Question

Description

In my C++ application class JNIXMLDocument which made some JAVA method calls. In the constructor of JNIXMLDocument class I attach current thread and set it to my class member JNIEnv* m_JavaEnv and then use it in all methods. Also in the constructor I am trying to find my JAVA class com/fido/android/framework/service/XMLDOMDocument and set it to class member m_XMLDocumentClass and also get that class object from the class and set it to class member m_XMLDocumentObject.

C++ Code

class JNIXMLDocument
{
    /* Constructor **/
    JNIXMLDocument()
    {
        /* Get JNI right version and set it. **/
        jint interface_id = JNI_VERSION_1_2;
        #ifdef JNI_VERSION_1_2
            interface_id = JNI_VERSION_1_2;
        #else
            interface_id = JNI_VERSION_1_1;
        #endif

        /* Trying to attach current thread. **/
        int res = g_JavaVirtualMachine->GetEnv(&m_JavaEnv, interface_id);
        if (res == JNI_EDETACHED || res == JNI_EVERSION) {
            res = g_JavaVirtualMachine->AttachCurrentThread(&m_JavaEnv, NULL);
        }

        /* Get Class from Java **/
        m_XMLDocumentClass = m_JavaEnv->FindClass("com/fido/android/framework/service/XMLDOMDocument");
        if (m_XMLDocumentClass != NULL) {
            /* Call java class constructor. **/
            jmethodID constructor = m_JavaEnv->GetMethodID(m_XMLDocumentClass , "<init>", "()V");
            m_XMLDocumentObject = m_JavaEnv->NewObject(m_XMLDocumentClass , constructor);    

        }

    }

    bool Initialize()
    {
        jmethodID method = m_JavaEnv->GetMethodID(m_XMLDocumentClass, "Initialize", "()Lorg/w3c/dom/Document;");
        jobject document = m_JavaEnv->CallObjectMethod(m_XMLDocumentObject , method);

    }

    private:
        JNIEnv* m_JavaEnv;
        jclass  m_XMLDocumentClass;
        jobject m_XMLDocumentObject;


};

C++ Code (Right way)

class JNIXMLDocument
{
    /* Constructor **/
    JNIXMLDocument()
    {
        /* Get JNI right version and set it. **/
        jint interface_id = JNI_VERSION_1_2;
        #ifdef JNI_VERSION_1_2
            interface_id = JNI_VERSION_1_2;
        #else
            interface_id = JNI_VERSION_1_1;
        #endif

        JNIEnv* env;
        /* Trying to attach current thread. **/
        int res = g_JavaVirtualMachine->GetEnv(&env, interface_id);
        if (res == JNI_EDETACHED || res == JNI_EVERSION) {
            res = g_JavaVirtualMachine->AttachCurrentThread(&env, NULL);
        }

        /* Get Class from Java **/
        jclass localClass = env->FindClass("com/fido/android/framework/service/XMLDOMDocument");
        if (localClass != NULL) {
            m_XMLDocumentClass = env->NewGlobalRef(localClass);
            /* Call java class constructor. **/
            jmethodID constructor = env->GetMethodID(localClass, "<init>", "()V");
            jobject localObject = env->NewObject(m_XMLDocumentClass , constructor);    
            m_XMLDocumentObject = env->NewGlobalRef(localObject );
        }

    }

    bool Initialize()
    {

    }

    private:
        jclass  m_XMLDocumentClass;
        jobject m_XMLDocumentObject;


};

Questions

1. Is it right to set JNI interface pointer (JNIEnv* m_JavaEnv) in the constructor once and use it in the whole code ?
2. Is it right to set jclass m_XMLDocumentClass in the constructor and than use that variable in the all methods ?
3. Is it right to set jobject m_XMLDocumentObject in the constructor in this way m_JavaEnv->NewObject(m_XMLDocumentClass , constructor); or maybe I must call NewGlobalRef.
4. What problems can appear if my application work not in the same thread (use many threads) ?

Answer 1

Is it right to set JNI interface pointer (JNIEnv* m_JavaEnv) in the constructor once and use it in the whole code ?

No. It is thread-specific. That’s what Attach/DetachCurrentThread are for. The only way this can work is if the C++ object is constructed and destroyed in the same thread.

Is it right to set jclass m_XMLDocumentClass in the constructor and than use that variable in the all methods ?

No. It is a local reference, and it expires when the JNI method it was acquired in returns. You must save as a global or weak reference unless it is only going to be used within a single JNI method.

Is it right to set jobject m_XMLDocumentObject in the constructor in this way m_JavaEnv->NewObject(m_XMLDocumentClass , constructor);

No: see above.

or maybe I must call NewGlobalRef.

Yes, as above.

What problems can appear if my application work not in the same thread (use many threads) ?

Mainly JVM crashes. The JVM assumes you follow all the rules in the JNI specification. So do that.