揭密springboot自動裝配（1）--ImportSelector

揭密springboot自動裝配

1. 揭密springboot自動裝配（1）--ImportSelector
2. 揭密springboot自動裝配（2）--AutoConfigurationImportSelector
3. 揭密springboot自動裝配（3）--ioc及調用selectImposts
4. 揭密springboot自動裝配（4）--ioc及創建beanFactory
5. 揭密springboot自動裝配（5）--ioc及@Autowired註解

在講這個之前，我們先來個例子熱熱身

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首先我們先來了解下ImportSelector這個接口的應用，ImportSelector接口是spring中導入外部配置的核心接口，在SpringBoot的自動化配置和@EnableXXX(功能性註解)都有它的存在，具體怎麼用下面走個例子看看

1.實現下ImportSelector

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

這裏我們看到有個selectImports方法，我們需要實現它，返回內容我們可以看到就是個數組，把需要裝配進spring容器中的bean的className放進返回數組即可

2.接着我們在啓動類中添加@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());


  }

}

這裏UserB我是沒有放在selectImports中的，目的是做下對比

3.跑下我們的程序看看結果

這裏你會發現UserA可以被拿到，證明已經交給spring容器中可以拿到，而UserB我沒有任何處理，是拿不到的這個毫無疑問到這裏肯定有人會問，直接@Import（UserA.class）不就行了，搞那麼複雜，嗯，這個沒錯，甚至我可以跑給你們看下

@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());
  }

}

那爲什麼搞那麼複雜？

ImportSelector主要是實現些比較複雜有邏輯性的bean裝載，我們可以在selectImports做下邏輯判斷，比如@ComponentScan像這個掃描器，我們自己來寫一個和它差不多的東西玩玩，起名@UserScan

1.創建註解@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.實現UserImportSelector 

public class UserImportSelector implements ImportSelector {
    @Override
    public 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));//這裏實現包含,相當@ComponentScan  includeFilters
        //scanner.addExcludeFilter(new AssignableTypeFilter(Object.class));//這裏可以實現排除，相當@ComponentScan  excludeFilters
        Set<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")，表示掃描包下的類

@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.我們跑起來看看結果

這樣可以看到我們是完全可以在spring容器裏面拿到UserA和UserB的

好了，熱身例子到這裏下文將會從源碼上分析springboot自動裝載的實現，主要和我們前面講的ImportSelector接口有關，其中有個叫做AutoConfigurationImportSelector的東西，下文將會提到