Create your own validator with Bean Validation

Overview

A memo of the procedure from writing a self-made validator with Bean Validation to writing a unit test.

Try

I will try to make a validator to judge whether it is a Michael Jackson song name.

Validator class implementation

package com.example.customvalidate;

import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;
import java.util.ArrayList;
import java.util.List;

public class MJValidator implements ConstraintValidator<MJ, String> {

    private final static List<String> songs = new ArrayList<String>() {
        {
            add("Remember The Time"); add("Stranger In Moscow"); add("You Are Not Alone");
        }
    };

    public void initialize(MJ constraintAnnotation) {
    }

    public boolean isValid(String value, ConstraintValidatorContext context) {
        return songs.stream().anyMatch(song -> song.equals(value));
    }
}

Commentary

--Inherit the ConstraintValidator interface. For the generics, specify the annotation to be created later and the type to be validated. --ʻIsValid` method is used to judge the actual validation. Makes an arbitrary judgment and returns boolean. --In this example, "Remember The Time", "Stranger In Moscow", or "You Are Not Alone" will pass the validation.

Annotation implementation

Make an annotation. Now you can use it like @ MJ

package com.example.customvalidate;

import javax.validation.Constraint;
import javax.validation.Payload;
import java.lang.annotation.*;

@Documented
@Constraint(validatedBy = {MJValidator.class})
@Target({ElementType.PARAMETER, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MJ {

    String message() default "you must specify one of Michael Jackson's songs.";

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};

    @Target({ElementType.FIELD})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @interface List {
        MJ[] value();
    }
}

Commentary

--Specify the class to actually validate in place of @Constraint (validatedBy =). Specify the MJ Validator created above. --For @Target, specify the target to which this annotation can be added. This time, I used arguments and fields. --For message, specify the message to be set in the exception object when validation is caught.

Write a test

package com.example.customvalidate;

import org.junit.Before;
import org.junit.Test;
import javax.validation.*;
import java.util.Set;
import static org.hamcrest.Matchers.is;
import static org.junit.Assert.assertThat;

public class MJValidatorTest {

    private Validator validator;

    @Before
    public void setUp() {
        validator = Validation.buildDefaultValidatorFactory().getValidator();
    }

    @Test
public void An error will occur if you pass an incorrect song title.() {
        TestBean bean = new TestBean("Remember a Time");
        Set<ConstraintViolation<Object>> violations = validator.validate(bean);
        assertThat(violations.isEmpty(), is(false));
        String expectedMessage = "you must specify one of Michael Jackson's songs.";
        violations.forEach(v -> assertThat(v.getMessage().equals(expectedMessage), is(true)));
    }

    @Test
public void Do not cause an error when passing the correct song title() {
        TestBean bean = new TestBean("Remember The Time");
        Set<ConstraintViolation<Object>> violations = validator.validate(bean);
        assertThat(violations.isEmpty(), is(true));
    }

    private static class TestBean {
        @MJ
        private String song;

        TestBean(String song) {
            this.song = song;
        }
    }
}

Run it and if the test passes ok.

Commentary

--Create an appropriate class as the inner class of the test class and set the annotation created this time in the field. --Running validation for an instance of that class