Java和JNI全局变量

Question

I need help, because i don't known what to do. I'm newbie in JNI.

I'm developing a Android application and it has to use a especific protocol (OpenIGTLink).

To use this protocol I use a C library. The problem is that I need to define a global variable to store the conection. You can see in the next example:

igtl::ClientSocket::Pointer socket;


Java_es_iac_iactec_infraredsend_Comunicacion_OpenIGTLink_connect(
    JNIEnv *env,
    jobject mjobject,
    jstring host,
    jint port) {
socket = igtl::ClientSocket::New();

const char *chost = env->GetStringUTFChars(host, 0);

int r = socket->ConnectToServer(chost, port);
if (r != 0) {

    return (jstring) "Cannot connect to the server.";
}

return (jstring) "ok";
}

and in other method iwant to do:

 Java_es_iac_iactec_infraredsend_Comunicacion_OpenIGTLink_send() {
    igtl::ImageMessage::Pointer imgMsg = igtl::ImageMessage::New();
    imgMsg->SetDimensions(size);
    imgMsg->SetSpacing(spacing);
    imgMsg->SetScalarType(scalarType);
    imgMsg->SetDeviceName("ImagerClient");
    imgMsg->SetSubVolume(svsize, svoffset);
    imgMsg->AllocateScalars();
    imgMsg->Pack();

    socket->Send(imgMsg->GetPackPointer(), imgMsg->GetPackSize());
 }

I don't know if I have to declare a java global variable to store de socket object and access it from JNI or if it's possible to define socket like a global variable in the cpp file.

Thank you to all and sorry for my english.

Answer 1

Maybe, it's not the best way but we are doing it so. We are returning a pointer from JNI to Java as jlong and passing it to every call. It works like a self pointer to the object in methods. In your case, it will look somehow like this

jlong Java_es_iac_iactec_infraredsend_Comunicacion_OpenIGTLink_connect(
     JNIEnv *env,
     jobject mjobject,
     jstring host,
     jint port) {
     /*some code*/
     return reinterpret_cast<jlong>(socket);
 }

and then

 Java_es_iac_iactec_infraredsend_Comunicacion_OpenIGTLink_send(jlong pointer) {
    /*some code*/
    igtl::ClientSocket::Pointer socket = reinterpret_cast<igtl::ClientSocket::Pointer>(pointer)
    socket -> Send(imgMsg->GetPackPointer(), imgMsg->GetPackSize());
 }