用于获取/释放资源的JNI C ++模板

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

You can use std::unique_ptr in C++11, which allows you to set the deleter yourself:

#include <memory>
#include <functional>

typedef std::unique_ptr<char const[], std::function<void(char const*)>> jni_string_ptr;
// the "std::function" is needed to store any callable†

extern "C"
JNIEXPORT void JNICALL Java_ClassName_MethodName
  (JNIEnv *env, jobject obj, jstring javaString)
{
    //Get the native string from javaString
    jni_string_ptr nativeString(env->GetStringUTFChars(javaString, 0),
        [=](char const* p) mutable{ env->ReleaseStringUTFChars(javaString, p); });
    // lambda as the deleter
    // mutable needed if "ReleaseStringUTFChars" is a non-const method

    //Do something with the nativeString

    // nativeString automatically calls the passed deleter
}

If you're stuck in C++03 and don't have access to std::unique_ptr , boost::shared_array provides a.. viable alternative.

#include <boost/shared_ptr.hpp>

typedef boost::shared_array<char const*> jni_string_ptr;

struct jni_string_deleter{
  jni_string_deleter(JNIEnv* e, jstring js)
    : _env(e), _java_string(js) {}

  void operator()(char const* p) const{
    _env->ReleaseStringUTFChars(_java_string, p);
  }

private:
  JNIEnv* _env;
  jstring _java_string;
};

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

    //Do something with the nativeString

    // nativeString automatically calls the passed deleter
}

---

† You can also use a specific delter type here, as with the shared_array example, to avoid the type-erasure overhead of std::function :

struct jni_string_deleter{
  jni_string_deleter(JNIEnv* e, jstring js)
    : _env(e), _java_string(js) {}

  void operator()(char const* p) const{
    _env->ReleaseStringUTFChars(_java_string, p);
  }

private:
  JNIEnv* _env;
  jstring _java_string;
};

typedef std::unique_ptr<char const[], jni_string_deleter> jni_string_ptr;

// create
jni_string_ptr(env->get_the_chars(), jni_string_deleter(env, javaString));

Answer 2

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
}