What is the best way to keep JNIEnv *
I have an Android project with JNI. There is an x () callback in the CPP file that implements the listener class. When the x () function is called, I want to call another function in the java class. However, in order to call this java function, I need to access the JNIEnv *.
I know there is a function in the same callback cpp file:
static jboolean init (JNIEnv* env, jobject obj) {...}
Should I save JNIEnv * as a member variable in the cpp file when called init(..)
? and use it later when the callback occurs?
Sorry, but I'm new to JNI.
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Caching a is JNIEnv*
not a particularly good idea as you cannot use the same JNIEnv*
for multiple threads and may not even be able to use it for multiple native calls on the same thread (see http://android-developers.blogspot.se /2011/11/jni-local-reference-changes-in-ics.html )
Writing a function that receives JNIEnv*
and optionally attaches the current thread to the VM isn't too difficult:
bool GetJniEnv(JavaVM *vm, JNIEnv **env) {
bool did_attach_thread = false;
*env = nullptr;
// Check if the current thread is attached to the VM
auto get_env_result = vm->GetEnv((void**)env, JNI_VERSION_1_6);
if (get_env_result == JNI_EDETACHED) {
if (vm->AttachCurrentThread(env, NULL) == JNI_OK) {
did_attach_thread = true;
} else {
// Failed to attach thread. Throw an exception if you want to.
}
} else if (get_env_result == JNI_EVERSION) {
// Unsupported JNI version. Throw an exception if you want to.
}
return did_attach_thread;
}
How to use:
JNIEnv *env;
bool did_attach = GetJniEnv(vm, &env);
// Use env...
// ...
if (did_attach) {
vm->DetachCurrentThread();
}
You can wrap this in a class that attaches to the construct and detaches from destructive, RAII-style:
class ScopedEnv {
public:
ScopedEnv() : attached_to_vm_(false) {
attached_to_vm_ = GetJniEnv(g_vm, &env_); // g_vm is a global
}
ScopedEnv(const ScopedEnv&) = delete;
ScopedEnv& operator=(const ScopedEnv&) = delete;
virtual ~ScopedEnv() {
if (attached_to_vm_) {
g_vm->DetachCurrentThread();
attached_to_vm_ = false;
}
}
JNIEnv *GetEnv() const { return env_; }
private:
bool attached_to_env_;
JNIEnv *env_;
};
// Usage:
{
ScopedEnv scoped_env;
scoped_env.GetEnv()->SomeJniFunction();
}
// scoped_env falls out of scope, the thread is automatically detached if necessary
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