Java ThreadPoolExecutor preStartAllCoreThreads() Method

The preStartAllCoreThreads() method of ThreadPoolExecutor class starts all the core threads, causing them to wait for the work idly. This overrides the default policy of starting core threads only when new tasks are executed.

Syntax

Parameters

Return

The number of threads started

Throw

Does not throw the exception.

Example 1

import java.util.concurrent.Executors;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.lang.Thread;
public class preStartAllCoreThreadExample1   {
  public static void main(final String[] arguments) throws InterruptedException {
             ThreadPoolExecutor executor = (ThreadPoolExecutor)Executors.newCachedThreadPool();
             executor.submit(new Task());
             executor.submit(new Task());
             executor.prestartAllCoreThreads();
             System.out.println("Core Pool size after execution: " + executor.getCorePoolSize());
             executor.shutdown();
             }  
 static class Task implements Runnable {
      @Override
      public void run() {
                     try {
                         Long duration = (long) (Math.random() * 5);
                         System.out.println("Running Thread Name: " + Thread.currentThread().getName());
                         TimeUnit.SECONDS.sleep(duration);
                         System.out.println("Completed Thread Name: " +Thread.currentThread().getName());
                         } catch (InterruptedException e) {  }
                     }
   }
}

Test it Now

Output:

Core Pool size after execution: 0
Running Thread Name: pool-1-thread-2
Running Thread Name: pool-1-thread-1
Completed Thread Name: pool-1-thread-1
Completed Thread Name: pool-1-thread-2

Example 2

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadPoolExecutor;
import java.lang.Thread;
import java.util.logging.Level;
import java.util.logging.Logger;
public class preStartAllCoreThreadExample2    {
                   public static void main(String[] args) {
                                  Task88 task1 = new Task88("t1");
                                  System.out.println("Starting threads");
                          ThreadPoolExecutor tExecutor = (ThreadPoolExecutor) Executors.newCachedThreadPool();
                          tExecutor.execute(task1); // start task1
                          tExecutor.prestartAllCoreThreads();
                          tExecutor.shutdown(); // shutdown worker threads
                          System.out.println("Threads started, main ends\n");
                            }
                   }
class Task88 implements Runnable {
             private final String threadName;
             public Task88(String threadName) {
             this.threadName = threadName;
             }
   @Override
  public void run() {
         for(int i=0 ; i<=10;i++)
             {
                if(i<5)
                   {
                    try {
                        System.out.println("Thread sleeping for 1000 i=" +i);
                        Thread.sleep(1000);
                         } catch (InterruptedException ex) {
                         Logger.getLogger(Task88.class.getName()).log(Level.SEVERE, null, ex);
                       }
                   }
                  else{
                       System.out.println(" now  i > 5 :");
                       break;
                    }
               }
                   }
}

Test it Now

Output:

Starting threads
Threads started, main ends
Thread sleeping for 1000 i=0
Thread sleeping for 1000 i=1
Thread sleeping for 1000 i=2
Thread sleeping for 1000 i=3
Thread sleeping for 1000 i=4
 now  i > 5 :

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