Sample JNI/C++ HelloWorld

.gitignore

build/
*.class

README.md

Example of using JNI with C++

On e.g. debian we can:

$ sudo apt-get install openjdk-21-jdk cmake g++
$ $ ./makerun.sh 
-- The CXX compiler identification is GNU 13.3.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found Java: /usr/lib/jvm/java-21-openjdk-amd64/bin/java (found version "21.0.9") found components: Development 
-- Found JNI: /usr/lib/jvm/java-21-openjdk-amd64/include  found components: AWT JVM 
-- Configuring done (1.3s)
-- Generating done (0.0s)
-- Build files have been written to: /home/bjorningvar/vc/HelloJNI.java/build
[ 33%] Building CXX object CMakeFiles/hello.dir/HelloJNIImpl.cpp.o
[ 66%] Linking CXX shared library libhello.so
[100%] Built target hello
[DEBUG] Releasing Dave
Hello, Dave

CMakeLists.txt

cmake_minimum_required(VERSION 3.28)
# set language to c++
project(HelloJNI.java LANGUAGES CXX)

find_package(Java REQUIRED COMPONENTS Development)
find_package(JNI REQUIRED)

# 2. Define locations for outputs
set(JAVA_SOURCE "${CMAKE_CURRENT_SOURCE_DIR}/HelloJNI.java")
set(CLASS_OUTPUT_DIR "${CMAKE_CURRENT_BINARY_DIR}/classes")
set(HEADER_OUTPUT_DIR "${CMAKE_CURRENT_BINARY_DIR}/generated_headers")

# Create these directories before running commands
file(MAKE_DIRECTORY ${CLASS_OUTPUT_DIR} ${HEADER_OUTPUT_DIR})

# 3. Define the path of the generated header
# Note: If your Java class is inside a package (e.g., com.example), 
# the file will be com_example_HelloJNI.h.
# Since we are using default package here:
set(GENERATED_HEADER "${HEADER_OUTPUT_DIR}/HelloJNI.h")

# 4. Create a custom command to run javac -h
add_custom_command(
    OUTPUT ${GENERATED_HEADER} "${CLASS_OUTPUT_DIR}/HelloJNI.class"
    COMMAND ${Java_JAVAC_EXECUTABLE} 
            -h ${HEADER_OUTPUT_DIR} 
            -d ${CLASS_OUTPUT_DIR} 
            ${JAVA_SOURCE}
    DEPENDS ${JAVA_SOURCE}
    COMMENT "Compiling Java source and generating JNI header"
    VERBATIM
)

# 5. Build the Native Library (Shared Object)
# We add GENERATED_HEADER to the sources so CMake knows to run the custom command first
add_library(hello SHARED HelloJNIImpl.cpp ${GENERATED_HEADER})

# 6. Include Directories
target_include_directories(hello PRIVATE
    ${JNI_INCLUDE_DIRS}   # Where jni.h and jni_md.h live
    ${HEADER_OUTPUT_DIR}  # Where our generated HelloJNI.h lives
)

# 7. Link Libraries (Optional on Linux usually, but good practice)
target_link_libraries(hello ${JNI_LIBRARIES})

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

#!/bin/bash
if [[ ! -v JAVA_HOME ]]; then
    JAVA_HOME="$(java -XshowSettings:properties -version 2>&1 > /dev/null \
                       | grep 'java.home' \
                       | awk '{print $3}')"
fi
if [[ ! -d "$JAVA_HOME/include" ]]; then
    >&2 echo "${BASH_SOURCE[0]}:${BASH_LINENO[0]}:No include/ directory under JAVA_HOME=$JAVA_HOME?"
    exit 1
fi
env JAVA_HOME="$JAVA_HOME" cmake -S . -B build/ --fresh
cmake --build build/
env -C ./build java -cp classes -Djava.library.path=. HelloJNI