Hello everyone, I am Xiaomi! Today we will talk about an interesting topic in the Spring framework: different types of events. The Spring framework is a shining pearl in Java development. It not only provides powerful functions such as dependency injection and aspect-oriented programming, but also introduces an event mechanism to achieve loosely coupled communication between components. But, you know what? There are many different types of events in the Spring framework, each with its own unique uses and characteristics. Today, we will reveal these events one by one to give you an in-depth understanding of the internal mechanism of the Spring framework.
ApplicationEvent
First, let's take a look at the most basic event type: application event (ApplicationEvent). Application events are the basis of the event system in the Spring framework and are the parent class of all other event types. When we trigger an event in a Spring application, we actually create an instance of a custom event class that inherits from ApplicationEvent.
Typical uses of application events include:
User-defined events: You can create your own event class, inherited from ApplicationEvent, to customize the triggering and processing of events in your application.
Spring built-in events: The Spring framework itself will also trigger some built-in events, such as ContextRefreshedEvent (application context refresh completion event) and ContextClosedEvent (application context closing event), etc. You can listen to these events to execute corresponding logic.
Here is a simple example showing how to create a custom application event class:
Application event listener (ApplicationListener)
With application events, you need an application event listener (ApplicationListener) to capture and process these events. An application event listener is an interface that you can implement to define your own event listeners. Typically, we will register a listener into Spring's application context so that it can sense and respond to specific types of events.
Here is a simple example showing how to create a custom event listener:
In the above example, we created a CustomEventListener class that implements the ApplicationListener interface and registered it as a Spring component using the @Component annotation. This listener will execute the logic in the onApplicationEvent method when receiving the CustomEvent event.
ContextEvent (ContextEvent)
In addition to application events, Spring also provides the context event (ContextEvent) type to represent state changes in the application context. These events are usually related to the life cycle of the application and allow us to perform some actions at specific times.
Some common contextual events include:
ContextRefreshedEvent: Application context refresh completion event, indicating that the application initialization is complete.
ContextClosedEvent: Application context closing event, indicating that the application is about to close.
You can perform some special initialization or cleanup work by listening to these events. Here's an example:
Ordered Event
Sometimes we need to ensure that multiple event listeners are executed in a specific order. For this purpose, the Spring framework allows us to set priorities for event listeners. Implementing Ordered Events ensures that listeners are executed in the defined order.
To implement ordered events, we need to let the event listener implement the org.springframework.core.Ordered interface and implement the getOrder() method to specify the priority. Listeners with smaller priority values will execute before listeners with larger priority values.
Here is an example showing how to create an ordered event listener:
Async Event
Some event processing may be time-consuming. In order not to block the main thread, Spring allows event listeners to be marked asynchronous. By using the @Async annotation, you can enable event listeners to handle events in a separate thread, thereby improving the responsiveness of your application.
Here is an example showing how to create an asynchronous event listener:
By adding @Async and @EventListener annotations on the method , we mark this listener as asynchronous and it will handle events in a separate thread.
Summarize
The event mechanism in the Spring framework is a powerful communication method that can help us achieve loose coupling between components. In this article, we introduced the different types of events, including application events, contextual events, ordered events, and asynchronous events, and how to create and use event listeners. I hope this knowledge can help you better understand and utilize the event mechanism of the Spring framework.
If you have more questions about events in the Spring framework or want to learn more about other aspects of knowledge, please leave a message in the comment area and I will try my best to answer your questions. Thank you everyone for reading!
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