вывод только определенных System.property

build.gradle

@@ -15,36 +15,17 @@ repositories {
 }
 
 ext {
-    slf4j_version = '1.7.30'
+    libs_dir = 'libs'
-    logback_version = '1.2.3'
+    jline_version = '3.14.1'
-    netty_version = '4.1.22.Final'
-
-    library = [
-            guice: ['com.google.inject:guice:4.1.0'],
-            logger: ["ch.qos.logback:logback-core:$logback_version",
-                     "ch.qos.logback:logback-classic:$logback_version"],
-            lombok: ['org.projectlombok:lombok:1.18.2'],
-            netty: ["io.netty:netty-transport:$netty_version",
-                    "io.netty:netty-codec:$netty_version",
-                    "io.netty:netty-handler:$netty_version" /*for logger handler*/],
-            slf4j: ["org.slf4j:slf4j-api:$slf4j_version",
-                    "org.slf4j:jcl-over-slf4j:$slf4j_version"],
-            commons: ['commons-io:commons-io:2.6']
-    ]
 }
 
 dependencies {
-    /* LOGGER */
+    compile group: 'org.jline', name: 'jline',                version: jline_version
-    implementation library.slf4j
+    compile group: 'org.jline', name: 'jline-terminal-jansi', version: jline_version
-    implementation library.logger
+}
 
-    /* LOMBOK */
+task copyDeps(type: Copy) {
-    annotationProcessor library.lombok
+    into project.buildDir.toPath().resolve('libs').resolve(libs_dir)
-    compileOnly library.lombok
+    from configurations.runtime
-
+}
-    /* COMPONENTS */
+jar.dependsOn(copyDeps)
-    implementation library.guice
-    implementation library.netty
-    implementation library.commons
-    implementation project(':protocol')
-}

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
 # suppress inspection "UnusedProperty" for whole file
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.5.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-5.3-all.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists

settings.gradle

@@ -1,4 +1 @@
-rootProject.name = projectName
+rootProject.name = projectName
-
-include ':protocol'
-project(':protocol').projectDir = new File(settingsDir, '../mc-protocol')

src/main/java/mc/server/Main.java

@@ -1,16 +1,48 @@
 package mc.server;
 
-import com.google.inject.Guice;
+import mc.server.shell.CommandService;
-import com.google.inject.Injector;
+import mc.server.shell.CommandServiceImpl;
-import mc.server.config.NetworkModule;
+import mc.server.shell.SystemPropertiesCommand;
-import mc.server.network.Server;
+import org.jline.builtins.CommandRegistry;
+import org.jline.reader.*;
+
+import java.io.PrintWriter;
 
 public class Main {
 
     public static void main(String[] args) {
-        final Injector injector = Guice.createInjector(new NetworkModule());
+        System.out.println("mc-server");
 
-        final Server server = injector.getInstance(Server.class);
+        CommandService commandService = new CommandServiceImpl();
-        server.start("127.0.0.1", 25565);
+        commandService.register("system-properties", new SystemPropertiesCommand());
+
+        final Completer completer = CommandRegistry.compileCompleters(commandService);
+
+        final LineReader reader = LineReaderBuilder.builder()
+                .completer(completer)
+                .build();
+
+        try {
+            final CommandRegistry.CommandSession session = new CommandRegistry.CommandSession(reader.getTerminal());
+            final PrintWriter terminalWriter = reader.getTerminal().writer();
+            //noinspection InfiniteLoopStatement
+            while (true) {
+                final String line = reader.readLine().trim();
+
+                final ParsedLine parsedLine = reader.getParser().parse(line, 0);
+                final String commandName = parsedLine.word();
+                final String[] argv = parsedLine.words().subList(1, parsedLine.words().size()).toArray(new String[0]);
+
+                if (commandService.hasCommand(commandName)) {
+                    commandService.execute(session, commandName, argv);
+                }
+
+                terminalWriter.println("line = " + String.join(", ", parsedLine.words()));
+            }
+        } catch (UserInterruptException ignore) {
+            // ignore
+        } catch (Exception e) {
+            e.printStackTrace();
+        }
     }
 }

