【Spring Boot】SpringBoot设计了哪些可拓展的机制？(spring boot s)

【Spring Boot】SpringBoot设计了哪些可拓展的机制？ 文章目录前言SpringBoot核心源码拓展Initializer拓展监听器ApplicationListenerBeanFactory的后置处理器 & Bean的后置处理器AOP其他的拓展点前言 当我们引入注册中心的依赖，比如nacos的时候，当我们启动springboot，这个服务就会根据配置文件自动注册到注册中心中，这个动作是如何完成的？ 注册中心使用了SpringBoot中的事件监听机制，在springboot初始化的时候完成服务注册SpringBoot核心源码public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) { ... this.primarySources = new LinkedHashSet(Arrays.asList(primarySources)); // Servlet this.webApplicationType = WebApplicationType.deduceFromClasspath(); this.bootstrapRegistryInitializers = new ArrayList(this.getSpringFactoriesInstances(BootstrapRegistryInitializer.class)); // 注意这里，Initializers this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class)); // 注意这里 Listeners this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class)); this.mainApplicationClass = this.deduceMainApplicationClass(); }

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我们可以看到空的SpringBoot项目有一些initializers以及一些listeners

注意这两行，换言之我们只要实现这两个类就可以自定义拓展SpringBoot了！

这里和手写Starter都是对SpringBoot的拓展，有兴趣的小伙伴可以看这篇文章

拓展Initializer

再看这张图

我们需要研究一下ApplicationContextInitializer这个类：

@FunctionalInterfacepublic interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> { /** * Initialize the given application context. * @param applicationContext the application to configure */ void initialize(C applicationContext); }

这样就很清晰了，我们尝试手写一个继承类：

public class DemoInitializer implements ApplicationContextInitializer { @Override public void initialize(ConfigurableApplicationContext applicationContext) { System.out.println("自定义初始化器执行..."); ConfigurableEnvironment environment = applicationContext.getEnvironment(); Map<String, Object> map = new HashMap<>(1); map.put("name", "sccccc"); environment.getPropertySources().addLast(new MapPropertySource("DemoInitializer", map)); System.out.println("DemoInitializer execute, and add some property"); } }

通过SPI机制将自定义初始化器交给list集合initializers

然后再debug，就会发现：

最后经过一次回调:

private void prepareContext(DefaultBootstrapContext bootstrapContext, ConfigurableApplicationContext context, ... applyInitializers(context); ... // Add boot specific singleton beans 下面是beanFactory的操作

遍历所有的初始化器，然后

/*** Apply any {@link ApplicationContextInitializer}s to the context before it is * refreshed. * @param context the configured ApplicationContext (not refreshed yet) * @see ConfigurableApplicationContext#refresh() */ @SuppressWarnings({ "rawtypes", "unchecked" }) protected void applyInitializers(ConfigurableApplicationContext context) { for (ApplicationContextInitializer initializer : getInitializers()) { Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(initializer.getClass(), ApplicationContextInitializer.class); Assert.isInstanceOf(requiredType, context, "Unable to call initializer."); initializer.initialize(context); } }

流程：

拓展监听器ApplicationListener

@FunctionalInterfacepublic interface ApplicationListener<E extends ApplicationEvent> extends EventListener { /** * Handle an application event. */ void onApplicationEvent(E event); /** * Create a new {@code ApplicationListener} for the given payload consumer. */ static <T> ApplicationListener<PayloadApplicationEvent<T>> forPayload(Consumer<T> consumer) { return event -> consumer.accept(event.getPayload()); } }

这里和上面initializer一样，就不演示了

BeanFactory的后置处理器 & Bean的后置处理器

Spring Boot解析配置成BeanDefinition的操作在invokeBeanFactoryPostProcessors方法中 自定义BeanFactory的后置处理器:

@Componentpublic class MyBeanFactoryPostProcessor implements BeanFactoryPostProcessor { @Override public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException { Arrays.asList(beanFactory.getBeanDefinitionNames()) .forEach(beanDefinitionName -> System.out.println(beanDefinitionName)); System.out.println("BeanFactoryPostProcessor..."); }}

自定义Bean的后置处理器：

@Componentpublic class MyBeanPostProcessor implements BeanPostProcessor { @Override public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { if(beanName.equals("userController")){ System.out.println("找到了userController: "+bean); } return null; }}AOP

这个相信大家用的比较多，可以自定义切面：

@Aspect@Componentpublic class LogAspect {// 切入点 Pointcut 可以对Service服务做切面@Pointcut("execution(* com.example.service.*.*(..))")public void mypointcut(){}// 前置通知@Before(value = "mypointcut()")public void before(JoinPoint joinPoint){ System.out.println("[前置通知] 准备开始记录日志..."); System.out.println("[前置通知] 目标类是: "+joinPoint.getTarget()); System.out.println("[前置通知] 目标方法是: "+joinPoint.getSignature().getName());}// 后置通知@AfterReturning(value = "mypointcut()")public void afterReturning(JoinPoint joinPoint){ System.out.println("[后置通知] 记录日志完成..."); System.out.println("[后置通知] 目标类是: "+joinPoint.getTarget()); System.out.println("[后置通知] 目标方法是: "+joinPoint.getSignature().getName());}/*@Around(value = "mypointcut()")public void around(ProceedingJoinPoint joinPoint){ System.out.println("[环绕通知] 日志记录前的操作..."); try { joinPoint.proceed(); System.out.println("[环绕通知] 日志记录后的操作..."); System.out.println("[环绕通知] "+joinPoint.getTarget()); System.out.println("[环绕通知] "+joinPoint.getSignature().getName()); } catch (Throwable throwable) { System.out.println("[环绕通知] 发生异常的操作..."); throwable.printStackTrace(); }finally { ... }}其他的拓展点Banner

方法地址： printBanner(env)->bannerPrinter.print->SpringBootBanner#printBanner 可以在resource目录下建立banner.txt文件夹实现自定义Banner

Runners

流程：

自定义：

@Componentpublic class JackApplicationRunner implements ApplicationRunner { @Override public void run(ApplicationArguments args) throws Exception { System.out.println("JackApplicationRunner..."); }}