I have an Android app that uses NDK - a regular Android Java app with regular UI and C++ core. There are places in the core where I need to call Java methods, which means I need a JNIEnv* for that thread, which in turn means that I need to call JavaVM->AttachCurrentThread() to get the valid env.
Previously, was just doing AttachCurrentThread and didn't bother to detach at all. It worked fine in Dalvik, but ART aborts the application as soon as a thread that has called AttachCurrentThread exits without calling DetachCurrentThread. So I've read the JNI reference, and indeed it says that I must call DetachCurrentThread. But when I do that, ART aborts the app with the following message:
attempting to detach while still running code
What's the problem here, and how to call DetachCurrentThread properly?
Dalvik will also abort if the thread exits without detaching. This is implemented through a pthread key -- see threadExitCheck() in Thread.cpp.
A thread may not detach unless its call stack is empty. The reasoning behind this is to ensure that any resources like monitor locks (i.e. synchronized statements) are properly released as the stack unwinds.
The second and subsequent attach calls are, as defined by the spec, low-cost no-ops. There's no reference counting, so detach always detaches, no matter how many attaches have happened. One solution is to add your own reference-counted wrapper.
Another approach is to attach and detach every time. This is used by the app framework on certain callbacks. This wasn't so much a deliberate choice as a side-effect of wrapping Java sources around code developed primarily in C++, and trying to shoe-horn the functionality in. If you look at SurfaceTexture.cpp, particularly JNISurfaceTextureContext::onFrameAvailable(), you can see that when SurfaceTexture needs to invoke a Java-language callback function, it will attach the thread, invoke the callback, and then if the thread was just attached it will immediately detach it. The "needsDetach" flag is set by calling GetEnv to see if the thread was previously attached.
This isn't a great thing performance-wise, as each attach needs to allocate a Thread object and do some internal VM housekeeping, but it does yield the correct behavior.