1. Getting Started
2. What you will need
3. Solution
4. Flyway endpoint
- 4.1. Test
5. Test Resources
6. Testing the Application
7. Running the Application
8. Generate a Micronaut Application Native Executable with GraalVM
- 8.1. Native executable generation
9. Next steps
10. Help with the Micronaut Framework

Schema Migration with Flyway

Learn how to use Flyway to manage your schema migrations

Authors: Sergio del Amo

Micronaut Version: 3.9.2

1. Getting Started

In this guide, we will create a Micronaut application written in Java.

You use Flyway:

Flyway is an open-source database migration tool. It strongly favors simplicity and convention over configuration.

2. What you will need

To complete this guide, you will need the following:

Some time on your hands
A decent text editor or IDE
JDK 1.8 or greater installed with JAVA_HOME configured appropriately

3. Solution

We recommend that you follow the instructions in the next sections and create the application step by step. However, you can go right to the completed example.

Download and unzip the source

Create an application using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app example.micronaut.micronautguide \
   --features=data-jdbc,mysql,flyway \
   --build=maven --lang=java

If you don’t specify the --build argument, Gradle is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.

The previous command creates a Micronaut application with the default package example.micronaut in a directory named micronautguide.

If you use Micronaut Launch, select "Micronaut Application" as application type and add mysql, data-jdbc, and flyway as features.

Setup IntelliJ IDEA to develop Micronaut Applications.

3.1. Create Entity

Create a @MappedEntity to save persons. Initially, consider name and age required. Use int primitive for the age.

src/main/java/example/micronaut/Person.java

package example.micronaut;

import io.micronaut.core.annotation.NonNull;
import io.micronaut.core.annotation.Nullable;
import io.micronaut.data.annotation.GeneratedValue;
import io.micronaut.data.annotation.Id;
import io.micronaut.data.annotation.MappedEntity;
import io.micronaut.data.annotation.Version;

import javax.validation.constraints.NotBlank;

@MappedEntity (1)
public class Person {

    @Id (2)
    @GeneratedValue (3)
    private Long id;

    @Version (4)
    private Long version;

    @NonNull
    @NotBlank
    private final String name;

    private final int age;

    public Person(@NonNull String name, int age) {
        this.name = name;
        this.age = age;
    }

    public int getAge() {
        return age;
    }


    @NonNull
    public String getName() {
        return name;
    }

    public Long getId() {
        return id;
    }

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

    public Long getVersion() {
        return version;
    }

    public void setVersion(Long version) {
        this.version = version;
    }
}

1	Annotate the class with `@MappedEntity` to map the class to the table defined in the schema.
2	Specifies the ID of an entity
3	Specifies that the property value is generated by the database and not included in inserts
4	Annotate the field with `@Version` to enable optimistic locking for your entity.

3.2. Database Migration with Flyway

We need a way to create the database schema. For that, we use Micronaut integration with Flyway.

Flyway automates schema changes, significantly simplifying schema management tasks, such as migrating, rolling back, and reproducing in multiple environments.

Add the following snippet to include the necessary dependencies:

pom.xml

<dependency>
    <groupId>io.micronaut.flyway</groupId>
    <artifactId>micronaut-flyway</artifactId>
    <scope>compile</scope>
</dependency>
<dependency>
    <groupId>org.flywaydb</groupId>
    <artifactId>flyway-mysql</artifactId>
    <scope>runtime</scope>
</dependency>

We will enable Flyway in application.yml and configure it to perform migrations on one of the defined data sources.

src/main/resources/application.yml

flyway:
  datasources:
    default:
      enabled: true (1)

1	Enable Flyway for the `default` datasource.

Configuring multiple data sources is as simple as enabling Flyway for each one. You can also specify directories that will be used for migrating each data source. Review the Micronaut Flyway documentation for additional details.

Flyway migration will be automatically triggered before your Micronaut application starts. Flyway will read migration commands in the resources/db/migration/ directory, execute them if necessary, and verify that the configured data source is consistent with them.

Create the following migration files with the database schema creation:

src/main/resources/db/migration/V1__create-person.sql

CREATE TABLE person(
    id   bigint primary key not null,
    name varchar(255)       not null,
    age  int                not null
)

During application startup, Flyway executes the SQL file and creates the schema needed for the application.

If you check the database schema, there are two tables:

person
flyway_scheme_history

Flyway uses the table flyway_scheme_history to keep track of database migrations.

