Building modern applications with Micronaut makes enforcing business rules super smooth. It ensures your data is solid and your app behaves just the way you want. One killer feature of Micronaut is the ability to validate stuff at compile time, which means catching potential issues early before they cause havoc in the wild.

Let’s dive into how you can use Micronaut’s automatic validation to keep your business logic tight and your application rock-solid.

Setting Up Your Micronaut App

First things first, getting your Micronaut project ready to roll with validation features requires adding some dependencies. If you’re rolling with Gradle, slap these bad boys into your build.gradle file:

dependencies {
    implementation "io.micronaut.validation:micronaut-validation"
    annotationProcessor "io.micronaut.validation:micronaut-validation-processor"
}

If Maven is more your thing, add these to your pom.xml:

<dependencies>
    <dependency>
        <groupId>io.micronaut.validation</groupId>
        <artifactId>micronaut-validation</artifactId>
    </dependency>
    <dependency>
        <groupId>io.micronaut.validation</groupId>
        <artifactId>micronaut-validation-processor</artifactId>
        <scope>annotationProcessor</scope>
    </dependency>
</dependencies>

Using Validation Annotations

Micronaut jives well with jakarta.validation annotations to validate beans. This is pretty handy. You’ll be using constraints like @NotNull, @Min, and @Max. Check out this example where we pimp out a simple Person class:

import jakarta.validation.constraints.Max;
import jakarta.validation.constraints.NotBlank;
import jakarta.validation.constraints.PositiveOrZero;

public class Person {
    @Max(10000)
    private Integer id;

    @NotBlank
    private String firstName;

    @NotBlank
    private String lastName;

    @PositiveOrZero
    private int age;

    // Getters and setters
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

Compile-Time Validation

Here’s where Micronaut shines. It performs validation checks at compile time. This means if you mess up with the annotations, it screams at you right then and there, halting the compilation process. This is possible thanks to micronaut-validation-processor in your build path.

If, for instance, you use a custom annotation incorrectly, it will get flagged during compilation:

@Retention(RetentionPolicy.RUNTIME)
public @interface TimeOff {
    @DurationPattern
    String duration();
}

// Incorrect usage
@TimeOff(duration = "nonsensicalString")
public class MyBean {
    // ...
}

The compiler won’t let you get away with using “nonsensicalString” if it doesn’t match the @DurationPattern rule.

Custom Validation Annotations

There will be times you need to enforce rules not covered by standard annotations. Creating custom validation annotations will be your jam. For example, validating phone numbers in E.164 format:

First, create the annotation:

import jakarta.validation.Constraint;
import jakarta.validation.Payload;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Repeatable;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Constraint(validatedBy = E164Validator.class)
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Repeatable(E164.List.class)
public @interface E164 {
    String message() default "must be a phone in E.164 format";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};

    @Target({ElementType.FIELD, ElementType.PARAMETER})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @interface List {
        E164[] value();
    }
}

Then, whip up the validator:

import io.micronaut.validation.validator.constraints.ConstraintValidator;
import jakarta.inject.Singleton;

@Singleton
public class E164Validator implements ConstraintValidator<E164, String> {
    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {
        // Validate the E.164 phone number format
        return value.matches("\\+\\d{1,3}[-\\.\\s]?\\(\\d{1,3}\\)?[-\\.\\s]?\\d{1,4}[-\\.\\s]?\\d{1,9}");
    }
}

Stress-Testing Your Validation

To make sure everything’s on point, you’ll want to test the validation rules. Here’s an example test case using Micronaut’s test framework:

import io.micronaut.test.annotation.MicronautTest;
import jakarta.inject.Inject;
import jakarta.validation.ConstraintViolation;
import jakarta.validation.Validator;

import java.util.Set;

@MicronautTest
public class PersonValidationTest {

    @Inject
    private Validator validator;

    @Test
    public void testValidPerson() {
        Person person = new Person();
        person.setId(123);
        person.setFirstName("John");
        person.setLastName("Doe");
        person.setAge(30);

        Set<ConstraintViolation<Person>> violations = validator.validate(person);
        Assertions.assertTrue(violations.isEmpty());
    }

    @Test
    public void testInvalidPerson() {
        Person person = new Person();
        person.setId(123);
        person.setFirstName("John");
        person.setLastName("Doe");
        person.setAge(-1); // Invalid age

        Set<ConstraintViolation<Person>> violations = validator.validate(person);
        Assertions.assertFalse(violations.isEmpty());
    }
}

Why Compile-Time Validation Rocks

Having validation checks at compile time is a game-changer. Here’s why it’s so awesome:

• Catch Errors Early: You’ll spot errors during development rather than runtime, saving a lot of headaches.
• Boost Performance: Since you avoid runtime validations, your app runs smoother and quicker.
• Smaller App Size: Micronaut’s validation approach trims down your JAR size, unlike beefier frameworks like Hibernate Validator.
• Lightning-Fast Startup: No clunky reflection-based APIs or proxies mean your app starts up much faster.
• Seamless GraalVM Compatibility: It just works natively with GraalVM!

Wrapping Up

Micronaut’s automatic validation at compile time is like having a safety net for your data integrity. Using jakarta.validation annotations and cooking up custom ones lets you clamp down on your business logic right from the get-go. This doesn’t just make your app more robust but also packs in those performance gains and keeps things lean and mean.

With Micronaut, building efficient and reliable applications is a walk in the park. Embrace it, and watch your apps run like a dream.