
## What is Flyway

[Flyway](https://flywaydb.org/) is an open-source database version control tool implemented in Java. It offers various methods such as CLI, Java API, Maven/Gradle Plugin, etc., making it convenient for developers to manage and deploy changes to database table structures and contents in traceable code form. Flyway supports most mainstream relational databases, including MySQL, SQL Server, Oracle Database, PostgreSQL, SQLite, TiDB, MariaDB, etc. Support for MongoDB is still in the preview stage (Mongock is recommended instead). Its counterpart, Liquibase, is more complex to use compared to Flyway, with additional concepts serving as a learning curve.

<div>
  <github user="flyway" repo="flyway"></github>
</div>

## Integration with Spring Boot

Following Spring Boot's well-known convention-over-configuration principle, the Spring team provides [Flyway Auto Configuration](https://github.com/spring-projects/spring-boot/tree/main/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/flyway).

```java {2-4} showLineNumbers
@AutoConfiguration(after = { DataSourceAutoConfiguration.class, JdbcTemplateAutoConfiguration.class,HibernateJpaAutoConfiguration.class })
@ConditionalOnClass(Flyway.class)
@Conditional(FlywayDataSourceCondition.class)
@ConditionalOnProperty(prefix = "spring.flyway", name = "enabled", matchIfMissing = true)
@Import(DatabaseInitializationDependencyConfigurer.class)
@ImportRuntimeHints(FlywayAutoConfigurationRuntimeHints.class)
public class FlywayAutoConfiguration { /*...*/ }
```

The fully qualified class name `org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration` will perform automatic configuration when the following conditions are detected:

1. The `Flyway` class exists (class name: `org.flywaydb.core.Flyway`)
2. Bean or properties conditions are met in the `FlywayDataSourceCondition` class, meaning connection configurations such as `DataSource` and database URL exist.
3. The property configuration `spring.flyway.enabled=true` exists.

For MySQL, besides the required JDBC dependency, you need to include Flyway's own dependencies. Spring Boot has already declared the version number in the POM, so no additional version field is needed here:

```xml
<dependency>
    <groupId>org.flywaydb</groupId>
    <artifactId>flyway-core</artifactId>
</dependency>
<dependency>
    <groupId>org.flywaydb</groupId>
    <artifactId>flyway-mysql</artifactId>
</dependency>
```

There are two ways to use it:

1. Native SQL
2. Java API

Flyway will discover version change files or implementation classes based on the default configured folder path `classpath:db/migration`.

```shell
V{VERSION}__{DESCRIPTION}.sql
V{VERSION}__{DESCRIPTION}.java
```

`VERSION` can be a string version containing dots and underscores, and `DESCRIPTION` is a simple description text. The separator in the middle is two underscores. Custom prefixes and suffixes can be configured through spring.flyway.

The `V{VERSION}__{DESCRIPTION}.sql` file needs to be placed in the `src/main/resources/db/migration` folder, while Java classes need to define the package name as db.migration and inherit the superclass `BaseJavaMigration`. We need to do is overriding the following method:

```java
void migrate(Context context) throws Exception;
```

According to the official documentation, we can implement database content version changes using Java code in the `migrate()` method. However, the drawback here is that we cannot perform dependency injection for `V{VERSION}__{DESCRIPTION}.java` in the Spring environment. We can only attempt to use plain SQL implementations through native JDBC methods, static methods, etc., rather than fully utilizing existing DAO interfaces for business data updates. Therefore, custom configuration of Flyway is required.

## Performing Dependency Injection into Flyway Class

```java
package me.lawrenceli.migration.config;

import jakarta.annotation.PostConstruct;
import org.flywaydb.core.Flyway;
import org.flywaydb.core.api.migration.JavaMigration;
import org.flywaydb.core.api.output.MigrateResult;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.Configuration;

import javax.sql.DataSource;

@Configuration
public class FlywayConfig {
    // Custom migration history table name
    private static final String SCHEMA_HISTORY_TABLE = "schema_changes";

    @Autowired
    private DataSource dataSource;

    @Autowired
    private ApplicationContext applicationContext;

    @PostConstruct
    public void migrate() {
        log.info("Flyway, starting!");
        // Retrieving all migration implementation classes through the Spring container
        // All implementation classes no longer need to be defined in the `db.migration` package,
        // they can be placed in any location that supports Bean scanning
        JavaMigration[] migrationBeans = applicationContext
                .getBeansOfType(JavaMigration.class)
                .values()
                .toArray(new JavaMigration[0]);
        Flyway flyway = Flyway.configure()
                .dataSource(dataSource) // No need to configure flyway JDBC URL by using the original DataSource
                .locations("db/migration") // Default migration script path
                .table(SCHEMA_HISTORY_TABLE) // Default migration history table is `flyway_schema_history`
                .baselineOnMigrate(true) // Default false, must be set to true for initial migration on existing databases
                .baselineVersion("0") // Default "1"
                .executeInTransaction(true) // Performing migration as a transaction
                .installedBy(applicationContext.getId()) // Using microservice name as migration executor
                .javaMigrations(migrationBeans) // Registering migration classes
                .load();
        MigrateResult migrate = flyway.migrate(); // Executing migration by calling subclass implementations sequentially
        log.info("Flyway migrated {} versions. {}", migrate.migrationsExecuted, migrate.success);
    }
}