src/main/java/mc/server/config/NetworkModule.java

@@ -1,65 +0,0 @@
-package mc.server.config;
-
-import com.google.inject.AbstractModule;
-import com.google.inject.Provides;
-import com.google.inject.Singleton;
-import com.google.inject.name.Named;
-import com.google.inject.name.Names;
-import io.netty.bootstrap.ServerBootstrap;
-import io.netty.channel.ChannelHandler;
-import io.netty.channel.EventLoopGroup;
-import io.netty.channel.nio.NioEventLoopGroup;
-import io.netty.channel.socket.nio.NioServerSocketChannel;
-import io.netty.handler.logging.LoggingHandler;
-import mc.protocol.PacketDirection;
-import mc.protocol.io.coder.ProtocolDecoder;
-import mc.protocol.io.coder.ProtocolEncoder;
-import mc.protocol.io.coder.ProtocolSplitter;
-import mc.server.network.Server;
-import mc.server.network.impl.ChannelInitializer;
-import mc.server.network.impl.NettyServer;
-import mc.server.network.impl.codec.PacketDecoder;
-import mc.server.network.impl.codec.PacketEncoder;
-import mc.server.network.impl.codec.PacketSplitter;
-import mc.server.network.impl.handler.HandshakeHandler;
-
-import java.util.LinkedHashMap;
-import java.util.Map;
-
-public class NetworkModule extends AbstractModule {
-
-    @Override
-    protected void configure() {
-        bind(Server.class).to(NettyServer.class).in(Singleton.class);
-        bind(EventLoopGroup.class).annotatedWith(Names.named("bossGroup")).toInstance(new NioEventLoopGroup(1));
-        bind(EventLoopGroup.class).annotatedWith(Names.named("workerGroup")).toInstance(new NioEventLoopGroup());
-    }
-
-    @Provides
-    @Singleton
-    ServerBootstrap serverBootstrap(@Named("bossGroup") EventLoopGroup bossGroup,
-                                    @Named("workerGroup") EventLoopGroup workerGroup,
-                                    ChannelInitializer channelChannelInitializer) {
-        ServerBootstrap bootstrap = new ServerBootstrap();
-
-        bootstrap.group(bossGroup, workerGroup)
-                .channel(NioServerSocketChannel.class)
-                .childHandler(channelChannelInitializer);
-
-        return bootstrap;
-    }
-
-    @Provides
-    @Named("channelHandlerMap")
-    Map<String, ChannelHandler> channelHandlerMap(HandshakeHandler handshakeHandler) {
-        final Map<String, ChannelHandler> map = new LinkedHashMap<>();
-
-        map.put("logger", new LoggingHandler());
-        map.put("packet_splitter", new PacketSplitter(new ProtocolSplitter()));
-        map.put("packet_decoder", new PacketDecoder(new ProtocolDecoder(PacketDirection.SERVER_BOUND)));
-        map.put("packet_encoder", new PacketEncoder(new ProtocolEncoder(PacketDirection.CLIENT_BOUND)));
-        map.put("handshake_handler", handshakeHandler);
-
-        return map;
-    }
-}

src/main/java/mc/server/network/Server.java

@@ -1,6 +0,0 @@
-package mc.server.network;
-
-public interface Server {
-
-    void start(String host, int port);
-}

src/main/java/mc/server/network/impl/ChannelInitializer.java

@@ -1,29 +0,0 @@
-package mc.server.network.impl;
-
-import com.google.inject.Inject;
-import com.google.inject.Provider;
-import com.google.inject.Singleton;
-import com.google.inject.name.Named;
-import io.netty.channel.ChannelHandler;
-import io.netty.channel.ChannelPipeline;
-import io.netty.channel.socket.SocketChannel;
-
-import java.util.Map;
-
-@Singleton
-public class ChannelInitializer extends io.netty.channel.ChannelInitializer<SocketChannel> {
-
-    private final Provider<Map<String, ChannelHandler>> channelHandlerMapProvider;
-
-    @Inject
-    public ChannelInitializer(
-            @Named("channelHandlerMap") Provider<Map<String, ChannelHandler>> channelHandlerMapProvider) {
-        this.channelHandlerMapProvider = channelHandlerMapProvider;
-    }
-
-    @Override
-    protected void initChannel(SocketChannel socketChannel) {
-        final ChannelPipeline pipeline = socketChannel.pipeline();
-        channelHandlerMapProvider.get().forEach(pipeline::addLast);
-    }
-}

