JNI Method Signatures

com_marakana_jniexamples_InstanceAccess.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class HelloJNI */
 
#ifndef _Included_HelloJNI
#define _Included_HelloJNI
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     HelloJNI
 * Method:    sayHello
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_HelloJNI_sayHello(JNIEnv *, jobject);
 
#ifdef __cplusplus
}
#endif
#endif

HelloJNI.cc

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// Save as "HelloJNI.cc"
#include <jni.h>       // JNI header provided by JDK
#include <iostream>    // C++ standard IO header
#include "HelloJNI.h"  // Generated
using namespace std;

// Implementation of the native method sayHello()
JNIEXPORT void JNICALL Java_HelloJNI_sayHello(JNIEnv *env, jobject thisObj) {
	cout << "Hello World from C++!" << endl;
   return;
}

jni_method_signatures.cc

#include <cstdio>
#include "com_marakana_jniexamples_InstanceAccess.h"

JNIEXPORT void JNICALL Java_com_marakana_jniexamples_InstanceAccess_propertyAccess(JNIEnv *env, jobject object){
    jfieldID fieldId;
    jstring jstr;
    const char *cString;

    /* Getting a reference to object class */
    jclass class = (*env)->GetObjectClass(env, object); /* 1 */

    /* Getting the field id in the class */
    fieldId = (*env)->GetFieldID(env, class, "name", "Ljava/lang/String;"); /* 2 */
    if (fieldId == NULL) {
        return; /* Error while getting field id */
    }

    /* Getting a jstring */
    jstr = (*env)->GetObjectField(env, object, fieldId); /* 3 */

    /* From that jstring we are getting a C string: char* */
    cString = (*env)->GetStringUTFChars(env, jstr, NULL); /* 4 */
    if (cString == NULL) {
        return; /* Out of memory */
    }
    printf("C: value of name before property modification = \"%s\"\n", cString);
    (*env)->ReleaseStringUTFChars(env, jstr, cString);

    /* Creating a new string containing the new name */
    jstr = (*env)->NewStringUTF(env, "Brian"); /* 5 */
    if (jstr == NULL) {
        return; /* Out of memory */
    }
    /* Overwrite the value of the name property */
    (*env)->SetObjectField(env, object, fieldId, jstr); /* 6 */
}

JNIEXPORT void JNICALL Java_com_marakana_jniexamples_InstanceAccess_methodAccess(JNIEnv *env, jobject object){
    jclass class = (*env)->GetObjectClass(env, object); /* 7 */
    jmethodID methodId = (*env)->GetMethodID(env, class, "setName", "(Ljava/lang/String;)V"); /* 8 */
    jstring jstr;
    if (methodId == NULL) {
        return; /* method not found */
    }
    /* Creating a new string containing the new name */
    jstr = (*env)->NewStringUTF(env, "Nick"); /* 9 */
    (*env)->CallVoidMethod(env, object, methodId, jstr); /* 10 */
}

TestJNIInstanceVariable.cc

#include <jni.h>
#include <stdio.h>
#include "TestJNIInstanceVariable.h"
 
JNIEXPORT void JNICALL Java_TestJNIInstanceVariable_modifyInstanceVariable
          (JNIEnv *env, jobject thisObj) {
   // Get a reference to this object's class
   jclass thisClass = (*env)->GetObjectClass(env, thisObj);
 
   // int
   // Get the Field ID of the instance variables "number"
   jfieldID fidNumber = (*env)->GetFieldID(env, thisClass, "number", "I");
   if (NULL == fidNumber) return;
 
   // Get the int given the Field ID
   jint number = (*env)->GetIntField(env, thisObj, fidNumber);
   printf("In C, the int is %d\n", number);
 
   // Change the variable
   number = 99;
   (*env)->SetIntField(env, thisObj, fidNumber, number);
 
   // Get the Field ID of the instance variables "message"
   jfieldID fidMessage = (*env)->GetFieldID(env, thisClass, "message", "Ljava/lang/String;");
   if (NULL == fidMessage) return;
 
   // String
   // Get the object given the Field ID
   jstring message = (*env)->GetObjectField(env, thisObj, fidMessage);
 
   // Create a C-string with the JNI String
   const char *cStr = (*env)->GetStringUTFChars(env, message, NULL);
   if (NULL == cStr) return;
 
   printf("In C++, the string is %s\n", cStr);
   (*env)->ReleaseStringUTFChars(env, message, cStr);
 
   // Create a new C-string and assign to the JNI string
   message = (*env)->NewStringUTF(env, "Hello from C++");
   if (NULL == message) return;
 
   // modify the instance variables
   (*env)->SetObjectField(env, thisObj, fidMessage, message);
}

Type erasure notes:

• https://javapapers.com/core-java/type-erasure/#Java%20Decompiler
• http://www.angelikalanger.com/GenericsFAQ/FAQSections/TechnicalDetails.html#FAQ110
• https://rgomes.info/using-typetokens-to-retrieve-generic-parameters/

The finished article:

• https://medium.com/@hopro/i-detest-the-python-static-typing-annotation-schemes-fbb0fa9b1f59