一、Spring Boot 核心之自动装配实现

一、简介和目标

• 简介：在 Spring Boot 场景下，基于约定大于配置的原则，实现 Spring 组件自动装配的目的。
• 目标：完成一个可通过配置和@EnableXXX 来控制的是否装配的Bean

二、底层装配技术简述

• Spring 模式注解装配

• Spring @Enable 模块装配

  • 注解驱动方式 eg:

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

    @Configuration 
    public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {
     
     }
    

  • 接口编程方式 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 条件装配

  • 配置方式 - @Profile，根据不同环境进行装配
  • 编程方式 - @Conditional

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

• Spring 工厂加载机制

  • 实现类： SpringFactoriesLoader
  • 配置资源： META-INF/spring.factories

三、实现

1、激活自动装配 @EnableAutoConfiguration

项目结构如下

添加核心Bean=>HelloWorld,只有一个hello方法用于测试输出结果

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

添加HelloWorldConfiguration用于注册HelloWorld

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

添加HelloWorldImportSelector实现ImportSelector，通过接口编程方式实现@Enable功能

@Slf4j
public class HelloWorldImportSelector implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        log.info("annotationMetadata.getAnnotationTypes():{}",annotationMetadata.getAnnotationTypes());
        // 此处可写分支条件，根据指定条件选择性注册某些类 或者返回null

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

添加@EnableHelloWorld用于控制是否装配HelloWorld

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
//@Import(HelloWorldConfiguration.class) // 基于注解驱动实现Sprig @Enable模块
@Import(HelloWorldImportSelector.class) // 基于接口驱动实现Spring @Enable模块
public @interface EnableHelloWorld {
}

注:如果直接使用@Import(HelloWorldConfiguration.class)注解方式实现，则不需要HelloWorldImportSelector类，但是注解方式无法添加分支判断，只能指定加载指定类

2、实现自动装配配置类 HelloWorldAutoConfiguration

@Configuration // 模式注解，声明是一个bean
@ConditionalOnSystemProperty(name = "user.name", value = "Administrator") // 正确的条件装配
//@ConditionalOnSystemProperty(name = "user.name", value = "lxt") // 错误的条件装配
@EnableHelloWorld // Spring @Enable 模块装配
public class HelloWorldAutoConfiguration {
}

• 先根据条件注解@ConditionalOnSystemProperty判断是否满足
• 满足则执行@EnableHelloWorld，加载HelloWorldImportSelector，注册HelloWorldConfiguration进而注册HelloWorld

3、配置自动装配实现 META-INF/spring.factories

在resources下添加META-INF/spring.factories配置文件，用于启动是通过工厂机制（SpringFactoriesLoader）加载

# 自动装配
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.lxt.springboot.autoconfigure.configuration.HelloWorldAutoConfiguration

4、测试

添加测试类HelloWorldService

@Component
public class HelloWorldService {

    @Autowired
    private HelloWorld helloWorld;

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

}

启动项目，控制台输出如下，测试开启情况成功

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 注解或者条件注解修改为ConditionalOnSystemProperty(name = “user.name”, value = “lxt”)，分别重启，控制台输出如下，测试关闭情况成功

***************************
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、源码

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