Question

What is recommended template set or library to acquire/release JNI resources from C++ code?

“Bad” example:

//C++ code
extern "C"
JNIEXPORT void JNICALL Java_ClassName_MethodName
  (JNIEnv *env, jobject obj, jstring javaString)
{
    //Get the native string from javaString
    const char *nativeString = env->GetStringUTFChars(javaString, 0);

    //Do something with the nativeString

    //DON'T FORGET THIS LINE!!!
    env->ReleaseStringUTFChars(javaString, nativeString);
}

Obviously, everybody uses a template set, not the code above.

For jstring it calls GetStringUTFChars to acquire resource and ReleaseStringUTFChars to release, when object goes “out of scope”.

Gotta be something similar to auto_ptr template, but tailored to JNI to call GetStringUTFChars/ReleaseStringUTFChars for jstring, for example.

Answer 1

The keyword here is RAII:

class jni_string {
public:
    jni_string(JNIEnv *env, jstring javaString)
        : env_(env), javaString_(javaString)
    { nativeString_ = env_->GetStringUTFChars(javaString_, 0); }

    ~jni_string() { env_->ReleaseStringUTFChars(javaString_, nativeString_); }

    operator const char *() const { return nativeString_; }
private:
    jni_string(const jni_string &x);
    jni_string &operator=(const jni_string &x);

    JNIEnv *env_;
    jstring javaString_;
    const char *nativeString_;
};

Usage would be:

//C++ code
extern "C"
JNIEXPORT void JNICALL Java_ClassName_MethodName
  (JNIEnv *env, jobject obj, jstring javaString)
{
    //Get the native string from javaString
    jni_string nativeString(env, javaString);

    //Do something with the nativeString
}