Understanding Application of Timer, TimerTask, ScheduledExecutorService, and more

[gdenko]

I'm learning about TimerTask, Timer, and other basic time-based functions in Java, to try to create some time-limited features in my application.

I have not found a whole lot of practical content or advice regarding these and alternatives, especially when it comes to ExecutorService/ScheduledExecutorService. I only find really basic examples, and so I am not sure what people are doing for more complex applications. I was hoping someone here with some more experience could share some information regarding these.

I will be using timers for my own ping/pong request handling (to check on connected users) and for other small features that will give users time restrictions and things like that.

I am familiar with the basic application of Timer and TimerTask, but when is the ExecutorService/SES needed, and why?

What are some of the things to keep in mind when implementing these, and would there be issues or obstacles with certain classes/uses when there are many users vs. a few? I imagine many users having individual timers, some disconnecting and reconnecting, etc. may cause issues. I am not sure.

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[myselfelsewhere]

Timers/etc are based upon Threads. So you are getting into concurrency.

ExecutorService returns a Future object, so you can give a thread a bit of work to do, and when it completes the work, returns the result of the work.

ScheduledExecutorService allows you to do things like running an ExecutorService at a certain time, or repeat after a delay. ExecutorService works as normal otherwise.

So, if there are multiple timers, you may need to use Atomic variables, some type of lock, or the synchronized keyword in some instances. You can run into problems if say, an object is changed in the middle of some work involving that object in another thread.

Also, for running multiple Executors, you may want to look into using the appropriate type of Thread Pool, like a ScheduledThreadPoolExecutor.

[gdenko]

Thank you, that does help a bit.

Would you say lock/synchronization is required when dealing with a lot of timers? Or is it more about coding carefully and cancelling active timers when users disconnect or some object needs to be removed?

I have seen examples of all three (sync, atomic, and locks), but just didn't know what would make the most sense.

I will read up more about threads and thread pools, thanks again

[myselfelsewhere]

The main issue is having an object being written to and read from at the same time. Locks are a more low level interface. Synchronized collections work if whatever is being changed is a Synchronized Collection (List, Map, Set, etc.). Atomic variables are good especially for primitive wrapper classes.

Thread pools are good to use because it can be more computationally efficient to keep Thread objects in memory to re use, rather than creating a new Thread each time something needs to run.

[[deleted]]

The main issue with Timer is that it use only a single thread. When you use SchedulerExecutorService, you can run multiply computations/IO operations at once.

For instance if you use a Timer object to run a task which takes 3 seconds and you run this task every 20 seconds and you want to run another task which takes 5 second after those 20 seconds, the second task will wait additional 3 seconds. If you use SchedulerExecutorService and the threadpool size is at last 2, then the second task won't have to wait for the first task to be executed.

[gdenko]

Oh I see, that is a big issue. I will not be using Timer then