How to convert java CharSequence to c++ char* through JNI

Question

I feel that this should have a simple answer as I can do it easily with a java String , but something is going wrong with the CharSequence . My java code initializes a cppGenerator , passes a functionBlock object to its generateImplementation method , and returns a CharSequence . Here is my code:

jobject fBlock = env->CallObjectMethod(structuredTextModel, getFunction);
jclass CPPGenerator = lookForClass(env, "Path/CPPGenerator");
jobject CPPGeneratorImpl = invokeClassObj(env, CPPGenerator);
    jmethodID generateImplementation = env->GetMethodID(CPPGenerator, "generateImplementation", "(LPath/FunctionBlock;)Ljava/lang/CharSequence;");
jobject mid = env->CallObjectMethod(CPPGeneratorImpl, generateImplementation, fBlock);
jstring jresult = (jstring)mid;
char* result = (char*)env->GetStringUTFChars(jresult, 0);

This logic has worked for me when using String but this function returns a CharSequence . No classes, objects, or methods are returning null. The program crashes in jni.h attempting to call GetStringUTFChars . So i know I cannot use this method for CharSequence. Is there just a jni method I am missing or do I have to do something completely different for a CharSequence?

Answer 1

You can't just cast mid to jstring and call GetStringUTFChars (since mid might not be a String , which seems to be the case). There is no convenience method like GetStringUTFChars for CharSequence s like there is for String s.

You'd have to do it the inconvenient way of calling charAt :

jclass cCharSequence = env->FindClass("java/lang/CharSequence");
jmethodID mLength = env->GetMethodID(cCharSequence, "length", "()I");
jmethodID mCharAt = env->GetMethodID(cCharSequence, "charAt", "(I)C");

int length = env->CallIntMethod(mid, mLength);
std::string str;
str.reserve(length);
for(int i = 0; i < length; i++) {
    str += env->CallCharMethod(mid, mCharAt, i);
}

// use 'str.c_str()'...

Answer 2

Why not use toString directly?

jclass clzCharSequence = env->GetObjectClass(mid);
jmethodID toString = env->GetMethodID(clzCharSequence, "toString", "()Ljava/lang/String;");
jobject s = env->CallObjectMethod(mid, toString);
const char* cstr = env->GetStringUTFChars(static_cast<jstring>(s), NULL);
// do something with cstr
env->ReleaseStringUTFChars(static_cast<jstring>(s), cstr);