Develop Reference

ThreadPoolExecutor : Pull out hanged tasks

I have implemented ThreadPoolExecutor to invoke a task.
And It seems tasks got hanged on I/O because of the core/queue size misconfiguration.
I want to pull the hanging thread out
so that the other threads in my queue starts executing.
Is there any way to list the threads inside a threadpoolexecutor and pull out the hanging thread?

You have control over the used executor, you could use the ThreadPoolExecutor's beforeExecute and afterExecute methods to keep track of running tasks and use that to create a getActiveTasks method.
import java.util.Set;
import java.util.concurrent.*;
public class ActiveTasksThreadPool extends ThreadPoolExecutor {
private final ConcurrentHashMap<Runnable, Boolean> activeTasks = new ConcurrentHashMap<>();
public ActiveTasksThreadPool(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) {
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
}
#Override
protected void beforeExecute(Thread t, Runnable r) {
activeTasks.put(r, Boolean.TRUE);
super.beforeExecute(t, r);
}
#Override
protected void afterExecute(Runnable r, Throwable t) {
super.afterExecute(r, t);
activeTasks.remove(r);
}
public Set<Runnable> getActiveTasks() {
// the returned set will not throw a ConcurrentModificationException.
return activeTasks.keySet();
}
In order to set a timeout on thread tasks with Future:
ActiveTasksThreadPool executor = new ActiveTasksThreadPool(maxTasks, maxTasks, 10, TimeUnit.SECONDS, new SynchronousQueue<Runnable>());Executors.newFixedThreadPool(2);
List<Future<Integer>> resultList = new ArrayList<>();
Random random = new Random();
for (int i=0; i<4; i++)
{
Integer number = random.nextInt(10);
FactorialCalculator calculator = new FactorialCalculator(number);
Future<Integer> result = executor.submit(calculator);
result .get(100, TimeUnit.MILLISECONDS); // here is a timeout of 100 milisecond
}

Executor wait for ever in a singleton class

I have implemented a singleton (manager) to manage some related tasks, inside this manager I am using an executor to handle 10 task at the same time, I was using linkedBlockingQueue with no limit, and that's working good so far, but now I need to set a limitation to my executor queue because I have a lot of tasks (hundreds of thousands tasks), and I don’t want to put them all in my queue that causing me a performance issues, so what I have done:
here is my Executor :
public class MyThreadPoolExecutor extends ThreadPoolExecutor {
public MyThreadPoolExecutor(int corePoolSize, BlockingQueue<Runnable> workQueue) {
super(corePoolSize, corePoolSize + 5, 500, TimeUnit.MILLISECONDS, workQueue);
}
#Override
protected void beforeExecute(Thread t, Runnable r) {
super.beforeExecute(t, r);
//Do something to my task
}
#Override
protected void afterExecute(Runnable r, Throwable t) {
super.afterExecute(r, t);
if(t != null) {
//
} else {
//Do something to my task
}
}
}
and here is my manager :
public final class MyManager {
private static MyManager manager = new MyManager();
public static final int queueMaxSize = 100;
private BlockingQueue<Runnable> workQueue = new ArrayBlockingQueue<Runnable>(queueMaxSize);
private ExecutorService executor = new MyThreadPoolExecutor(10, workQueue);
/**
* constructor
*/
private MyManager() {}
public static MyManager getInstance(){
if (manager == null){
synchronized(MyManager.class){
if (manager == null){
manager = new MyManager();
}
}
}
return manager;
}
/**
*/
public void executeTask(Integer key){
executeTask(key, Locale.getDefault());
}
/**
*/
public void executeTask(Integer key, Locale locale) {
Tasker task = new Tasker(key, locale);
executor.execute(task);
}
}
and here the class that asking to do the tasks :
public class MyClass {
public void doTasks() {
//geting my tasks in array of list, its holding more than 900 000 tasks,
//sometimes its holding up to 10 million task like :
MyManager.getInstance().isFull() {\\wait, then ask again}
ArrayList<Integer> myTasks = getAllTasksIds();
for(Integer id : myTasks) {
//if i perform a waiting here it will be waiting for ever.
MyManaget.getInstance().executeTask(id);
}
}
}
What I want exactly to wait the executor until finish his queue tasks, then re-full it again.
But the problem is when I try to wait based on queue size, the executor won’t work, and its wait forever because the queue still full.

Why wouldn't you just use a bounded blocking queue (i.e. specify a bound of a BlockingQueue)? If you use a bounded blocking queue (of which size you can choose yourself), your producer will block when the queue is full, and will resume publishing tasks when a task is consumed from a queue. This way, you can avoid putting too much stuff too quickly onto the queue, but also avoid putting too less on the queue. That's kind of the point of blocking queues...

