I need to execute some amount of tasks 4 at a time, something like this: ExecutorService taskExecutor = Executors.newFixedThreadPool(4); while(…) { taskExecutor.execute(new MyTask()); } //…wait for completion somehow How can I get notified once all of them are complete? For now I can’t think about anything better than setting some global task counter and decrease … Read more
Is it possible to specify a custom thread pool for Java 8 parallel stream? I can not find it anywhere. Imagine that I have a server application and I would like to use parallel streams. But the application is large and multi-threaded so I want to compartmentalize it. I do not want a slow running … Read more
I’m learning about concurrent programming for iOS. So far I’ve read about NSOperation/NSOperationQueue and GCD. What are the reasons for using NSOperationQueue over GCD and vice versa? Sounds like both GCD and NSOperationQueue abstract away the explicit creation of NSThreads from the user. However the relationship between the two approaches isn’t clear to me so … Read more
I’ve heard these words related to concurrent programming, but what’s the difference between lock, mutex and semaphore? 10 s 10 A lock allows only one thread to enter the part that’s locked and the lock is not shared with any other processes. A mutex is the same as a lock but it can be system … Read more
I have a Map which is to be modified by several threads concurrently. There seem to be three different synchronized Map implementations in the Java API: Hashtable Collections.synchronizedMap(Map) ConcurrentHashMap From what I understand, Hashtable is an old implementation (extending the obsolete Dictionary class), which has been adapted later to fit the Map interface. While it … Read more
HashSet is based on HashMap. If we look at HashSet<E> implementation, everything is been managed under HashMap<E,Object>. <E> is used as a key of HashMap. And we know that HashMap is not thread safe. That is why we have ConcurrentHashMap in Java. Based on this, I am confused that why we don’t have a ConcurrentHashSet … Read more
JavaScript is known to be single-threaded in all modern browser implementations, but is that specified in any standard or is it just by tradition? Is it totally safe to assume that JavaScript is always single-threaded? 12 s 12 That’s a good question. I’d love to say “yes”. I can’t. JavaScript is usually considered to have … Read more
A mutex is a programming concept that is frequently used to solve multi-threading problems. My question to the community: What is a mutex and how do you use it? 10 s 10 When I am having a big heated discussion at work, I use a rubber chicken which I keep in my desk for just … Read more
When should I use a ThreadLocal variable? How is it used? 26 s 26 One possible (and common) use is when you have some object that is not thread-safe, but you want to avoid synchronizing access to that object (I’m looking at you, SimpleDateFormat). Instead, give each thread its own instance of the object. For … Read more
I can’t get to the bottom of this error, because when the debugger is attached, it does not seem to occur. Collection was modified; enumeration operation may not execute Below is the code. This is a WCF server in a Windows service. The method NotifySubscribers() is called by the service whenever there is a data … Read more
