Part 11: java.util.concurrent - CachedThreadPool Example

This article will discuss about Thread pool that can reuse previously constructed threads when they are available. From Java 5.0+ one can get such pool from Executors using following method –

public static ExecutorService newCachedThreadPool()

Creates a thread pool that creates new threads as needed, but will reuse previously constructed threads when they are available. These pools will typically improve the performance of programs that execute many short-lived asynchronous tasks. Calls to execute will reuse previously constructed threads if available. If no existing thread is available, a new thread will be created and added to the pool. Threads that have not been used for sixty seconds are terminated and removed from the cache. Thus, a pool that remains idle for long enough will not consume any resources. Note that pools with similar properties but different details (for example, timeout parameters) may be created using ThreadPoolExecutor constructors.

Example-

Suppose we have 100 properties files in an application. We have one thread that can read properties file and return a map value. We want to optimize the time to read all 100 properties file by using concurrent reading. Here optimize means – we need a perfect balance between CPU Utilization and total time consumed by reading process.

Pseudo code – READER THREAD

Config Reader implements Callable<Map<String, String>

try{

    // Get the file name in the constructor of thread

    // Check if File exists

    // Read the file and retrun the map object

}catch(Exception e){

        //release all the resource

        //return null

}

Main THREAD-

        // Get a Cached thread pool from Executors

 try{

    // Get the list of all properties file in the directory

    // Create a READER THREAD by passing the name of file

    // store the READER thread in the a list

    //release all the thread in one go and get the Map objects

}catch(Exception e){

                //release all the resources

                // print the stack trace

}finally{

        //shutdown the thread pool

}

package com.jovialjava.blog.threads;

import java.io.File;
import java.io.FileInputStream;
import java.util.ArrayList;
import java.util.List;
import java.util.Properties;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;


class CachedReader implements Callable  <  Properties  > {
   
  private String name = null; 
  public CachedReader(String name){
   this.name = name;  
  } 
  public Properties call(){
   try{
    File f = new File(name); 
    Properties prop = new Properties();
          if(f.exists() && f.canRead() && f.isFile()){
           FileInputStream in = new FileInputStream(f);         
           prop.load(in);          
           return prop;
          }else{
           System.err.println("Please check about this file:["+ f.getAbsolutePath()+"]");
           return null;
          }   
   }catch(Exception e){
     e.printStackTrace();
     return null;
   }
  }
 }
public class CachedThreadPoolExample {
 public static String directory = "config";
   private static ExecutorService executorPool=null;
   private static int MAX_THREADS = 20;
    
   public static void main(String...args){
    try{
     File dir = new File(directory);
     if(dir.isDirectory()){
       List < Callable < Properties >  >  fileList = new ArrayList < Callable < Properties >  > ();
       String[] files = dir.list();
       /**
        * Optimization - Reuse the the threads.
        */
       executorPool = Executors.newCachedThreadPool();
       
      for(String file : files){
        Callable < Properties >  reader = new CachedReader(dir.getAbsolutePath()+ File.separator+ file);
        fileList.add(reader);      
      }
      List < Future < Properties >  >  results = executorPool.invokeAll(fileList);
      /**
       * Check how many success and how many failure
       */
      int success =0, failure = 0;
      for(Future < Properties >  result : results){
       if(result.get() == null){
        failure++;
       }else{
        success++;
       }
      }
      System.out.println("Total number of files ["+ fileList.size()+"]");
      System.out.println("Success Count ["+ success+"]");
      System.out.println("Failure Count ["+ failure+"]");
     }else{
       throw new IllegalArgumentException("There is no such directory name -"+ directory);
     }
    }catch(Exception e){
      e.printStackTrace();
    }finally{
     if(executorPool!=null){
       executorPool.shutdown();
     }
     }
   }
}