src/main/java/mc/server/network/impl/NettyConstants.java

@@ -1,12 +0,0 @@
-package mc.server.network.impl;
-
-import io.netty.util.AttributeKey;
-import lombok.AccessLevel;
-import lombok.NoArgsConstructor;
-import mc.protocol.State;
-
-@NoArgsConstructor(access = AccessLevel.PRIVATE)
-public class NettyConstants {
-
-    public static final AttributeKey<State> ATTR_STATE = AttributeKey.newInstance("ATTR_STATE");
-}

src/main/java/mc/server/network/impl/NettyServer.java

@@ -1,29 +0,0 @@
-package mc.server.network.impl;
-
-import com.google.inject.Inject;
-import com.google.inject.Provider;
-import io.netty.bootstrap.ServerBootstrap;
-import lombok.extern.slf4j.Slf4j;
-import mc.server.network.Server;
-
-
-@Slf4j
-public class NettyServer implements Server {
-
-    @Inject
-    private Provider<ServerBootstrap> serverBootstrapProvider;
-
-    @Override
-    public void start(String host, int port) {
-        try {
-            log.info("Network starting: {}:{}", host, port);
-            serverBootstrapProvider.get()
-                    .bind(host, port)
-                    .sync().channel().closeFuture().sync();
-        } catch (InterruptedException e) {
-            if (log.isTraceEnabled()) {
-                log.trace("{}: {}", e.getClass().getSimpleName(), e.getMessage(), e);
-            }
-        }
-    }
-}

src/main/java/mc/server/network/impl/codec/PacketDecoder.java

@@ -1,43 +0,0 @@
-package mc.server.network.impl.codec;
-
-import io.netty.buffer.ByteBuf;
-import io.netty.channel.ChannelHandlerContext;
-import io.netty.handler.codec.ByteToMessageDecoder;
-import lombok.RequiredArgsConstructor;
-import mc.protocol.Packet;
-import mc.protocol.State;
-import mc.protocol.io.coder.ProtocolDecoder;
-import mc.server.network.impl.io.ByteBufNetInputStream;
-
-import java.util.List;
-import java.util.Objects;
-
-import static mc.server.network.impl.NettyConstants.ATTR_STATE;
-
-@RequiredArgsConstructor
-public class PacketDecoder extends ByteToMessageDecoder {
-
-    private final ProtocolDecoder protocolDecoder;
-
-    @Override
-    public void channelActive(ChannelHandlerContext ctx) throws Exception {
-        ctx.channel().attr(ATTR_STATE).set(State.HANDSHAKING);
-        super.channelActive(ctx);
-    }
-
-    @Override
-    public void channelInactive(ChannelHandlerContext ctx) throws Exception {
-        ctx.channel().attr(ATTR_STATE).set(null);
-        super.channelInactive(ctx);
-    }
-
-    @Override
-    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
-        final State state = Objects.requireNonNull(ctx.channel().attr(ATTR_STATE).get());
-
-        final Packet packet = protocolDecoder.decode(state, new ByteBufNetInputStream(in));
-        if (packet != null) {
-            out.add(packet);
-        }
-    }
-}

src/main/java/mc/server/network/impl/codec/PacketEncoder.java

@@ -1,24 +0,0 @@
-package mc.server.network.impl.codec;
-
-import io.netty.buffer.ByteBuf;
-import io.netty.channel.ChannelHandlerContext;
-import io.netty.handler.codec.MessageToByteEncoder;
-import lombok.RequiredArgsConstructor;
-import mc.protocol.Packet;
-import mc.protocol.State;
-import mc.protocol.io.coder.ProtocolEncoder;
-import mc.server.network.impl.io.ByteBufNetOutputStream;
-
-import static mc.server.network.impl.NettyConstants.ATTR_STATE;
-
-@RequiredArgsConstructor
-public class PacketEncoder extends MessageToByteEncoder<Packet> {
-
-    private final ProtocolEncoder protocolEncoder;
-
-    @Override
-    protected void encode(ChannelHandlerContext ctx, Packet packet, ByteBuf out) {
-        final State state = ctx.channel().attr(ATTR_STATE).get();
-        protocolEncoder.encode(state, packet, new ByteBufNetOutputStream(out));
-    }
-}

