Implementing Unix Domain Socket Communication in Java with junixsocket

The article begins by noting the author's previous work on Unix socket support in Go and introduces the need for a similar implementation in Java, which requires adding an external dependency.

<dependency>
    <groupId>com.kohlschutter.junixsocket</groupId>
    <artifactId>junixsocket-core</artifactId>
    <version>2.3.3</version>
</dependency>

Java Implementation

Compared with Go, the Java code is more verbose, but the author is comfortable with Java.

Server

The server mirrors the functionality described earlier: it starts, listens for connections, reads messages, and replies.

package com.funtest.unix_socket;

import org.newsclub.net.unix.AFUNIXServerSocket;
import org.newsclub.net.unix.AFUNIXSocketAddress;

import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;

/**
 * Unix Socket server
 */
public class UnixSocketServer {
    public static void main(String[] args) throws IOException {
        File socketFile = new File("/Users/oker/logs/temp/unix_socket_example"); // server listening file
        try (ServerSocket server = AFUNIXServerSocket.newInstance()) { // create server
            server.bind(new AFUNIXSocketAddress(socketFile)); // bind file
            System.out.println("Listening file: " + socketFile);
            while (true) {
                try (Socket socket = server.accept()) { // accept client
                    try (BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                         PrintWriter writer = new PrintWriter(socket.getOutputStream(), true)) { // reply to client
                        String line;
                        while ((line = reader.readLine()) != null) { // read client messages
                            System.out.println("Received: " + line);
                            writer.println("Reply: " + line); // send reply
                        }
                    }
                }
            }
        }
    }
}

Client

The client simply sends a message and prints the response.

package com.funtest.unix_socket;

import org.newsclub.net.unix.AFUNIXSocket;
import org.newsclub.net.unix.AFUNIXSocketAddress;

import java.io.*;
import java.net.Socket;

/**
 * Unix Socket client
 */
public class UnixScoketClient {
    public static void main(String[] args) throws IOException {
        File socketFile = new File("/Users/oker/logs/temp/unix_socket_example"); // server listening file
        try (Socket socket = AFUNIXSocket.newInstance()) { // create client
            socket.connect(new AFUNIXSocketAddress(socketFile)); // connect to server
            try (BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                 PrintWriter writer = new PrintWriter(socket.getOutputStream(), true)) { // send message
                writer.println("Hello FunTester");
                String response = reader.readLine(); // read server response
                System.out.println("Received response: " + response);
            }
        }
    }
}

Testing Interoperability

Running the Java server and client shows the server printing received messages and the client printing the echoed response. When the same client talks to a Go server, the Go server initially does not send a response, resulting in a null reply on the Java side.

Listening file: /Users/oker/logs/temp/unix_socket_example
Received message: Hello FunTester
Received message: Hello FunTester
Received message: Hello FunTester

Received response: Reply: Hello FunTester

Process exited with code 0

To make the Go server return a response, the author adds a simple handler that reads the incoming data and writes back a greeting.

//handleConnection processes a connection
func handleConnection(conn net.Conn) {
    defer conn.Close()           // close connection
    buffer := make([]byte, 1024) // create buffer
    n, err := conn.Read(buffer)   // read data
    if err != nil {              // read error
        fmt.Println("Read error:", err)
        return
    }
    conn.Write([]byte("Hello FunTester"))        // write data
    fmt.Printf("Received message: %s\n", string(buffer[:n])) // print received data
}