I tested your code but instead of using ArrayBlockingQueue I extended it with this... And it works. Try it:
public static class MyBlockingQueue extends ArrayBlockingQueue<Runnable> {
private static final long serialVersionUID= -9016421283603545618L;
public static Lock lock= new ReentrantLock();
public static Condition condition= lock.newCondition();
public static volatile Boolean isWaiting= false;
public MyBlockingQueue(int capacity) {
super(capacity, true);
}
#Override
public boolean offer(Runnable e) {
if (remainingCapacity() == 0) {
try {
isWaiting= true;
condition.await();
} catch (InterruptedException e1) {
e1.printStackTrace();
}
}
return super.offer(e);
}
#Override
public Runnable take() throws InterruptedException {
Runnable take= super.take();
if (remainingCapacity() > 0 && isWaiting) {
isWaiting= false;
condition.signal();
}
return take;
}
}

How to make ScheduledThreadPool report errors?

After a painful debugging experience, I tracked down this issue: ScheduledThreadPool does not report if a task fails, and does not execute a task that failed once ever again. Therefore, it is hard to keep track of the liveness of periodic jobs, short of checking them with yet other periodic tasks (via dead man's switch or the ScheduledFuture).
Now we can hand a ScheduledThreadPool an UncaughtExceptionHandler, but not even that seems to work:
import java.util.concurrent.*;
class Test {
public static void main(String[] args) {
final ThreadFactory tf = new ThreadFactory() {
private final ThreadFactory delegate = Executors.defaultThreadFactory();
#Override public Thread newThread(final Runnable r) {
final Thread res = delegate.newThread(r);
res.setUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
#Override
public void uncaughtException(final Thread t, final Throwable e) {
e.printStackTrace();
}
});
return res;
}
};
final ScheduledThreadPoolExecutor exec = new ScheduledThreadPoolExecutor(1, tf);
final Runnable task = new Runnable() {
private int c = 0;
#Override
public void run() {
if ( c++ == 5 ) {
throw new ArrayIndexOutOfBoundsException("Runtime error!");
}
System.out.println("Reached " + c);
}
};
exec.scheduleWithFixedDelay(task, 1, 1, TimeUnit.SECONDS);
}
}
The output of this program is simply (Oracle Java SE (64-Bit Server) 1.7.0_06-b24)
Reached 1
Reached 2
Reached 3
Reached 4
Reached 5
and then it hangs (by design).
I can always try-catch the whole task, but that feels ugly; the UncaughtExceptionHandler should do that already!
Is there an API-solution for this issue? Did I do something wrong, or is it a bug?

The currency thread pools capture all exceptions and place then in the Future object for you to inspect. UncaughtExceptionHandler is only for exception the thread doesn't catch and kills the thread, which in this case would only be for exception thrown by the thread pool code.
A simple way around this is to wrap your runnable.
public class ExceptionHandlingScheduledExecutor extends ScheduledThreadPoolExecutor {
private final Thread.UncaughtExceptionHandler ueh;
public ExceptionHandlingScheduledExecutor(int corePoolSize, Thread.UncaughtExceptionHandler ueh) {
super(corePoolSize);
this.ueh = ueh;
}
#Override
public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit) {
return super.schedule(wrap(command), delay, unit);
}
#Override
public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) {
return super.schedule(wrap(callable), delay, unit);
}
#Override
public ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit) {
return super.scheduleAtFixedRate(wrap(command), initialDelay, period, unit);
}
#Override
public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit) {
return super.scheduleWithFixedDelay(wrap(command), initialDelay, delay, unit);
}
#Override
public void execute(Runnable command) {
super.execute(wrap(command));
}
#Override
public Future<?> submit(Runnable task) {
return super.submit(wrap(task));
}
#Override
public <T> Future<T> submit(Runnable task, T result) {
return super.submit(wrap(task), result);
}
#Override
public <T> Future<T> submit(Callable<T> task) {
return super.submit(wrap(task));
}
private Runnable wrap(final Runnable runnable) {
return new Runnable() {
#Override
public void run() {
try {
runnable.run();
} catch (final Throwable t) {
ueh.uncaughtException(Thread.currentThread(), t);
throw t;
}
}
};
}
private <T> Callable<T> wrap(final Callable<T> callable) {
return new Callable<T>() {
#Override
public T call() throws Exception {
try {
return callable.call();
} catch (Throwable t) {
ueh.uncaughtException(Thread.currentThread(), t);
throw t;
}
}
};
}
}
You can sub-class the ThreadPoolExecutor to do this transparently.
You can also use a cached thread pool to handle exception but this is is more complicated.
One way to use the returned Future in a transparent way is to sub-class ScheduledThreadPoolExecutor (or any Executor, for that matter):
class MyScheduledExecutor extends ScheduledThreadPoolExecutor {
private final Thread.UncaughtExceptionHandler ueh;
private final ExecutorService futureService = Executors.newCachedThreadPool();
public MyScheduledExecutor(int corePoolSize, Thread.UncaughtExceptionHandler ueh) {
super(corePoolSize);
this.ueh = ueh;
}
// Copy other constructors
#Override
public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
long initialDelay,
long delay,
TimeUnit unit) {
final ScheduledFuture<?> f = super.scheduleWithFixedDelay(command, initialDelay, delay, unit);
futureService.submit(new Runnable() {
#Override
public void run() {
try {
f.get();
} catch (Throwable t ) {
ueh.uncaughtException(null, t.getCause());
}
}
};
return f;
}
// Do similarly for other submit/schedule methods
}
And use it like this:
final ScheduledThreadPoolExecutor exec = new MyScheduledExecutor(1, new Thread.UncaughtExceptionHandler() {
#Override
public void uncaughtException(final Thread t, final Throwable e) {
e.printStackTrace();
}
});
Now the output is as desired:
Reached 1
Reached 2
Reached 3
Reached 4
Reached 5
java.lang.ArrayIndexOutOfBoundsException: Runtime error!
...

