SpringBoot设计了哪些可拓展的机制？
SpringBoot核心源码
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) { ...this.primarySources = new LinkedHashSet(Arrays.asList(primarySources));// Servletthis.webApplicationType = WebApplicationType.deduceFromClasspath(); this.bootstrapRegistryInitializers = new ArrayList(this.getSpringFactoriesInstances(BootstrapRegistryInitializer.class)); // 注意这里，Initializersthis.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class)); // 注意这里 Listenersthis.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
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);
}
复制代码这样就很清晰了，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
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
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
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()); } }
复制代码这里和上面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 {@Overridepublic void postProcessBeanFactory(ConfigurableListableBeanFactorybeanFactory) throws BeansException {Arrays.asList(beanFactory.getBeanDefinitionNames()).forEach(beanDefinitionName ->System.out.println(beanDefinitionName));System.out.println("BeanFactoryPostProcessor...");}
}
复制代码自定义Bean的后置处理器：
@Component
public class MyBeanPostProcessor implements BeanPostProcessor {@Overridepublic Object postProcessBeforeInitialization(Object bean, String beanName)throws BeansException {if(beanName.equals("userController")){System.out.println("找到了userController: "+bean);}return null;}
}
复制代码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
- 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：
@Component
public class JackApplicationRunner implements ApplicationRunner {@Overridepublic void run(ApplicationArguments args) throws Exception {System.out.println("JackApplicationRunner...");}
}