update

pom.xml

@@ -25,7 +25,10 @@
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-web</artifactId>
 		</dependency>
-
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-starter-validation</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>com.mysql</groupId>
 			<artifactId>mysql-connector-j</artifactId>

src/main/java/com/devteria/identityservice/controller/UserController.java

@@ -4,6 +4,7 @@ import com.devteria.identityservice.dto.request.UserCreationRequest;
 import com.devteria.identityservice.dto.request.UserUpdateRequest;
 import com.devteria.identityservice.entity.User;
 import com.devteria.identityservice.service.UserService;
+import jakarta.validation.Valid;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.web.bind.annotation.*;
 
@@ -16,7 +17,7 @@ public class UserController {
     private UserService userService;
 
     @PostMapping
-    User createUser(@RequestBody UserCreationRequest request){
+    User createUser(@RequestBody @Valid UserCreationRequest request){
         return userService.createUser(request);
     }
 

src/main/java/com/devteria/identityservice/dto/request/UserCreationRequest.java

 package com.devteria.identityservice.dto.request;
 
+import jakarta.validation.constraints.Size;
+
 import java.time.LocalDate;
 
 public class UserCreationRequest {
+    @Size(min = 3,message = "Username must be at least 3 characters")
     private String username;
+
+    @Size(min = 8, message = "Password must be at least 8 characters")
     private String password;
     private String firstName;
     private String lastName;
+
     private LocalDate dob;
 
     public String getUsername() {
@@ -25,14 +31,14 @@ public class UserCreationRequest {
         this.password = password;
     }
 
-    public String getFirstName() {
-        return firstName;
-    }
-
     public void setFirstName(String firstName) {
         this.firstName = firstName;
     }
 
+    public String getFirstName() {
+        return firstName;
+    }
+
     public String getLastName() {
         return lastName;
     }

src/main/java/com/devteria/identityservice/exception/GlobalExceptionHandler.java

+package com.devteria.identityservice.exception;
+
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.MethodArgumentNotValidException;
+import org.springframework.web.bind.annotation.ControllerAdvice;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+
+@ControllerAdvice
+public class GlobalExceptionHandler {
+
+    @ExceptionHandler(value = RuntimeException.class)
+    ResponseEntity<String> handlingRuntimeException(RuntimeException exception){
+        return ResponseEntity.badRequest().body(exception.getMessage());
+    }
+
+    @ExceptionHandler(value = MethodArgumentNotValidException.class)
+    ResponseEntity<String> handlingValidation(MethodArgumentNotValidException exception){
+        return ResponseEntity.badRequest().body(exception.getFieldError().getDefaultMessage());
+    }
+}

src/main/java/com/devteria/identityservice/repository/UserRepository.java

@@ -6,4 +6,5 @@ import org.springframework.stereotype.Repository;
 
 @Repository
 public interface UserRepository extends JpaRepository<User, String> {
+    boolean existsByUsername(String username);
 }

src/main/java/com/devteria/identityservice/service/UserService.java

@@ -17,6 +17,9 @@ public class UserService {
     public User createUser(UserCreationRequest request){
         User user = new User();
 
+        if (userRepository.existsByUsername(request.getUsername()))
+            throw new RuntimeException("User existed.");
+
         user.setUsername(request.getUsername());
         user.setPassword(request.getPassword());
         user.setFirstName(request.getFirstName());