hello.cpp

#include <jni/jni.hpp>

// The global references created during registration are purposefully leaked. Due to
// the design of Java/JNI, there is virtually no way to reliably release them. See
// http://bleaklow.com/2006/02/18/jni_onunload_mostly_useless.html for details.

static void RegisterLowLevel(JavaVM* vm)
   {
    jni::JNIEnv& env = jni::GetEnv(*vm);

    static jni::jclass* systemClass = jni::NewGlobalRef(env, &jni::FindClass(env, "java/lang/System")).release();
    static jni::jclass* printStreamClass = jni::NewGlobalRef(env, &jni::FindClass(env, "java/io/PrintStream")).release();

    static jni::jfieldID& outFieldID = jni::GetStaticFieldID(env, *systemClass, "out", "Ljava/io/PrintStream;");
    static jni::jmethodID& printlnMethodID = jni::GetMethodID(env, *printStreamClass, "println", "(Ljava/lang/String;)V");

    auto greet = [] (jni::JNIEnv* env, jni::jobject*, jni::jarray<jni::jobject>* args)
       {
        jni::NullCheck(*env, args);

        jni::jstring& name = jni::SafeDereference(*env,
            reinterpret_cast<jni::jstring*>(
                jni::GetObjectArrayElement(*env, *args, 0)));

        auto greeting = std::u16string(u"Hello, ") +
            std::get<0>(jni::GetStringChars(*env, name)).get() +
            u" (Native Low-Level)";

        jni::CallMethod<void>(*env,
            jni::GetStaticField<jni::jobject*>(*env, *systemClass, outFieldID),
            printlnMethodID,
            jni::NewString(*env, greeting));
       };

    jni::RegisterNatives(env, jni::FindClass(env, "LowLevelGreeter"),
        jni::MakeNativeMethod("greet", "([Ljava/lang/String;)V", greet));
   }

static void RegisterHighLevel(JavaVM* vm)
   {
    struct Greeter     { static constexpr auto Name() { return "HighLevelGreeter"; } };
    struct System      { static constexpr auto Name() { return "java/lang/System"; } };
    struct PrintStream { static constexpr auto Name() { return "java/io/PrintStream"; } };

    jni::JNIEnv& env { jni::GetEnv(*vm) };

    static auto& system = jni::Class<System>::Singleton(env);
    static auto& printStream = jni::Class<PrintStream>::Singleton(env);

    static auto out = system.GetStaticField<jni::Object<PrintStream>>(env, "out");
    static auto println = printStream.GetMethod<void (jni::String)>(env, "println");

    auto greet = [] (jni::JNIEnv& env, jni::Object<Greeter>&, jni::Array<jni::String>& args)
       {
        system.Get(env, out).Call(env, println,
            jni::Make<jni::String>(env,
                u"Hello, " + jni::Make<std::u16string>(env, args.Get(env, 0)) + u" (Native High-Level)"));
       };

    jni::RegisterNatives(env, *jni::Class<Greeter>::Find(env),
         jni::MakeNativeMethod("greet", greet));
   }

extern "C" JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm, void*)
   {
    RegisterLowLevel(vm);
    RegisterHighLevel(vm);
    return jni::Unwrap(jni::jni_version_1_2);
   }