Initial commit

.gitignore

+# Project-specific
+# none (for now)
+
+# Mac file finder metadata
+.DS_Store
+# MS Office temporary file
+~*
+# Emacs backup file
+*~
+
+# Java files
+*.class
+javadoc/
+
+# JetBrains (IntelliJ IDEA) files
+.idea/
+out/
+bin/
+*.iml
+*.iws
+*.ipr
+
+# Eclipse files
+bin/
+.settings/
+.classpath
+.project
+
+# Visual Studio / VS Code files
+.vs*/
+*.rsuser
+*.suo
+*.user
+*.userosscache
+*.sln.docstates
+*_i.c
+*_p.c
+*_h.h
+*.ilk
+*.meta
+*.obj
+*.iobj
+*.pch
+*.pdb
+*.ipdb
+*.pgc
+*.pgd
+*.rsp
+*.sbr
+*.tlb
+*.tli
+*.tlh
+*.tmp_proj
+*_wpftmp.csproj
+*.log
+*.vspscc
+*.vssscc
+.builds
+*.pidb
+*.svclog
+*.scc
+
+# Netbeans files
+nbproject/private/
+build/
+nbbuild/
+dist/
+nbdist/
+nbactions.xml
+nb-configuration.xml
+
+# Xcode files
+*.xcodeproj/
+xcuserdata/
+.build/
+
+# Maven
+target/
+
+# Miscellaneous
+tmp/
+*.tmp
+*.bak
+*.bk
+*.swp
+*.gcno
+

pom.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>edu.unl.cse.csce361</groupId>
+    <artifactId>RelayServer</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <build>
+        <plugins>
+            <plugin>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <version>3.8.1</version>
+                <configuration>
+                    <source>8</source>
+                    <target>8</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <dependencies>
+
+        <!-- FOR PRODUCTION CODE -->
+
+            <!-- n/a -->
+
+        <!-- FOR TEST CODE -->
+
+        <!-- https://mvnrepository.com/artifact/junit/junit -->
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <version>4.13</version>
+            <scope>test</scope>
+        </dependency>
+
+    </dependencies>
+
+</project>
\ No newline at end of file

src/main/java/edu/unl/cse/csce361/relay/RelayServer.java

+package edu.unl.cse.csce361.relay;
+
+import java.io.*;
+import java.net.ServerSocket;
+import java.net.Socket;
+import java.util.HashSet;
+import java.util.NoSuchElementException;
+import java.util.Scanner;
+import java.util.Set;
+
+import static java.lang.System.exit;
+
+public class RelayServer {
+    public static void main(String[] args) {
+        if (args.length == 0 || args.length > 2) {
+            printUsage();
+            exit(1);
+        }
+        int portNumber = 0;
+        try {
+            portNumber = Integer.parseInt(args[0]);
+        } catch (NumberFormatException ignored) {
+            printUsage();
+            exit(1);
+        }
+        boolean alternatingClients = false;
+        if (args.length == 2 && args[1].equalsIgnoreCase("alternatingClients")) {
+            alternatingClients = true;
+        }
+        new RelayServer().run(portNumber, alternatingClients);
+    }
+
+    private static void printUsage() {
+        System.err.println("Usage: java edu.unl.cse.csce361.relay.RelayServer <portNumber> (alternatingClients)");
+        System.err.println("  Where <portNumber> is the required network port the relay server will listen to,");
+        System.err.println("  and `alternatingClients` is the optional argument to prompt one client to be \"first.\"");
+    }
+
+    private Set<ClientHandler> clients;
+
+    public RelayServer() {
+        clients = new HashSet<>();
+    }
+
+    public void run(int portNumber, boolean alternatingClients) {
+        try {
+            ServerSocket serverSocket = new ServerSocket(portNumber);
+//            int counter = 0;
+            //noinspection InfiniteLoopStatement
+            while (true) {
+                Socket clientSocket = serverSocket.accept();
+//                System.out.print("Connection #" + ++counter);
+                ClientHandler clientHandler = new ClientHandler(clientSocket, clients);
+                clients.forEach(client -> client.addRelaySink(clientHandler));
+//                System.out.println(" connected to " + clients.size() + " others.");
+                clients.add(clientHandler);
+                clientHandler.start();
+                if (alternatingClients && clients.size() == 1) {
+                    clientHandler.sendMessage("You are the first client.");
+                }
+            }
+        } catch (IOException ioException) {
+            ioException.printStackTrace();
+        }
+    }
+
+    private class ClientHandler extends Thread {
+        private final BufferedReader in;
+        private final PrintStream out;
+        private final Set<ClientHandler> relaySinks;
+
+        public ClientHandler(Socket clientSocket, Set<ClientHandler> otherClients) throws IOException {
+            in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
+            out = new PrintStream(clientSocket.getOutputStream());
+            relaySinks = new HashSet<>(otherClients);
+        }
+
+        @SuppressWarnings("UnusedReturnValue")
+        public boolean addRelaySink(ClientHandler otherClient) {
+            return relaySinks.add(otherClient);
+        }
+
+        @SuppressWarnings("UnusedReturnValue")
+        public boolean removeRelaySink(ClientHandler otherClient) {
+            return relaySinks.remove(otherClient);
+        }
+
+        public void sendMessage(String message) {
+            out.println(message);
+        }
+
+        @SuppressWarnings("unused")
+        public void sendObject(Object object) {
+            out.print(object);
+        }
+
+        public void run() {
+//            System.out.println("Thread started.");
+            Scanner scanner = new Scanner(in);
+            boolean running = true;
+            while (running) {
+                try {
+                    String message = scanner.nextLine();
+                    relaySinks.forEach(sink -> sink.sendMessage(message));
+                } catch (NoSuchElementException exception) {
+                    // the client dropped, remove this handler from the other handlers and terminate
+                    relaySinks.forEach((sink -> sink.sendMessage("Remote client dropped.")));
+                    relaySinks.forEach(sink -> sink.removeRelaySink(this));
+                    clients.remove(this);   // accessing outer class
+                    running = false;
+                }
+            }
+        }
+    }
+}