.gitignore

@@ -48,6 +48,9 @@ gradle-app.setting
 # JDT-specific (Eclipse Java Development Tools)
 .classpath
 
+# Gradle properties
+gradle.properties
+
 ## MacOS
 # General
 .DS_Store
@@ -122,4 +125,28 @@ $RECYCLE.BIN/
 
 ## bin
 bin/
-gradle.properties
+
+# LaTeX (TeX)
+## Core latex/pdflatex auxiliary files:
+*.aux
+*.lof
+*.log
+*.lot
+*.fls
+*.out
+*.toc
+*.fmt
+*.fot
+*.cb
+*.cb2
+.*.lb
+
+## Build tool auxiliary files:
+*.fdb_latexmk
+*.synctex
+*.synctex(busy)
+*.synctex.gz
+*.synctex.gz(busy)
+*.pdfsync
+*.rubbercache
+rubber.cache

documents/docs/networking/commands/commands-deep-dive.md

@@ -64,11 +64,31 @@ Concrete subclasses add command-specific fields, all set via constructor. Reques
 ### `CommandHandler<T extends Request>`
 <img src="../../../images/docs/networking/commands/command_handler.png" alt="Class diagram of the CommandHandler" />
 
-The `CommandHandler` is a single-method interface responsible for executing a typed request.
+The `CommandHandler` is an abstract base class responsible for executing a typed request.
 The handler contains the domain logic: reading from registries and managers, modifying state, and dispatching a response via the `ResponseDispatcher`.
 
 Handlers receive their dependencies (the `ResponseDispatcher`, registries and managers etc.) through constructor injection.
-This keeps them fully testable without the network layer.
+They can also register reusable pre-execution checks via `addCheck(...)`. These checks are stored on the handler and are evaluated before `execute(...)` runs.
+
+When a piece of trivial validation is shared by multiple handlers, it should be extracted into a dedicated `HandlerCheck` instead of being duplicated in each handler.
+
+### `HandlerCheck`
+<img src="../../../images/docs/networking/commands/handler_check.png" alt="Class diagram of HandlerCheck and CommandHandlerExecutor" />
+
+The `HandlerCheck` is a functional interface for reusable pre-execution validation.
+Its `check(...)` method receives the incoming `Request` and returns an empty `Optional` if the request may continue.
+If the check fails, it returns an `ErrorResponse` wrapped in the `Optional`, which will be dispatched to the client instead of calling the handler.
+
+This is the right place for small shared checks such as "is the user logged in?" or other simple preconditions that multiple handlers need.
+
+### `CommandHandlerExecutor`
+<img src="../../../images/docs/networking/commands/handler_check_executor.png" alt="Class diagram of HandlerCheck and CommandHandlerExecutor" />
+
+The `CommandHandlerExecutor` runs all checks registered on a handler before invoking `execute(...)`.
+If any `HandlerCheck` returns a response, the executor dispatches it immediately and aborts execution.
+Otherwise, the handler is executed normally.
+
+This keeps precondition handling separate from the actual domain logic inside the handler.
 
 ### `CommandRouter`
 <img src="../../../images/docs/networking/commands/command_router.png" alt="Class diagram of the CommandRouter" />

documents/docs/networking/commands/guide-on-implementing-a-command.md

@@ -159,6 +159,9 @@ public class GreetHandler implements CommandHandler<GreetRequest> {
 **Rules for the Handler class:**
 
 - Declare all dependencies as `private final` fields, injected through the constructor.
+- Use `addCheck(...)` to attach pre-execution checks that should run before `execute(...)`.
+- If several handlers share the same trivial logic, such as verifying that the user is logged in, 
+  extract that logic into a separate `HandlerCheck` and reuse it instead of duplicating the code.
 - Always dispatch exactly one response per execution path. Every branch must end with a
   `responseDispatcher.dispatch(...)` call.
 - Use `ErrorResponse` for domain-level failures (e.g. entity not found, precondition not met).

