Springboot中@Import的使用原理

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最近我看了看Springboot的源碼，借著周末分享@Import這一小知識點，也正好體驗一下CSDN博客最新的編輯器。編輯起來很好用，舒服。

再看源碼之前先寫一個小的demo，試著看看@Import是怎么用的。（為避免文章過長，下面代碼均有一定程度的代碼刪減，可以自行補全或查看源碼哦?。?/span>

1.創(chuàng)建一個maven項目，pom.xml文件配置如下：

??org.springframework.boot
??spring-boot-starter-parent
??1.5.6.RELEASE
?
?
??
???org.springframework.boot
???spring-boot-starter-web
??
?
?
??
???
????org.springframework.boot
????spring-boot-maven-plugin
???
??
?

2.創(chuàng)建一個實體類

package?com.entity;
?
public?class?User?implements?Serializable{
?
?private?String?name;
?private?String?age;
?
?public?User()?{
??this.name?=?"xiaochen";
??this.age?=?"6";
?}
}

3.創(chuàng)建ImportUser類

package?com.demo;
?
public?class?ImportUser?implements?ImportSelector?{
?@Override
?public?String[]?selectImports(AnnotationMetadata?importingClassMetadata)?{
??return?new?String[]?{"com.entity.User"};
?}
}

這里看到ImportSelector接口，是不是好像在哪里見過呢？

4.創(chuàng)建配置類

package?com.demo;
?
@Import(ImportUser.class)
@Configuration
public?class?ImportConfiguration?{}

5.創(chuàng)建啟動類

package?com.demo;
public?class?ImportDemo?{
?public?static?void?main(String[]?args)?{
??AnnotationConfigApplicationContext?applicationContext?=?new?AnnotationConfigApplicationContext();
//這里使用register()和scan()方法都可,用scan()方法就不用在配置類中使用@Configuration注解了。
//??applicationContext.register(ImportConfiguration.class);
??applicationContext.scan("com.demo");
??applicationContext.refresh();
??User?user?=?applicationContext.getBean(User.class);
?????System.out.println(user);
?}
}

6，運行結(jié)果

上邊的代碼很簡單，就不過多解釋，下面直接看Springboot中是怎么使用的。

我們知道@SpringbootApplication注解主要是由@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan組成。而@EnableAutoConfiguration中就用到了@Import注解，用于引入

EnableAutoConfigurationImportSelector類，代碼如下：

@Import(EnableAutoConfigurationImportSelector.class)
public?@interface?EnableAutoConfiguration?{}

?這里我們先創(chuàng)建一個Springboot啟動類

@SpringBootApplication
public?class?ApplicationStart?{
?public?static?void?main(String[]?args)?{
??SpringApplication.run(ApplicationStart.class,?args);
?}
}

?點擊run()方法進入SpringApplication類中，依次查看調(diào)用方法；

public?ConfigurableApplicationContext?run(String...?args)?{
???refreshContext(context);
}
private?void?refreshContext(ConfigurableApplicationContext?context)?{
??refresh(context);
}
protected?void?refresh(ApplicationContext?applicationContext)?{
??((AbstractApplicationContext)?applicationContext).refresh();
}
?
@Override
public?void?refresh()?throws?BeansException,?IllegalStateException?{
??invokeBeanFactoryPostProcessors(beanFactory);
}
?
protected?void?invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory?beanFactory)?{
?????PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory,?getBeanFactoryPostProcessors());
}?

到此再點擊進入org.springframework.context.support.PostProcessorRegistrationDelegate類中；

public?static?void?invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory?beanFactory,?List?beanFactoryPostProcessors)?{
?????invokeBeanDefinitionRegistryPostProcessors(priorityOrderedPostProcessors,?registry);
}
?
private?static?void?invokeBeanDefinitionRegistryPostProcessors(
???Collection?postProcessors,?BeanDefinitionRegistry?registry)?{
?for?(BeanDefinitionRegistryPostProcessor?postProcessor?:?postProcessors)?{
???postProcessor.postProcessBeanDefinitionRegistry(registry);
?}
}

再點擊postProcessBeanDefinitionRegistry()方法進入org.springframework.context.annotation.ConfigurationClassPostProcessor類中；

public?void?postProcessBeanDefinitionRegistry(BeanDefinitionRegistry?registry)?{
??processConfigBeanDefinitions(registry);
}
?
public?void?processConfigBeanDefinitions(BeanDefinitionRegistry?registry)?{
//?Parse?each?@Configuration?class
?ConfigurationClassParser?parser?=?new?ConfigurationClassParser(
????this.metadataReaderFactory,?this.problemReporter,?this.environment,
????this.resourceLoader,?this.componentScanBeanNameGenerator,?registry);
?????parser.parse(candidates);
}

