Allow specifying bind port via command line

adds #140

proxy/build.gradle

@@ -38,6 +38,7 @@ dependencies {
     compile "org.apache.logging.log4j:log4j-core:${log4jVersion}"
     compile "org.apache.logging.log4j:log4j-slf4j-impl:${log4jVersion}"
 
+    compile 'net.sf.jopt-simple:jopt-simple:5.0.4' // command-line options
     compile 'net.minecrell:terminalconsoleappender:1.1.1'
     runtime 'net.java.dev.jna:jna:4.5.2' // Needed for JLine
     runtime 'com.lmax:disruptor:3.4.2' // Async loggers

proxy/src/main/java/com/velocitypowered/proxy/ProxyOptions.java

@@ -0,0 +1,44 @@
+package com.velocitypowered.proxy;
+
+import joptsimple.OptionParser;
+import joptsimple.OptionSet;
+import joptsimple.OptionSpec;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+import java.io.IOException;
+import java.util.Arrays;
+
+public final class ProxyOptions {
+  private static final Logger logger = LogManager.getLogger(ProxyOptions.class);
+  private final boolean help;
+  private final @Nullable Integer port;
+
+  ProxyOptions(final String[] args) {
+    final OptionParser parser = new OptionParser();
+
+    final OptionSpec<Void> help = parser.acceptsAll(Arrays.asList("h", "help"), "Print help").forHelp();
+    final OptionSpec<Integer> port = parser.acceptsAll(Arrays.asList("p", "port"), "Specify the bind port to be used. The configuration bind port will be ignored.").withRequiredArg().ofType(Integer.class);
+    final OptionSet set = parser.parse(args);
+
+    this.help = set.has(help);
+    this.port = port.value(set);
+
+    if (this.help) {
+      try {
+        parser.printHelpOn(System.out);
+      } catch (final IOException e) {
+        logger.error("Could not print help", e);
+      }
+    }
+  }
+
+  boolean isHelp() {
+    return this.help;
+  }
+
+  public @Nullable Integer getPort() {
+    return this.port;
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/Velocity.java

@@ -17,7 +17,13 @@ public class Velocity {
   public static void main(String... args) {
     long startTime = System.currentTimeMillis();
 
-    VelocityServer server = new VelocityServer();
+    final ProxyOptions options = new ProxyOptions(args);
+
+    if (options.isHelp()) {
+      return;
+    }
+
+    VelocityServer server = new VelocityServer(options);
     server.start();
 
     Runtime.getRuntime().addShutdownHook(new Thread(() -> server.shutdown(false), "Shutdown thread"));

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -70,6 +70,7 @@ public class VelocityServer implements ProxyServer {
       .registerTypeHierarchyAdapter(GameProfile.class, new GameProfileSerializer())
       .create();
 
+  private final ProxyOptions options;
   private @MonotonicNonNull ConnectionManager cm;
   private @MonotonicNonNull VelocityConfiguration configuration;
   private @MonotonicNonNull NettyHttpClient httpClient;
@@ -88,6 +89,10 @@ public class VelocityServer implements ProxyServer {
   private @MonotonicNonNull VelocityScheduler scheduler;
   private final VelocityChannelRegistrar channelRegistrar = new VelocityChannelRegistrar();
 
+  public VelocityServer(final ProxyOptions options) {
+    this.options = options;
+  }
+
   public KeyPair getServerKeyPair() {
     if (serverKeyPair == null) {
       throw new AssertionError();
@@ -181,7 +186,13 @@ public class VelocityServer implements ProxyServer {
     // init console permissions after plugins are loaded
     console.setupPermissions();
 
+    final Integer port = this.options.getPort();
+    if (port != null) {
+      logger.debug("Overriding bind port to {} from command line option", port);
+      this.cm.bind(new InetSocketAddress(configuration.getBind().getHostString(), port));
+    } else {
       this.cm.bind(configuration.getBind());
+    }
 
     if (configuration.isQueryEnabled()) {
       this.cm.queryBind(configuration.getBind().getHostString(), configuration.getQueryPort());