documents/images/docs/networking/commands/handler_check.puml

@@ -0,0 +1,8 @@
+@startuml
+skinparam backgroundColor transparent
+
+interface HandlerCheck {
+    + check(request: Request): Optional<Response>
+}
+
+@enduml

documents/images/docs/networking/commands/handler_check_executor.puml

@@ -0,0 +1,31 @@
+@startuml
+skinparam backgroundColor transparent
+
+class CommandHandlerExecutor {
+    - responseDispatcher: ResponseDispatcher
+    + CommandHandlerExecutor(responseDispatcher: ResponseDispatcher)
+    + execute(handler: CommandHandler<Request>, request: Request): void
+}
+
+interface HandlerCheck {
+    + check(request: Request): Optional<Response>
+}
+
+abstract class CommandHandler<T extends Request> {
+    - checks: List<HandlerCheck>
+    # addCheck(check: HandlerCheck): void
+    + getChecks(): List<HandlerCheck>
+    + execute(request: T): void
+}
+
+interface ResponseDispatcher
+class Request
+class Response
+
+CommandHandlerExecutor --> CommandHandler : executes
+CommandHandler "1" o-- "0..*" HandlerCheck : registered checks
+CommandHandlerExecutor --> HandlerCheck : evaluates
+HandlerCheck ..> Request : inspects
+HandlerCheck ..> Response : returns failure response
+CommandHandlerExecutor --> ResponseDispatcher : dispatches failures
+@enduml

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/ServerApp.java

@@ -3,12 +3,19 @@ package ch.unibas.dmi.dbis.cs108.casono.server;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick.CheckUsernameHandler;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick.CheckUsernameParser;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.check_nick.CheckUsernameRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.login.LoginHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.login.LoginParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.login.LoginRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.logout.LogoutHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.logout.LogoutParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.logout.LogoutRequest;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.ping.PingHandler;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.ping.PingParser;
 import ch.unibas.dmi.dbis.cs108.casono.server.app.commands.ping.PingRequest;
 import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserCleanupJob;
 import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserRegistry;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.NetworkManager;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandlerExecutor;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandRouter;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.command.parsing.CommandParserDispatcher;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.events.DisconnectEvent;
@@ -40,11 +47,12 @@ public class ServerApp {
 
         EventBus eventBus = new EventBus();
         CommandParserDispatcher dispatcher = new CommandParserDispatcher();
-        CommandRouter router = new CommandRouter();
+        SessionManager sessionManager = new SessionManager(eventBus, dispatcher);
+        ResponseDispatcher responseDispatcher = new ResponseDispatcher(sessionManager);
+        CommandHandlerExecutor handlerExecutor = new CommandHandlerExecutor(responseDispatcher);
+        CommandRouter router = new CommandRouter(handlerExecutor);
 
-        SessionManager sessionManager = new SessionManager(eventBus, dispatcher, router);
         eventBus.subscribe(DisconnectEvent.class, event -> sessionManager.onDisconnect(event));
-        NetworkManager networkManager = new NetworkManager(port, sessionManager);
 
         UserRegistry userRegistry = new UserRegistry();
         eventBus.subscribe(
@@ -65,10 +73,9 @@ public class ServerApp {
                 SESSION_DISCONNECT_JOB_PERIOD,
                 TimeUnit.SECONDS);
 
-        ResponseDispatcher responseDispatcher = new ResponseDispatcher(sessionManager);
-
         registerCommands(dispatcher, router, responseDispatcher, userRegistry);
 
+        NetworkManager networkManager = new NetworkManager(port, sessionManager, router);
         networkManager.start();
     }
 
@@ -91,5 +98,13 @@ public class ServerApp {
         commandRouter.register(
                 CheckUsernameRequest.class,
                 new CheckUsernameHandler(responseDispatcher, userRegistry));
+
+        parserDispatcher.register("LOGIN", new LoginParser());
+        commandRouter.register(
+                LoginRequest.class, new LoginHandler(responseDispatcher, userRegistry));
+
+        parserDispatcher.register("LOGOUT", new LogoutParser());
+        commandRouter.register(
+                LogoutRequest.class, new LogoutHandler(responseDispatcher, userRegistry));
     }
 }

