一、Spring Boot 核心之自動裝配實現

一、簡介和目標

• 簡介：在 Spring Boot 場景下，基於約定大於配置的原則，實現 Spring 組件自動裝配的目的。
• 目標：完成一個可通過配置和@EnableXXX 來控制的是否裝配的Bean

二、底層裝配技術簡述

• Spring 模式註解裝配

• Spring @Enable 模塊裝配

  • 註解驅動方式 eg:

    @Retention(RetentionPolicy.RUNTIME)
    @Target(ElementType.TYPE) @Documented
    @Import(DelegatingWebMvcConfiguration.class) 
    public @interface EnableWebMvc {
    
    }
    

    @Configuration 
    public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {
     
     }
    

  • 接口編程方式 eg:

    @Target(ElementType.TYPE) 
    @Retention(RetentionPolicy.RUNTIME) 
    @Documented 
    @Import(CachingConfigurationSelector.class) 
    public @interface EnableCaching { 
     
     }
    

    public class CachingConfigurationSelector extends AdviceModeImportSelector<EnableCaching> {
        public String[] selectImports(AdviceMode adviceMode) {
            switch(adviceMode) {
            case PROXY:
                return this.getProxyImports();
            case ASPECTJ:
                return this.getAspectJImports();
            default:
                return null;
            }
        }
     }
    

• Spring 條件裝配

  • 配置方式 - @Profile，根據不同環境進行裝配
  • 編程方式 - @Conditional

    @Target({ElementType.TYPE, ElementType.METHOD})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Conditional({OnClassCondition.class})
    public @interface ConditionalOnClass {
        Class<?>[] value() default {};
    
        String[] name() default {};
    }
    

• Spring 工廠加載機制

  • 實現類： SpringFactoriesLoader
  • 配置資源： META-INF/spring.factories

三、實現

1、激活自動裝配 @EnableAutoConfiguration

項目結構如下

添加核心Bean=>HelloWorld,只有一個hello方法用於測試輸出結果

@Slf4j
public class HelloWorld {
    public void hello(){
        log.info("hello world 2019!");
    }
}

添加HelloWorldConfiguration用於註冊HelloWorld

@Slf4j
public class HelloWorldConfiguration {
    @Bean
    public HelloWorld hello(){
        log.info("Load HelloWorld");
        return new HelloWorld();
    }
}

添加HelloWorldImportSelector實現ImportSelector，通過接口編程方式實現@Enable功能

@Slf4j
public class HelloWorldImportSelector implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        log.info("annotationMetadata.getAnnotationTypes():{}",annotationMetadata.getAnnotationTypes());
        // 此處可寫分支條件，根據指定條件選擇性註冊某些類 或者返回null

        return new String[]{HelloWorldConfiguration.class.getName()};
    }
}

添加@EnableHelloWorld用於控制是否裝配HelloWorld

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
//@Import(HelloWorldConfiguration.class) // 基於註解驅動實現Sprig @Enable模塊
@Import(HelloWorldImportSelector.class) // 基於接口驅動實現Spring @Enable模塊
public @interface EnableHelloWorld {
}

注:如果直接使用@Import(HelloWorldConfiguration.class)註解方式實現，則不需要HelloWorldImportSelector類，但是註解方式無法添加分支判斷，只能指定加載指定類

2、實現自動裝配配置類 HelloWorldAutoConfiguration

@Configuration // 模式註解，聲明是一個bean
@ConditionalOnSystemProperty(name = "user.name", value = "Administrator") // 正確的條件裝配
//@ConditionalOnSystemProperty(name = "user.name", value = "lxt") // 錯誤的條件裝配
@EnableHelloWorld // Spring @Enable 模塊裝配
public class HelloWorldAutoConfiguration {
}

• 先根據條件註解@ConditionalOnSystemProperty判斷是否滿足
• 滿足則執行@EnableHelloWorld，加載HelloWorldImportSelector，註冊HelloWorldConfiguration進而註冊HelloWorld

3、配置自動裝配實現 META-INF/spring.factories

在resources下添加META-INF/spring.factories配置文件，用於啓動是通過工廠機制（SpringFactoriesLoader）加載

# 自動裝配
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.lxt.springboot.autoconfigure.configuration.HelloWorldAutoConfiguration

4、測試

添加測試類HelloWorldService

@Component
public class HelloWorldService {

    @Autowired
    private HelloWorld helloWorld;

    @PostConstruct
    public void init(){
        helloWorld.hello();
    }

}

啓動項目，控制檯輸出如下，測試開啓情況成功

2019-11-24 21:25:02.299  INFO 13880 --- [           main] c.l.s.a.a.HelloWorldImportSelector       : annotationMetadata.getAnnotationTypes():[com.lxt.springboot.autoconfigure.condition.ConditionalOnSystemProperty, com.lxt.springboot.autoconfigure.annotation.EnableHelloWorld]
2019-11-24 21:25:02.710  INFO 13880 --- [           main] c.l.s.a.c.HelloWorldConfiguration        : Load HelloWorld
2019-11-24 21:25:02.712  INFO 13880 --- [           main] c.l.s.autoconfigure.entity.HelloWorld    : hello world 2019!

去掉@EnableHelloWorld 註解或者條件註解修改爲ConditionalOnSystemProperty(name = “user.name”, value = “lxt”)，分別重啓，控制檯輸出如下，測試關閉情況成功

***************************
APPLICATION FAILED TO START
***************************

Description:

Field helloWorld in com.lxt.springboot.autoconfigure.service.HelloWorldService required a bean of type 'com.lxt.springboot.autoconfigure.entity.HelloWorld' that could not be found.

The injection point has the following annotations:
	- @org.springframework.beans.factory.annotation.Autowired(required=true)


Action:

Consider defining a bean of type 'com.lxt.springboot.autoconfigure.entity.HelloWorld' in your configuration.

五、源碼

• https://github.com/hdlxt/dive-in-spring-boot