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Spring Framework - Spring - Spring JSR 303 Bean Validation
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  JSR303 Annotation based validations in Springs examples

Add any JSR 303 implementation to your classpath. Popular one used is Hibernate validator from Hibernate.

<dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-validator</artifactId>
    <version>4.2.0.Final</version>
</dependency>

Lets say the there is a rest api to create user in the system

@RequestMapping(value="/registeruser", method=RequestMethod.POST)
public String registerUser(User user);

The input json sample would look like as below

{"username" : "[email protected]", "password" : "password1", "password2":"password1"}

User.java

public class User {

    private String username;
    private String password;
    private String password2;

    getXXX and setXXX

}

We can define JSR 303 validations on User Class as below.

public class User {

    @NotEmpty
    @Size(min=5)
    @Email
    private String username;
    
    @NotEmpty
    private String password;
    
    @NotEmpty
    private String password2;

}

We may also need to have a business validator like password and password2(confirm password) are same, for this we can add a custom validator as below. Write a custom annotation for annotating the data field.

@Target({ ElementType.FIELD })
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = PasswordValidator.class)
public @interface GoodPassword {
    String message() default "Passwords wont match.";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

Write a Validator class for applying Validation logic.

public class PastValidator implements ConstraintValidator<GoodPassword, User> {
    @Override
    public void initialize(GoodPassword annotation) {}
    
    @Override
    public boolean isValid(User user, ConstraintValidatorContext context) {
        return user.getPassword().equals(user.getPassword2());
    }
}

Adding this validation to User Class

@GoodPassword
public class User {

    @NotEmpty
    @Size(min=5)
    @Email
    private String username;
    
    @NotEmpty
    private String password;
    
    @NotEmpty
    private String password2;
}

@Valid triggers validation in Spring. BindingResult is an object injected by spring which has list of errors after validation.

public String registerUser(@Valid User user, BindingResult result);

JSR 303 annotation has message attributes on them which can be used for providing custom messages.

@GoodPassword
public class User {

    @NotEmpty(message="Username Cant be empty")
    @Size(min=5, message="Username cant be les than 5 chars")
    @Email(message="Should be in email format")
    private String username;
    
    @NotEmpty(message="Password cant be empty")
    private String password;
    
    @NotEmpty(message="Password2 cant be empty")
    private String password2;

}
  Spring JSR 303 Validation - Customize error messages

Suppose we have a simple class with validation annotations

public class UserDTO {
    @NotEmpty
    private String name;

    @Min(18)
    private int age;

//getters/setters
}

A controller to check the UserDTO validity.

@RestController
public class ValidationController {

    @RequestMapping(value = "/validate", method = RequestMethod.POST)
    public ResponseEntity<String> check(@Valid @RequestBody UserDTO userDTO,
           BindingResult bindingResult) {
        return new ResponseEntity<>("ok" , HttpStatus.OK);
    }
}

And a test.

@Test
public void testValid() throws Exception {
    TestRestTemplate template = new TestRestTemplate();
    String url = base + contextPath + "/validate";
    Map<String, Object> params = new HashMap<>();
    params.put("name", "");
    params.put("age", "10");

    MultiValueMap<String, String> headers = new LinkedMultiValueMap<>();
    headers.add("Content-Type", "application/json");

    HttpEntity<Map<String, Object>> request = new HttpEntity<>(params, headers);
    String res = template.postForObject(url, request, String.class);

    assertThat(res, equalTo("ok"));
}

Both name and age are invalid so in the BindingResult we have two validation errors. Each has array of codes.

Codes for Min check

0 = "Min.userDTO.age"
1 = "Min.age"
2 = "Min.int"
3 = "Min"

And for NotEmpty check

0 = "NotEmpty.userDTO.name"
1 = "NotEmpty.name"
2 = "NotEmpty.java.lang.String"
3 = "NotEmpty"

Let's add a custom.properties file to substitute default messages.

@SpringBootApplication
@Configuration
public class DemoApplication {

    @Bean(name = "messageSource")
    public MessageSource messageSource() {
        ReloadableResourceBundleMessageSource bean = new ReloadableResourceBundleMessageSource();
        bean.setBasename("classpath:custom");
        bean.setDefaultEncoding("UTF-8");
        return bean;
    }

    @Bean(name = "validator")
    public LocalValidatorFactoryBean validator() {
        LocalValidatorFactoryBean bean = new LocalValidatorFactoryBean();
        bean.setValidationMessageSource(messageSource());
        return bean;
    }

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

If we add to the custom.properties file the line

NotEmpty=The field must not be empty!

The new value is shown for the error. To resolve message validator looks through the codes starting from the beginning to find proper messages.

Thus when we define NotEmpty key in the .properties file for all cases where the @NotEmpty annotation is used our message is applied.

If we define a message

Min.int=Some custom message here.

All annotations where we app min check to integer values use the newly defined message.

The same logic could be applied if we need to localize the validation error messages.
  @Valid usage to validate nested POJOs

Suppose we have a POJO class User we need to validate.

public class User {

    @NotEmpty
    @Size(min=5)
    @Email
    private String email;
}

and a controller method to validate the user instance

public String registerUser(@Valid User user, BindingResult result);

Let's extend the User with a nested POJO Address we also need to validate.

public class Address {

    @NotEmpty
    @Size(min=2, max=3)
    private String countryCode;
}

Just add @Valid annotation on address field to run validation of nested POJOs.

public class User {

    @NotEmpty
    @Size(min=5)
    @Email
    private String email;

    @Valid
    private Address address;
}



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