You can use VerboseRunnable class from jcabi-log, which does the wrapping suggested above:
import com.jcabi.log.VerboseRunnable;
Runnable runnable = new VerboseRunnable(
Runnable() {
public void run() {
// do business logic, may Exception occurs
}
},
true // it means that all exceptions will be swallowed and logged
);
Now, when executor calls runnable.run() no exceptions are thrown. Instead, they are swallowed and logged (to SLF4J). Thus, the executor won't stop because of exception and you will see what's going on.

How to implement PriorityBlockingQueue with ThreadPoolExecutor and custom tasks

I've searched a lot but could not find a solutuion to my problem.
I have my own class, BaseTask, that uses a ThreadPoolExecutor to handle tasks. I want task prioritization, but when I try to use a PriorityBlockingQueue I get ClassCastException because the ThreadPoolExecutor wraps my Tasks into a FutureTask object.
This obviously makes sense because the FutureTask does not implement Comparable, but how would I go on to solve the priority problem? I've read that you could override newTaskFor() in ThreadPoolExecutor, but I can not seem to find this method at all...?
Any suggestions would be much appreciated!
Some code to help:
In my BaseTask class I have
private static final BlockingQueue<Runnable> sWorkQueue = new PriorityBlockingQueue<Runnable>();
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1);
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
private static final BaseThreadPoolExecutor sExecutor = new BaseThreadPoolExecutor(
1, Integer.MAX_VALUE, 10, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);
private final BaseFutureTask<Result> mFuture;
public BaseTask(int priority) {
mFuture = new BaseFutureTask<Result>(mWorker, priority);
}
public final BaseTask<Params, Progress, Result> execute(Params... params) {
/* Some unimportant code here */
sExecutor.execute(mFuture);
}
In BaseFutureTask class
#Override
public int compareTo(BaseFutureTask another) {
long diff = this.priority - another.priority;
return Long.signum(diff);
}
In BaseThreadPoolExecutor class i override the 3 submit methods... The constructor in this class gets called, but none of the submit methods

public class ExecutorPriority {
public static void main(String[] args) {
PriorityBlockingQueue<Runnable> pq = new PriorityBlockingQueue<Runnable>(20, new ComparePriority());
Executor exe = new ThreadPoolExecutor(1, 2, 10, TimeUnit.SECONDS, pq);
exe.execute(new RunWithPriority(2) {
#Override
public void run() {
System.out.println(this.getPriority() + " started");
try {
Thread.sleep(3000);
} catch (InterruptedException ex) {
Logger.getLogger(ExecutorPriority.class.getName()).log(Level.SEVERE, null, ex);
}
System.out.println(this.getPriority() + " finished");
}
});
exe.execute(new RunWithPriority(10) {
#Override
public void run() {
System.out.println(this.getPriority() + " started");
try {
Thread.sleep(3000);
} catch (InterruptedException ex) {
Logger.getLogger(ExecutorPriority.class.getName()).log(Level.SEVERE, null, ex);
}
System.out.println(this.getPriority() + " finished");
}
});
}
private static class ComparePriority<T extends RunWithPriority> implements Comparator<T> {
#Override
public int compare(T o1, T o2) {
return o1.getPriority().compareTo(o2.getPriority());
}
}
}
as you can guess RunWithPriority is an abstract class that is Runnable and has a Integer priority field

