Let's dive into creating custom Spring Boot starters with conditional auto-configuration. This is a powerful way to extend Spring Boot's capabilities and make your own reusable components.

At its core, a Spring Boot starter is a convenient way to add a bunch of related dependencies and auto-configuration to your project. When you're building your own starter, you're essentially packaging up a set of libraries and configuration that can be easily added to any Spring Boot project.

The magic happens with auto-configuration. This is where Spring Boot automatically sets up beans and configurations based on what's in your classpath and what properties you've defined. When you're creating a custom starter, you can leverage this to make your component super easy to use.

Let's start with a basic example. Say we're creating a starter for a custom logging framework. Here's what the main configuration class might look like:

@Configuration
@ConditionalOnClass(CustomLogger.class)
@EnableConfigurationProperties(CustomLoggerProperties.class)
public class CustomLoggerAutoConfiguration {

    @Autowired
    private CustomLoggerProperties properties;

    @Bean
    @ConditionalOnMissingBean
    public CustomLogger customLogger() {
        return new CustomLogger(properties.getLogLevel());
    }
}

This configuration class does a few key things:

1. It's only activated if the CustomLogger class is on the classpath.
2. It enables configuration properties, allowing users to customize the logger through their application.properties file.
3. It creates a CustomLogger bean, but only if one doesn't already exist.

The @ConditionalOnClass and @ConditionalOnMissingBean annotations are crucial here. They allow your auto-configuration to play nicely with existing configurations and avoid conflicts.

Now, let's talk about the properties. You'll typically want to create a separate class for these:

@ConfigurationProperties(prefix = "custom.logger")
public class CustomLoggerProperties {
    private String logLevel = "INFO";

    // getters and setters
}

This allows users of your starter to configure the log level in their application.properties file like this:

custom.logger.log-level=DEBUG

But what if you want to provide more complex configuration options? That's where conditional beans come in handy. Let's say we want to offer different types of loggers based on the environment:

@Configuration
public class CustomLoggerAutoConfiguration {

    @Bean
    @ConditionalOnProperty(name = "custom.logger.type", havingValue = "file")
    public CustomLogger fileLogger(CustomLoggerProperties properties) {
        return new FileLogger(properties.getLogLevel());
    }

    @Bean
    @ConditionalOnProperty(name = "custom.logger.type", havingValue = "console", matchIfMissing = true)
    public CustomLogger consoleLogger(CustomLoggerProperties properties) {
        return new ConsoleLogger(properties.getLogLevel());
    }
}

Now users can choose between a file logger and a console logger (defaulting to console if not specified) simply by setting a property.

One of the trickier aspects of creating a custom starter is handling dependencies. You'll want to declare the dependencies your starter needs, but be careful not to force unnecessary dependencies on your users. Use optional dependencies where appropriate.

In your starter's pom.xml, you might have something like this:

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-autoconfigure</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-configuration-processor</artifactId>
        <optional>true</optional>
    </dependency>
    <!-- Other dependencies your starter needs -->
</dependencies>

The spring-boot-configuration-processor is marked as optional because it's only needed at compile time to process your @ConfigurationProperties classes.

Now, let's look at a more complex example. Say we're creating a starter for a custom database connection pool. We might want to auto-configure based on the presence of certain classes and properties:

@Configuration
@ConditionalOnClass(CustomConnectionPool.class)
@EnableConfigurationProperties(CustomConnectionPoolProperties.class)
public class CustomConnectionPoolAutoConfiguration {

    @Autowired
    private CustomConnectionPoolProperties properties;

    @Bean
    @ConditionalOnMissingBean
    @ConditionalOnProperty(prefix = "custom.pool", name = "enabled", havingValue = "true", matchIfMissing = true)
    public CustomConnectionPool customConnectionPool() {
        CustomConnectionPool pool = new CustomConnectionPool();
        pool.setMaxConnections(properties.getMaxConnections());
        pool.setConnectionTimeout(properties.getConnectionTimeout());
        // Set other properties...
        return pool;
    }

    @Bean
    @ConditionalOnMissingBean
    public DataSource dataSource(CustomConnectionPool pool) {
        return new CustomDataSource(pool);
    }
}

This configuration sets up a custom connection pool and a DataSource that uses it. It's only activated if the CustomConnectionPool class is available and the custom.pool.enabled property is true (or not set).

One powerful feature of Spring Boot's auto-configuration is the ability to order your configurations. This is useful when you have multiple related configurations that need to be applied in a specific order. You can use the @AutoConfigureOrder annotation for this:

@Configuration
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
public class HighPriorityConfig {
    // ...
}

@Configuration
@AutoConfigureOrder(Ordered.LOWEST_PRECEDENCE)
public class LowPriorityConfig {
    // ...
}

This ensures that HighPriorityConfig is processed before LowPriorityConfig.

