Support native code only usage of JVM

[jpphillips]

Here is what I have:

HelloWorld.java

package com.test;

public class HelloWorld {

    public void sayHello(String msg) {
        System.out.println("Hello from Java: " + msg);
    }
}

Compile it:

$ javac com/test/HelloWorld.java

The Google JNI-BIND C++ code:

#include "jni_bind_release.h"


// 1: Setup JNI Bind in your OnLoad call (needed only once).
std::unique_ptr < jni::JvmRef < jni::kDefaultJvm >> jvm;

extern "C" {
    JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM * pjvm, void * ) {
        jvm.reset(new jni::JvmRef < jni::kDefaultJvm > (pjvm));
        return JNI_VERSION_1_6;
    }
}

int main() {

    // 2: Define the class
    static constexpr jni::Class kClass {
        "com.test.HelloWorld",
        jni::Method {
            "sayHello",
            jni::Return < void > {},
            jni::Params < jstring > {}
        }
    };

    // 3: Use the class
    jni::LocalObject < kClass > obj {}; // Constructs new instance.

    obj("sayHello", "a cool string from C++");

    return 0;
}

I have the JNI-BIND sources inside a local jni_bind folder. So to compile the code above I do on a Mac:

$ tree -L 1
.
├── HelloWorld.cpp
├── com
└── jni_bind

$ tree com
com
└── test
    ├── HelloWorld.class
    └── HelloWorld.java

C++ compilation:

$ clang++ -std=c++17 -I./jni_bind -I"$JAVA_HOME/include"
-I"$JAVA_HOME/include/darwin/" HelloWorld.cpp -o HelloWorld

And it compiles fine, great!

But now when I run it, I get a segmentation fault because it is not able to find the ./com/test/HelloWorld.class file to load.

$ ./HelloWorld
zsh: segmentation fault  ./HelloWorld

Setting the CLASSPATH does not work either:

$ CLASSPATH=. ./HelloWorld
zsh: segmentation fault  CLASSPATH=. ./HelloWorld

Perhaps I have to set JavaVMOption with -Djava.class.path=. to pass to its embedded JVM? But how would I go about doing that?

[jwhpryor]

Ah wow, so, it's great that you're able to compile and run, although, it looks like you're driving control of the binary entirely through native.

While this is actually possible, I don't have the sample released yet. Unfortunately, I think you need to initiate control from Java, and send it to native.

If you really want this as native only (i.e. you deliberately wanted control to emanate from native), please change the bug name and file it against me. It would be helpful for me to understand what folks would find most useful to help adoption of the library.

[coralblocks]

Very nice project! Congratulations for the hard and meticulous work you did there. I'm maintaining a GitHub repo (JavaToCppAndBack) with an assortment of examples of Java to C++ and C++ to Java integration. I would love to include a JNI-BIND HelloWorld there eventually!

[jwhpryor]

That is very kind, I appreciate the compliment! It has been a tremendous amount of work so it is nice that it is getting closer to being in people's hands. I'd love to be used by other projects, although to start i imagine that would be done most simply through the Bazel/WORKSPACE mechanisms.

If you would like to file a new issues for "Distribution to third party projects" that would be great, although, realistically I'm afraid I need to address other issues for the 1.0 release. In the meantime, any mentions/stars help the project gain traction which I would be very grateful for 🙂

[ltungv]

For anyone trying to use this project for native code only usage and needs to get the classpath to work, I guess we can initiate the JavaVM ourselves with the correct classpath.

void run(JavaVM *jvm) {
  jni::JvmRef<jni::kDefaultJvm> jvm_ref = jni::JvmRef<jni::kDefaultJvm>(jvm);
  // ...
}

int main(int argc, char **argv) {
  std::string classpath = "-Djava.class.path=example.jar";

  JavaVMOption options[1];
  options[0].optionString = classpath.data();

  JavaVMInitArgs init_args = {
      .version = JNI_VERSION,
      .nOptions = 1,
      .options = options,
      .ignoreUnrecognized = false,
  };

  JavaVM *jvm;
  JNIEnv *env;
  if (JNI_CreateJavaVM(&jvm, (void **)&env, (void *)&init_args) != JNI_OK) {
    return 1;
  }

  run(jvm);
  return 0;
}

I'm not 100% percent sure if this is the correct use of the library, but it works for my purpose of using Java classes in C++.

[jwhpryor]

Ah, this is similar to the unreleased binary I have. If you manage to add it as a binary in the tree as a pull request I'd love to add your work to the tree. Realistically I don't think I have time to do this myself.

(Also, apologies, I missed this comment when you posted it).