Spring Boot - Create a Custom Auto-Configuration

Creating a custom auto-configuration in Spring Boot allows you to simplify the process of configuring and setting up beans in various projects by providing default beans and configurations. This is useful when you have a library or a set of beans that you use across multiple projects.

Here's a step-by-step guide to creating a custom auto-configuration:

1. Project Setup

Start by creating a new Spring Boot project or use an existing library project where you want to add auto-configuration.

Add the required dependency:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-autoconfigure</artifactId>
</dependency>

2. Define Your Configuration

Create a configuration class that contains the beans and properties you want to auto-configure.

@Configuration
public class MyCustomAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean
    public MyService myService() {
        return new MyServiceImpl();
    }

    // Add more beans and configurations as needed
}

@ConditionalOnMissingBean ensures that the bean is only created if there isn't already one of the same type in the Spring context.

3. Register Your Auto-Configuration

Create a file named spring.factories inside src/main/resources/META-INF and register your configuration class:

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.example.MyCustomAutoConfiguration

4. Conditional Configuration

Often, you'll want your auto-configuration to be conditional, i.e., to only apply under certain conditions.

Spring Boot provides various annotations, such as:

@ConditionalOnClass: The configuration will load if a specific class is on the classpath.
@ConditionalOnProperty: The configuration will load based on a specific property value.
@ConditionalOnBean: The configuration will load if a specific bean is defined.

For example, to only create a bean if a certain class exists on the classpath:

@Bean
@ConditionalOnClass(SomeClass.class)
public MyService myService() {
    return new MyServiceImpl();
}

5. Configuration Properties

To allow customization of your auto-configuration, you can use @ConfigurationProperties:

Create a properties class:

@ConfigurationProperties(prefix = "myservice")
public class MyServiceProperties {

    private String someProperty;

    // getters and setters
}

@EnableConfigurationProperties(MyServiceProperties.class)

Now, you can use properties like myservice.some-property in your application.properties or application.yml to customize your auto-configuration.

6. Package and Distribute

Finally, package your project as a JAR and include it as a dependency in any project where you want this auto-configuration to apply.

7. Testing

You can also add integration tests to ensure your auto-configuration works as expected using @SpringBootTest and setting specific properties for the test context.

Remember, the order of auto-configuration can matter. Spring Boot allows you to control the ordering using @AutoConfigureBefore, @AutoConfigureAfter, and @AutoConfigureOrder annotations if needed.

This is a basic introduction to creating a custom auto-configuration in Spring Boot. Depending on your requirements, you might need to delve deeper into the rich set of features and annotations provided by Spring Boot.

Auto-configuring beans in Spring Boot with custom annotations:

Define custom annotations and use them to conditionally enable or disable auto-configured beans.

Example:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(MyAutoConfiguration.class)
public @interface EnableMyCustomFeature {
    // Marker annotation to enable custom feature
}

@Configuration
public class MyAutoConfiguration {
    // Auto-configure beans based on the presence of EnableMyCustomFeature
}

Conditional custom auto-configuration in Spring Boot:

Use @ConditionalOnProperty, @ConditionalOnClass, or other conditions to conditionally apply auto-configuration.

Example:

@Configuration
@ConditionalOnProperty(name = "my.custom.feature.enabled", havingValue = "true")
public class MyAutoConfiguration {
    // Auto-configure beans if my.custom.feature.enabled property is true
}

Externalizing properties for custom auto-configuration in Spring Boot:
- Externalize properties for configuration using application.properties or application.yml.
- Example (application.properties):
```
my.custom.feature.enabled=true
```
Ordering and precedence in custom auto-configuration classes:
- Use @AutoConfigureOrder or implement Ordered to control the order of custom auto-configuration classes.
- Example:
```
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration
public class MyHighPriorityAutoConfiguration {
    // Auto-configure with highest precedence
}
```

Testing custom auto-configuration in Spring Boot:

Write tests to ensure that auto-configuration classes work as expected.

Example:

@SpringBootTest(classes = MyTestConfiguration.class)
public class MyAutoConfigurationTests {
    // Test auto-configured beans
}

Integrating custom auto-configuration with existing Spring Boot projects:
- Include the custom starter as a dependency in existing Spring Boot projects.
- Annotate the main class with @EnableMyCustomFeature to activate the custom feature.
- Example:
```
@SpringBootApplication
@EnableMyCustomFeature
public class MyApplication {
    // Existing Spring Boot application
}
```