src/main/java/mc/server/network/impl/codec/PacketSplitter.java

@@ -1,29 +0,0 @@
-package mc.server.network.impl.codec;
-
-import io.netty.buffer.ByteBuf;
-import io.netty.buffer.Unpooled;
-import io.netty.channel.ChannelHandlerContext;
-import io.netty.handler.codec.ByteToMessageDecoder;
-import lombok.RequiredArgsConstructor;
-import mc.protocol.io.NetInputStream;
-import mc.protocol.io.coder.ProtocolSplitter;
-import mc.server.network.impl.io.ByteBufNetInputStream;
-
-import java.util.List;
-
-@RequiredArgsConstructor
-public class PacketSplitter extends ByteToMessageDecoder {
-
-    private final ProtocolSplitter protocolSplitter;
-
-    @Override
-    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) {
-        final NetInputStream netInputStream = protocolSplitter.split(new ByteBufNetInputStream(in));
-
-        if (netInputStream != null) {
-            byte[] buff = new byte[netInputStream.readableBytes()];
-            netInputStream.readBytes(buff);
-            out.add(Unpooled.wrappedBuffer(buff));
-        }
-    }
-}

src/main/java/mc/server/network/impl/handler/AbstractPacketHandler.java

@@ -1,15 +0,0 @@
-package mc.server.network.impl.handler;
-
-import io.netty.channel.ChannelHandlerContext;
-import io.netty.channel.SimpleChannelInboundHandler;
-import mc.protocol.Packet;
-
-public abstract class AbstractPacketHandler<P extends Packet> extends SimpleChannelInboundHandler<P> {
-
-    @Override
-    protected void channelRead0(ChannelHandlerContext ctx, P msg) throws Exception {
-        channelRead1(ctx, msg);
-    }
-
-    protected abstract void channelRead1(ChannelHandlerContext ctx, P packet) throws Exception;
-}

src/main/java/mc/server/network/impl/handler/HandshakeHandler.java

@@ -1,34 +0,0 @@
-package mc.server.network.impl.handler;
-
-import com.google.inject.Inject;
-import com.google.inject.Provider;
-import io.netty.channel.ChannelHandlerContext;
-import lombok.RequiredArgsConstructor;
-import lombok.extern.slf4j.Slf4j;
-import mc.protocol.State;
-import mc.protocol.handshake.client.HandshakePacket;
-
-import static mc.server.network.impl.NettyConstants.ATTR_STATE;
-
-@Slf4j
-@RequiredArgsConstructor(onConstructor = @__({ @Inject }))
-public class HandshakeHandler extends AbstractPacketHandler<HandshakePacket> {
-
-    private final Provider<StatusHandler> statusHandlerProvider;
-    private final Provider<PingHandler> pingHandlerProvider;
-    private final Provider<LoginHandler> loginHandlerProvider;
-
-    @Override
-    protected void channelRead1(ChannelHandlerContext ctx, HandshakePacket packet) {
-        log.info("{}", packet);
-
-        ctx.channel().attr(ATTR_STATE).set(packet.getNextState());
-
-        if (packet.getNextState() == State.STATUS) {
-            ctx.channel().pipeline().replace("handshake_handler", "status_handler", statusHandlerProvider.get());
-            ctx.channel().pipeline().addAfter("status_handler", "ping_handler", pingHandlerProvider.get());
-        } else if (packet.getNextState() == State.LOGIN) {
-            ctx.channel().pipeline().replace("handshake_handler", "login_handler", loginHandlerProvider.get());
-        }
-    }
-}

src/main/java/mc/server/network/impl/handler/LoginHandler.java

@@ -1,23 +0,0 @@
-package mc.server.network.impl.handler;
-
-import io.netty.channel.ChannelHandlerContext;
-import mc.protocol.login.client.LoginStartPacket;
-import mc.protocol.login.server.DisconnectPacket;
-import mc.protocol.text.Text;
-import mc.protocol.text.TextColor;
-import mc.protocol.text.TextStyle;
-
-public class LoginHandler extends AbstractPacketHandler<LoginStartPacket> {
-
-    @Override
-    protected void channelRead1(ChannelHandlerContext ctx, LoginStartPacket packet) {
-        DisconnectPacket disconnectPacket = new DisconnectPacket();
-        disconnectPacket.setReason(Text.builder()
-                .append(Text.of(TextColor.WHITE, "Server is "))
-                .color(TextColor.RED).style(TextStyle.BOLD).append("NOT ")
-                .color(TextColor.WHITE).append("available.")
-                .build());
-
-        ctx.channel().writeAndFlush(disconnectPacket).channel().disconnect();
-    }
-}

