In this tutorial, you will learn to implement an REST API Validation + Error Handling Example in Spring Boot
REST API validation can be implemented by using Java Bean Validation API, Hibernate Validator and Unified Expression Language. Check this tutorial if you're new to Java Bean Validation API
Error Handling for REST in Spring can be implemented by extending ResponseEntityExceptionHandler and using @ControllerAdvice class annotation to apply globally to all controllers
What you'll need
- JDK 8+ or OpenJDK 8+
- Maven 3+
Stack
- Java Bean Validation API, Hibernate Validator and Unified Expression Language
- Spring Boot with @RestController, @RequestMapping, @Validated, ResponseEntityExceptionHandler, @ControllerAdvice and @ExceptionHandler
- JPA, Hibernate and HSQL
- Lombok with @Builder
Project structure
├── src
│ └── main
│ ├── java
│ │ └── com
│ │ └── hellokoding
│ │ └── springboot
│ │ └── restful
│ │ ├── product
│ │ │ ├── Product.java
│ │ │ ├── ProductAPI.java
│ │ │ ├── ProductIDExisting.java
│ │ │ ├── ProductIDExistingValidator.java
│ │ │ ├── ProductRepository.java
│ │ │ └── ProductService.java
│ │ ├── Application.java
│ │ ├── ExceptionHandler.java
│ │ └── ResponseDTO.java
│ └── resources
│ └── ValidationMessages.properties
└── pom.xml
Dependencies
Include spring-boot-starter-validation into your pom.xml
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-validation</artifactId>
</dependency>
spring-boot-starter-validation contains
hibernate-validator: an implementation of Java Bean Validation APItomcat-embed-el: an implementation of Java Unified Expression Language (EL)
Full content of pom.xml as below
Define data model and validation constraints
Define data model
@NotNull, @Size and @Min are built-in constraints
@ProductIDExisting is a custom constraint defined as below
{NotNull} is message templates, its value will be replaced at run time by the below ValidationMessages.properties
Define custom constraint
{ProductIDExisting} is a message template. Its value will be replaced at run time by the below ValidationMessages.properties
Define custom constraint validator
Custom the validation messages
Define ResponseDTO, REST API and API Exception Handler
ResponseDTO is defined to unify REST API response data format to client
REST APIs are defined with @RestController, @RequestMapping, @GetMapping, @PostMapping, @@PutMapping and @DeleteMapping
@Valid indicates validation cascading: constraints defined on the object and its properties are be validated when the property, method parameter or method return type is validated
@Validated is a variant of @Valid indicating that a specific class is supposed to be validated at the method level. In this example, it is using to trigger validation for @ProductIDExisting method parameters
ResponseEntityExceptionHandler provides centralized exception handling across all @RequestMapping methods through @ExceptionHandler methods which return ResponseEntity
@ControllerAdvice is a specialization for classes that declare @ExceptionHandler, @InitBinder, or @ModelAttribute methods to be shared across multiple @Controller classes
Run and Test
Run with Maven
Type command mvn clean spring-boot:run at your project root directory to run the application
Test with cURL
curl -i -X PUT -H "Content-Type:application/json" -d "{\"id\": 1, \"name\" : \"Hello Koding\", \"description\": \"Practical Coding Courses, Tutorials and Examples\", \"price\":1}" http://localhost:8080/api/v1/products/1
Expected output
{"status":"400 BAD_REQUEST","message":"Product ID 1 is not existing","body": null}