Spring Boot Rest API JSON validation

[Greg Abel]

I have a rest API set up that accepts a JSON request and maps it to a POJO.  My question is, what is the best practice or how do I validate that the JSON that is being sent contains valid data before continuing on and processing further logic?  If it's invalid, I want to send a message back to the user to indicate that the request was bad.  Do I have to iterate through every possible field and check them?  For example, let's say I'm expecting a value for a field that will end up being a main data element for my java object, how can I verify it is there?  Thanks in advance for the help.

[Miroslav Lehotsky]

What you are describing is commonly known as (Java Bean Validation).

[Himai Minh]

Here is my brainstorming suggestion.
Support you have a POJO called Student:

...
import javax.validation.constraints.NotBlank;
import javax.persistence.Entity;
.....
public class Student{
   @Id
   @GeneratedValue(strategy=IDENTITY)
   private Integer id;
   @NotBlank(message="name should not be blank.")
    private String name;

 ......
}

In your controller, you may have something like this:

//suppose you have a Thymleaf front end that has model attribute for student
@PostMapping("/students/add")
public String addStudent(@ModelAttribute("student") @Valid Student student){
      ....
    //Assume you have interface StudentRepository extends CrudRepository<Student, Integer>
     studentRepository.save(student);
    return "/student/home"
} 

You can create a Test package using JUnit 5:

//Suppose you are writing a @SpringBootApplication 
@SpringBootTest
@AutoConfigureMock
class MyStudentTest{
 @Autowired 
  MockMvc mvc;

 @Test
  public void testAdd() throws Exception{
   Student student = new Student();
   student.setId(1);
   student.setName("John Smith");

    mvc.perform(post("/students/add")
                       .flashAttr("student", student)
    ).andExpect(view().name("/students/home") );
  
  }

 @Test
  public void testErrorAdd() throws Exception{
   Student student = new Student();
   student.setId(1);
   student.setName("");

    mvc.perform(post("/students/add")
                       .flashAttr("student", student)
    ).andExpect(view().name("/students/home") );
  
  }
}

[Greg Abel]

Thank you for the help