When Java is calling into a C function via JNI, we know that any local references to classes or objects may become invalid once the C function returns to java.
However when we only have C calling into Javam there is no "return to java". So it's not clear to me when these references will become invalid.
For example this is bad since aClass, and anObject will potentially become invalid once the call is complete.
jclass aClass;
jobject anObject;
JNIEXPORT void JNICALL Java_example_method1(JNIEnv *env) {
aClass = env->FindClass("AClass");
}
JNIEXPORT void JNICALL Java_example_method2(JNIEnv * env) {
jmethodID constructor = env->GetMethodID( aClass, "<init>", "()V");
anObject = env->NewObject(klass, constructor );
}
However in the following example, what can cause aClass and anObject to become invalid? Is it only an explicit call to DeleteLocalRef?
jclass aClass;
jobject anObject;
int main() {
JavaVMInitArgs vm_args;
JavaVMOption options[1];
vm_args.version = JNI_VERSION_1_6;
vm_args.nOptions = 1;
options[0].optionString = "-Djava.class.path=.";
vm_args.options = options;
vm_args.ignoreUnrecognized = JNI_FALSE;
JNI_CreateJavaVM(&jvm, (void **)&env, &vm_args);
aClass = env->FindClass("AClass");
jmethodID constructor = env->GetMethodID( aClass, "<init>", "()V");
anObject = env->NewObject(klass, constructor );
}
That's correct. When Java calls JNI it allocates a table where local refs to, that are all released when the JNI returns. Wen you're calling Java there is no such mechanism.
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