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/checks/UserLoggedInCheck.java

@@ -0,0 +1,30 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.checks;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.User;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserRegistry;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.checks.HandlerCheck;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.Request;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.ErrorResponse;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.Response;
+import java.util.Optional;
+
+public class UserLoggedInCheck implements HandlerCheck {
+    private final UserRegistry userRegistry;
+
+    public UserLoggedInCheck(UserRegistry userRegistry) {
+        this.userRegistry = userRegistry;
+    }
+
+    @Override
+    public Optional<Response> check(Request request) {
+        Optional<User> user = userRegistry.getBySessionId(request.getSessionId());
+        if (user.isPresent()) {
+            return Optional.empty();
+        }
+        return Optional.of(
+                new ErrorResponse(
+                        request.getContext(),
+                        "USER_NOT_LOGGED_IN",
+                        "The execution of this command requires the user to be logged in"));
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/check_nick/CheckUsernameHandler.java

@@ -7,8 +7,7 @@ import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatch
 import java.util.Optional;
 
 /** Handles {@link CheckUsernameRequest}s to check whether a username is available. */
-public class CheckUsernameHandler implements CommandHandler<CheckUsernameRequest> {
-    private final ResponseDispatcher responseDispatcher;
+public class CheckUsernameHandler extends CommandHandler<CheckUsernameRequest> {
     private final UserRegistry userRegistry;
 
     /**
@@ -18,7 +17,7 @@ public class CheckUsernameHandler implements CommandHandler<CheckUsernameRequest
      * @param userRegistry the registry used to look up existing users
      */
     public CheckUsernameHandler(ResponseDispatcher responseDispatcher, UserRegistry userRegistry) {
-        this.responseDispatcher = responseDispatcher;
+        super(responseDispatcher);
         this.userRegistry = userRegistry;
     }
 

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/login/LoginHandler.java

@@ -0,0 +1,57 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.login;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.User;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserFactory;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserId;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserRegistry;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.ErrorResponse;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
+import java.util.Optional;
+
+/**
+ * Handles {@link LoginRequest}s to create a user for a session, if the session has not assigned one
+ * already
+ */
+public class LoginHandler extends CommandHandler<LoginRequest> {
+    private final UserRegistry userRegistry;
+    private final UserFactory userFactory;
+
+    /**
+     * Creates a new handler for checking for existing user and creating a new one
+     *
+     * @param responseDispatcher the dispatcher used to send the response
+     * @param userRegistry the registry used to look up existing users and create the new one
+     */
+    public LoginHandler(ResponseDispatcher responseDispatcher, UserRegistry userRegistry) {
+        super(responseDispatcher);
+        this.userRegistry = userRegistry;
+        this.userFactory = new UserFactory(userRegistry);
+    }
+
+    /**
+     * Executes the login request
+     *
+     * <p>If no user is already assigned to the session, a new user is created and its name and
+     * {@link UserId} returned in the response. If the session has already a user assigned, a {@code
+     * ALREADY_LOGGED_IN} error is responded with.
+     *
+     * @param request the request to execute
+     */
+    @Override
+    public void execute(LoginRequest request) {
+        Optional<User> existingUser = userRegistry.getBySessionId(request.getSessionId());
+        if (existingUser.isPresent()) {
+            responseDispatcher.dispatch(
+                    new ErrorResponse(
+                            request.getContext(),
+                            "ALREADY_LOGGED_IN",
+                            "This session is already associated with an active user."));
+            return;
+        }
+
+        User user = userFactory.create(request.getUsername(), request.getSessionId());
+        responseDispatcher.dispatch(
+                new LoginResponse(request.getContext(), user.getName(), user.getId()));
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/login/LoginParser.java

