ThreadPoolExecutor is a feature added by java concurrent api to maintain and reuse threads efficiently , so that our programs don't have to worry about creating and destroying threads and focus on the core functionality. I have created a custom thread pool executor to get better understanding of how thread pool executor would work .
Functionality :
- It maintains a fixed thread pool ,and creates threads and start the threads even if no task is submitted whereas ThreadPoolExecutor creates threads on demand , i.e. whenever a runnable is submitted to pool and the number of threads are less than core pool size .
- In ThreadPoolExecutor, we provide a waiting queue ,where new runnable task waits when all threads are busy running existing task. Once the queue is filled , new threads will be created up to maximum pool size. In MyThreadPool , i am storing the runnable in a linked list , so every task will wait in the list and it is unbounded , so no usage of maxPoolSize in this .
- In ThreadPoolExecutor , we use Future Objects to get the result from task , future.get() method will block if the result is not available , or we use CompletionService . In MyThreadPoolExecutor , i have created a simple interface called ResultListener , user has to provide a implementation of this as to how he wants the output to be processed . After every task is completed , the ResultListener will get callback with the output of task or error method will be called in case of any exception.
- When shutdown method is called , MyThreadPoolExecutor will stop accepting new tasks and complete the remaining tasks .
- I have provided very basic functionality as compared to ThreadPoolExecutor , i have used simple thread mechanism like wait() , notify() , notifyAll(), and join().
- Performance wise it is similar to ThreadPoolExecutor , some times better in some cases. Do let me know if you find any interesting results or ways to improve it .
package com.util;
import java.util.concurrent.Callable;
/**
* Run submitted task of {@link MyThreadPool} After running the task , It calls
* on {@link ResultListener}object with {@link Output}which contains returned
* result of {@link Callable}task. Waits if the pool is empty.
*
* @author abhishek
*
* @param
*/
import java.util.concurrent.Callable;
/**
* Run submitted task of {@link MyThreadPool} After running the task , It calls
* on {@link ResultListener}object with {@link Output}which contains returned
* result of {@link Callable}task. Waits if the pool is empty.
*
* @author abhishek
*
* @param <V>
*/
public class MyThread<V> extends Thread {
/**
* MyThreadPool object, from which the task to be run
*/
private MyThreadPool<V> pool;
private boolean active = true;
public boolean isActive() {
return active;
}
public void setPool(MyThreadPool<V> p) {
pool = p;
}
/**
* Checks if there are any unfinished tasks left. if there are , then runs
* the task and call back with output on resultListner Waits if there are no
* tasks available to run If shutDown is called on MyThreadPool, all waiting
* threads will exit and all running threads will exit after finishing the
* task
*/
public void run() {
ResultListener<V> result = pool.getResultListener();
Callable<V> task;
while (true)
{
task = pool.removeFromQueue();
if (task != null)
{
try
{
V output = task.call();
result.finish(output);
} catch (Exception e)
{
result.error(e);
}
} else
{
if (!isActive())
break;
else
{
synchronized (pool.getWaitLock())
{
try
{
pool.getWaitLock().wait();
} catch (InterruptedException e)
{
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
}
}
void shutdown() {
active = false;
}
}
package com.util;
import java.util.LinkedList;
import java.util.concurrent.Callable;
/**
* This class is used to execute submitted {@link Callable} tasks. this class
* creates and manages fixed number of threads User will provide a
* {@link ResultListener}object in order to get the Result of submitted task
*
* @author abhishek
*
*
*/
public class MyThreadPool<V> {
private Object waitLock = new Object();
public Object getWaitLock() {
return waitLock;
}
/**
* list of threads for completing submitted tasks
*/
private final LinkedList<MyThread<V>> threads;
/**
* submitted task will be kept in this list untill they run by one of
* threads in pool
*/
private final LinkedList<Callable<V>> tasks;
/**
* shutDown flag to shut Down service
*/
private volatile boolean shutDown;
/**
* ResultListener to get back the result of submitted tasks
*/
private ResultListener<V> resultListener;
/**
* initializes the threadPool by starting the threads threads will wait till
* tasks are not submitted
*
* @param size
* Number of threads to be created and maintained in pool