```

Since Flyway's configuration is based on manual configuration like this, it's necessary to exclude the original auto-configuration classes in the Spring Boot startup class to avoid conflicts in auto-configuration loading.

```java
@SpringBootApplication(exclude = FlywayAutoConfiguration.class)
public class MyApplication {

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

}
```

Finally, define a Component Bean to implement `BaseJavaMigration`:

```java {12} showLineNumbers caption="Example Migration Component"
package me.lawrenceli.balabala.migration;

import org.flywaydb.core.api.migration.BaseJavaMigration;
import org.flywaydb.core.api.migration.Context;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class V2__QueryExample extends BaseJavaMigration {

    @Autowired
    private MyMapper myMapper; // Bean of DAO

    @Override
    public void migrate(Context context) throws Exception {
        Data data = myMapper.selectById(2024L);
        // ... other CRUD codes with Java
    }
}
```

Now, all migration classes can easily use dependency injection to perform CRUD operations. In practice, Flyway will execute all migrations synchronously after database connection configuration and before HTTP service exposure, ensuring that timing does not affect online services.

## Reference

- <https://github.com/flyway/flyway/issues/1062>


Increase reliability of deployments by versioning your database

Dependency Injection with Flyway


## Flyway 是什么

[Flyway](https://flywaydb.org/) 是一款开源的，基于 Java 实现的数据库内容变更控制工具。它提供了 CLI、Java API、Maven/Gradle Plugin 等多种方式方便开发人员将数据库的表结构改动、内容变动以可追溯的代码形式进行管理和部署。Flyway 支持包括大多数主流的关系型数据库：MySQL、SQL Server、Oracle Database、PostgreSQL、SQLite、TiDB、MariaDB 等，对于 MongoDB 的支持尚在预览阶段（更推荐 Mongock），同类竞品有 Liquibase，但 Liquibase 的使用相比 Flyway 更复杂，有额外的概念作为学习成本。

<div>
  <github user="flyway" repo="flyway"></github>
</div>

## 配合 Spring Boot 使用

遵循 SpringBoot 广为人知的约定大于配置，Spring 官方提供了 [Flyway 的自动配置实现](https://github.com/spring-projects/spring-boot/tree/main/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/flyway)。

```java {2-4} showLineNumbers
@AutoConfiguration(after = { DataSourceAutoConfiguration.class,JdbcTemplateAutoConfiguration.class,HibernateJpaAutoConfiguration.class })
@ConditionalOnClass(Flyway.class)
@Conditional(FlywayDataSourceCondition.class)
@ConditionalOnProperty(prefix = "spring.flyway", name = "enabled", matchIfMissing = true)
@Import(DatabaseInitializationDependencyConfigurer.class)
@ImportRuntimeHints(FlywayAutoConfigurationRuntimeHints.class)
public class FlywayAutoConfiguration { /*...*/ }
```

全限定类名 `org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration` 会在检测到以下几种状态下实现自动配置：

1. 存在 `Flyway` 类 (即 `org.flywaydb.core.Flyway`)
2. 满足 `FlywayDataSourceCondition` 类中的 Bean 或 Properties 条件，其实就是 `DataSource` 和数据库 URL 等连接配置存在
3. 存在 `spring.flyway.enabled=true` 属性配置

对于 MySQL 而言，除了必须的 JDBC 依赖之外，需要引入 Flyway 自身依赖，SpringBoot 已经在 pom 中声明过版本号，因此此处无需额外定义版本字段:

```xml
<dependency>
    <groupId>org.flywaydb</groupId>
    <artifactId>flyway-core</artifactId>
</dependency>
<dependency>
    <groupId>org.flywaydb</groupId>
    <artifactId>flyway-mysql</artifactId>
</dependency>
```

