How to call same method of a different class in multithreaded way

问题

I have a method named process in two of my Classes, lets say CLASS-A and CLASS-B. Now in the below loop, I am calling process method of both of my classes sequentially meaning one by one and it works fine but that is the not the way I am looking for.

for (ModuleRegistration.ModulesHolderEntry entry : ModuleRegistration.getInstance()) {
    final Map<String, String> response = entry.getPlugin().process(outputs);

    // write to database
    System.out.println(response);
}

Is there any way, I can call the process method of both of my classes in a multithreaded way. Meaning one thread will call process method of CLASS-A and second thread will call process method of CLASS-B.

And then after that I was thinking to write the data that is being returned by the process method into the database. So I can have one more thread for writing into database.

Below is the code that I came up with in a multithreaded way but somehow it is not running at all.

public void writeEvents(final Map<String, Object> data) {

    // Three threads: one thread for the database writer, two threads for the plugin processors
    final ExecutorService executor = Executors.newFixedThreadPool(3);

    final BlockingQueue<Map<String, String>> queue = new LinkedBlockingQueue<Map<String, String>>();

    @SuppressWarnings("unchecked")
    final Map<String, String> outputs = (Map<String, String>)data.get(ModelConstants.EVENT_HOLDER);

    for (final ModuleRegistration.ModulesHolderEntry entry : ModuleRegistration.getInstance()) {
        executor.submit(new Runnable () {
            public void run() {
                final Map<String, String> response = entry.getPlugin().process(outputs);
                // put the response map in the queue for the database to read
                queue.offer(response);
            }
        });
    }

    Future<?> future = executor.submit(new Runnable () {
        public void run() {
            Map<String, String> map;
            try {
                while(true) {
                    // blocks until a map is available in the queue, or until interrupted
                    map = queue.take();
                    // write map to database
                    System.out.println(map);
                }
            } catch (InterruptedException ex) {
                // IF we're catching InterruptedException then this means that future.cancel(true)
                // was called, which means that the plugin processors are finished;
                // process the rest of the queue and then exit
                while((map = queue.poll()) != null) {
                    // write map to database
                    System.out.println(map);
                }
            }
        }
    });

    // this interrupts the database thread, which sends it into its catch block
    // where it processes the rest of the queue and exits
    future.cancel(true); // interrupt database thread

    // wait for the threads to finish
    try {
        executor.awaitTermination(5, TimeUnit.MINUTES);
    } catch (InterruptedException e) {
        //log error here
    }
}

But If I remove the last line executor.awaitTermination(5, TimeUnit.MINUTES); then it start running fine and after some time, I always get error like this-

JVMDUMP006I Processing dump event "systhrow", detail "java/lang/OutOfMemoryError" - please wait.
JVMDUMP032I JVM requested Heap dump using 'S:\GitViews\Stream\goldseye\heapdump.20130827.142415.16456.0001.phd' in response to an event
JVMDUMP010I Heap dump written to S:\GitViews\Stream\goldseye\heapdump.20130827.142415.16456.0001.phd
JVMDUMP006I Processing dump event "systhrow", detail "java/lang/OutOfMemoryError" - please wait.

Can anybody help me in figuring out what's the problem and what wrong I am doing in my above code? if I am running sequentially then I don't get any errors and it works fine.

And also is there any better way of doing this as compared to the way I am doing? Because in future I can have multiple plugin processor as compared to two.

What I am trying to do is- Call the process method of both of my classes in a multithreaded way and then write into the database bcoz my process method will return back a Map.

Any help will be appreciated on this.. And I am looking for a workable example on this if possible. Thanks for the help,

回答1:

The code snippet you pasted has few issues, if you fix them, this should work.
1. You are using an infinite loop to fetch element from the blocking queue and trying to break this using future. This is definitely not a good approach. The problem with this approach is it is possible that your database thread would never run because it could be cancelled by the future task running in the caller thread even before it runs. This is error-prone.
- You should run the while loop fixed number of times (you already know how many producers are there or how many times you are going to get the response).

Also, tasks submitted to executor service should be independent tasks...here your database task is dependent on the execution of other tasks..this can also lead to deadlock if your execution policy changes..for example if you use single thread pool executor and if database thread is scheduled it would just block waiting for producers to add data in the queue.

A good way is to create task that retrieves data and update the database in the same thread.
Or retrieve all the responses first and then execute database operations in parallel

public void writeEvents(final Map data) {

final ExecutorService executor = Executors.newFixedThreadPool(3);       
@SuppressWarnings("unchecked")
final Map<String, String> outputs = (Map<String, String>)data.get(ModelConstants.EVENT_HOLDER);

for (final ModuleRegistration.ModulesHolderEntry entry : ModuleRegistration.getInstance()) {
    executor.submit(new Runnable () {
        public void run() {
            try {
                final Map<String, String> response = entry.getPlugin().process(outputs);
                //process the response and update database.
                System.out.println(map);
            } catch (Throwable e) {
                //handle execption
            } finally {
                //clean up resources
            }               
        }
    });
}

// This will wait for running threads to complete ..it's an orderly shutdown. executor.shutdown(); }

回答2:

OK, here's some code for the comments I suggested above. Disclaimer: I'm not sure whether it works or even compiles, or whether it solves the problem. But the idea is to take control of the cancellation process instead of relying on future.cancel which I suspect could cause problems.

class CheckQueue implements Runnable {
    private volatile boolean cancelled = false;
    public void cancel() { cancelled = true; }
    public void run() {
        Map<String, String> map;
        try {
            while(!cancelled) {
                // blocks until a map is available in the queue, or until interrupted
                map = queue.take();
                if (cancelled) break;
                // write map to database
                System.out.println(map);
        } catch (InterruptedException e) {
        }
        while((map = queue.poll()) != null) {
            // write map to database
            System.out.println(map);
        }
    }
}

CheckQueue queueChecker = new CheckQueue ();
Future<?> future = executor.submit(queueChecker);

// this interrupts the database thread, which sends it into its catch block
// where it processes the rest of the queue and exits
queueChecker.cancel();

来源：https://stackoverflow.com/questions/18475772/how-to-call-same-method-of-a-different-class-in-multithreaded-way

标签

java

multithreading

threadpool

executorservice

blockingqueue