src/main/java/mc/server/network/impl/handler/PingHandler.java

@@ -1,12 +0,0 @@
-package mc.server.network.impl.handler;
-
-import io.netty.channel.ChannelHandlerContext;
-import mc.protocol.status.PingPacket;
-
-public class PingHandler extends AbstractPacketHandler<PingPacket> {
-
-    @Override
-    protected void channelRead1(ChannelHandlerContext ctx, PingPacket packet) {
-        ctx.writeAndFlush(packet).channel().disconnect();
-    }
-}

src/main/java/mc/server/network/impl/handler/StatusHandler.java

@@ -1,50 +0,0 @@
-package mc.server.network.impl.handler;
-
-import io.netty.channel.ChannelHandlerContext;
-import lombok.extern.slf4j.Slf4j;
-import mc.protocol.ProtocolConstant;
-import mc.protocol.dto.ServerInfo;
-import mc.protocol.status.client.StatusServerRequest;
-import mc.protocol.status.server.StatusServerResponse;
-import mc.protocol.text.Text;
-import org.apache.commons.io.IOUtils;
-
-import java.io.IOException;
-import java.util.Base64;
-import java.util.Collections;
-
-@Slf4j
-public class StatusHandler extends AbstractPacketHandler<StatusServerRequest> {
-
-    private static final String FAVICON_HEADER = "data:image/png;base64,";
-
-    @Override
-    protected void channelRead1(ChannelHandlerContext ctx, StatusServerRequest packet) throws Exception {
-        log.info("{}", packet);
-
-        final ServerInfo.Version version = new ServerInfo.Version();
-        version.setName(ProtocolConstant.PROTOCOL_VERSION_VALUE);
-        version.setProtocol(ProtocolConstant.PROTOCOL_VERSION);
-
-        final ServerInfo.PlayersInfo playersInfo = new ServerInfo.PlayersInfo();
-        playersInfo.setMax(20);
-        playersInfo.setOnline(0);
-        playersInfo.setSamplePlayers(Collections.emptyList());
-
-        final ServerInfo serverInfo = new ServerInfo();
-        serverInfo.setVersion(version);
-        serverInfo.setDescription(Text.of("MC-SERVER 1.8.8"));
-        serverInfo.setFaviconBase64(getEmbeddedIconBase64());
-        serverInfo.setPlayersInfo(playersInfo);
-
-        StatusServerResponse response = new StatusServerResponse();
-        response.setServerInfoDto(serverInfo);
-
-        ctx.channel().writeAndFlush(response);
-    }
-
-    private String getEmbeddedIconBase64() throws IOException {
-        return FAVICON_HEADER + Base64.getEncoder()
-                .encodeToString(IOUtils.toByteArray(getClass().getResourceAsStream("/icon.png")));
-    }
-}

src/main/java/mc/server/network/impl/io/ByteBufNetInputStream.java

@@ -1,61 +0,0 @@
-package mc.server.network.impl.io;
-
-import io.netty.buffer.ByteBuf;
-import lombok.RequiredArgsConstructor;
-import mc.protocol.io.NetInputStream;
-
-@RequiredArgsConstructor
-public class ByteBufNetInputStream extends NetInputStream {
-
-    private final ByteBuf byteBuf;
-
-    @Override
-    public void markReadIndex() {
-        byteBuf.markReaderIndex();
-    }
-
-    @Override
-    public void resetReadIndex() {
-        byteBuf.resetReaderIndex();
-    }
-
-    @Override
-    public int readableBytes() {
-        return byteBuf.readableBytes();
-    }
-
-    @Override
-    public byte readByte() {
-        return byteBuf.readByte();
-    }
-
-    @Override
-    public int readBytes(byte[] buffer, int offset, int lengtn) {
-        return byteBuf.readBytes(buffer, offset, lengtn).readableBytes();
-    }
-
-    @Override
-    public int readShort() {
-        return byteBuf.readShort();
-    }
-
-    @Override
-    public int readInt() {
-        return byteBuf.readInt();
-    }
-
-    @Override
-    public long readLong() {
-        return byteBuf.readLong();
-    }
-
-    @Override
-    public float readFloat() {
-        return byteBuf.readFloat();
-    }
-
-    @Override
-    public double readDouble() {
-        return byteBuf.readDouble();
-    }
-}

