Java Concurrency : ExecutorService
- 1 Java thread = 1 OS thread
- 1 OS thread is running on 1 core
- usually create fixed number of threads and assign task to the threads
- code in the task need to be thread-safe
- For CPU intensive task, number of thread should= number CPU cores
- For IO intensive task, number of thread should be high to accommodate the task submission rate/ wait time (SLA). But too many threads will increase memory consumption
4 Types of Thread Pool
- FixedThreadPool (Blocking Queue is used)
2. SingleThreadPool (Blocking Queue is used) (recreate thread if killed because of the task) (tasks will be executed in sequence.
3. CachedThreadPool (Synchronous Queue, only hold 1 task) (create new one if needed, and put the thread to the pool. If a thread idle for more than 60 seconds, will be killed)
4. ScheduledThreadPool (Delay Queue)
- schedule(): can schedule to start the task after certain delay
- scheduleAtFixedRate(new Task(), $initialDelay, TimeUnit): start after certain delay and start every xx period
- scheduleAtFixedDelay(new Task(), $initialDelay, $delayAfterComplete, TimeUnit): run the task after certain delay, wait for another delay period after the task to complete, and rerun the task
Pool Size
Queue Type
Rejection Policy
Lifecycle Management
service.shutdown() ;// not accept new task, will complete all submitted tasks
List<Runnable> runnables = service.shutdownNow(); // start shutdown and return all queued tasks.
service.awaitTermination(xx,TimeUnit);//block until all tasks are completed, or if timeout
service.isShutdown();// check if shutdown has begun
service.isTerminated();//
Task that return result : Callable
Interface : runnable vs callable
Future<Integer> future =service.submit(new Task());
Integer result = future.get(); // blocking method: like await
