如何取消具有本机代码（C / C ++）的任务？

Question

Background

I have a short yet heavy task that uses NDK (JNI) in order to do some bitmap manipulation.

On some cases, I need to cancel the task and go to other things.

However, since the task uses a lot of memory too, and in the end also creates a large bitmap, this could cause out-of-memory errors.

The question

How should I cancel a task that has native code in it? Is there a best practice for this?

Maybe some kind of interrupted exception that I can use for C/C++, so that as soon as it is caught, I can at least free all of the memory?

Maybe I should add some kind of listener (observer) to the native code? Or maybe I should call a method that I need to assume the caller has?

Answer 1

The simplest way?

Spin the task on a thread. You may spin the thread in Java and call into JNI from there. It's easier than pthreads.

Introduce a volatile flag (boolean variable), set it to false on task startup.

In the task, check the stop flag as often as it's practical. If true, abandon the task (you might throw an exception to return through several levels of function nesting, but you don't have to).

When you want to cancel, set the flag from the main thread. Wait for the task to notice that.

This won't work well if the slowest part of the task is calling a library/OS function - that function won't check for the flag.

There's no safe builtin mechanism in C++ for correctly terminating a thread of code that does not want to be terminated (maybe signals, but they kill the whole process, not a thread). Periodically checking for the flag is what I mean by "wanting to be terminated".

Answer 2

I've seen this approach used before, and I think the best approach would be to create a isCancelled() method that you can call periodically from your native code in order to determine when to stop. If you're using AsyncTask , that method should already exist.