Bean Validation in Spring

@Entity
public class User {
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private long id;
 
  @NotNull(message = ”Name must not be null.”)
  private String name;
 
  @Size(min = 10, max = 500, message = ”Some violation message.”)
  private String aboutMe;
 
  @Min(value = 18, message = “Age must be higher than 18”)
  @Max(value = 130, message = “Age must be lower than 130”)
  private int age;
 
  @Email(message = “Invalid e-mail address format”)
  private String email;
 
}

Out of the box validation is achieved via annotations. For the most part these are pretty self-explanatory, though it should be taken into account that some could have different meanings for different data types (@Size for example, validates the length of a List if such element is annotated).

@Entity, @Id, @GeneratedValue – are annotations from  javax.persistence package and are used if we persist our data in a database.

@NotNull, @Size, @Max, @Min, @Email – are bean validation annotations used in our example. It should be mentioned that these are only some of the available validators, but they are the most common. Some other annotations are: @NotEmpty, @NotBlank, @Positive, @Past and so on.

Custom constraint annotations

It is also possible to write custom validators. This can be done through annotations as well. First we write a custom annotation like this WithoutA.java (we dislike the letter “a” for some reason

).

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = WithoutAValidator.class)
public @interface WithoutA {
    String message() default "Hey, there is the letter a there";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

Now we write the actual validator WithoutAValidator.class:

// imports
public class WithoutAValidator implements ConstraintValidator<WithoutA, String> {
  
  @Override
  public boolean isValid(String value, ConstraintValidatorContext context)
{
        return !value.contains(“a”);
}
}

By adding our annotation, to the aboutMe field, we would then need to write an about that does not have the letter “a” in it:

@Size(min = 10, max = 500, message = ”Some violation message.”)
@WithoutA
private String aboutMe;

This is all there is to it. Validators do not require additional configuration, the only thing that needs to be done for them to take effect is adding @Valid annotation in our Controllers and Services.