How to Use @NotNull in Spring Boot for Validation

In Spring Boot, use the @NotNull annotation from jakarta.validation.constraints on fields or method parameters to ensure they are not null during validation. Combine it with @Valid in controller methods to trigger validation automatically.

Syntax

The @NotNull annotation is placed on a field, method, or parameter to indicate it must not be null. It is part of Jakarta Bean Validation and requires a validation framework like Hibernate Validator.

• @NotNull: Ensures the annotated element is not null.
• Used with @Valid in Spring controllers to trigger validation.
• Works on fields, method parameters, and return values.

java

import jakarta.validation.constraints.NotNull;

public class User {
    @NotNull(message = "Name must not be null")
    private String name;

    // getters and setters
}

Example

This example shows a Spring Boot REST controller that validates a request body with a @NotNull field. If the field is null, Spring returns a 400 Bad Request with the validation message.

java

import jakarta.validation.Valid;
import jakarta.validation.constraints.NotNull;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class UserController {

    public static class User {
        @NotNull(message = "Name must not be null")
        private String name;

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }
    }

    @PostMapping("/users")
    public ResponseEntity<String> createUser(@Valid @RequestBody User user) {
        return ResponseEntity.ok("User created: " + user.getName());
    }
}

Output

POST /users with JSON {"name":null} returns 400 Bad Request with message: "Name must not be null"
POST /users with JSON {"name":"Alice"} returns 200 OK with body: "User created: Alice"

Common Pitfalls

• Forgetting to add @Valid on the controller method parameter, so validation does not run.
• Using @NotNull but sending empty strings or blank values; @NotNull only checks for null, not emptiness.
• Not including a validation provider like Hibernate Validator in your project dependencies.
• Confusing @NotNull with @NotEmpty or @NotBlank which check for empty or blank strings.

java

/* Wrong: Missing @Valid, validation won't trigger */
@PostMapping("/users")
public ResponseEntity<String> createUser(@RequestBody User user) {
    return ResponseEntity.ok("User created: " + user.getName());
}

/* Right: Add @Valid to trigger validation */
@PostMapping("/users")
public ResponseEntity<String> createUser(@Valid @RequestBody User user) {
    return ResponseEntity.ok("User created: " + user.getName());
}

Quick Reference

• @NotNull: Ensures value is not null.
• @Valid: Triggers validation on method parameters or fields.
• Use with Spring Boot's spring-boot-starter-validation dependency.
• Validation errors return HTTP 400 with messages by default.

Key Takeaways

Use @NotNull on fields or parameters to enforce non-null values in Spring Boot.

Always add @Valid on controller method parameters to activate validation.

Include spring-boot-starter-validation dependency for validation support.

@NotNull checks only for null, not empty or blank strings.

Validation failures automatically return HTTP 400 with error details.