具体使用有两种方式：

1. 原生 SQL
2. Java API

Flyway 会基于默认配置的文件夹路径 classpath:db/migration 发现版本变更文件或实现类。

```shell
V{VERSION}__{DESCRIPTION}.sql
V{VERSION}__{DESCRIPTION}.java
```

`VERSION` 可使用包含小数点、下划线的字符串版本，`DESCRIPTION` 则是简单的描述文本。中间的分割符是两个下划线。可通过 spring.flyway 配置自定义前后缀。

`V{VERSION}__{DESCRIPTION}.sql` 文件需要存放在 src/main/resources/db/migration 文件夹下; Java 类则需要定义包名 `db.migration` 并且继承父类 `BaseJavaMigration`，重写如下方法:

```java
void migrate(Context context) throws Exception;
```

按照官方的写法，我们就可以在 `migrate()` 方法中去用 Java 代码实现数据库内容版本变动了，但是这里有一个缺点，那就是我们在 Spring 环境下无法针对 `V{VERSION}__{DESCRIPTION}.java` 进行依赖注入，只能尝试通过原生 JDBC 的方式、静态方法等去写比较朴素的 SQL 实现，而不能充分成分利用现有的 DAO 接口来做业务数据的更新。因此需要自定义配置 Flyway。

## 向 Flyway Migration 实现类进行依赖注入

```java
package me.lawrenceli.migration.config;

import jakarta.annotation.PostConstruct;
import org.flywaydb.core.Flyway;
import org.flywaydb.core.api.migration.JavaMigration;
import org.flywaydb.core.api.output.MigrateResult;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.Configuration;

import javax.sql.DataSource;

@Configuration
public class FlywayConfig {
    // 自定义迁移历史表名
    private static final String SCHEMA_HISTORY_TABLE = "schema_changes";

    @Autowired
    private DataSource dataSource;

    @Autowired
    private ApplicationContext applicationContext;

    @PostConstruct
    public void migrate() {
        log.info("Flyway, 启动!");
        // 通过 Spring 容器获取所有迁移实现类
        // 这样一来，所有实现类就不再需要定义在 package `db.migration` 下，可以放在任何支持 Bean 扫描的位置。
        JavaMigration[] migrationBeans = applicationContext
                .getBeansOfType(JavaMigration.class)
                .values()
                .toArray(new JavaMigration[0]);
        Flyway flyway = Flyway.configure()
                .dataSource(dataSource) // 通过原本的 DataSource Bean 实现无需配置 flyway 自身的 JDBC URL
                .locations("db/migration") // 默认迁移脚本路径
                .table(SCHEMA_HISTORY_TABLE) // 默认迁移历史表为 `flyway_schema_history`
                .baselineOnMigrate(true) // 默认 false, 对以存在的数据库做首次迁移必须设置开启
                .baselineVersion("0") // 默认 "1"
                .executeInTransaction(true) // 将迁移作为事务，你懂的
                .installedBy(applicationContext.getId()) // 将微服务名作为迁移执行者
                .javaMigrations(migrationBeans) // 注册迁移类
                .load();
        MigrateResult migrate = flyway.migrate(); // 执行迁移，依次调用子类实现
        log.info("Flyway 迁移了 {} 版. {}", migrate.migrationsExecuted, migrate.success);
    }
}
```

由于 Flyway 的配置基于这种手动配置，因此需要在 SpringBoot 启动类上排除原有的自动配置类，以防止自动配置存在加载冲突。

```java
@SpringBootApplication(exclude = FlywayAutoConfiguration.class)
public class MyApplication {

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

}
```

最后，定义一个 Component Bean 去实现 `BaseJavaMigration`:

```java {12} showLineNumbers caption="Example Migration Component"
package me.lawrenceli.balabala.migration;

import org.flywaydb.core.api.migration.BaseJavaMigration;
import org.flywaydb.core.api.migration.Context;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class V2__QueryExample extends BaseJavaMigration {

    @Autowired
    private MyMapper myMapper; // Bean of DAO

    @Override
    public void migrate(Context context) throws Exception {
        Data data = myMapper.selectById(2024L);
        // ... other CRUD codes with Java
    }
}
```

这样，所有的迁移类都可以方便地使用依赖注入来愉快地做 CRUD 了。经过实践，Flyway 会在数据库连接配置后、HTTP 服务暴露(也就是 Servlet 容器监听端口)前同步地执行完所有迁移，因此无需担心执行时机影响线上服务。

## 参考

- <https://github.com/flyway/flyway/issues/1062>


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