@@ -0,0 +1,21 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.login;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.parsing.CommandParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.PrimitiveRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.accessor.RequestParameterAccessor;
+
+/** Parses a primitive request into a {@link LoginRequest}. */
+public class LoginParser implements CommandParser<LoginRequest> {
+    /**
+     * Extracts the required {@code USERNAME} parameter from the incoming request.
+     *
+     * @param primitiveRequest the request to parse
+     * @return {@link LoginRequest} containing the username
+     */
+    @Override
+    public LoginRequest parse(PrimitiveRequest primitiveRequest) {
+        RequestParameterAccessor accessor =
+                new RequestParameterAccessor(primitiveRequest.parameters());
+        return new LoginRequest(primitiveRequest.context(), accessor.require("USERNAME"));
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/login/LoginRequest.java

@@ -0,0 +1,30 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.login;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.Request;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.RequestContext;
+
+/** Request implementation used to create server-side user instance based on provided username */
+public class LoginRequest extends Request {
+    private final String username;
+
+    /**
+     * Constructs a new LoginRequest with the given context and desired username
+     *
+     * @param context the {@link RequestContext} containing information for responding to the
+     *     request
+     * @param username the desired username when creating the user
+     */
+    public LoginRequest(RequestContext context, String username) {
+        super(context);
+        this.username = username;
+    }
+
+    /**
+     * Returns the desired username requested for login
+     *
+     * @return the desired username
+     */
+    public String getUsername() {
+        return username;
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/login/LoginResponse.java

@@ -0,0 +1,23 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.login;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserId;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.RequestContext;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.SuccessResponse;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.builder.ResponseBodyBuilder;
+
+/** Response containing the assigned username and id of said user */
+public class LoginResponse extends SuccessResponse {
+    /**
+     * @param context the {@link RequestContext} associated with the request
+     * @param assignedUsername the assigned username to this user
+     * @param id of the created user
+     */
+    public LoginResponse(RequestContext context, String assignedUsername, UserId id) {
+        super(
+                context,
+                new ResponseBodyBuilder()
+                        .param("USERNAME", assignedUsername)
+                        .param("ID", id.value())
+                        .build());
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/logout/LogoutHandler.java

@@ -0,0 +1,44 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.logout;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserRegistry;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.ErrorResponse;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.OkResponse;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
+
+/** Handles {@link LogoutRequest} to logout connected user. */
+public class LogoutHandler extends CommandHandler<LogoutRequest> {
+    private final UserRegistry userRegistry;
+
+    /**
+     * Creates a new logout handler to logout user.
+     *
+     * @param responseDispatcher dispatcher used to send the logout result back to the client
+     * @param userRegistry registry responsible for tracking connected user sessions
+     */
+    public LogoutHandler(ResponseDispatcher responseDispatcher, UserRegistry userRegistry) {
+        super(responseDispatcher);
+        this.userRegistry = userRegistry;
+    }
+
+    /**
+     * Executes the logout request for the given request.
+     *
+     * <p>The user is removed from the {@link UserRegistry}.
+     *
+     * @param request the request to execute
+     */
+    @Override
+    public void execute(LogoutRequest request) {
+        boolean wasRemoved = userRegistry.removeBySessionId(request.getSessionId());
+        if (wasRemoved) {
+            responseDispatcher.dispatch(new OkResponse(request.getContext()));
+        } else {
+            responseDispatcher.dispatch(
+                    new ErrorResponse(
+                            request.getContext(),
+                            "NO_USER_ASSOCIATED",
+                            "No user is associated with your session. Did you login before?"));
+        }
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/logout/LogoutParser.java

@@ -0,0 +1,19 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.logout;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.parsing.CommandParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.PrimitiveRequest;
+
+/** Parses a primitive request into a {@link LogoutRequest}. */
+public class LogoutParser implements CommandParser<LogoutRequest> {
+
+    /**
+     * Parses a primitive request into a LogoutRequest.
+     *
+     * @param primitiveRequest the request to parse
+     * @return the created {@link LogoutRequest}
+     */
+    @Override
+    public LogoutRequest parse(PrimitiveRequest primitiveRequest) {
+        return new LogoutRequest(primitiveRequest.context());
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/logout/LogoutRequest.java