這里再點擊parse()方法進入org.springframework.context.annotation.ConfigurationClassParser類當中；

public?void?parse(Set?configCandidates)?{
??processDeferredImportSelectors();
}
private?void?processDeferredImportSelectors()?{
????????String[]?imports?=?deferredImport.getImportSelector().selectImports(configClass.getMetadata());
????????processImports(configClass,?asSourceClass(configClass),?asSourceClasses(imports),?false);
}

這里看到selectImports()方法就在上面demo中出現(xiàn)過，再查看一下之前使用@Import注解導入的EnableAutoConfigurationImportSelector類，其就間接實現(xiàn)了ImportSelector接口；這和demo中是不是很相似。

這里的?selectImports()方法再往里就是讀取各個jar包中有含?META-INF/spring.factories.文件的信息了，這里就不多做分析了。

但這里會不會有個疑問，@Import是怎么起作用的，這里就要看看deferredImport是怎么實例化的?。?！，也是在上面的方法中有這樣一段代碼；

private?void?processDeferredImportSelectors()?{
List?deferredImports?=?this.deferredImportSelectors;
this.deferredImportSelectors?=?null;
Collections.sort(deferredImports,?DEFERRED_IMPORT_COMPARATOR);
for?(DeferredImportSelectorHolder?deferredImport?:?deferredImports)?{
?ConfigurationClass?configClass?=?deferredImport.getConfigurationClass();
}
}

代碼中可以看出是deferredImportSelectors對象間接賦值?, 那就找它實例化的位置；

private?void?processImports(ConfigurationClass?configClass,?SourceClass?currentSourceClass,Collection?importCandidates,
???????boolean?checkForCircularImports)?throws?IOException?{
????for?(SourceClass?candidate?:?importCandidates)?{
??if?(candidate.isAssignable(ImportSelector.class))?{
//?Candidate?class?is?an?ImportSelector?->?delegate?to?it?to?determine?imports
?????Class?candidateClass?=?candidate.loadClass();
????????????//這里用于實例化ImportSelector的實現(xiàn)類
?????ImportSelector?selector?=?BeanUtils.instantiateClass(candidateClass,ImportSelector.class);
?????ParserStrategyUtils.invokeAwareMethods(selector,?this.environment,this.resourceLoader,?this.registry);
?????if?(this.deferredImportSelectors?!=?null?&&?selector?instanceof?DeferredImportSelector)?{
????this.deferredImportSelectors.add(new?DeferredImportSelectorHolder(configClass,?(DeferredImportSelector)?selector));
???}??
?????}
????}
}

上面也就是判斷并實例化ImportSelector接口的實現(xiàn)類，而怎么識別的呢? importCandidates又是如何得到的？接著找。。

protected?final?SourceClass?doProcessConfigurationClass(ConfigurationClass?configClass,?SourceClass?sourceClass)?throws?IOException?{
//?Process?any?@Import?annotations
?processImports(configClass,?sourceClass,?getImports(sourceClass),?true);
}
private?Set?getImports(SourceClass?sourceClass)?throws?IOException?{
??Set?imports?=?new?LinkedHashSet();
??Set?visited?=?new?LinkedHashSet();
??collectImports(sourceClass,?imports,?visited);
??return?imports;
}
private?void?collectImports(SourceClass?sourceClass,?Set?imports,Set?visited)?throws?IOException?{
?if?(visited.add(sourceClass))?{
??for?(SourceClass?annotation?:?sourceClass.getAnnotations())?{
???String?annName?=?annotation.getMetadata().getClassName();
???if?(!annName.startsWith("java")?&&?!annName.equals(Import.class.getName()))?{
????collectImports(annotation,?imports,?visited);
???}
??}
????????imports.addAll(sourceClass.getAnnotationAttributes(Import.class.getName(),?"value"));
?}
}

看到這里的判斷也大體明白了吧。

這里就不做總結(jié)了，加個最近覺得很有道理的一句話，學習其實是個低成本，高回報的方式。

版權聲明：本文為博主原創(chuàng)文章，遵循 CC 4.0 BY-SA 版權協(xié)議，轉(zhuǎn)載請附上原文出處鏈接和本聲明。

本文鏈接：

https://blog.csdn.net/qq_21299835/article/details/109695279

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