Analysis of @ConditionalOnBean annotation scenario in Spring Boot

1. Preface

In Spring Boot,Conditional annotationis a powerful feature that allows us to dynamically register or skip specific beans based on certain conditions. in,@ConditionalOnBeanIt is one of the most commonly used conditional annotations, and its function is:The current bean is registered only when the specified bean exists in the Spring container.。

This article will introduce in detail@ConditionalOnBeanUse scenarios, principles, and provide multiple examples to help understand.

2. @ConditionalOnBean Functions and Basic Usage

2.1 The role of @ConditionalOnBean

@ConditionalOnBeanMainly used in the following scenarios:

• Loading beans on demand: Only when a bean exists, another bean is created.
• Modular design: Some functional modules need to rely on specific beans to enable, for exampleAutomatic configuration will only take effect if a component exists。
• Avoid Bean Conflicts: If a bean depends on other beans, it can be used@ConditionalOnBeanMake sure it does not fail to load due to lack of dependencies.

2.2 Basic usage

Example: WhenDataSourceCreated when a bean existsMyService Bean

import ;
import ;
import ;
import ;
@Configuration
public class MyConfig {
    @Bean
    public DataSource dataSource() {
        // Simulate DataSource instance here, and HikariDataSource, Druid, etc. are actually available        return new FakeDataSource();
    }
    @Bean
    @ConditionalOnBean()  // Create MyService only if DataSource exists    public MyService myService() {
        return new MyService();
    }
}
class MyService {
    public MyService() {
        ("MyService was created");
    }
}
class FakeDataSource implements DataSource {
    // Here you can simulate the DataSource method}

Execution results：

MyService was created

ifdataSource()The method is commented out, thenMyServiceWill not be created.

3. @ConditionalOnBean Details

@ConditionalOnBeanProvides multiple attributes to enable more flexible control of the creation of beans.

3.1 value and type attributes (specify the Bean type)

Used to specify that a certain type of bean exists, the current bean will be registered.

@Bean
@ConditionalOnBean(value = )  // Take effect only if DataSource existspublic MyRepository myRepository() {
    return new MyRepository();
}

Equivalent to:

@Bean
@ConditionalOnBean(type = "")  // Use fully qualified class namespublic MyRepository myRepository() {
    return new MyRepository();
}

the difference：

• value: Use the Class type directly, and it is safer to check when compiling.
• type: Use strings, which can be used to avoid some classes being not found (such as optional dependencies).

3.2 name attribute (specify the bean name)

ForSpecify whether a bean name existsto determine whether the current bean is loaded.

@Bean
@ConditionalOnBean(name = "customBean")  // Register only if a bean named customBean existspublic MyComponent myComponent() {
    return new MyComponent();
}

3.3 annotation attribute (specify the annotation that the bean needs to be annotated)

You can specify whether certain beans contain specific annotations, and if so, the current bean will be registered.

@Bean
@ConditionalOnBean(annotation = )  // Effective only if a bean with @Repository annotation existspublic MyService myService() {
    return new MyService();
}

3.4 search attributes (search scope)

By default,@ConditionalOnBeanWill only be inCurrent application contextFind beans, not searchFather context(i.e. Spring Boot'sApplicationContextlevel).

searchOptions can specify the search range:

• ALL: in all fathers and sonsApplicationContextSearch in.
• CURRENT(Default): Search only for the currentApplicationContext。

@Bean
@ConditionalOnBean(value = , search = ) // Search in all contextspublic MyService myService() {
    return new MyService();
}

4.@ConditionalOnBean usage scenario

Scenario 1: Loading database-related beans on demand

If in the applicationUsed the database, then provide aDatabaseService, otherwise it will not be created:

@Bean
@ConditionalOnBean()
public DatabaseService databaseService() {
    return new DatabaseService();
}

Scenario 2: Enable some automatic configurations

Spring Boot'sspring-boot-autoconfigureMassive use of modules@ConditionalOnBeanto control automatic configuration. For example:

Only whenDispatcherServletWhen it exists,Spring MVC-related automatic configuration will only take effect。

@Configuration
@ConditionalOnBean()
public class MvcAutoConfiguration {
    // Spring MVC configuration takes effect only if the DispatcherServlet exists}

Scenario 3: Optional dependency components

Sometimes, certain features are optional, such as creating a cache manager when a Redis component exists:

@Bean
@ConditionalOnBean(name = "redisTemplate")  // Load only when redisTemplate existspublic CacheManager cacheManager() {
    return new RedisCacheManager();
}

5. @ConditionalOnBean vs @ConditionalOnMissingBean

Example:

@Bean
@ConditionalOnMissingBean()  // Create only if DataSource does not existpublic DataSource defaultDataSource() {
    return new DefaultDataSource();
}

6. Conclusion

In Spring Boot,@ConditionalOnBeanCan help us accordingIs there a specific beanComeDynamic registration Bean, widely usedLoading on demand, automatic configurationetc.

Summarize:

✅ Specify the bean type：@ConditionalOnBean()
✅ Specify the Bean name：@ConditionalOnBean(name = "customBean")
✅ Specify Bean Annotation：@ConditionalOnBean(annotation = )
✅ Search range：@ConditionalOnBean(search = )

You've used it in the project@ConditionalOnBeanIs it? Welcome to leave a message to share your experience! 🚀

This is the end of this article about the detailed explanation of @ConditionalOnBean annotation in Spring Boot. For more related Spring Boot @ConditionalOnBean annotation, please search for my previous articles or continue browsing the related articles below. I hope everyone will support me in the future!