SpringBoot基本原理

SpringBoot可以帮助我们更好的使用Spring&＃xff0c;更快的配置一个可以运行的Spring应用。

SpringBoot实现自动配置实现的入口是&＃64;EnableAutoConfiguration注解&＃xff0c;该注解又通过&＃64;Import注解导入了AutoConfigurationImportSelector&＃xff0c;在该类中实现了对META-INF/spring.factories中 org.springframework.boot.autoconfigure.EnableAutoConfiguration属性指定的所有配置&＃xff0c;如org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration。

//文件&＃xff1a;spring-boot-autoconfigure-2.1.3.RELEASE.jar!/META-INF/spring.factories# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration&＃61;\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\


在CacheAutoConfiguration中&＃xff0c;通过条件注解&＃64;ConditionalOnXXX&＃xff0c;以及&＃64;Import注解来实现对缓存功能的相关Bean的自动配置。

// 文件&＃xff1a;spring-boot-autoconfigure-2.1.3.RELEASE-sources.jar!/org/springframework/boot/autoconfigure/cache/CacheAutoConfiguration.java&＃64;Configuration
&＃64;ConditionalOnClass(CacheManager.class)
&＃64;ConditionalOnBean(CacheAspectSupport.class)
&＃64;ConditionalOnMissingBean(value &＃61; CacheManager.class, name &＃61; "cacheResolver")
&＃64;EnableConfigurationProperties(CacheProperties.class)
&＃64;AutoConfigureAfter({ CouchbaseAutoConfiguration.class, HazelcastAutoConfiguration.class,HibernateJpaAutoConfiguration.class, RedisAutoConfiguration.class })
&＃64;Import(CacheConfigurationImportSelector.class)
public class CacheAutoConfiguration {&＃64;Bean&＃64;ConditionalOnMissingBeanpublic CacheManagerCustomizers cacheManagerCustomizers(ObjectProvider> customizers) {return new CacheManagerCustomizers(customizers.orderedStream().collect(Collectors.toList()));}


实现自己的AutoConfiguration

在实际开发过程中&＃xff0c;我们可能需要将公司内部的&＃xff0c;或者Spring官方还不支持的第三方类库集成到我们的项目中。但是为了重用&＃xff0c;最好能够按照SpringBoot的自动配置来配置。

第一步&＃xff1a;引入Spring官方依赖

项目的pom文件&＃xff1a;

"1.0" encoding&＃61;"UTF-8"?>
"http://maven.apache.org/POM/4.0.0" xmlns:xsi&＃61;"http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation&＃61;"http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">4.0.0geektime.spring.hellogeektime-spring-boot-autoconfigure0.0.1-SNAPSHOTgeektime-spring-boot-autoconfigureDemo project for Spring Boot1.82.1.3.RELEASEorg.springframework.bootspring-boot-dependencies${spring-boot.version}<type>pomtype>importorg.springframework.bootspring-boot-autoconfiguregeektime.spring.hellogreeting0.0.1-SNAPSHOTprovided


第二步&＃xff1a;编写Java Configuration&＃xff0c;并添加条件注解

Spring支持的条件注解&＃xff1a;

• 条件注解&＃64;Conditional
• 类条件&＃64;ConditionalOnClass&＃xff0c;&＃64;ConditionalOnMissingClass
• 属性条件&＃64;ConditionalOnProperty
• Bean 条件&＃64;ConditionalOnBean&＃xff0c;&＃64;ConditionalOnMissingBean&＃xff0c;&＃64;ConditionalOnSingleCandidate
• 资源条件&＃64;ConditionalOnResource
• Web 应⽤用条件&＃64;ConditionalOnWebApplication&＃xff0c;&＃64;ConditionalOnNotWebApplication
• 其他条件&＃64;ConditionalOnExpression&＃xff0c;&＃64;ConditionalOnJava&＃xff0c;&＃64;ConditionalOnJndi

Java Configuration代码如下&＃xff1a;

&＃64;Configuration
&＃64;ConditionalOnClass(GreetingApplicationRunner.class)
public class GreetingAutoConfiguration {&＃64;Bean&＃64;ConditionalOnMissingBean(GreetingApplicationRunner.class)&＃64;ConditionalOnProperty(name &＃61; "greeting.enabled", havingValue &＃61; "true", matchIfMissing &＃61; true)public GreetingApplicationRunner greetingApplicationRunner() {return new GreetingApplicationRunner();}
}


第三步&＃xff1a;定位自动配置

在当前项目的resource目录下的META-INF目录下创建spring.factories文件&＃xff0c;内容如下&＃xff1a;

org.springframework.boot.autoconfigure.EnableAutoConfiguration&＃61;\
geektime.spring.hello.greeting.GreetingAutoConfiguration


第四步&＃xff1a;定义自动配置的执行顺序

主要的注解有&＃xff1a;

• &＃64;AutoConfigureBefore
• &＃64;AutoConfigureAfter
• &＃64;AutoConfigureOrder

一般不需要&＃xff0c;具体使用可以参考Spring官方的自动配置。

如何在不支持SpringBoot的老版本Spring中实现SpringBoot

如果在老版本的Spring中实现类似SpringBoot的自动配置功能&＃xff0c;则需要使用Spring的扩展点机制&＃xff0c;原理可以参考博文Spring框架的扩展点。

添加一个Configuration&＃xff1a;

&＃64;Configuration
public class GreetingAutoConfiguration {&＃64;Beanpublic static GreetingBeanFactoryPostProcessor greetingBeanFactoryPostProcessor() {return new GreetingBeanFactoryPostProcessor();}
}


自定义一个BeanFactoryPostProcessor实现类似条件注解的功能&＃xff1a;

&＃64;Slf4j
public class GreetingBeanFactoryPostProcessor implements BeanFactoryPostProcessor {public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {boolean hasClass &＃61; ClassUtils.isPresent("geektime.spring.hello.greeting.GreetingApplicationRunner",GreetingBeanFactoryPostProcessor.class.getClassLoader());if (!hasClass) {log.info("GreetingApplicationRunner is NOT present in CLASSPATH.");return;}if (beanFactory.containsBeanDefinition("greetingApplicationRunner")) {log.info("We already have a greetingApplicationRunner bean registered.");return;}register(beanFactory);}private void register(ConfigurableListableBeanFactory beanFactory) {if (beanFactory instanceof BeanDefinitionRegistry) {GenericBeanDefinition beanDefinition &＃61; new GenericBeanDefinition();beanDefinition.setBeanClass(GreetingApplicationRunner.class);((BeanDefinitionRegistry) beanFactory).registerBeanDefinition("greetingApplicationRunner",beanDefinition);} else {beanFactory.registerSingleton("greetingApplicationRunner",new GreetingApplicationRunner());}}
}


参考资料

1. Spring框架的扩展点
2. Spring全家桶视频课程之SpringBoot
3. Spring全家桶课程代码