread {
	public static void main(String[] args) {
		ReadFile objReadFileThread1 = new ReadFile();
		ReadFile objReadFileThread2 = new ReadFile();
		ReadFile objReadFileThread3 = new ReadFile();
				
		objReadFileThread1.start();
		objReadFileThread2.start();
		objReadFileThread3.start();
	}
}

ReadFile.java

package com.mugil.runnables;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;

public class ReadFile extends Thread {

 public void run() {

  try (BufferedReader reader = new BufferedReader(new FileReader(new File("E:\\JavaProjects\\JavaThreads\\src\\Sample.txt")))) {
   String line = null;

   while ((line = reader.readLine()) != null) {
    System.out.println(Thread.currentThread().getName() + " reading line " + line);
   }

  } catch (IOException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }

 }
}

Output

Thread-2 reading line Line1
Thread-0 reading line Line1
Thread-0 reading line Line2
Thread-0 reading line Line3
Thread-1 reading line Line1
Thread-1 reading line Line2
Thread-2 reading line Line2
Thread-1 reading line Line3
Thread-1 reading line Line4
Thread-1 reading line Line5
Thread-0 reading line Line4
Thread-0 reading line Line5
Thread-2 reading line Line3
Thread-2 reading line Line4
Thread-2 reading line Line5

Reading a File implementing Runnable API

  1. Now in the below code the runnable API is implemented rather than extending like Thread
  2. The run() is called over instance of ReadFile rather than start() method
  3. Calling run() method will start the execution of thread in the present running thread rather than creating new Thread for execution which can been seen in output main reading line rather than Thread-N reading line

TestThread.java

package com.mugil.test;

import com.mugil.runnables.ReadFile;

public class TestThread {
	public static void main(String[] args) {
		ReadFile objReadFileThread1 = new ReadFile();
		ReadFile objReadFileThread2 = new ReadFile();
		ReadFile objReadFileThread3 = new ReadFile();
				
		objReadFileThread1.run();
		objReadFileThread2.run();
		objReadFileThread3.run();
	}
}

ReadFile.java

public class ReadFile implements Runnable {

 public void run() {

  try (BufferedReader reader = new BufferedReader(new FileReader(new File("E:\\JavaProjects\\JavaThreads\\src\\Sample.txt")))) {
   String line = null;

   while ((line = reader.readLine()) != null) {
    System.out.println(Thread.currentThread().getName() + " reading line " + line);
   }

  } catch (IOException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
 }
}

Output

main reading line Line1
main reading line Line2
main reading line Line3
main reading line Line4
main reading line Line5
main reading line Line1
main reading line Line2
main reading line Line3
main reading line Line4
main reading line Line5
main reading line Line1
main reading line Line2
main reading line Line3
main reading line Line4
main reading line Line5

Methods to Manage thread are available on Thread class not in Runnable. So we can pass the runnable instance as parameter like one below
TestThread.java

.
.
.
Thread objThread = new Thread(runObj);
objThread.start();
.
.