You can use these helper classes:
public class PriorityFuture<T> implements RunnableFuture<T> {
private RunnableFuture<T> src;
private int priority;
public PriorityFuture(RunnableFuture<T> other, int priority) {
this.src = other;
this.priority = priority;
}
public int getPriority() {
return priority;
}
public boolean cancel(boolean mayInterruptIfRunning) {
return src.cancel(mayInterruptIfRunning);
}
public boolean isCancelled() {
return src.isCancelled();
}
public boolean isDone() {
return src.isDone();
}
public T get() throws InterruptedException, ExecutionException {
return src.get();
}
public T get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
return src.get();
}
public void run() {
src.run();
}
public static Comparator<Runnable> COMP = new Comparator<Runnable>() {
public int compare(Runnable o1, Runnable o2) {
if (o1 == null && o2 == null)
return 0;
else if (o1 == null)
return -1;
else if (o2 == null)
return 1;
else {
int p1 = ((PriorityFuture<?>) o1).getPriority();
int p2 = ((PriorityFuture<?>) o2).getPriority();
return p1 > p2 ? 1 : (p1 == p2 ? 0 : -1);
}
}
};
}
AND
public interface PriorityCallable<T> extends Callable<T> {
int getPriority();
}
AND this helper method:
public static ThreadPoolExecutor getPriorityExecutor(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS,
new PriorityBlockingQueue<Runnable>(10, PriorityFuture.COMP)) {
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
RunnableFuture<T> newTaskFor = super.newTaskFor(callable);
return new PriorityFuture<T>(newTaskFor, ((PriorityCallable<T>) callable).getPriority());
}
};
}
AND then use it like this:
class LenthyJob implements PriorityCallable<Long> {
private int priority;
public LenthyJob(int priority) {
this.priority = priority;
}
public Long call() throws Exception {
System.out.println("Executing: " + priority);
long num = 1000000;
for (int i = 0; i < 1000000; i++) {
num *= Math.random() * 1000;
num /= Math.random() * 1000;
if (num == 0)
num = 1000000;
}
return num;
}
public int getPriority() {
return priority;
}
}
public class TestPQ {
public static void main(String[] args) throws InterruptedException, ExecutionException {
ThreadPoolExecutor exec = getPriorityExecutor(2);
for (int i = 0; i < 20; i++) {
int priority = (int) (Math.random() * 100);
System.out.println("Scheduling: " + priority);
LenthyJob job = new LenthyJob(priority);
exec.submit(job);
}
}
}

I will try to explain this problem with a fully functional code. But before diving into the code I would like to explain about PriorityBlockingQueue
PriorityBlockingQueue : PriorityBlockingQueue is an implementation of BlockingQueue. It accepts the tasks along with their priority and submits the task with the highest priority for execution first. If any two tasks have same priority, then we need to provide some custom logic to decide which task goes first.
Now lets get into the code straightaway.
Driver class : This class creates an executor which accepts tasks and later submits them for execution. Here we create two tasks one with LOW priority and the other with HIGH priority. Here we tell the executor to run a MAX of 1 threads and use the PriorityBlockingQueue.
public static void main(String[] args) {
/*
Minimum number of threads that must be running : 0
Maximium number of threads that can be created : 1
If a thread is idle, then the minimum time to keep it alive : 1000
Which queue to use : PriorityBlockingQueue
*/
PriorityBlockingQueue queue = new PriorityBlockingQueue();
ThreadPoolExecutor executor = new ThreadPoolExecutor(0,1,
1000, TimeUnit.MILLISECONDS,queue);
MyTask task = new MyTask(Priority.LOW,"Low");
executor.execute(new MyFutureTask(task));
task = new MyTask(Priority.HIGH,"High");
executor.execute(new MyFutureTask(task));
}
MyTask class : MyTask implements Runnable and accepts priority as an argument in the constructor. When this task runs, it prints a message and then puts the thread to sleep for 1 second.
public class MyTask implements Runnable {
public int getPriority() {
return priority.getValue();
}
private Priority priority;
public String getName() {
return name;
}
private String name;
public MyTask(Priority priority,String name){
this.priority = priority;
this.name = name;
}
#Override
public void run() {
System.out.println("The following Runnable is getting executed "+getName());
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
MyFutureTask class : Since we are using PriorityBlocingQueue for holding our tasks, our tasks must be wrapped inside FutureTask and our implementation of FutureTask must implement Comparable interface. The Comparable interface compares the priority of 2 different tasks and submits the task with the highest priority for execution.
public class MyFutureTask extends FutureTask<MyFutureTask>
implements Comparable<MyFutureTask> {
private MyTask task = null;
public MyFutureTask(MyTask task){
super(task,null);
this.task = task;
}
#Override
public int compareTo(MyFutureTask another) {
return task.getPriority() - another.task.getPriority();
}
}
Priority class : Self explanatory Priority class.
public enum Priority {
HIGHEST(0),
HIGH(1),
MEDIUM(2),
LOW(3),
LOWEST(4);
int value;
Priority(int val) {
this.value = val;
}
public int getValue(){
return value;
}
}
Now when we run this example, we get the following output
The following Runnable is getting executed High
The following Runnable is getting executed Low
Even though we submitted the LOW priority first, but HIGH priority task later, but since we are using a PriorityBlockingQueue, an task with a higher priority will execute first.

