SpringBoot中的条件注解使用示例详解

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

@Configuration
public class DatabaseConfig {
    // 当类路径中存在DataSource类时才创建这个Bean
    @Bean
    @ConditionalOnClass(DataSource.class)
    public DataSource dataSource() {
        return new HikariDataSource();
    }
    // 当类路径中不存在Redis相关类时才创建内存缓存
    @Bean
    @ConditionalOnMissingClass("org.springframework.data.redis.core.RedisTemplate")
    public CacheManager memoryCache() {
        return new ConcurrentMapCacheManager();
    }
}

@Configuration
public class ServiceConfig {
    // 当容器中存在UserRepository Bean时才创建UserService
    @Bean
    @ConditionalOnBean(UserRepository.class)
    public UserService userService(UserRepository userRepository) {
        return new UserServiceImpl(userRepository);
    }
    // 当容器中不存在CacheManager时才创建默认的
    @Bean
    @ConditionalOnMissingBean(CacheManager.class)
    public CacheManager defaultCacheManager() {
        return new NoOpCacheManager();
    }
}

@Configuration
public class FeatureConfig {
    // 当spring.feature.enabled=true时才启用这个功能
    @Bean
    @ConditionalOnProperty(
        prefix = "spring.feature",
        name = "enabled",
        havingValue = "true",
        matchIfMissing = false // 默认false，即属性不存在时不匹配
    )
    public FeatureService featureService() {
        return new FeatureServiceImpl();
    }
}

@Configuration
public class DataSourceConfig {
    @Primary
    @Bean
    @ConditionalOnProperty(name = "app.datasource.primary.enabled", havingValue = "true")
    public DataSource primaryDataSource() {
        return DataSourceBuilder.create()
            .url("jdbc:mysql://localhost:3306/primary")
            .build();
    }
    @Bean
    @ConditionalOnProperty(name = "app.datasource.secondary.enabled", havingValue = "true")
    public DataSource secondaryDataSource() {
        return DataSourceBuilder.create()
            .url("jdbc:mysql://localhost:3306/secondary")
            .build();
    }
}

# META-INF/spring.factories示例
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(RedisOperations.class)  // Redis类存在时才配置
@EnableConfigurationProperties(RedisProperties.class)
@Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class })
public class RedisAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean(name = "redisTemplate")  // 用户没有自定义时才创建
    @ConditionalOnSingleCandidate(RedisConnectionFactory.class)
    public RedisTemplate<Object, Object> redisTemplate(RedisConnectionFactory redisConnectionFactory) {
        RedisTemplate<Object, Object> template = new RedisTemplate<>();
        template.setConnectionFactory(redisConnectionFactory);
        return template;
    }
    @Bean
    @ConditionalOnMissingBean
    @ConditionalOnSingleCandidate(RedisConnectionFactory.class)
    public StringRedisTemplate stringRedisTemplate(RedisConnectionFactory redisConnectionFactory) {
        return new StringRedisTemplate(redisConnectionFactory);
    }
}

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ DataSource.class, EmbeddedDatabaseType.class })
@ConditionalOnMissingBean(type = "javax.sql.DataSource")  // 没有数据源时才自动配置
@EnableConfigurationProperties(DataSourceProperties.class)
@Import({ DataSourcePoolMetadataProvidersConfiguration.class,
         DataSourceInitializationConfiguration.class })
public class DataSourceAutoConfiguration {
    @Configuration(proxyBeanMethods = false)
    @Conditional(EmbeddedDatabaseCondition.class)  // 自定义条件判断
    @ConditionalOnMissingBean({ DataSource.class, XADataSource.class })
    @Import(EmbeddedDataSourceConfiguration.class)
    protected static class EmbeddedDatabaseConfiguration {
    }
    @Configuration(proxyBeanMethods = false)
    @Conditional(PooledDataSourceCondition.class)
    @ConditionalOnMissingBean({ DataSource.class, XADataSource.class })
    @Import({ HikariConfiguration.class, TomcatConfiguration.class,
             DbcpConfiguration.class, OracleUcpConfiguration.class,
             GenericConfiguration.class, DataSourceJmxConfiguration.class })
    protected static class PooledDataSourceConfiguration {
    }
}