Spring @ConditionalOnMissingBean 注解的主要作用解析

@Configuration
public class MyAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean  // 只有在用户没有定义自己的 DataSource 时，才创建这个默认的
    public DataSource dataSource() {
        return new HikariDataSource();
    }
}

@Configuration
public class MyCustomConfig {
    @Bean
    public DataSource dataSource() {
        // 用户自定义的 DataSource
        return new MyCustomDataSource();
        // 由于这个 Bean 存在，上面的自动配置的 DataSource 就不会被创建
    }
}

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

@Bean
@ConditionalOnMissingBean(name = "customBean")
public MyService myService() {
    return new DefaultMyService();
}

@Bean
@ConditionalOnMissingBean({DataSource.class, XADataSource.class})
public DataSource dataSource() {
    return new EmbeddedDataSource();
}

@Configuration
@ConditionalOnClass(DataSource.class)
public class DataSourceAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean
    @ConditionalOnProperty(prefix = "spring.datasource", name = "type")
    public DataSource dataSource(DataSourceProperties properties) {
        // 创建默认数据源
    }
}

@Configuration
@ConditionalOnClass(CacheManager.class)
public class CacheAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean
    public CacheManager cacheManager() {
        // 提供默认的缓存管理器
    }
}

@Configuration
@ConditionalOnClass(MyService.class)  // 类路径存在时才生效
@ConditionalOnProperty(prefix = "app", name = "enabled", havingValue = "true")  // 属性条件
public class MyAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean  // 用户没有自定义时才生效
    public MyService myService() {
        return new DefaultMyService();
    }
}

@Bean
@ConditionalOnMissingBean
@Primary  // 当有多个 DataSource 时，这个作为主要的
public DataSource primaryDataSource() {
    return new PrimaryDataSource();
}