How to instantiate shared pointer with JNI to call java implementation

Question

I'm new in JNI and C++. I have to call lib function with shared pointer. My code:

JNIEXPORT jint JNICALL Java_com_test_NativeClient_subscribe(JNIEnv* env, jobject thisObj, jobject handler) {
jclass handlerClass = env->GetObjectClass(handler);
jmethodID starts = env->GetMethodID(handlerClass, "starts", "(I)V");
jmethodID joins = env->GetMethodID(handlerClass, "joins", "(Ljava/lang/String;Ljava/lang/String;)V");

// int subscribe(std::shared_ptr< SomeHandler > handler) // I need implement this
 
 std::shared_ptr<?> sharedPointer = new std::shared_ptr<?>;
 
return some::lib::subscribe(sharedPointer);
}

SomeHandler it is an interface from lib - some::lib::SomeHamdler, but also I pass java implementation in the method (jobject handler). How I can properly define sharedPointer to call java implementation after subscribe method performed? Thanks in advance.

UPD: Java code:

public native int subscribe(SomeHandler handler); // native method in NativeClient

SomeHandler interface:

public interface SomeHandler {

void starts(int uptime);

void joins(String mac, String name);

SomeHandlerImpl class:

public class SomeHandlerImpl implements SomeHandler {

@Override
public void starts(int uptime) {
    System.out.println("uptime is " + uptime);
}

@Override
public void joins(String mac, String name) {
    System.out.println("mac: " + mac + ", nName: " + name);
}

It sounds like you need to create a class which derives from SomeHandler, and when your class's handler is called, it calls the Java handler. — user253751, Jul 17 '20 at 10:09
Yes, I already have java interface SomeHandler and its implementation SomeHandlerImpl. I'll add java code. — Valeriy K., Jul 17 '20 at 10:15
Okay, but you're talking about C++ code here so obviously you need a class that `some::lib::subscribe` can use, which is a C++ class. — user253751, Jul 17 '20 at 10:18
As I understand from my task, it is required to call java implementation after subscription. I.e. subscribe will invoke starts and joins methods. — Valeriy K., Jul 17 '20 at 10:24
It is a function to subscribe to some events using the notification handler (SOmeHandler). some::lib is a placeholder. It is a quite real implementation but I can't share it here. some::lib::subscribe calls int subscribe(std::shared_ptr< SomeHandler > handler) function — Valeriy K., Jul 17 '20 at 10:36
So i need something like std::shared_ptr sharedPointer = GetSharedPointer(handlerClass); But i don't know how to write GetSharedPointer... in c++. — Valeriy K., Jul 17 '20 at 10:41

Botje · Accepted Answer · 2020-07-22T09:02:41.900

1

All you need to do is store a global reference to the jobject and write some wrapper code:

class JavaWrapperHandler : public some::lib::callback {
    jobject java_handler;

public:
    JavaWrapperHandler(jobject handler) {
        JNIEnv *env = nullptr;
        vm->GetEnv(&env, JNI_VERSION_1_6);
        java_handler = env->NewGlobalRef(handler);
    }

    ~JavaWrapperHandler() {
        JNIEnv *env = nullptr;
        vm->GetEnv(&env, JNI_VERSION_1_6);
        env->DeleteGlobalRef(java_handler);
    }

    virtual joins(std::string mac, std::string name) {
        JNIEnv *env = nullptr;
        vm->GetEnv(&env, JNI_VERSION_1_6);
        jclass handlerClass = env->GetObjectClass(java_handler);
        jmethodID joins = env->GetMethodID(handlerClass, "joins", "(Ljava/lang/String;Ljava/lang/String;)V");
        env->CallVoidMethod(java_handler, joins, ...);
    };
};

And you can instantiate this as follows in your JNI method:

std::make_shared<JavaWrapperHandler>(handler);

Note that you still need to store the shared_ptr again somewhere, otherwise it will immediately be freed. You could for example store it in a std::map<long, shared_ptr<JavaWrapperHandler>> and return the long as a jlong.

Points of note:

This code keeps a global reference to prevent the Java handler object from being garbage collected.
The global reference is freed when the handler is destroyed. Make sure to unregister the callback at some point if you want to free the Java object.
We use the GetEnv method from the JNI Invocation API. It will only produce a useful value if the current (C++) thread has already been attached to the JVM. If it fails, you need to call vm->AttachCurrentThread or vm->AttachCurrentThreadAsDaemon.

edited Jul 22 '20 at 09:02

answered Jul 17 '20 at 10:56

Botje

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The quite simple and obvious solution. Thanks a lot! Why you instantiate *env in constructor and method? And where you get VM variable? Can I use env from Java_com_test_NativeClient_subscribe in JavaWrapperHandler constructor and methods? – Valeriy K. Jul 20 '20 at 10:52
No, you cannot share `JNIEnv* env` across threads, that is why I retrieve it every time for the current thread. It was unclear from your explanation, so I assumed your "somelib" uses a separate thread to invoke callbacks, which is different from the subscribing thread. Retrieving the `env` associated with the current thread solves that. As I said, you may need to attach it the thread the first time a callback fires. If you did not create the VM yourself you can use [`env->GetJavaVM`](https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/functions.html#wp23168) – Botje Jul 20 '20 at 11:05
I get error - " 'GetEnv' was not declared in this scope". And also -" note: suggested alternative: ‘setenv’" How I can fix it? – Valeriy K. Jul 22 '20 at 08:22
It should be part of the jni header. Did you not include it? – Botje Jul 22 '20 at 08:44
Did you mean #include ? I tried and got the same. – Valeriy K. Jul 22 '20 at 08:53
Sorry, I was too quick. `GetEnv` is a method of the VM object. So you need to do `vm->GetEnv(...)`. Updated answer. – Botje Jul 22 '20 at 09:03
Yes, it works. I wrote working code here - https://stackoverflow.com/a/63032634/7804595 – Valeriy K. Jul 22 '20 at 10:59

How to instantiate shared pointer with JNI to call java implementation

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