SpringBoot设计的可拓展机制有哪些？

快来打我* 2024-03-23 13:18 56阅读 0赞

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();  
    }
    复制代码

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

![format_png][]

![format_png 1][]

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！

![format_png 2][]

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][Link 1]

### 拓展Initializer ###

再看这张图

![format_png 3][]

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);  
    }
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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");  
        }  
    }
    复制代码

通过SPI机制将自定义初始化器交给list集合`initializers`

![format_png 4][]

然后再debug，就会发现：

![format_png 5][]

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
    复制代码

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);  
        }  
    }
    复制代码

![format_png 6][]

流程：

![format_png 7][]

### 拓展监听器ApplicationListener ###

![format_png 8][]

@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());  
        }  
      
    }
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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 ###

![format_png 9][]

`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...");
        }
    }
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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;
        }
    }
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### 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 {
            ...
        }
    }
    复制代码

### 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

1.  Runners

流程：

![format_png 10][]

自定义：

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

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[Link 1]: https://juejin.cn/post/7195890214211534903
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