Spring Boot条件化 Bean 注册机制实战案例解析

@Configuration
public class DataSourceConfig {
    @Bean
    public DataSource dataSource() {
        if ("dev".equals(env.getProperty("app.profile"))) {
            return new H2DataSource();
        } else {
            return new MySQLDataSource();
        }
    }
}

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {
    Class<? extends Condition>[] value();
}

public interface Condition {
    boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);
}

@Configuration
public class FeatureConfig {
    @Bean
    @ConditionalOnProperty(name = "app.feature.report.enabled", havingValue = "true")
    public ReportService reportService() {
        return new AdvancedReportService();
    }
    @Bean
    @ConditionalOnMissingBean(ReportService.class) // 若未启用高级功能，提供基础实现
    public ReportService defaultReportService() {
        return new BasicReportService();
    }
}

app:
  feature:
    report:
      enabled: true  # 设为 false 则使用 BasicReportService

// 仅当 MyBatis 存在且用户配置了 secondary 数据源时启用
@Configuration
@ConditionalOnClass(SqlSessionFactory.class)
@ConditionalOnProperty(prefix = "spring.datasource.secondary", name = "url")
public class SecondaryDataSourceAutoConfiguration {
    @Bean
    @ConfigurationProperties("spring.datasource.secondary")
    @ConditionalOnMissingBean(name = "secondaryDataSource")
    public DataSource secondaryDataSource() {
        return DataSourceBuilder.create().build();
    }
    @Bean
    @ConditionalOnMissingBean
    public SqlSessionFactory secondarySqlSessionFactory(
            @Qualifier("secondaryDataSource") DataSource dataSource) throws Exception {
        SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
        factory.setDataSource(dataSource);
        return factory.getObject();
    }
}

@Configuration
public class DatabaseConfig {
    @Bean
    @ConditionalOnMissingBean(DataSource.class) // 用户未自定义 DataSource
    @ConditionalOnClass(H2.class)              // H2 在类路径中（通常是 test scope）
    @Profile("test")
    public DataSource embeddedH2DataSource() {
        return new EmbeddedDatabaseBuilder()
                .setType(EmbeddedDatabaseType.H2)
                .addScript("schema.sql")
                .build();
    }
    @Bean
    @ConditionalOnMissingBean(DataSource.class)
    @ConditionalOnProperty(prefix = "spring.datasource", name = "url")
    public DataSource productionDataSource() {
        // 从配置创建真实数据源
        return DataSourceBuilder.create().build();
    }
}

public class OnLinuxCondition implements Condition {
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        String osName = context.getEnvironment().getProperty("os.name");
        return osName != null && osName.toLowerCase().contains("linux");
    }
}
// 使用
@Bean
@Conditional(OnLinuxCondition.class)
public SystemMonitor linuxSystemMonitor() {
    return new LinuxSystemMonitor();
}

public class RedisAvailableCondition extends AllNestedConditions {
    public RedisAvailableCondition() {
        super(ConfigurationPhase.REGISTER_BEAN);
    }
    @ConditionalOnClass(RedisTemplate.class)
    static class RedisClientPresent {}
    @ConditionalOnProperty(name = "spring.redis.host")
    static class RedisHostConfigured {}
}

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(RedisOperations.class)
@EnableConfigurationProperties(RedisProperties.class)
@Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class })
public class RedisAutoConfiguration {
    // ...
}