1. @ConditionalOnMissingBean

案例：
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration

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

    @Bean
    @ConditionalOnMissingBean(name = "redisTemplate")
    public RedisTemplate<Object, Object> redisTemplate(
            RedisConnectionFactory redisConnectionFactory) throws UnknownHostException {
        RedisTemplate<Object, Object> template = new RedisTemplate<>();
        template.setConnectionFactory(redisConnectionFactory);
        return template;
    }

    @Bean
    @ConditionalOnMissingBean
    public StringRedisTemplate stringRedisTemplate(
            RedisConnectionFactory redisConnectionFactory) throws UnknownHostException {
        StringRedisTemplate template = new StringRedisTemplate();
        template.setConnectionFactory(redisConnectionFactory);
        return template;
    }

}

2. @ConditionalOnProperty

注意点：

1. @Bean的方法名字不能相同，否则@ConditionalOnProperty配置失效；
2. 配置中使用枚举类型时，@ConditionalOnProperty中的值是小写；
3. 当配置文件中没有对应配置项时，可以使用@ConditionalOnProperty的matchIfMissing = true表示，未配置时默认初始化某bean。

场景：实现了Cache，可以根据配置文件的值灵活的切换缓存。

@Getter
public enum CacheEnum {
    MEMORY, REDIS;
}

配置文件：

@Data
@ConfigurationProperties("base")
public class BaseConfigProperties {
    /**
     * 缓存类型，默认内存级别的缓存
     * 默认支持本地缓存和Redis分布式缓存
     */
    private CacheEnum cache;

}

@Slf4j
@Configuration
@EnableConfigurationProperties(value = BaseConfigProperties.class)
public class OSAutoConfiguration {


    @Configuration
    @ConditionalOnClass(org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration.class)
    @ConditionalOnProperty(name = "base.cache", havingValue = "redis")
    protected static class RedisCacheManagerConfiguration {

        @Bean
        @ConditionalOnMissingBean
        public CacheManager redisCacheManager(StringRedisTemplate stringRedisTemplate) {
            log.info("启用redis缓存...");
            return new CacheManager() {
                @Override
                public void put(String key, String value, long time) {
                    stringRedisTemplate.opsForValue().set(key, value, time, TimeUnit.MILLISECONDS);
                }

                @Override
                public String get(String key) {
                    return stringRedisTemplate.opsForValue().get(key);
                }

                @Override
                public void remove(String key) {
                    stringRedisTemplate.delete(key);
                }
            };
        }
    }

    /**
     * 内存级别的缓存
     */
    @Bean
    @ConditionalOnMissingBean
    @ConditionalOnProperty(name = "base.cache", havingValue = "memory", matchIfMissing = true)
    public CacheManager localCacheManager() {
        log.info("启用memory缓存");
        return new MemoryCacheManager();
    }

}

如上述代码：当配置文件存在base.cache: redis时，则初始化Redis缓存，若不配置，则初始化memory缓存。

3. @ConditionalOnClass

注意该类注解需要用在类上，即不允许JVM加载该类。若用在方法上，且项目没有对应class，则项目启动时出现异常。

案例：如2中所示，可以构造静态内部类实现。这也是为什么源码的Config中存在大量静态内部类声明bean的原因。

项目实战：

    /**
     * 原生Redis的缓存
     */
    @Configuration
    @ConditionalOnClass(org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration.class)
    @ConditionalOnProperty(name = "seal.os.cache", havingValue = "redis")
    protected static class RedisCacheManagerConfiguration {

        @Bean
        @ConditionalOnMissingBean
        public CacheManager redisCacheManager(StringRedisTemplate stringRedisTemplate) {
            return new RedisCacheManager(stringRedisTemplate);
        }
    }

4. @ConditionalOnBean

5. @EnableConfigurationProperties

案例：如2中所示。

6. @Import

案例：
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration

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

    @Bean
    @ConditionalOnMissingBean(name = "redisTemplate")
    public RedisTemplate<Object, Object> redisTemplate(
            RedisConnectionFactory redisConnectionFactory) throws UnknownHostException {
        RedisTemplate<Object, Object> template = new RedisTemplate<>();
        template.setConnectionFactory(redisConnectionFactory);
        return template;
    }

    @Bean
    @ConditionalOnMissingBean
    public StringRedisTemplate stringRedisTemplate(
            RedisConnectionFactory redisConnectionFactory) throws UnknownHostException {
        StringRedisTemplate template = new StringRedisTemplate();
        template.setConnectionFactory(redisConnectionFactory);
        return template;
    }

}

上述注解：@Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class })会按照顺序导入LettuceConnectionConfiguration和JedisConnectionConfiguration配置。

LettuceConnectionConfiguration的优先级高于JedisConnectionConfiguration。
最终是由了LettuceConnectionConfiguration的RedisConnectionFactory。