nng-java

An idiomatic Java wrapper around nng using JNA.

Goals

1. Wrap nng in a way that can be used "naturally" via Java idioms so users can quickly spin up sockets, dial/listen, and send/receive data without all the pomp and circumstance seen in the C code (because Java has enough pomp and circumstance to deal with, amiright).
2. Keep the wrapping (via JNA) at just the public API surface allowing it to track releases that might have internal (i.e. nni_*) changes only.
3. Use a (somewhat) strong typing using JNA's extensible type system to keep usage of the library somewhat safe from memory issues.
4. On the topic of memory issues, provide guardrails around protecting the native memory managed by nng from Java's garbage collector.
5. Lastly, provide all the scalability constructs from nng (e.g. Aio) in a way that let's Java developers utilize nng's performance and thread safety.

My downline goal is to write my own protocol in an nng extension and use it in a Java application...but we'll see if/when I get there.

Current State

As of 8 Feb 2021, the core nng primitives are implemented allowing a Java developer to:

• instantiate and use Socket's that implement the Scalability Protocols
• use Context's for multi-threaded, async use of a single Socket
• use the Aio framework in 3 different ways for async operations:
  1. in blocking (i.e. non-async) manner...the trivial use of a Context
  2. in non-blocking manner using the Java CompletableFuture API
  3. in an event-driver manner using AioCallbacks
• allocated Messages without worrying too much about memory management in some simpler cases

The AIO stuff needs some tire kicking, is a bit messy, and I'm not too happy with it yet. In short, the design lets you keep as much state in the JVM as possible (for callbacks and the like), but seems overly complicated. (Maybe it's just that way because of Java?)

Things not yet working or implemented:

• the http client api
• the http server api
• stream wrangling
• TLS support has not been tested

It's not quite clear how much of the HTTP stuff I want to implement, buf if there's anything required for WebSocket support it will be on the list.

Building

1. I've done nothing (yet) to package nng, so you'll need to build and install that yourself. It's really not that hard.
2. A simple gradlew build should suffice, using the supplied Gradle wrapper scripts.

Using

Finding the nng Library

Once you have nng installed somewhere, if it's not installed in a sane default location, you might need to set one of the following to point JNA to the correct location:

• Java Property: jna.library.path
• Environment variable: JNA_LIBRARY_PATH

Note: The Java Property will override any value set in the environment

Using the native API

In general, most users should only need to utilize classes in the io.sisu.nng and io.sisu.nng.aio namespaces. The direct calling of the nng api should be wrapped for you.

For those looking to do low-level nng programming, most of the nng api is accessible via io.sisu.nng.Nng. Everything you need for low-level usage should be in the io.sisu.nng.internal namespace, with a few exceptions.

The Java NNG API

As part of my Java-fication of nng, I'm currently marrying Sockets to their protocols explicitly. (Maybe I'll go full on OOP and go overboard here...tbd.)

For detailed examples, check the demo project. The goal is to provide translations of the existing NNG demos into Java versions to illustrate any similarities or differences in usage (as well to provide a way to validate if the Java implementation is working).

A simple example of how the Java API works:

package io.sisu.nng;
import io.sisu.nng.reqrep.*;

import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;

class Example {
    public static void main(String[] argv) throws NngException {
        final String url = "inproc://example";
        Socket req = new Req0Socket();
        Socket rep = new Rep0Socket();

        rep.listen(url);
        req.dial(url);

        Message msg = new Message();
        msg.append("hey man".getBytes(StandardCharsets.UTF_8));
        req.sendMessage(msg);

        // After sending, we no longer own the Message
        assert(!msg.isValid());

        Message msg2 = rep.receiveMessage();
        assert(msg2.isValid());

        String msg2Str = Charset.defaultCharset()
                .decode(msg2.getBody()).toString();
        assert("hey man".equalsIgnoreCase(msg2Str));
        System.out.println("Rep socket heard: " + msg2Str);
    }
}

And a simple example of using a Context for asynchronous operations that is the Java analog to nng's async server.c demo code:

package io.sisu.nng.demo.async;

import io.sisu.nng.*;

import java.util.ArrayList;
import java.util.List;

/**
 * Java implementation of the NNG async demo server program.
 *
 * Unlike the C demo, the Java version uses the asynchronous event handler approach provided via
 * the io.sisu.nng.aio.Context class.
 */
public class Server {
    private static final int PARALLEL = 128;
    private final String url;

    public Server(String url) {
        this.url = url;
    }

    public void start() throws Exception {
        try (Socket socket = new Rep0Socket()) {
            // keep Context references to prevent any possible gc
            List<Context> contexts = new ArrayList<>(PARALLEL);

            for (int i = 0; i < PARALLEL; i++) {
                Context ctx = new Context(socket);
                ctx.setRecvHandler((ctxProxy, msg) -> {
                    try {
                        int when = msg.trim32Bits();
                        ctxProxy.sleep(when);
                        ctxProxy.put("reply", msg);
                    } catch (NngException e) {
                        ctxProxy.receive();
                    }
                });
                ctx.setSendHandler((ctxProxy) -> ctxProxy.receive());
                ctx.setWakeHandler((ctxProxy) -> ctxProxy.send((Message) ctxProxy.get("reply")));

                // perform the initial receive operation to start the "event loop"
                ctx.receiveMessage();
            }

            socket.listen(this.url);
            System.out.println("Listening on " + this.url);

            Thread.sleep(1000 * 60 * 20);
        }
    }

    public static void main(String[] argv) {
        if (argv.length != 1) {
            System.err.println(String.format("Usage: server <url>"));
            System.exit(1);
        }

        Server server = new Server(argv[0]);
        try {
            server.start();
        } catch (Exception e) {
            System.err.println(e);
            e.printStackTrace();
        }
    }
}

Note: you'll notice the above uses the baked in event handling capabilities in the Context class. For a more like-for-like example, see the "raw" demo in the demos project that shows how to use your own AioCallback to make a like-for-like version of the nng raw.c demo.

About Memory Management and Safety

Currently, the bare minimum to safely allocate/free native memory for nng constructs exists, but it's not optimal in preventing things like native heap fragmentation if a developer just lets the JVM garbage collect Messages and call the message's free() method.

While it will probably only occur in long-running, server-based applications, using the tire-kicking benchmarks project it's been observed that services running that allocate hundreds of megabytes or gigabytes of messages via the Java api that do not manually call free() and rely on the garbage collector will experience excessive memory pressure as nng_msg objects stay allocated on the native heap, causing it to grow, and leading to fragmentation that is not recoverable.

tl;dr: if you know you're done with a Message, just call free() for now.