How to create and use ExecutorService for Java? 1. Use the Executors factory class to create a thread pool, such as newFixedThreadPool(4) to create a fixed-size thread pool; 2. Submit a task without return value through execute(), or submit a task with return value and get the Future result; 3. When closing the thread pool, call shutdown() first, cooperate with awaitTermination() to wait for the task to complete, and call shutdownNow() to interrupt execution if necessary.
Java's ExecutorService is a very practical tool in concurrent programming, used to manage thread pools and execute tasks, and avoid the overhead caused by frequent creation and destruction of threads. If you just want to quickly look at an example and understand how to use it, then let’s just add the useful information.
How to create a basic ExecutorService
The easiest way to do this is to use the Executors factory class to create a thread pool. For example, if you want to create a fixed-size thread pool, you can write it like this:
ExecutorService executorService = Executors.newFixedThreadPool(4);
This means you have created a thread pool that can run up to 4 tasks simultaneously. You can adjust the numbers according to actual needs, such as the number of CPU cores or the expected concurrency.
In addition to newFixedThreadPool , there are several commonly used types:
-
newCachedThreadPool(): Create threads on demand, suitable for short-term tasks. -
newSingleThreadExecutor(): There is only one thread, which ensures that tasks are executed sequentially. -
newScheduledThreadPool(n): Supports timing and periodic tasks.
Several ways to submit tasks
After creating the thread pool, you can submit tasks in it. Commonly used methods are execute() and submit() .
execute() method
Suitable for tasks that do not require return values ??(Runnable):
executorService.execute(() -> {
System.out.println("Execute a task");
});This method is simple and direct, but cannot obtain task results.
submit() method
If you want to get the result of the task, you can use submit() , which accepts Callable or Runnable and returns a Future object:
Future<Integer> future = executorService.submit(() -> {
return 123;
});
try {
Integer result = future.get(); // Get the result, which may block System.out.println("Task return value:" result);
} catch (Exception e) {
e.printStackTrace();
} Note that calling get() here will block the current thread until the task is completed. If the task takes a long time, you may want to consider adding timeout processing.
Close the thread pool correctly
Many people use ExecutorService but don’t close it, which causes the program to hang and not exit. So remember to close the thread pool at the appropriate time.
shutdown()
executorService.shutdown();
After the call, the thread pool no longer accepts new tasks, but will wait for the submitted tasks to be executed.
shutdownNow()
executorService.shutdownNow();
This is even more ruthless. Not only will it reject new tasks, it will also try to interrupt the task being executed.
It is usually recommended to call shutdown() first, and then wait for all tasks to complete with awaitTermination() :
executorService.shutdown();
try {
if (!executorService.awaitTermination(60, TimeUnit.SECONDS)) {
executorService.shutdownNow();
}
} catch (InterruptedException e) {
executorService.shutdownNow();
}This is both elegant and safe.
Tips in actual development
- Don't use unbounded queues : the default
newFixedThreadPooluses unbounded queues, too many tasks may lead to OOM. - Thread naming is very important : custom thread factories can name threads, making it much easier to troubleshoot problems.
- Be careful when handling exceptions : task throwing exceptions will not be automatically printed, remember to try-catch or set up an uncaught exception handler.
- Don't forget to close : especially in web applications or Spring Boot, closing timing is also critical.
Basically that's it. ExecutorService is not complicated, but details are easily overlooked, especially the closing and exception handling parts. As long as it is used correctly, it can greatly simplify the management of concurrent tasks.
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