My solution:
public class XThreadPoolExecutor extends ThreadPoolExecutor
{
public XThreadPoolExecutor(int corePoolSize, int maximumPoolSize,
long keepAliveTime, TimeUnit unit, PriorityBlockingQueue<Runnable> workQueue)
{
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
}
public XThreadPoolExecutor(int corePoolSize, int maximumPoolSize,
long keepAliveTime, TimeUnit unit, PriorityBlockingQueue<Runnable> workQueue,
RejectedExecutionHandler handler)
{
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, handler);
}
public XThreadPoolExecutor(int corePoolSize, int maximumPoolSize,
long keepAliveTime, TimeUnit unit, PriorityBlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory)
{
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory);
}
public XThreadPoolExecutor(int corePoolSize, int maximumPoolSize,
long keepAliveTime, TimeUnit unit, PriorityBlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory, RejectedExecutionHandler handler)
{
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, handler);
}
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value)
{
return new ComparableFutureTask<>(runnable, value);
}
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable)
{
return new ComparableFutureTask<>(callable);
}
protected class ComparableFutureTask<V>
extends FutureTask<V> implements Comparable<ComparableFutureTask<V>>
{
private Object object;
public ComparableFutureTask(Callable<V> callable)
{
super(callable);
object = callable;
}
public ComparableFutureTask(Runnable runnable, V result)
{
super(runnable, result);
object = runnable;
}
#Override
#SuppressWarnings("unchecked")
public int compareTo(ComparableFutureTask<V> o)
{
if (this == o)
{
return 0;
}
if (o == null)
{
return -1; // high priority
}
if (object != null && o.object != null)
{
if (object.getClass().equals(o.object.getClass()))
{
if (object instanceof Comparable)
{
return ((Comparable) object).compareTo(o.object);
}
}
}
return 0;
}
}
}

It looks like they left that out of apache harmony. There is a svn commit log about a year ago fixing the absence of newTaskFor. You can probably just override the submit functions in an extended ThreadPoolExecutor to create an extended FutureTask that is Comparable. They are not very long.

To answer your question: The newTaskFor() method is found in ThreadPoolExecutor's superclass, AbstractExecutorService. You can simply override it in ThreadPoolExecutor, however.

This answer is a simplified version of #StanislavVitvitskyy's answer. Thanks to him.
I wanted to make the jobs that I submitted be Comparable. I created an ExecutorService with a PriorityBlockingQueue and extend it to handle the newTaskFor(...) methods:
ExecutorService pool = new ThreadPoolExecutor(corePoolSize, maximumPoolSize,
keepAliveTime, timeUnit, new PriorityBlockingQueue<Runnable>()) {
#Override
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new ComparableFutureTask<T>(runnable, value);
}
#Override
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new ComparableFutureTask<T>(callable);
};
};
I defined a ComparableFutureTask which extends FutureTask and implements Comparable by delegating to the job.compareTo(...) that are submitted to the pool.
public class ComparableFutureTask<T> extends FutureTask<T>
implements Comparable<Object> {
private final Comparable<Object> comparableJob;
#SuppressWarnings("unchecked")
public ComparableFutureTask(Runnable runnable, T value) {
super(runnable, value);
this.comparableJob = (Comparable<Object>) runnable;
}
#SuppressWarnings("unchecked")
public ComparableFutureTask(Callable<T> callable) {
super(callable);
this.comparableJob = (Comparable<Object>) callable;
}
#Override
public int compareTo(Object o) {
return this.comparableJob
.compareTo(((ComparableFutureTask<?>) o).comparable);
}
}
This ExecutorService then can handle Runnable or Callable jobs that are also Comparable. For example:
public class MyJob implements Runnable, Comparable<MyJob> {
private int priority;
...
#Override
public int compareTo(MyJob other) {
// we want higher priority to go first
return other.priority - this.priority;
}
...
}
It is important to note that if you submit a job that is not Comparable to this queue, it will throw a ClassCastException.

How do I implement task prioritization using an ExecutorService in Java 5?

I am implementing a thread pooling mechanism in which I'd like to execute tasks of varying priorities. I'd like to have a nice mechanism whereby I can submit a high priority task to the service and have it be scheduled before other tasks. The priority of the task is an intrinsic property of the task itself (whether I express that task as a Callable or a Runnable is not important to me).
Now, superficially it looks like I could use a PriorityBlockingQueue as the task queue in my ThreadPoolExecutor, but that queue contains Runnable objects, which may or may not be the Runnable tasks I've submitted to it. Moreover, if I've submitted Callable tasks, it's not clear how this would ever map.
Is there a way to do this? I'd really rather not roll my own for this, since I'm far more likely to get it wrong that way.
(An aside; yes, I'm aware of the possibility of starvation for lower-priority jobs in something like this. Extra points (?!) for solutions that have a reasonable guarantee of fairness)