@@ -0,0 +1,18 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.logout;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.Request;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.RequestContext;
+
+/** Request implementation used to logout a currently logged in user */
+public class LogoutRequest extends Request {
+
+    /**
+     * Constructs a new LogoutRequest with the given context.
+     *
+     * @param context the {@link RequestContext} containing information for responding to the
+     *     request
+     */
+    public LogoutRequest(RequestContext context) {
+        super(context);
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/ping/PingHandler.java

@@ -5,8 +5,7 @@ import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.OkRespon
 import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
 
 /** Handler for {@link PingRequest}. */
-public class PingHandler implements CommandHandler<PingRequest> {
-    private final ResponseDispatcher responseDispatcher;
+public class PingHandler extends CommandHandler<PingRequest> {
 
     /**
      * Create a new PingHandler to execute {@link PingRequest}s
@@ -14,7 +13,7 @@ public class PingHandler implements CommandHandler<PingRequest> {
      * @param responseDispatcher dispatcher used to send responses back to clients
      */
     public PingHandler(ResponseDispatcher responseDispatcher) {
-        this.responseDispatcher = responseDispatcher;
+        super(responseDispatcher);
     }
 
     /**

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/user/UserRegistry.java

@@ -31,6 +31,66 @@ public class UserRegistry {
         return Optional.of(user);
     }
 
+    /**
+     * Removes a user from the registry by user ID.
+     *
+     * @param userId the ID of the user to remove
+     * @return true if a user was removed, false if no user with that ID exists
+     */
+    public synchronized boolean removeByUserId(UserId userId) {
+        User user = byId.get(userId);
+        if (user == null) {
+            return false;
+        }
+
+        byName.remove(user.getName());
+        user.getSessionId().ifPresent(bySessionId::remove);
+        return true;
+    }
+
+    /**
+     * Removes a user from the registry by session ID.
+     *
+     * @param sessionId the session ID associated with the user to remove
+     * @return true if a user was removed, false if no user with that session exists
+     */
+    public synchronized boolean removeBySessionId(SessionId sessionId) {
+        User user = bySessionId.get(sessionId);
+        if (user == null) {
+            return false;
+        }
+
+        remove(user);
+        return true;
+    }
+
+    /**
+     * Removes a user from the registry by username.
+     *
+     * @param username the username associated with the user to remove
+     * @return true if a user was removed, false if no user with that session exists
+     */
+    public synchronized boolean removeByUsername(String username) {
+        User user = byName.get(username);
+        if (user == null) {
+            return false;
+        }
+
+        remove(user);
+        return true;
+    }
+
+    /**
+     * Removes a user from the internals of the registry.
+     *
+     * @param user the user to remove
+     */
+    private synchronized void remove(User user) {
+        byId.remove(user.getId());
+        byName.remove(user.getName());
+        user.getSessionId().ifPresent(bySessionId::remove);
+    }
+
     /**
      * Removes the user with the given ID, but only if they are still disconnected. This prevents
      * removing a user who has reconnected between the cleanup job's check and its removal call.
@@ -47,8 +107,7 @@ public class UserRegistry {
             return false;
         }
 
-        byId.remove(user.getId());
-        byName.remove(user.getName());
+        remove(user);
         return true;
     }
 

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/network/NetworkManager.java

@@ -1,5 +1,6 @@
 package ch.unibas.dmi.dbis.cs108.casono.server.network;
 
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandRouter;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.sessions.SessionManager;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.transport.TcpTransport;
 import java.io.IOException;
@@ -15,6 +16,7 @@ public class NetworkManager implements Runnable {
     private Thread thread;
     private Boolean running;
     private SessionManager sessionManager;
+    private CommandRouter router;
 
