Return object from java native method

Question

I would like to call a method through a native java interface, which returns an object.

This is my native method

public native Node getProjectionPoint(double lat, double lon);

Node class

 public class Node {        
    private String id;
    private double latitude;
    private double longitude;
}

C header file

JNIEXPORT jobject JNICALL Java_org_smartcar_serverdatainterface_shared_services_CppConnector_getProjectionPoint (JNIEnv *env, jobject obj, jdouble lat, jdouble lon);

How could I create an object and return it to java?

Take a look to : http://stackoverflow.com/a/4325678/3315914 – rpax Mar 10 '14 at 13:04 — rpax, Mar 10 '14 at 13:04

score 7 · Accepted Answer · answered Mar 10 '14 at 18:22

I sort out the problem

JNIEXPORT jobject JNICALL Java_org_smartcar_serverdatainterface_shared_services_CppConnector_getProjectionPoint
  (JNIEnv *env, jobject obj, jdouble lat, jdouble lon)
{
    jclass class = (*env)->FindClass(env,"org/smartcar/serverdatainterface/shared/businessentities/Node");

    if (NULL == class)
        PrintError ("class");

    jmethodID cid = (*env)->GetMethodID(env,class, "<init>", "(DD)V");

   if (NULL == cid)
       PrintError ("method");

   return (*env)->NewObject(env, class, cid, lat, lon);
}

this works perfectly

ebasconp · Answer 2 · 2014-03-10T13:22:14.600

3

In JNI you have a method

JNIEnv->NewObject() that invokes the actual constructor of a given Java class.

Maybe something like:

JNIEXPORT jobject JNICALL Java_org_smartcar_serverdatainterface_shared_services_CppConnector_getProjectionPoint (JNIEnv *env, jobject obj, jdouble lat, jdouble lon)
{
  jclass cls = env->GetObjectClass(obj);
  jmethodID constructor = env->GetMethodID(cls, "<init>", "(DD)V");
  return env->NewObject(cls, constructor, lat, lon);
}

You should modify your class constructor to receive two parameters. You can also initialize field by field, but it requires to invoke GetFieldID two times in C++.

edited Mar 10 '14 at 13:22

answered Mar 10 '14 at 13:13

ebasconp

1,608
2
17
27

Buff... Maybe it's better to copy it, don't you think? – rpax Mar 10 '14 at 13:14
Sorry, I misunderstood your question; I edited my answer accordingly. – ebasconp Mar 10 '14 at 13:15
I'm getting this error "error: request for member 'GetObjectClass' in something not a structure or union" – JanithOCoder Mar 10 '14 at 13:32
Are you including jni.h ? – ebasconp Mar 10 '14 at 13:33
i think this is the correct way "jclass cls = (*env)->GetObjectClass(env, obj);" :P – JanithOCoder Mar 10 '14 at 14:01
@JanithOCoder Your question is tagged C++ but you've discovered the C syntax. Either would work if you are compiling as C++ instead of C. – Tom Blodget Mar 12 '14 at 02:38
1

`GetObjectClass` would work in this case because the method is an instance method in the same class as the object to be created. I would find it confusing. `FindClass` is clearer. – Tom Blodget Mar 12 '14 at 02:40

Return object from java native method

2 Answers2