I have solved this problem in a reasonable fashion, and I'll describe it below for future reference to myself and anyone else who runs into this problem with the Java Concurrent libraries.
Using a PriorityBlockingQueue as the means for holding onto tasks for later execution is indeed a movement in the correct direction. The problem is that the PriorityBlockingQueue must be generically instantiated to contain Runnable instances, and it is impossible to call compareTo (or similiar) on a Runnable interface.
Onto solving the problem. When creating the Executor, it must be given a PriorityBlockingQueue. The queue should further be given a custom Comparator to do proper in place sorting:
new PriorityBlockingQueue<Runnable>(size, new CustomTaskComparator());
Now, a peek at CustomTaskComparator:
public class CustomTaskComparator implements Comparator<MyType> {
#Override
public int compare(MyType first, MyType second) {
return comparison;
}
}
Everything looking pretty straight forward up to this point. It gets a bit sticky here. Our next problem is to deal with the creation of FutureTasks from the Executor. In the Executor, we must override newTaskFor as so:
#Override
protected <V> RunnableFuture<V> newTaskFor(Callable<V> c) {
//Override the default FutureTask creation and retrofit it with
//a custom task. This is done so that prioritization can be accomplished.
return new CustomFutureTask(c);
}
Where c is the Callable task that we're trying to execute. Now, let's have a peek at CustomFutureTask:
public class CustomFutureTask extends FutureTask {
private CustomTask task;
public CustomFutureTask(Callable callable) {
super(callable);
this.task = (CustomTask) callable;
}
public CustomTask getTask() {
return task;
}
}
Notice the getTask method. We're gonna use that later to grab the original task out of this CustomFutureTask that we've created.
And finally, let's modify the original task that we were trying to execute:
public class CustomTask implements Callable<MyType>, Comparable<CustomTask> {
private final MyType myType;
public CustomTask(MyType myType) {
this.myType = myType;
}
#Override
public MyType call() {
//Do some things, return something for FutureTask implementation of `call`.
return myType;
}
#Override
public int compareTo(MyType task2) {
return new CustomTaskComparator().compare(this.myType, task2.myType);
}
}
You can see that we implement Comparable in the task to delegate to the actual Comparator for MyType.
And there you have it, customized prioritization for an Executor using the Java libraries! It takes some bit of bending, but it's the cleanest that I've been able to come up with. I hope this is helpful to someone!

At first blush it would seem you could define an interface for your tasks that extends Runnable or Callable<T> and Comparable. Then wrap a ThreadPoolExecutor with a PriorityBlockingQueue as the queue, and only accept tasks that implement your interface.
Taking your comment into account, it looks like one option is to extend ThreadPoolExecutor, and override the submit() methods. Refer to AbstractExecutorService to see what the default ones look like; all they do is wrap the Runnable or Callable in a FutureTask and execute() it. I'd probably do this by writing a wrapper class that implements ExecutorService and delegates to an anonymous inner ThreadPoolExecutor. Wrap them in something that has your priority, so that your Comparator can get at it.

You can use these helper classes:
public class PriorityFuture<T> implements RunnableFuture<T> {
private RunnableFuture<T> src;
private int priority;
public PriorityFuture(RunnableFuture<T> other, int priority) {
this.src = other;
this.priority = priority;
}
public int getPriority() {
return priority;
}
public boolean cancel(boolean mayInterruptIfRunning) {
return src.cancel(mayInterruptIfRunning);
}
public boolean isCancelled() {
return src.isCancelled();
}
public boolean isDone() {
return src.isDone();
}
public T get() throws InterruptedException, ExecutionException {
return src.get();
}
public T get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
return src.get(timeout, unit);
}
public void run() {
src.run();
}
public static Comparator<Runnable> COMP = new Comparator<Runnable>() {
public int compare(Runnable o1, Runnable o2) {
if (o1 == null && o2 == null)
return 0;
else if (o1 == null)
return -1;
else if (o2 == null)
return 1;
else {
int p1 = ((PriorityFuture<?>) o1).getPriority();
int p2 = ((PriorityFuture<?>) o2).getPriority();
return p1 > p2 ? 1 : (p1 == p2 ? 0 : -1);
}
}
};
}
AND
public interface PriorityCallable<T> extends Callable<T> {
int getPriority();
}
AND this helper method:
public static ThreadPoolExecutor getPriorityExecutor(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS,
new PriorityBlockingQueue<Runnable>(10, PriorityFuture.COMP)) {
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
RunnableFuture<T> newTaskFor = super.newTaskFor(callable);
return new PriorityFuture<T>(newTaskFor, ((PriorityCallable<T>) callable).getPriority());
}
};
}
AND then use it like this:
class LenthyJob implements PriorityCallable<Long> {
private int priority;
public LenthyJob(int priority) {
this.priority = priority;
}
public Long call() throws Exception {
System.out.println("Executing: " + priority);
long num = 1000000;
for (int i = 0; i < 1000000; i++) {
num *= Math.random() * 1000;
num /= Math.random() * 1000;
if (num == 0)
num = 1000000;
}
return num;
}
public int getPriority() {
return priority;
}
}
public class TestPQ {
public static void main(String[] args) throws InterruptedException, ExecutionException {
ThreadPoolExecutor exec = getPriorityExecutor(2);
for (int i = 0; i < 20; i++) {
int priority = (int) (Math.random() * 100);
System.out.println("Scheduling: " + priority);
LenthyJob job = new LenthyJob(priority);
exec.submit(job);
}
}
}

