It works !

ChatApp/app/src/main/java/client/ClientGuiController.java

@@ -1,8 +1,15 @@
 package client;
 
+import javafx.application.Platform;
 import javafx.fxml.FXML;
+import javafx.scene.control.Alert;
+import javafx.scene.layout.Border;
+import javafx.scene.layout.BorderPane;
+import javafx.stage.Stage;
 import network.protocol.packets.ServerResponsePacket;
 
+import java.net.InetSocketAddress;
+import java.net.SocketException;
 import java.time.Instant;
 import java.util.List;
 
@@ -10,30 +17,44 @@ public class ClientGuiController implements ClientListener {
 
     private Client client;
 
+    @FXML
+    private BorderPane vueContainer;
+
     public void setClient(Client client) {
         this.client = client;
     }
 
-    @FXML
-    public void initialize() {
-
-    }
+@FXML
+public void initialize() throws SocketException {
+    client = new Client(new InetSocketAddress("localhost", 6665), this, UsernameSingleton.getInstance().getUsername());
+    Platform.runLater(() -> {
+        Stage stage = (Stage) vueContainer.getScene().getWindow();
+        stage.setOnCloseRequest(event -> {
+            client.close();
+        });
+    });
+}
 
     @Override
     public void handleDisconnect() {
-
+        System.out.println("Disconnected");
     }
 
     @Override
     public void handleConnexionError() {
-
+        System.out.println("Connection error");
+        System.out.println(UsernameSingleton.getInstance().getUsername());
     }
 
     @Override
     public void handleConnect() {
-
+        Platform.runLater(() -> {
+            Alert alert = new Alert(Alert.AlertType.INFORMATION);
+            alert.setTitle("Connection");
+            alert.setHeaderText("Connection to the server successful");
+            alert.showAndWait();
+        });
     }
-
     @Override
     public void handleChatMessage(Instant time, String chatter, String content) {