Spring Boot 中的 @ConditionalOnBean 注解作用及基本用法

import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import javax.sql.DataSource;
@Configuration
public class MyConfig {
    @Bean
    public DataSource dataSource() {
        // 这里模拟 DataSource 实例，实际可用 HikariDataSource、Druid 等
        return new FakeDataSource();
    }
    @Bean
    @ConditionalOnBean(DataSource.class)  // 仅当 DataSource 存在时，才创建 MyService
    public MyService myService() {
        return new MyService();
    }
}
class MyService {
    public MyService() {
        System.out.println("MyService 被创建");
    }
}
class FakeDataSource implements DataSource {
    // 这里可以模拟 DataSource 方法
}

@Bean
@ConditionalOnBean(value = DataSource.class)  // 仅当 DataSource 存在时生效
public MyRepository myRepository() {
    return new MyRepository();
}

@Bean
@ConditionalOnBean(type = "javax.sql.DataSource")  // 使用全限定类名
public MyRepository myRepository() {
    return new MyRepository();
}

@Bean
@ConditionalOnBean(annotation = Repository.class)  // 仅当存在 @Repository 注解的 Bean 时生效
public MyService myService() {
    return new MyService();
}

@Bean
@ConditionalOnBean(annotation = Repository.class)  // 仅当存在 @Repository 注解的 Bean 时生效
public MyService myService() {
    return new MyService();
}

@Bean
@ConditionalOnBean(value = DataSource.class, search = SearchStrategy.ALL) // 在所有上下文中搜索
public MyService myService() {
    return new MyService();
}

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

@Configuration
@ConditionalOnBean(DispatcherServlet.class)
public class MvcAutoConfiguration {
    // 仅当 DispatcherServlet 存在时，Spring MVC 配置生效
}

@Bean
@ConditionalOnBean(name = "redisTemplate")  // 只有当 redisTemplate 存在时才加载
public CacheManager cacheManager() {
    return new RedisCacheManager();
}

@Bean
@ConditionalOnMissingBean(DataSource.class)  // 仅当 DataSource 不存在时才创建
public DataSource defaultDataSource() {
    return new DefaultDataSource();
}