Nick1407 的个人博客

一、原理
1、 基于添加的 JAR 依赖⾃动对 Spring Boot 应⽤程序进⾏配置
spring-boot-autoconfifiguration

2、开启自动配置
@EnableAutoConfiguration
exclude = Class<?>[]
@SpringBootApplication（包含有EnableAutoConfiguration）

AutoConfigurationImportSelector

getAutoConfigurationEntry自动配置元数据

getCandidateConfigurations根据条件进行判断


自动配置项

二、条件注解
@Conditional
1、当满足某个条件时配置生效
条件注解
@Conditional
类条件
@ConditionalOnClass
@ConditionalOnMissingClass
属性条件
@ConditionalOnProperty
Bean 条件
@ConditionalOnBean
@ConditionalOnMissingBean
@ConditionalOnSingleCandidate
资源条件
@ConditionalOnResource
Web 应⽤条件
@ConditionalOnWebApplication
@ConditionalOnNotWebApplication

其他条件
@ConditionalOnExpression
@ConditionalOnJava
@ConditionalOnJndi

三、实现自己的自动配置
定义日志输出类，当自动配置生效时打印日志

@Slf4j

public class GreetingApplicationRunner implements ApplicationRunner {

    public GreetingApplicationRunner() {

        log.info("Initializing GreetingApplicationRunner.");

    }

    public GreetingApplicationRunner(String  name) {

        log.info("Initializing GreetingApplicationRunner."+name);

    }
    public void run(ApplicationArguments args) throws Exception {
        log.info("Hello everyone! We all like Spring! ");
    }
}

定位⾃动配置，resources目录新建META-INF新增spring.factories文件，内容：

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
geektime.spring.hello.greeting.GreetingAutoConfiguration

定义自动配置类

@Configuration

@ConditionalOnClass(GreetingApplicationRunner.class)
public class GreetingAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean(GreetingApplicationRunner.class)
    @ConditionalOnProperty(name = "greeting.enabled", havingValue = "true", matchIfMissing = true)
    public GreetingApplicationRunner greetingApplicationRunner() {
        return new GreetingApplicationRunner("test");
    }
}

当满足以下几个条件时配置生效：
1、存在GreetingApplicationRunner类时生效
2、GreetingApplicationRunner对象不存在生效
3、属性greeting.enabled为true生效，默认为true

测试结果：

代码：
demo.zip