Spring 50例常见错误（七）

文章整理来源：Spring编程常见错误50例_spring_spring编程_bean_AOP_SpringCloud_SpringWeb_测试_事务_Data-极客时间

案例15：试图处理并不会抛出的事件 ContextStartedEvent

        定义了一个监听器 MyContextStartedEventListener，试图拦截 ContextStartedEvent

@Slf4j
@Component
public class MyContextStartedEventListener implements ApplicationListener {

  public void onApplicationEvent(final ContextStartedEvent event) {
    log.info("{} received: {}", this.toString(), event);
  }

}

        解析：在 Spring Boot 中，这个事件的抛出只发生在一处，即位于方法 AbstractApplicationContext 的 start() 方法中

@Override
public void start() {
   getLifecycleProcessor().start();
   publishEvent(new ContextStartedEvent(this));
}

        Spring 启动最终调用的是 AbstractApplicationContext#refresh，并不是 AbstractApplicationContext#start。因此，ContextStartedEvent 自然不会被抛出，自然也不可能被捕获。

        解决：1. 将 ContextStartedEvent 改为 ContextRefreshedEvent 

@Component
public class MyContextRefreshedEventListener implements ApplicationListener {

  public void onApplicationEvent(final ContextRefreshedEvent event) {
    log.info("{} received: {}", this.toString(), event);
  }

}

                2. 调用 AbstractApplicationContext#start 方法

@RestController
public class HelloWorldController {

    @Autowired
    private AbstractApplicationContext applicationContext;

    @RequestMapping(path = "publishEvent", method = RequestMethod.GET)
    public String notifyEvent(){
        applicationContext.start();       
        return "ok";
    };
}

--------------------------------------------
public abstract class AbstractApplicationContext extends DefaultResourceLoader implements ConfigurableApplicationContext {

    @Override
    public void start() {
       getLifecycleProcessor().start();
       publishEvent(new ContextStartedEvent(this));
    }
}

案例16：自定义监听器没有被采用

        和上述案例相似，事件没有被监听到

@Slf4j
@Component
public class MyApplicationEnvironmentPreparedEventListener implements ApplicationListener {

    public void onApplicationEvent(final ApplicationEnvironmentPreparedEvent event) {
        log.info("{} received: {}", this.toString(), event);
    }

}

        解析：与 ApplicationEnvironmentPreparedEvent 的处理，它相关的两大组件 1. 广播器是 EventPublishingRunListener 的 initialMulticaster  2. 监听器  EventPublishingRunListener 

        通过 Debug 会发现这个事件的监听器就存储在 SpringApplication#Listeners 中，但其中没有 上述定义的 MyApplicationEnvironmentPreparedEventListener 

         实际上 定义的监听器并没有被放置在 META-INF/spring.factories 中

        解决：1.在构建 Spring Boot 时，添加 MyApplicationEnvironmentPreparedEventListener

@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        MyApplicationEnvironmentPreparedEventListener myApplicationEnvironmentPreparedEventListener = new MyApplicationEnvironmentPreparedEventListener();
        SpringApplication springApplication = new SpringApplicationBuilder(Application.class).listeners(myApplicationEnvironmentPreparedEventListener).build();
        springApplication.run(args);
    }
}

         2. 使用 META-INF/spring.factories，即在 /src/main/resources 下面新建目录 META-INF，然后新建一个对应的 spring.factories 文件

org.springframework.context.ApplicationListener=\
com.spring.puzzle.listener.example2.MyApplicationEnvironmentPreparedEventListener

 案例17：部分事件监听器失效

        部分事件监听器一直失效或偶尔失效，监听器 MySecondEventListener 有一半的概率并没有接收到任何事件

public class MyEvent extends ApplicationEvent {
    public MyEvent(Object source) {
        super(source);
    }
}
--------------------------------------------------
@Component
@Order(1)
public class MyFirstEventListener implements ApplicationListener {

    Random random = new Random();

    @Override
    public void onApplicationEvent(MyEvent event) {
        log.info("{} received: {}", this.toString(), event);
        //模拟部分失效
        if(random.nextInt(10) % 2 == 1)
            throw new RuntimeException("exception happen on first listener");
    }
}
--------------------------------------------------
@Component
@Order(2)
public class MySecondEventListener implements ApplicationListener {
    @Override
    public void onApplicationEvent(MyEvent event) {
        log.info("{} received: {}", this.toString(), event);
    }
}

         解析：处理器的执行是顺序执行的，在执行过程中，如果一个监听器执行抛出了异常，则后续监听器就得不到被执行的机会了

@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
   ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
   Executor executor = getTaskExecutor();
   for (ApplicationListener listener : getApplicationListeners(event, type)) {
      if (executor != null) {
         executor.execute(() -> invokeListener(listener, event));
      }
      else {
         invokeListener(listener, event);
      }
   }
}
---------------------------------------------------------------

protected void invokeListener(ApplicationListener listener, ApplicationEvent event) {
   ErrorHandler errorHandler = getErrorHandler();
   if (errorHandler != null) {
      try {
         doInvokeListener(listener, event);
      }
      catch (Throwable err) {
         errorHandler.handleError(err);
      }
   }
   else {
      doInvokeListener(listener, event);
   }
}

private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
   try {
      listener.onApplicationEvent(event);
   }
   catch (ClassCastException ex) {
        //省略非关键代码
      }
      else {
         throw ex;
      }
   }
}

        因为没有设置什么 org.springframework.util.ErrorHandler，也没有绑定 Executor 来执行任务。因此，事件的执行是由同一个线程按顺序来完成的，任何一个报错，都会导致后续的监听器执行不了

        解决：1. 在监听中使用 try catch 去处理

                   2. 定义 ErrorHandler 

public static final ErrorHandler LOG_AND_SUPPRESS_ERROR_HANDLER = new LoggingErrorHandler();

private static class LoggingErrorHandler implements ErrorHandler {

   private final Log logger = LogFactory.getLog(LoggingErrorHandler.class);

   @Override
   public void handleError(Throwable t) {
      logger.error("Unexpected error occurred in scheduled task", t);
   }
}