HandlerThread vs Executor - When is one more appropriate over the other?

Question

I'm just curious about whether there are times in which I should choose an Executor over a HandlerThread. Are there times that one is superior over the other, or should I really just stick with the HandlerThread? In my case, I'm currently listening to a ServerSocket for connections, and handling each request on a separate thread created by an Executor. Even though I gave a specific example, I'm really just looking for cases in which one is more appropriate than the other. However, I welcome comments about my design.

Answer 1

The Executor class is more powerful and can use a pool of threads, whereas each Handler references a single thread. The Executor allows you to get all the scheduled tasks and cancel them if you'd like. The Handler on the other hand will not answer simple questions such as, how many tasks are waiting or give me a reference to all waiting tasks. I believe one reason that the Handler is more limited is because Android gives you access to the main Handler it uses for the UI and you could really screw up the OS if you started canceling OS tasks.

In general if you need a pool of threads or lots of power use the Executor. If you just need a nice background thread to run one task at a time use a Handler. As an example when I want to query my database I only really want one query to occur at a time and I don't want to generate an ANR so I use a Handler running on a background thread to run my queries.

I believe your choice of executor sounds appropriate since you want to handle multiple incoming requests simultaneously and a Handler can only do one at a time.

UPDATE: How to create a Handler that runs on a background thread:

In your constructor or onCreate write the following, obviously you can set the priority to whatever you like:

public class MyClass {

    private Handler mBgHandler;

    public MyClass() {
        HandlerThread bgThread = new HandlerThread("My-Background-Handler");
        bgThread.start();
        mBgHandler = new Handler(bgThread.getLooper());
    }
}

UPDATE: Don't forget to quit() or quitSafely() your HandlerThread when you are done with it otherwise it will remain waiting forever

Answer 2

I wouldn't follow the sample code in satur9nine's answer as of 2011-Dec-22.

Thread.MIN_PRIOROTY is mapped to android.os.Process.THREAD_PRIORITY_LOWEST. Quote:

Lowest available thread priority. Only for those who really, really don't want to run if anything else is happening.

I would at least use android.os.Process.THREAD_PRIORITY_BACKGROUND, like so:

HandlerThread bgThread = new HandlerThread("handler name");
Process.setThreadPriority(bgThread.getThreadId(), Process.THREAD_PRIORITY_BACKGROUND);
bgThread.start();
mBgHandler = new Handler(bgThread.getLooper());

This assigns the default Android background priority to the thread.

Currently, threads of priority Process.THREAD_PRIORITY_BACKGROUND and lower share an artificially limited amount of CPU time by means of a Linux cgroup, see for example here. If a background task does not just wait for I/O but performs real computations, I'd consider increasing its priority by android.os.Process.THREAD_PRIORITY_MORE_FAVORABLE which (currently) moves it out of the background cgroup while still not substantially endangering UI and real time activities.

Update: satur9nine's answer was silently revised on 2013-Jan-08 to not set the lowest possible priority any more. The HandlerThread will now implicitly have a priority of android.os.Process.THREAD_PRIORITY_BACKGROUND. This means that it now gets the default background task priority but it is still limited to consume an artificial maximum of 10% CPU time along with all another background tasks which may exist. If that's not desired, use my code above e.g. with

Process.setThreadPriority(bgThread.getThreadId(),
                          Process.THREAD_PRIORITY_BACKGROUND + Process.THREAD_PRIORITY_MORE_FAVORABLE);

to lift your background thread out of the cgroup.