The person table looks like:

Column Nullable

Column	Nullable
`id`	NO
`version`	NO
`name`	NO
`age`	NO

id

version

name

age

3.3. Drop Not Null Constraint

Applications change. Make age optional:

src/main/java/example/micronaut/Person.java

    @Nullable
    private final Integer age;

    public Person(@NonNull String name,
                  @Nullable Integer age) {
        this.name = name;
        this.age = age;
    }

    @Nullable
    public Integer getAge() {
        return age;
    }

Add a new migration to drop the null constraint:

src/main/resources/db/migration/V2__nullable-age.sql

ALTER TABLE person MODIFY age int default null;

After the migration, the person table looks like:

Column Nullable

Column	Nullable
`id`	NO
`version`	NO
`name`	NO
`age`	YES

id

version

name

age

YES

4. Flyway endpoint

To enable the Flyway endpoint, add the management dependency on your classpath.

pom.xml

<dependency>
    <groupId>io.micronaut</groupId>
    <artifactId>micronaut-management</artifactId>
    <scope>compile</scope>
</dependency>

Enable the Flyway endpoint:

src/main/resources/application.yml

endpoints:
  flyway:
    enabled: true
    sensitive: false

4.1. Test

Create a test that invokes the Flyway endpoint.

src/test/java/example/micronaut/FlywayEndpointTest.java

package example.micronaut;

import io.micronaut.core.type.Argument;
import io.micronaut.http.HttpRequest;
import io.micronaut.http.HttpResponse;
import io.micronaut.http.client.BlockingHttpClient;
import io.micronaut.http.client.HttpClient;
import io.micronaut.http.client.annotation.Client;
import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
import jakarta.inject.Inject;
import org.junit.jupiter.api.Test;

import java.util.List;

import static io.micronaut.http.HttpStatus.OK;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertNotNull;

@MicronautTest (1)
public class FlywayEndpointTest {

    @Inject
    @Client("/")  (2)
    HttpClient httpClient;

    @Test
    void migrationsAreExposedViaAndEndpoint() {
        BlockingHttpClient client = httpClient.toBlocking();

        HttpResponse<List<FlywayReport>> response = client.exchange(
                HttpRequest.GET("/flyway"),
                Argument.listOf(FlywayReport.class));
        assertEquals(OK, response.status());

        List<FlywayReport> flywayReports = response.body();
        assertNotNull(flywayReports);
        assertEquals(1, flywayReports.size());

        FlywayReport flywayReport = flywayReports.get(0);
        assertNotNull(flywayReport);
        assertNotNull(flywayReport.getMigrations());
        assertEquals(2, flywayReport.getMigrations().size());
    }

    static class FlywayReport {
        private List<Migration> migrations;

        public void setMigrations(List<Migration> migrations) {
            this.migrations = migrations;
        }

        public List<Migration> getMigrations() {
            return migrations;
        }
    }

    static class Migration {
        private String script;

        public void setId(String script) {
            this.script = script;
        }

        public String getScript() {
            return script;
        }
    }
}

1	Annotate the class with `@MicronautTest` so the Micronaut framework will initialize the application context and the embedded server. More info.
2	Inject the `HttpClient` bean and point it to the embedded server.

5. Test Resources

When the application is started locally — either under test or by running the application — resolution of the datasource URL is detected and the Test Resources service will start a local MySQL docker container, and inject the properties required to use this as the datasource.

For more information, see the JDBC section or R2DBC section of the Test Resources documentation.

6. Testing the Application

To run the tests:

./mvnw test

7. Running the Application

To run the application, use the ./mvnw mn:run command, which starts the application on port 8080.

You can run a cURL command to test the application:

curl http://localhost:8080/flyway

You will see information about migrations.

8. Generate a Micronaut Application Native Executable with GraalVM

We will use GraalVM, the polyglot embeddable virtual machine, to generate a native executable of our Micronaut application.

Compiling native executables ahead of time with GraalVM improves startup time and reduces the memory footprint of JVM-based applications.

Only Java and Kotlin projects support using GraalVM’s native-image tool. Groovy relies heavily on reflection, which is only partially supported by GraalVM.

8.1. Native executable generation

The easiest way to install GraalVM on Linux or Mac is to use SDKMan.io.

Java 11

sdk install java 22.3.r11-grl

If you still use Java 8, use the JDK11 version of GraalVM.

Java 17

sdk install java 22.3.r17-grl

For installation on Windows, or for manual installation on Linux or Mac, see the GraalVM Getting Started documentation.

After installing GraalVM, install the native-image component, which is not installed by default:

gu install native-image

To generate a native executable using Maven, run:

./mvnw package -Dpackaging=native-image

The native executable is created in the target directory and can be run with target/micronautguide.