     /**
      * Creates a new NetworkManager with the given port, session manager, and event bus.
@@ -22,12 +24,13 @@ public class NetworkManager implements Runnable {
      * @param port the port to listen on
      * @param sessionManager the session manager to use
      */
-    public NetworkManager(Integer port, SessionManager sessionManager) {
+    public NetworkManager(Integer port, SessionManager sessionManager, CommandRouter router) {
         this.port = port;
         this.logger = LogManager.getLogger(NetworkManager.class);
         this.thread = new Thread(this, "networkManager");
         this.running = true;
         this.sessionManager = sessionManager;
+        this.router = router;
     }
 
     /** Starts the internal thread to accept new connections. */
@@ -45,7 +48,7 @@ public class NetworkManager implements Runnable {
 
                 logger.debug("Accepted connection from {}", clientSocket.getRemoteSocketAddress());
 
-                sessionManager.create(new TcpTransport(clientSocket));
+                sessionManager.create(new TcpTransport(clientSocket), router);
             }
 
         } catch (IOException e) {

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/network/command/execution/CommandHandler.java

@@ -1,7 +1,56 @@
 package ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution;
 
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.checks.HandlerCheck;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.Request;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
 
-public interface CommandHandler<T extends Request> {
-    void execute(T request);
+/**
+ * Abstract base class for handling requests of a specific type.
+ *
+ * @param <T> the type of request this handler processes, must extend {@link Request}
+ */
+public abstract class CommandHandler<T extends Request> {
+    private final List<HandlerCheck> checks = new ArrayList<>();
+    protected final ResponseDispatcher responseDispatcher;
+
+    /**
+     * Constructs a CommandHandler with the specified ResponseDispatcher.
+     *
+     * @param responseDispatcher the ResponseDispatcher instance used to send responses to clients.
+     */
+    public CommandHandler(ResponseDispatcher responseDispatcher) {
+        this.responseDispatcher = responseDispatcher;
+    }
+
+    /**
+     * Adds a handler check to be performed before request execution.
+     *
+     * <p>The {@code execute} Method is only invoked, if all specified checks passed.
+     *
+     * @param check the {@link HandlerCheck} to add
+     */
+    protected final void addCheck(HandlerCheck check) {
+        checks.add(check);
+    }
+
+    /**
+     * Returns an unmodifiable list of all registered handler checks.
+     *
+     * @return an unmodifiable list of {@link HandlerCheck} objects
+     */
+    public final List<HandlerCheck> getChecks() {
+        return Collections.unmodifiableList(checks);
+    }
+
+    /**
+     * Executes the request.
+     *
+     * <p>Subclasses should override this method to implement their specific request handling logic.
+     *
+     * @param request the request to execute, of type T
+     */
+    public void execute(T request) {}
 }

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/network/command/execution/CommandHandlerExecutor.java

@@ -0,0 +1,46 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.checks.HandlerCheck;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.Request;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.Response;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
+import java.util.Optional;
+
+/**
+ * Invokes execution of the {@link CommandHandler} if all defined pre-execution checks pass.
+ *
+ * <p>If any check fails, the response returned by the check is dispatched immediately and execution
+ * is aborted. Otherwise, the handler will invoke execution normally.
+ */
+public class CommandHandlerExecutor {
+    private final ResponseDispatcher responseDispatcher;
+
+    /**
+     * Creates a new CommandHandlerExecutor to execute pre-execution checks and invoke the {@link
+     * CommandHandler} if all checks pass.
+     *
+     * @param responseDispatcher to dispatch the response with if an check fails
+     */
+    public CommandHandlerExecutor(ResponseDispatcher responseDispatcher) {
+        this.responseDispatcher = responseDispatcher;
+    }
+
+    /**
+     * Executes a command handler if all defined pre-execution checks pass.
+     *
+     * @param handler the command handler containing checks and execution logic
+     * @param request the incoming request to validate and process
+     * @throws NullPointerException if handler or request is null
+     */
+    public void execute(CommandHandler<Request> handler, Request request) {
+        for (HandlerCheck check : handler.getChecks()) {
+            Optional<Response> result = check.check(request);
+            if (result.isPresent()) {
+                responseDispatcher.dispatch(result.get());
+                return;
+            }
+        }
+
+        handler.execute(request);
+    }
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/network/command/execution/CommandRouter.java