src/main/java/mc/server/network/impl/io/ByteBufNetOutputStream.java

@@ -1,47 +0,0 @@
-package mc.server.network.impl.io;
-
-import io.netty.buffer.ByteBuf;
-import lombok.RequiredArgsConstructor;
-import mc.protocol.io.NetOutputStream;
-
-@RequiredArgsConstructor
-public class ByteBufNetOutputStream extends NetOutputStream {
-
-    private final ByteBuf byteBuf;
-
-    @Override
-    public void writeByte(int value) {
-        byteBuf.writeByte(value);
-    }
-
-    @Override
-    public void writeBytes(byte[] buffer, int offset, int lengtn) {
-        byteBuf.writeBytes(buffer, offset, lengtn);
-    }
-
-    @Override
-    public void writeShort(int value) {
-        byteBuf.writeShort(value);
-    }
-
-    @Override
-    public void writeInt(int value) {
-        byteBuf.writeInt(value);
-    }
-
-    @Override
-    public void writeLong(long value) {
-        byteBuf.writeLong(value);
-    }
-
-    @Override
-    public void writeFloat(float value) {
-        byteBuf.writeFloat(value);
-    }
-
-    @Override
-    public void writeDouble(double value) {
-        byteBuf.writeDouble(value);
-    }
-}
-

src/main/java/mc/server/shell/Command.java

@@ -0,0 +1,8 @@
+package mc.server.shell;
+
+import org.jline.builtins.Builtins;
+
+public interface Command {
+
+    void execute(Builtins.CommandInput commandInput);
+}

src/main/java/mc/server/shell/CommandService.java

@@ -0,0 +1,8 @@
+package mc.server.shell;
+
+import org.jline.builtins.CommandRegistry;
+
+public interface CommandService extends CommandRegistry {
+
+    void register(String commandName, Command command);
+}

src/main/java/mc/server/shell/CommandServiceImpl.java

@@ -0,0 +1,57 @@
+package mc.server.shell;
+
+import org.jline.builtins.Builtins;
+import org.jline.builtins.Builtins.CommandMethods;
+import org.jline.builtins.Completers.SystemCompleter;
+import org.jline.reader.impl.completer.NullCompleter;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Set;
+
+public class CommandServiceImpl implements CommandService {
+
+    private final Map<String, CommandMethods> commands = new HashMap<>();
+
+    public void register(String commandName, Command command) {
+        commands.put(commandName, new CommandMethods(
+                command::execute,
+                cmdName -> Collections.singletonList(NullCompleter.INSTANCE)
+        ));
+    }
+
+    public Object execute(CommandSession session, String command) {
+        return execute(session, command, new String[0]);
+    }
+
+    @Override
+    public Object execute(CommandSession session, String command, String[] args) {
+        commands.get(command).execute().accept(new Builtins.CommandInput(command, args, session));
+        return null;
+    }
+
+    @Override
+    public Set<String> commandNames() {
+        return commands.keySet();
+    }
+
+    @Override
+    public Map<String, String> commandAliases() {
+        return Collections.emptyMap();
+    }
+
+    @Override
+    public boolean hasCommand(String commandName) {
+        return commands.containsKey(commandName);
+    }
+
+    @Override
+    public SystemCompleter compileCompleters() {
+        final SystemCompleter systemCompleter = new SystemCompleter();
+        systemCompleter.addAliases(commandAliases());
+        commands.forEach((commandName, command) -> systemCompleter.add(commandName, NullCompleter.INSTANCE));
+
+        return systemCompleter;
+    }
+}

src/main/java/mc/server/shell/SystemPropertiesCommand.java

@@ -0,0 +1,28 @@
+package mc.server.shell;
+
+import org.jline.builtins.Builtins.CommandInput;
+
+import java.io.PrintWriter;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Stream;
+
+public class SystemPropertiesCommand implements Command {
+
+    @Override
+    public void execute(CommandInput input) {
+        final PrintWriter writer = input.terminal().writer();
+        Stream<Map.Entry<Object, Object>> stream = System.getProperties().entrySet().stream();
+
+        if (input.args().length > 0) {
+            final List<String> argv = Arrays.asList(input.args());
+            stream = stream.filter(entry -> {
+                //noinspection SuspiciousMethodCalls
+                return argv.contains(entry.getKey());
+            });
+        }
+
+        stream.forEach(entry -> writer.printf("%s = %s\n", entry.getKey(), entry.getValue().toString()));
+    }
+}