Question

I am currently studying the multithreaded programming and I am having a hard time mastering shared resources between threads.

I have two shared resources, time and alarm

I have three threads, one keeps updating time every second, one monitors time against alarm and finally one changes time and alarm at user request.

this is to be done using mutual exclusion semaphore, I have a simple setup described below:

1. Main thread, program begins here, hosts time and alarm common variables and instantiates sub threads.
2. Tick sub-thread, sleeps every 1000ms, then modifies Mother thread’s time variable
3. UserInput sub-thread, is blocked until a user input is caught, and modifies either time or alarm in Main thread.
4. Alarm sub-thread, sleeps every 1000ms, goes off when alarm == time

The Main thread keeps a mutex for each resource so only one thread can access it at a time.

Is such design reasonable or am I just over complicating things?

Answer 1

My first comment would be on the language you use, which may be an indication of some misunderstanding on your part. A thread doesn’t own any variables; this may mean that you have declared several custom subclasses of Thread. If that is so, this would not be an advisable approach. Note that Thread is just the name of a class, it is not the actual thread—exactly the same way as File is not a file. You haven’t posted any code so I must resort to guesswork, but if what I describe is true then you should redesign. Use plain classes (that don’t subclass anything) and instantiate Thread with a Runnable.

Second, it is not very clear what you imply by “mutex” as this is not a term defined by the Java Language Specification. If you mean something from the java.util.concurrent package, such as Semaphore, this would be overkill; if you mean just plain Java Object instances that you use as locks, that would be a simple and straightforward approach. Note, however, that you don’t really need any locking at all: volatile variables would be enough, or, at most, AtomicLongs (or whatever type you use) for the two timestamp values, so you can update them atomically.