When creating a custom starter, it's also important to think about how it will interact with other starters and configurations. Spring Boot provides several annotations to help with this:

• @AutoConfigureBefore and @AutoConfigureAfter let you specify that your configuration should be processed before or after certain other configurations.
• @AutoConfigureOrder gives you fine-grained control over the order of configuration processing.

Here's an example:

@Configuration
@AutoConfigureBefore(DataSourceAutoConfiguration.class)
public class CustomDataSourceConfiguration {
    // ...
}

This ensures that your custom DataSource configuration is processed before Spring Boot's standard DataSource auto-configuration.

Testing is crucial when developing a custom starter. Spring Boot provides the @ImportAutoConfiguration annotation to help with this. You can use it in your test classes to import just the auto-configuration you're testing:

@RunWith(SpringRunner.class)
@ImportAutoConfiguration(CustomLoggerAutoConfiguration.class)
public class CustomLoggerAutoConfigurationTest {

    @Autowired
    private CustomLogger logger;

    @Test
    public void testCustomLoggerCreated() {
        assertNotNull(logger);
        assertEquals("INFO", logger.getLogLevel());
    }
}

This allows you to test your auto-configuration in isolation, without loading your entire application context.

Another important aspect of creating a custom starter is providing good documentation. You should clearly explain what your starter does, what dependencies it brings in, what properties can be configured, and any other important details. Consider creating a separate README file for your starter with this information.

Remember, the goal of a starter is to make it easy for developers to add a particular piece of functionality to their Spring Boot applications. By leveraging conditional auto-configuration, you can create starters that are flexible and adapt to different environments and configurations.

As you develop more complex starters, you might find yourself needing to auto-configure multiple related beans. Let's look at an example where we're creating a starter for a custom caching solution:

@Configuration
@ConditionalOnClass(CustomCache.class)
@EnableConfigurationProperties(CustomCacheProperties.class)
public class CustomCacheAutoConfiguration {

    @Autowired
    private CustomCacheProperties properties;

    @Bean
    @ConditionalOnMissingBean
    public CustomCacheManager cacheManager() {
        return new CustomCacheManager(properties.getCacheNames());
    }

    @Bean
    @ConditionalOnMissingBean
    public CustomKeyGenerator keyGenerator() {
        return new CustomKeyGenerator();
    }

    @Bean
    @ConditionalOnProperty(prefix = "custom.cache", name = "use-custom-serializer", havingValue = "true")
    public CustomCacheSerializer cacheSerializer() {
        return new CustomCacheSerializer();
    }
}

In this example, we're setting up multiple beans related to our caching solution. The CacheManager and KeyGenerator are always created if they don't already exist, while the CacheSerializer is only created if a specific property is set.

When creating a starter, it's also important to consider how it will behave in different environments. Spring Boot's @profile annotation can be useful for this:

@Configuration
@Profile("production")
public class ProductionCacheConfig {
    @Bean
    public CustomCache productionCache() {
        return new CustomCache(1000); // larger cache for production
    }
}

@Configuration
@Profile("!production")
public class DevelopmentCacheConfig {
    @Bean
    public CustomCache developmentCache() {
        return new CustomCache(100); // smaller cache for development
    }
}

This allows your starter to adapt its configuration based on the active profile.

As your starter grows more complex, you might want to split your auto-configuration into multiple classes. Spring Boot supports this through the @import annotation:

@Configuration
@Import({CustomCacheAutoConfiguration.class, CustomSerializerAutoConfiguration.class})
public class CustomCacheStarterAutoConfiguration {
}

This main configuration class imports the other configuration classes, keeping your code organized and modular.

One often overlooked aspect of creating a custom starter is providing proper metadata for configuration properties. Spring Boot can use this metadata to provide auto-completion and documentation in IDEs. You can generate this metadata by adding the spring-boot-configuration-processor to your project and creating a META-INF/spring-configuration-metadata.json file:

{
  "properties": [
    {
      "name": "custom.cache.use-custom-serializer",
      "type": "java.lang.Boolean",
      "description": "Whether to use the custom cache serializer.",
      "defaultValue": false
    }
  ]
}

This metadata helps developers understand and use your starter more easily.

Finally, let's talk about graceful degradation. Your starter should handle cases where certain dependencies or conditions aren't met. For example:

@Configuration
@ConditionalOnClass(CustomCache.class)
public class CustomCacheAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean
    public CacheManager cacheManager() {
        try {
            return new CustomCacheManager();
        } catch (UnsupportedOperationException e) {
            // Custom cache not supported in this environment
            return new SimpleCacheManager();
        }
    }
}

This configuration attempts to create a CustomCacheManager, but falls back to a SimpleCacheManager if the custom implementation isn't supported.

Creating a custom Spring Boot starter with conditional auto-configuration is a powerful way to package and share functionality across multiple projects. By leveraging Spring Boot's extensive set of conditional annotations and configuration options, you can create starters that are flexible, adaptable, and easy to use. Remember to consider different use cases, provide good documentation, and test thoroughly. With these techniques, you'll be well on your way to creating robust and useful Spring Boot starters.

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