feat: implemented JSON validation

src/test/java/uulm/teamname/marvelous/gamelibrary/messages/MessageValidationTest.java

@@ -0,0 +1,54 @@
+package uulm.teamname.marvelous.gamelibrary.messages;
+
+import jakarta.validation.ConstraintViolation;
+import jakarta.validation.ValidatorFactory;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import uulm.teamname.marvelous.gamelibrary.messages.client.CharacterSelectionMessage;
+
+import jakarta.validation.Validation;
+import jakarta.validation.Validator;
+
+import java.util.stream.Collectors;
+
+import static org.mockito.Mockito.*;
+import static org.junit.jupiter.api.Assertions.*;
+import static org.assertj.core.api.Assertions.*;
+
+class MessageValidationTest {
+
+    ValidatorFactory factory;
+    Validator validator;
+
+    @BeforeEach
+    void beforeEach() {
+        factory = Validation.buildDefaultValidatorFactory();
+        validator = factory.getValidator();
+    }
+
+    // TODO: more tests here
+
+    @Test
+    void CharacterSelectionMessageTest() {
+        var message = new CharacterSelectionMessage();
+
+        var violations = validator.validate(message);
+
+        assertThat(violations).size().isOne();
+
+        message.characters = new Boolean[] {true, false, true, false, true, false}; // too little
+
+        violations = validator.validate(message);
+
+        var violationsString = violations.stream()
+                .map(violation -> violation.getPropertyPath() + " " + violation.getMessage())
+                .collect(Collectors.joining(", "));
+
+//        System.out.println(violationsString);
+
+        assertThat(violations).isNotEmpty();
+        assertThat(violationsString)
+                .isEqualTo("characters doesn't have twelve elements");
+    }
+
+}