ExecutorService is an important tool in Java to manage thread execution tasks. You can create fixed-size thread pools, cache thread pools, and single thread pools through the Executors factory class. 1. Use the submit() or execute() method to submit a task. Submit() can return the Future object to obtain results or exceptions; 2. Process the return value and can use Future.get() to block the result, or use invokeAll() to uniformly handle the return values ??of multiple tasks; 3. Close the ExecutorService, you should first call shutdown() to stop receiving new tasks and wait for completion. If the timeout, call shutdownNow() to force stop; 4. When using it, pay attention to avoid resource leakage, reasonably set the thread pool size, properly handle task exceptions, and try to reuse the thread pool to reduce overhead.
ExecutorService is an important tool in Java to manage thread execution tasks, and it simplifies the complexity of multi-threaded programming. If you are still manually creating Thread objects to handle concurrent tasks, it's time to learn about ExecutorService.
How to create and start an ExecutorService?
Java's Executors factory class provides several common thread pool implementation methods. The most commonly used ones include:
- Fixed-size thread pool :
Executors.newFixedThreadPool(nThreads) - Cache thread pool :
Executors.newCachedThreadPool() - Single thread pool :
Executors.newSingleThreadExecutor()
For example, if you want to create a fixed thread pool with 4 threads, you can write it like this:
ExecutorService executor = Executors.newFixedThreadPool(4);
Once the creation is complete, you can submit the task through submit() or execute() method. The difference is that submit() can return a Future object to get task results or catch exceptions, while execute() is just used to execute tasks without return values.
How to submit a task and process the return value?
You can submit a Runnable or Callable task to the ExecutorService. Runnable has no return value, Callable can return a result.
For example, use Callable to submit a task:
Future<Integer> future = executor.submit(() -> {
return 42; // Return value }); After that, you can get the result through future.get() , but note that this method will block the current thread until the task is completed.
If there are many tasks that need to be executed concurrently and you need to process the results uniformly, you can use the invokeAll() method, which accepts a set of tasks and returns a list of all futures.
How to correctly close the ExecutorService?
When you no longer need ExecutorService, you must explicitly close it, otherwise the threads in the thread pool may run all the time, causing the program to fail to exit normally.
There are two common methods to close:
-
shutdown(): Stop receiving new tasks and wait for the submitted tasks to be executed. -
shutdownNow(): Try to immediately stop all tasks being executed and return the list of tasks waiting to be executed.
The usual way is to call shutdown() first, and then set a timeout time with awaitTermination() :
executor.shutdown();
try {
if (!executor.awaitTermination(60, TimeUnit.SECONDS)) {
executor.shutdownNow();
}
} catch (InterruptedException e) {
executor.shutdownNow();
}The meaning of this code is: try to close gracefully first, and force it to close if it has not been completed within 60 seconds.
What are the common precautions during use?
- Avoid resource leaks : Not closing the ExecutorService will cause the thread to continue running and occupy resources.
- Task exception handling : Exceptions thrown in the submitted task will not be printed directly to the console. They need to be wrapped with try-catch or captured through Future.get().
- The thread pool size is set reasonably : too large wastes resources, too small affects performance. Adjust the number of threads according to the task type (CPU-intensive, IO-intensive).
- Do not create thread pools frequently : thread pools should be reused as much as possible. Frequent creation and destruction will increase overhead.
Basically that's it. ExecutorService is not difficult to use, but it is easy to ignore the details, especially the closure process and exception handling, which will cause hidden dangers if you are not careful.
The above is the detailed content of How to use the ExecutorService in Java?. For more information, please follow other related articles on the PHP Chinese website!
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