Sample JNI/C++ HelloWorld

HelloJNI.java

public class HelloJNI {
    static {
        System.loadLibrary("hello");    // loads libhello.so
    }

    private native void sayHello(String name);

    public static void main(String[] args) {
        new HelloJNI().sayHello("Dave");
    }
}

HelloJNIImpl.cpp

#include "HelloJNIImpl.h"

#include <memory>
#include <functional>
#include <iostream>

#include <jni.h>
#include "HelloJNI.h"   // auto-generated by `javah HelloJNI`


using std::string;
using std::function;
using std::unique_ptr;
using std::shared_ptr;
using std::cout;
using std::endl;


class jstring_deleter
{
    JNIEnv *m_env;
    jstring m_jstr;

public:

    jstring_deleter(JNIEnv *env, jstring jstr)
        : m_env(env)
        , m_jstr(jstr)
    {
    }

    void operator()(const char *cstr)
    {
        cout << "[DEBUG] Releasing " << cstr << endl;
        m_env->ReleaseStringUTFChars(m_jstr, cstr);
    }

};

const string ToString(JNIEnv *env, jstring jstr)
{
    jstring_deleter deleter(env, jstr);     // using a function object
    unique_ptr<const char, jstring_deleter> pcstr(
            env->GetStringUTFChars(jstr, JNI_FALSE),
            deleter );

    return string( pcstr.get() );
}


shared_ptr<const char> ToStringPtr(JNIEnv *env, jstring jstr)
{
    function<void(const char*)> deleter =   // using a lambda
        [env, jstr](const char *cstr) -> void
        {
            cout << "[DEBUG] Releasing " << cstr << endl;
            env->ReleaseStringUTFChars(jstr, cstr);
        };

    return shared_ptr<const char>(
            env->GetStringUTFChars(jstr, JNI_FALSE),
            deleter );
}


/*
 * Class:     HelloJNI
 * Method:    sayHello
 * Signature: (Ljava/lang/String;)V
 */
JNIEXPORT void JNICALL Java_HelloJNI_sayHello
  (JNIEnv *env, jobject thisObj, jstring arg)
{
    DoSayHello(ToString(env, arg));

    //const string name = ToStringPtr(env, arg).get();
    //DoSayHello(name);
}


void DoSayHello(const string &name)
{
    cout << "Hello, " << name << endl;
}

HelloJNIImpl.h

#ifndef _HELLO_JNI_IMPL_H
#define _HELLO_JNI_IMPL_H

#include <string>


void DoSayHello(const std::string &name);


#endif//_HELLO_JNI_IMPL_H

makerun.sh

export JAVA_INC=/usr/lib/jvm/java-7-oracle/include

# step 1: compile the .class file with invocation to a native method
javac HelloJNI.java
# step 2: auto-generate a .h header file from said Java source
javah HelloJNI

# step 3: make the shared library with the name linked in said Java source, and implementing said native method
g++ -std=c++11 -shared -fPIC -I$JAVA_INC -I$JAVA_INC/linux HelloJNIImpl.cpp -o libhello.so

# step 4: run JVM with java.library.path set to include said shared library
java -Djava.library.path=. HelloJNI

Thanks a lot for this guide! It is also very interesting how use this with Gradle.

Nice

Thanks

This gist helped me a lot.
Thanks

Thanks, this hist helped me.

This is outdated at this point. javah was removed in JDK 10. The modern equivalent of line 6 (JDK >= 10) in makerun.sh is javac -h . HelloJNI.