I will try to explain this problem with a fully functional code. But before diving into the code I would like to explain about PriorityBlockingQueue
PriorityBlockingQueue : PriorityBlockingQueue is an implementation of BlockingQueue. It accepts the tasks along with their priority and submits the task with the highest priority for execution first. If any two tasks have same priority, then we need to provide some custom logic to decide which task goes first.
Now lets get into the code straightaway.
Driver class : This class creates an executor which accepts tasks and later submits them for execution. Here we create two tasks one with LOW priority and the other with HIGH priority. Here we tell the executor to run a MAX of 1 threads and use the PriorityBlockingQueue.
public static void main(String[] args) {
/*
Minimum number of threads that must be running : 0
Maximium number of threads that can be created : 1
If a thread is idle, then the minimum time to keep it alive : 1000
Which queue to use : PriorityBlockingQueue
*/
PriorityBlockingQueue queue = new PriorityBlockingQueue();
ThreadPoolExecutor executor = new ThreadPoolExecutor(0,1,
1000, TimeUnit.MILLISECONDS,queue);
MyTask task = new MyTask(Priority.LOW,"Low");
executor.execute(new MyFutureTask(task));
task = new MyTask(Priority.HIGH,"High");
executor.execute(new MyFutureTask(task));
task = new MyTask(Priority.MEDIUM,"Medium");
executor.execute(new MyFutureTask(task));
}
MyTask class : MyTask implements Runnable and accepts priority as an argument in the constructor. When this task runs, it prints a message and then puts the thread to sleep for 1 second.
public class MyTask implements Runnable {
public int getPriority() {
return priority.getValue();
}
private Priority priority;
public String getName() {
return name;
}
private String name;
public MyTask(Priority priority,String name){
this.priority = priority;
this.name = name;
}
#Override
public void run() {
System.out.println("The following Runnable is getting executed "+getName());
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
MyFutureTask class : Since we are using PriorityBlocingQueue for holding our tasks, our tasks must be wrapped inside FutureTask and our implementation of FutureTask must implement Comparable interface. The Comparable interface compares the priority of 2 different tasks and submits the task with the highest priority for execution.
public class MyFutureTask extends FutureTask<MyFutureTask>
implements Comparable<MyFutureTask> {
private MyTask task = null;
public MyFutureTask(MyTask task){
super(task,null);
this.task = task;
}
#Override
public int compareTo(MyFutureTask another) {
return task.getPriority() - another.task.getPriority();
}
}
Priority class : Self explanatory Priority class.
public enum Priority {
HIGHEST(0),
HIGH(1),
MEDIUM(2),
LOW(3),
LOWEST(4);
int value;
Priority(int val) {
this.value = val;
}
public int getValue(){
return value;
}
}
Now when we run this example, we get the following output
The following Runnable is getting executed High
The following Runnable is getting executed Medium
The following Runnable is getting executed Low
Even though we submitted the LOW priority first, but HIGH priority task later, but since we are using a PriorityBlockingQueue, any task with a higher priority will execute first.