@@ -4,13 +4,46 @@ import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.Request;
 import java.util.HashMap;
 import java.util.Map;
 
+/** Routes incoming requests to their corresponding {@link CommandHandler} implementations. */
 public class CommandRouter {
     private final Map<Class<? extends Request>, CommandHandler<?>> handlers = new HashMap<>();
+    private final CommandHandlerExecutor handlerExecutor;
 
+    /**
+     * Creates a new CommandRouter with the specified handler executor.
+     *
+     * @param commandHandlerExecutor to delegate execution of checks and invocation of {@link
+     *     CommandHandler} to
+     */
+    public CommandRouter(CommandHandlerExecutor commandHandlerExecutor) {
+        this.handlerExecutor = commandHandlerExecutor;
+    }
+
+    /**
+     * Registers a {@link CommandHandler} for a specific request type.
+     *
+     * <p>This method establishes a type-safe mapping between a request class and its handler.
+     *
+     * <p>Only one handler can be registered per request type; subsequent registrations will
+     * overwrite previous ones.
+     *
+     * @param <T> the type of request handled by the provided handler
+     * @param request the request class to associate with the given command handler
+     * @param handler the command handler for the given request type
+     */
     public <T extends Request> void register(Class<T> request, CommandHandler<T> handler) {
         handlers.put(request, handler);
     }
 
+    /**
+     * Executes the appropriate handler for the given request.
+     *
+     * <p>Looks up the handler registered for the request's class and executes it through the
+     * configured {@link CommandHandlerExecutor}.
+     *
+     * @param request the request to be executed
+     * @throws UnknownRequestException if no handler is registered for the request type
+     */
     // Safe, because during registration, it's ensured that the provided CommandHandler only
     // receives requests it can handle.
     @SuppressWarnings("unchecked")
@@ -23,6 +56,7 @@ public class CommandRouter {
             throw new UnknownRequestException(
                     "Unable to execute request " + requestName + ". Type unknown", requestName);
         }
-        handler.execute(request);
+
+        handlerExecutor.execute(handler, request);
     }
 }

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/network/command/execution/checks/HandlerCheck.java

@@ -0,0 +1,23 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.checks;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.Request;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.ErrorResponse;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.Response;
+import java.util.Optional;
+
+/**
+ * Functional interface used by pre-execution checks. Executed before the execute method of the
+ * {@link CommandHandler} is called
+ */
+@FunctionalInterface
+public interface HandlerCheck {
+    /**
+     * Checks that the requirements outlined by this HandlerCheck are fulfilled.
+     *
+     * @param request to execute the check with
+     * @return Optional containing no value if the check was successful. And a {@link ErrorResponse}
+     *     if the check failed, the response will be dispatched to the client.
+     */
+    Optional<Response> check(Request request);
+}

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/network/sessions/SessionManager.java

@@ -19,16 +19,13 @@ public class SessionManager {
     private final EventBus eventBus;
     private final Logger logger;
     private final CommandParserDispatcher dispatcher;
-    private final CommandRouter router;
 
     /** Constructs a new SessionManager. */
-    public SessionManager(
-            EventBus eventBus, CommandParserDispatcher dispatcher, CommandRouter router) {
+    public SessionManager(EventBus eventBus, CommandParserDispatcher dispatcher) {
         this.sessions = new ConcurrentHashMap<>();
         this.eventBus = eventBus;
         this.logger = LogManager.getLogger(SessionManager.class);
         this.dispatcher = dispatcher;
-        this.router = router;
     }
 
     /**
@@ -37,9 +34,10 @@ public class SessionManager {
      * <p>Will create both worker threads and start them.
      *
      * @param transport to create session from
+     * @param router the command router used by the created session
      * @return newly created session
      */
-    public Session create(TransportLayer transport) {
+    public Session create(TransportLayer transport, CommandRouter router) {
         Session session = new Session(transport, eventBus, dispatcher, router);
         SessionReader reader = new SessionReader(session, eventBus);
         SessionWriter writer = new SessionWriter(session);