Spring Boot中starter的原理是什么？如何实现一些starter?

原理：核心就是@EnableAutoConfiguration注解，在该注解中有一个@Import注解。@Import注解导入了配置类：AutoConfigurationImportSelector.class。在该类中使用SpringFactoriesLoader.class加载配置文件META-INF/spring.factories。

实现也starter需要实现一下步骤：

1. autoconfigure模块
2. 实现RedisAutoConfiguration.class
3. 实现RedisProperties.class
4. 实现配置文件META-INF/spring.factories
5. 实现starter模块
   只需要在starter模块的pom文件中引入autoconfigure模块的依赖即可。

RedisAutoConfiguration.class

@Configuration
@EnableConfigurationProperties(RedisProperties.class)
public class RedisAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean(name = "jedis")
    public Jedis jedis(RedisProperties redisProperties){
        System.out.println("RedisAutoConfiguraton...");
        return new Jedis(redisProperties.getHost(), redisProperties.getPort());
    }
}

RedisProperties.class

@ConfigurationProperties(prefix = "redis")
public class RedisProperties {
    private String host = "localhost";
    private int port = 6379;

    public String getHost() {
        return host;
    }

    public void setHost(String host) {
        this.host = host;
    }

    public int getPort() {
        return port;
    }

    public void setPort(int port) {
        this.port = port;
    }
}

META-INF/spring.factories

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  com.itheima.redis.config.RedisAutoConfiguration