Calling dll implemented JNI from C++

Question

Situation: I have a dll which implemented JNI,I want to call the methods in it from a Cpp application.

Current Status: According to my understanding, a dll which implemented JNI actually has nothing to do with JAVA, for example: In Test.java, I wrote
public native int Add(int a,int b);
And realized it with Cpp In TestDll.Cpp
JNIEXPORT jint JNICALL Java_SomeNamespace_Add(JNIEnv* _Env, jobject _Object, jint a, jint b) { return a+b; }
I think that such a program has nothing to do with JVM, the jint structure seems have been fully defined in jni.h.
So, I wonder if it's possible to call Java_SomeNamespace_Add directly without creating a VM from a Cpp application, if it do is possible,:

What should the `JNIEnv*` and the `jobject` in the parameter list be?
How can i convert a `jint` variable to a standard `int` variable?
How can i convert a `jstring` variable to a standard `string` variable without using `_Env->FindClass("Ljava/lang/String;")` and a bunch of following code?

1. They would be null. 2. jint and int32_t is the same thing. 3. jstring cannot be converted to std::string without Java JNI code.. You can create a JVM using JNI and then do the conversion.. — Brandon
@Brandon Can I just include jni.h and do something then it magically works? — Ragnarokkr Xia
Did you verify that your actual DLL doesn't do anything with the env pointer and/or the receiver object or possible parameter objects? I don't think you can generalize this from your test function. — user2543253

Carlo B. Carlo B. · Accepted Answer · 2018-08-08T22:50:04

For number 1 I think it would be a good idea to just create a simple alternative using a bit of refactoring.

Instead of putting the logic of the algorithm into the JNI call itself, move the logic to a separate c++ function and invoke that function from both use cases.

ie:

// MyJNIWrapper.cpp
#include "MyMathFunctions.h"
JNIEXPORT jint JNICALL Java_SomeNamespace_Add(JNIEnv* _Env, jobject _Object, jint a, jint b)
{
   return (jint)add((int)a, (int)b);
}

// MyMathFunctions.h
int add(int a, int b)
{
    return a + b;
}

// MyCppProgram.cpp
#include "MyMathFunctions.h"
int main()
{
    int c = add(a, b)
}

For number 2, you can convert between jint and int using standard C style casts:

int a = 10;
jint b = (jint) a;
int c = (int) b

As for number 3:

If you are calling the function from within JNI, you would use the pair of functions described here:

JNIEXPORT void JNICALLJava_MyJavaClass_printName(JNIEnv *env, jobject obj, jstring name)
{
    const char *str= (*env)->GetStringUTFChars(env,name,0);
    printf(“%s”, str);
    //need to release this string when done with it in order to
    //avoid memory leak
    (*env)->ReleaseStringUTFChars(env, name, str);
}

To convert from a C string like above to std::string you can do the following:

std::string cpp_string = str;

If you are not calling the function using JNI, it looks like you will need to try and roll your own conversion. I would see if you can use an existing VM implementation, figure out how they do it and mimic that.

For example you could try the Android VM with these as a starting point: https://android.googlesource.com/platform/dalvik/+/donut-release/vm/Jni.c#2230 https://android.googlesource.com/platform/dalvik.git/+/android-4.3_r3/vm/UtfString.cpp#284

Calling dll implemented JNI from C++

2 Answers