* @param myResultListener
* ResultListener to get back result
*/
public MyThreadPool(int size, ResultListener<V> myResultListener) {
tasks = new LinkedList<Callable<V>>();
threads = new LinkedList<MyThread<V>>();
shutDown = false;
resultListener = myResultListener;
for (int i = 0; i < size; i++) {
MyThread<V> myThread = new MyThread<V>();
myThread.setPool(this);
threads.add(myThread);
myThread.start();
}
}
public ResultListener<V> getResultListener() {
return resultListener;
}
public void setResultListener(ResultListener<V> resultListener) {
this.resultListener = resultListener;
}
public boolean isShutDown() {
return shutDown;
}
public int getThreadPoolSize() {
return threads.size();
}
public synchronized Callable<V> removeFromQueue() {
return tasks.poll();
}
public synchronized void addToTasks(Callable<V> callable) {
tasks.add(callable);
}
/**
* submits the task to threadPool. will not accept any new task if shutDown
* is called Adds the task to the list and notify any waiting threads
*
* @param callable
*/
public void submit(Callable<V> callable) {
if (!shutDown) {
addToTasks(callable);
synchronized (this.waitLock) {
waitLock.notify();
}
} else {
System.out.println("task is rejected.. Pool shutDown executed");
}
}
/**
* Initiates a shutdown in which previously submitted tasks are executed,
* but no new tasks will be accepted. Waits if there are unfinished tasks
* remaining
*
*/
public void stop() {
for (MyThread<V> mythread : threads) {
mythread.shutdown();
}
synchronized (this.waitLock) {
waitLock.notifyAll();
}
for (MyThread<V> mythread : threads) {
try {
mythread.join();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
package com.util;
/**
* This interface imposes finish method
* which is used to get the {@link Output} object
* of finished task
* @author abhishek
*
* @param
*/
public interface ResultListener {
public void finish(T obj);
public void error(Exception ex);
}
you can implement this class as you want to get back and process the result returned by tasks.
package com.util;
public class DefaultResultListener implements ResultListener{
@Override
public void finish(Object obj) {
}
@Override
public void error(Exception ex) {
ex.printStackTrace();
}
}
For example this class will add the number returned by tasks .
package com.util;
import java.util.concurrent.atomic.AtomicInteger;
/**
* ResultListener class to keep track of total matched count
* @author abhishek
*
* @param
*/
public class MatchedCountResultListener implements ResultListener {
/**
* matchedCount to keep track of the number of matches returned by submitted
* task
*/
AtomicInteger matchedCount = new AtomicInteger();
/**
* this method is called by ThreadPool to give back the result of callable
* task. if the task completed successfully then increment the matchedCount by
* result count
*/
@Override
public void finish(V obj) {
//System.out.println("count is "+obj);
matchedCount.addAndGet((Integer)obj);
}
/**
* print exception thrown in running the task
*/
@Override
public void error(Exception ex) {
ex.printStackTrace();
}
/**
* returns the final matched count of all the finished tasks
*
* @return
*/
public int getFinalCount() {
return matchedCount.get();
}
}
This is a test class which runs simple for loop using CompletionService and MyThreadPoolExecutor
package test;
import java.util.concurrent.Callable;
import java.util.concurrent.CompletionService;
import java.util.concurrent.ExecutorCompletionService;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import com.util.DefaultResultListener;
import com.util.MyThreadPool;
public class TestClass {
public static void main(String[] args) throws InterruptedException {
CompletionService threadService;
ExecutorService service = Executors.newFixedThreadPool(2);
threadService = new ExecutorCompletionService(service);
long b = System.currentTimeMillis();
for(int i =0;i<50000;i++){
threadService.submit(new MyRunable (i));
}
service.shutdown();
System.out.println("time taken by Completion Service " + (System.currentTimeMillis()-b));
DefaultResultListener result = new DefaultResultListener();
MyThreadPool newPool = new MyThreadPool(2,result);
long a = System.currentTimeMillis();
int cc =0;
for(int i =0;i<50000;i++)
{
cc = cc+i;
}
System.out.println("time taken without any pool " + (System.currentTimeMillis()-a));
a= System.currentTimeMillis();
for(int i =0;i<5000;i++){
newPool.submit(new MyRunable (i));
}
newPool.stop();
System.out.println("time taken by myThreadPool " + (System.currentTimeMillis()-a));
}
}
class MyRunable implements Callable
{
int index = -1;
public MyRunable(int index)
{
this.index = index;
}
@Override
public Integer call() throws Exception {
return index;
}
}
