SpringBoot设计了哪些可拓展的机制？

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();  
}

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：

@FunctionalInterface  
public 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

@FunctionalInterface  
public 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:

@Component
public 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：

@Component
public 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
@Component
public 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

1. 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

2. Runners

流程：

自定义：

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