How to call class method from JNI and cast the return to a custom class

Question

First day working with JNI and in my search I have not found a solution that quite matches the problem I am having. I have three classes, and a class method I am trying use like this:

Class1
- EobjectClass Method1("some text")
Class2
EobjectClass

What my execution looked like in source code was:

Class1 class1Instance = new Class1();
// class1Instance.Method1() returns an EobjectClass which is cast to Class2
Class2 result = (Class2) class1Instance.Method1("Some string of text");

result then had the object I desired. I am struggling with how to do this from the JNI interface. Here is what I have so far.

jclass lookForClass(JNIEnv* env, char* name)
{
    jclass clazz = env->FindClass(name);

    if (!clazz) {
        printf("Unable to find class %s\n", name);
        exit(1);
    }

    printf("Class %s found\n", name);
    fflush(stdout);

    return clazz;
}
jobject invokeClassObj(JNIEnv* env, jclass classInDll) {
    jmethodID init;
    jobject result;
    init = env->GetMethodID(classInDll, "<init>", "()V");
    if (!init) {
        printf("Error: Could not find class Constructor\n");
        return;
    }
    result = env->NewObject(classInDll, init);
    if (!result) {
        printf("Error: failed to allocate an object\n");
        return;
    }
    return result;
}
jclass Parser = lookForClass(env, "com/Path/Parser");
jclass TextModel = lookForClass(env, "com/Path/TextModel");
jobject ParserInstance = invokeClassObj(env, Parser);
jmethodID parse = GetMethodID(Parser, "parse", "(Ljava/lang/String;)Lorg/Path/Eobject;");

Now here is where I lose what I am supposed to do. What it looks like logically to me is:

TextModel model = static_cast<TextModel> (ParserInstance.parse("some text here"));

But how would I execute that in this JNI environment? If there is any information or clarifications I missed please comment.

Christophe · Accepted Answer · 2017-06-23 19:27:23Z

1

As your method returns an object, you may invoke it as follows:

jobject model = env->CallObjectMethod(ParserInstance, parse, env->NewStringUTF("some text here"));

For a general tutorial on invoking methods and static methods from C++, you can look at this step-by-step CodeProject article.

For the different ways to call a method you may look in Oracle's JNI reference. You'll see that alternatively, you can also pass the parameters using an array or a var arg list.

For the different ways to construct a string object, you can look here. In the code above, I assumed your original string on the C++ side was UTF8 encoded.

answered Jun 23, 2017 at 19:27

Christophe

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11 Comments

Zannith Over a year ago

I don't know if it makes a difference but my method returns an interface Eobject, which I am then casting to textmodel. When I execute the line above with valid text my model is resulting in null, when it should be a structured tree.

Christophe Over a year ago

But if you receive an object from java, it's be a jobject containing a reference to the real object. Have you checked that the returend object is ok before your case ? For example using env->GetObjectClass(env, model); and looking at the name of that class ? Second question do you use your object immediately, or do you keep it, do a lot of stuff and use it later (because in this case i could have been dereferenced unless you locked it) ?

Zannith Over a year ago

Neither my class or my object are returning NULL which I assumed meant it was working okay. I am not sure how to get the actual name of the class from the GetObjectClass function as it returns a memory location. And yes i use the object immediately. on invokeclassobject function I call its constructor which does a short initialization, I then immediately call the parse method on that.

Zannith Over a year ago

-update, I am now getting an Access violation reading location 0x00A20004 within the NewStringUTF function.

Christophe Over a year ago

The newStringUTF expects a char* pointer to a valid UTF C-string which is null terminated. It may trhow if out of memory. It may also return NULL if it didn't work. May be first do a jstring x= .... to check that evrything went fien at this stage.

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