SpringBoot自动配置xxxAutoConfiguration 的使用

https://sdqali.in/blog/2016/07/16/controlling-redis-auto-configuration-for-spring-boot-session/

常用的类：

@ConditionalOnProperty(name = "use.redis.session.store", havingValue = "true")
@ConditionalOnClass(Session.class)
@AutoConfigureAfter(RedisAutoConfiguration.class)
@ConditionalOnWebApplication
@ConditionalOnMissingBean(RedisHttpSessionConfiguration.class)
@SpringBootApplication(exclude ={SessionAutoConfiguration.class})

举例：以MybatisAutoConfiguration为例
1. 在jar包中/META-INF/spring.factories中配置
org.springframework.boot.autoconfigure.EnableAutoConfiguration=org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration
spring在启动时会去自动加载这个bean

2. MybatisAutoConfiguration.java上加上@Configuration

3. 如果需要通过自定义构造函数来构造bean，则加上构造函数

public MybatisAutoConfiguration(MybatisProperties properties,
                                  ObjectProvider<Interceptor[]> interceptorsProvider,
                                  ResourceLoader resourceLoader,
                                  ObjectProvider<DatabaseIdProvider> databaseIdProvider,
                                  ObjectProvider<List<ConfigurationCustomizer>> configurationCustomizersProvider) {
    this.properties = properties;
    this.interceptors = interceptorsProvider.getIfAvailable();
    this.resourceLoader = resourceLoader;
    this.databaseIdProvider = databaseIdProvider.getIfAvailable();
    this.configurationCustomizers = configurationCustomizersProvider.getIfAvailable();
}

构造函数中依赖的Bean，Spring会自动从Spring beanFactory容器中去找到适配的bean来传入

参数：
MybatisProperties properties --> 依赖MybatisProperties 这个bean，而MybatisProperties 是一个@ConfigurationProperties。则加上@EnableConfigurationProperties(MybatisProperties.class)，让bean MybatisProperties 先加载
ObjectProvider<Interceptor[]> interceptorsProvider --> 依赖 org.apache.ibatis.plugin.Interceptor[]，但又不确定 Interceptor 的bean是否存在，则通过ObjectProvider<Interceptor[]> 去获取，interceptorsProvider.getIfAvailable()
ResourceLoader resourceLoader --> 依赖 bean ResourceLoader，直接在 Spring 容器中找到相应的 bean
ObjectProvider<DatabaseIdProvider> databaseIdProvider --> 同上
ObjectProvider<List<ConfigurationCustomizer>> configurationCustomizersProvider --> 同上

附：断点打在MybatisAutoConfiguration 的构造函数上，查看调用栈，便可以看出端倪