SpringBoot手写启动器

starter原理

1. spring在启动过程中会去扫描启动类上带有@Import注解，会将@Import导入的类注册到BeanFactoryRegistry中，Spring会帮我们去实例化类，放入IOC容器中。

   

   /**
   	 * Recursively collect all declared {@code @Import} values. Unlike most
   	 * meta-annotations it is valid to have several {@code @Import}s declared with
   	 * different values; the usual process of returning values from the first
   	 * meta-annotation on a class is not sufficient.
   	 * <p>For example, it is common for a {@code @Configuration} class to declare direct
   	 * {@code @Import}s in addition to meta-imports originating from an {@code @Enable}
   	 * annotation.
   	 * @param sourceClass the class to search
   	 * @param imports the imports collected so far
   	 * @param visited used to track visited classes to prevent infinite recursion
   	 * @throws IOException if there is any problem reading metadata from the named class
   	 */
   private void collectImports(SourceClass sourceClass, Set<SourceClass> imports, Set<SourceClass> visited)
     throws IOException {
   
     if (visited.add(sourceClass)) {
       for (SourceClass annotation : sourceClass.getAnnotations()) {
         String annName = annotation.getMetadata().getClassName();
         if (!annName.equals(Import.class.getName())) {
           collectImports(annotation, imports, visited);
         }
       }
       imports.addAll(sourceClass.getAnnotationAttributes(Import.class.getName(), "value"));
     }
   }
   

   • getImports()方法会将@Imports注解的值收集起来放入一个集合中，在之后的过程中帮我们实例化。

     

手写starter

1. 创建一个maven项目，引入Spring-Context 依赖

   <dependency>
     <groupId>org.springframework</groupId>
     <artifactId>spring-context</artifactId>
     <version>5.3.7</version>
   </dependency>
   

2. 首先定义一个类：JalivvTemplate 主要用来实现功能的类

   public class JalivvTemplate {
     public void say() {
       System.out.println("jalivv is so cool!");
     }
   }
   

3. 创建一个配置类，将 JalivvTemplate 注入的IOC容器中

   public class JalivvAutoConfiguration {
     @Bean
     public JalivvTemplate jalivvTemplate() {
       return new JalivvTemplate();
     }
   }
   

4. 创建一个 MyImportSelector 实现 ImportSelector 接口

   public class MyImportSelector implements ImportSelector {
     @Override
     public String[] selectImports(AnnotationMetadata importingClassMetadata) {
       return new String[]{JalivvAutoConfiguration.class.getName()};
     }
   }
   

5. 定义一个注解 EnableJalivv，引入 MyImportSelector

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

测试

1. 在我们项目中引入这个starter

   <dependency>
     <groupId>com.jalivv.starter</groupId>
     <artifactId>jalivv-spring-boot-starter</artifactId>
     <version>1.0.0</version>
   </dependency>
   

2. 在启动类上标上注解 EnableJalivv

   @EnableJalivv
   @SpringBootApplication
   public class SpringbootProjectsApplication 
   

3. 我们可以从IOC容器中获取到 JalivvTemplate 对象，执行他的 say() 方法

   ConfigurableApplicationContext run = SpringApplication.run(SpringbootProjectsApplication.class, args);
   JalivvTemplate jalivvTemplate = run.getBean("jalivvTemplate", JalivvTemplate.class);
   jalivvTemplate.say();
   

4. 输出