My solution preserves submition order of tasks for same priorities. It's an improvement of this answer
Task execution order is based on:
Priority
Submit order (within same priority)
Tester class:
public class Main {
public static void main(String[] args) throws InterruptedException, ExecutionException {
ExecutorService executorService = PriorityExecutors.newFixedThreadPool(1);
//Priority=0
executorService.submit(newCallable("A1", 200)); //Defaults to priority=0
executorService.execute(newRunnable("A2", 200)); //Defaults to priority=0
executorService.submit(PriorityCallable.of(newCallable("A3", 200), 0));
executorService.submit(PriorityRunnable.of(newRunnable("A4", 200), 0));
executorService.execute(PriorityRunnable.of(newRunnable("A5", 200), 0));
executorService.submit(PriorityRunnable.of(newRunnable("A6", 200), 0));
executorService.execute(PriorityRunnable.of(newRunnable("A7", 200), 0));
executorService.execute(PriorityRunnable.of(newRunnable("A8", 200), 0));
//Priority=1
executorService.submit(PriorityRunnable.of(newRunnable("B1", 200), 1));
executorService.submit(PriorityRunnable.of(newRunnable("B2", 200), 1));
executorService.submit(PriorityCallable.of(newCallable("B3", 200), 1));
executorService.execute(PriorityRunnable.of(newRunnable("B4", 200), 1));
executorService.submit(PriorityRunnable.of(newRunnable("B5", 200), 1));
executorService.shutdown();
}
private static Runnable newRunnable(String name, int delay) {
return new Runnable() {
#Override
public void run() {
System.out.println(name);
sleep(delay);
}
};
}
private static Callable<Integer> newCallable(String name, int delay) {
return new Callable<Integer>() {
#Override
public Integer call() throws Exception {
System.out.println(name);
sleep(delay);
return 10;
}
};
}
private static void sleep(long millis) {
try {
Thread.sleep(millis);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
throw new RuntimeException(e);
}
}
}
Result:
A1 B1 B2 B3 B4 B5 A2 A3 A4 A5 A6 A7 A8
First task is A1 because there were no higher priority in the queue when it was inserted. B tasks are 1 priority so executed earlier, A tasks are 0 priority so executed later, but execution order is follows submition order: B1, B2, B3, ... A2, A3, A4 ...
The solution:
public class PriorityExecutors {
public static ExecutorService newFixedThreadPool(int nThreads) {
return new PriorityExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS);
}
private static class PriorityExecutor extends ThreadPoolExecutor {
private static final int DEFAULT_PRIORITY = 0;
private static AtomicLong instanceCounter = new AtomicLong();
#SuppressWarnings({"unchecked"})
public PriorityExecutor(int corePoolSize, int maximumPoolSize,
long keepAliveTime, TimeUnit unit) {
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, (BlockingQueue) new PriorityBlockingQueue<ComparableTask>(10,
ComparableTask.comparatorByPriorityAndSequentialOrder()));
}
#Override
public void execute(Runnable command) {
// If this is ugly then delegator pattern needed
if (command instanceof ComparableTask) //Already wrapped
super.execute(command);
else {
super.execute(newComparableRunnableFor(command));
}
}
private Runnable newComparableRunnableFor(Runnable runnable) {
return new ComparableRunnable(ensurePriorityRunnable(runnable));
}
#Override
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new ComparableFutureTask<>(ensurePriorityCallable(callable));
}
#Override
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new ComparableFutureTask<>(ensurePriorityRunnable(runnable), value);
}
private <T> PriorityCallable<T> ensurePriorityCallable(Callable<T> callable) {
return (callable instanceof PriorityCallable) ? (PriorityCallable<T>) callable
: PriorityCallable.of(callable, DEFAULT_PRIORITY);
}
private PriorityRunnable ensurePriorityRunnable(Runnable runnable) {
return (runnable instanceof PriorityRunnable) ? (PriorityRunnable) runnable
: PriorityRunnable.of(runnable, DEFAULT_PRIORITY);
}
private class ComparableFutureTask<T> extends FutureTask<T> implements ComparableTask {
private Long sequentialOrder = instanceCounter.getAndIncrement();
private HasPriority hasPriority;
public ComparableFutureTask(PriorityCallable<T> priorityCallable) {
super(priorityCallable);
this.hasPriority = priorityCallable;
}
public ComparableFutureTask(PriorityRunnable priorityRunnable, T result) {
super(priorityRunnable, result);
this.hasPriority = priorityRunnable;
}
#Override
public long getInstanceCount() {
return sequentialOrder;
}
#Override
public int getPriority() {
return hasPriority.getPriority();
}
}
private static class ComparableRunnable implements Runnable, ComparableTask {
private Long instanceCount = instanceCounter.getAndIncrement();
private HasPriority hasPriority;
private Runnable runnable;
public ComparableRunnable(PriorityRunnable priorityRunnable) {
this.runnable = priorityRunnable;
this.hasPriority = priorityRunnable;
}
#Override
public void run() {
runnable.run();
}
#Override
public int getPriority() {
return hasPriority.getPriority();
}
#Override
public long getInstanceCount() {
return instanceCount;
}
}
private interface ComparableTask extends Runnable {
int getPriority();
long getInstanceCount();
public static Comparator<ComparableTask> comparatorByPriorityAndSequentialOrder() {
return (o1, o2) -> {
int priorityResult = o2.getPriority() - o1.getPriority();
return priorityResult != 0 ? priorityResult
: (int) (o1.getInstanceCount() - o2.getInstanceCount());
};
}
}
}
private static interface HasPriority {
int getPriority();
}
public interface PriorityCallable<V> extends Callable<V>, HasPriority {
public static <V> PriorityCallable<V> of(Callable<V> callable, int priority) {
return new PriorityCallable<V>() {
#Override
public V call() throws Exception {
return callable.call();
}
#Override
public int getPriority() {
return priority;
}
};
}
}
public interface PriorityRunnable extends Runnable, HasPriority {
public static PriorityRunnable of(Runnable runnable, int priority) {
return new PriorityRunnable() {
#Override
public void run() {
runnable.run();
}
#Override
public int getPriority() {
return priority;
}
};
}
}
}

Would it be possible to have one ThreadPoolExecutor for each level of priority? A ThreadPoolExecutor can be instanciated with a ThreadFactory and you could have your own implementation of a ThreadFactory to set the different priority levels.
class MaxPriorityThreadFactory implements ThreadFactory {
public Thread newThread(Runnable r) {
Thread thread = new Thread(r);
thread.setPriority(Thread.MAX_PRIORITY);
}
}