SpringBoot上下文初始器加载过程详解

利用 Spring 工厂加载机制，实例化 ApplicationContextInitializer 实现类，并排序对象集合。

关键方法

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
			Class<?>[] parameterTypes, Object... args) {
		ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
		// Use names and ensure unique to protect against duplicates
		Set<String> names = new LinkedHashSet<>(
				SpringFactoriesLoader.loadFactoryNames(type, classLoader));
		List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
				classLoader, args, names);
		AnnotationAwareOrderComparator.sort(instances);
		return instances;
}

技术实现步骤

• 实现类： org.springframework.core.io.support.SpringFactoriesLoader
  
• 配置资源： META-INF/spring.factories
• 排序： AnnotationAwareOrderComparator#sort
  

自定义初始化器

编写类实现ApplicationContextInitializer接口

有两种指定顺序的方法,第一种:类上加注解@Order(Ordered.HIGHEST_PRECEDENCE),第二种则是实现Ordered接口

@Order(Ordered.HIGHEST_PRECEDENCE)
public class HelloWorldApplicationContextInitializer<C extends ConfigurableApplicationContext>
    implements ApplicationContextInitializer<C> {
  @Override
  public void initialize(C applicationContext) {
    System.out.println("ConfigurableApplicationContext.id = "+ applicationContext.getId());
  }
}

public class AfterHelloWorldApplicationContextInitializer implements ApplicationContextInitializer, Ordered {
  @Override
  public void initialize(ConfigurableApplicationContext applicationContext) {
    System.out.println("After application.id = " + applicationContext.getId());
  }
  @Override
  public int getOrder() {
    return Ordered.LOWEST_PRECEDENCE;
  }
}

在spring.properties中配置

# ApplicationContextInitializer
org.springframework.context.ApplicationContextInitializer=\
com.imooc.diveinspringboot.context.AfterHelloWorldApplicationContextInitializer,\
com.imooc.diveinspringboot.context.HelloWorldApplicationContextInitializer

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