揭密springboot自动装配（1）--ImportSelector

本文主要是介绍揭密springboot自动装配（1）--ImportSelector，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

揭密springboot自动装配

揭密springboot自动装配（1）--ImportSelector
揭密springboot自动装配（2）--AutoConfigurationImportSelector
揭密springboot自动装配（3）--ioc及调用selectImposts
揭密springboot自动装配（4）--ioc及创建beanFactory
揭密springboot自动装配（5）--ioc及@Autowired注解

在讲这个之前，我们先来个例子热热身

首先我们先来了解下ImportSelector这个接口的应用，ImportSelector接口是spring中导入外部配置的核心接口，在SpringBoot的自动化配置和@EnableXXX(功能性注解)都有它的存在，具体怎么用下面走个例子看看

1.实现下ImportSelector

public class UserImportSelector implements ImportSelector {@Overridepublic String[] selectImports(AnnotationMetadata annotationMetadata) {return new String[]{UserA.class.getName()};}
}

这里我们看到有个selectImports方法，我们需要实现它，返回内容我们可以看到就是个数组，把需要装配进spring容器中的bean的className放进返回数组即可

2.接着我们在启动类中添加@Import(UserImportSelector.class)

@SpringBootApplication
@Import(UserImportSelector.class)
public class DemoApplication {public static void main(String[] args) {ConfigurableApplicationContext run = SpringApplication.run(DemoApplication.class, args);Object userA = run.getBeanFactory().getBean(UserA.class);System.out.println(userA.toString());Object userB = run.getBeanFactory().getBean(UserB.class);System.out.println(userB.toString());}}

这里UserB我是没有放在selectImports中的，目的是做下对比

3.跑下我们的程序看看结果

这里你会发现UserA可以被拿到，证明已经交给spring容器中可以拿到，而UserB我没有任何处理，是拿不到的这个毫无疑问到这里肯定有人会问，直接@Import（UserA.class）不就行了，搞那么复杂，嗯，这个没错，甚至我可以跑给你们看下

@SpringBootApplication
@Import({UserImportSelector.class,UserB.class})
public class DemoApplication {public static void main(String[] args) {ConfigurableApplicationContext run = SpringApplication.run(DemoApplication.class, args);Object userA = run.getBeanFactory().getBean(UserA.class);System.out.println(userA.toString());Object userB = run.getBeanFactory().getBean(UserB.class);System.out.println(userB.toString());}}

那为什么搞那么复杂？

ImportSelector主要是实现些比较复杂有逻辑性的bean装载，我们可以在selectImports做下逻辑判断，比如@ComponentScan像这个扫描器，我们自己来写一个和它差不多的东西玩玩，起名@UserScan

1.创建注解@UserScan，引用@Import(UserImportSelector.class)

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
@Documented
@Import(UserImportSelector.class)
public @interface UserScan {@AliasFor("basePackages")String[] value() default {};@AliasFor("value")String[] basePackages() default {};
}

2.实现UserImportSelector

public class UserImportSelector implements ImportSelector {@Overridepublic String[] selectImports(AnnotationMetadata annotationMetadata) {Map<String, Object> annotationAttributes = annotationMetadata.getAnnotationAttributes(UserScan.class.getName());if(CollectionUtils.isEmpty(annotationAttributes))return new String[0];String[] basePackages = (String[]) annotationAttributes.get("basePackages");ClassPathScanningCandidateComponentProvider scanner = new ClassPathScanningCandidateComponentProvider(false);scanner.addIncludeFilter(new AssignableTypeFilter(Object.class));//这里实现包含,相当@ComponentScan  includeFilters//scanner.addExcludeFilter(new AssignableTypeFilter(Object.class));//这里可以实现排除，相当@ComponentScan  excludeFiltersSet<String> classes = new HashSet<>();for (String basePackage : basePackages) {Set<BeanDefinition> candidateComponents = scanner.findCandidateComponents(basePackage);candidateComponents.forEach(e-> {classes.add(e.getBeanClassName());});}return classes.toArray(new String[classes.size()]);//return new String[]{UserA.class.getName()};}
}

3.使用@UserScan("com.example.demo.service")，表示扫描包下的类

@SpringBootApplication
@UserScan("com.example.demo.service")
public class DemoApplication {public static void main(String[] args) {ConfigurableApplicationContext run = SpringApplication.run(DemoApplication.class, args);Object userA = run.getBeanFactory().getBean(UserA.class);System.out.println(userA.toString());Object userB = run.getBeanFactory().getBean(UserB.class);System.out.println(userB.toString());}
}

4.我们跑起来看看结果