Spring boot application event tutorial - reflecting

Jiedao jdon 2021-05-04 12:35:47
spring boot application event tutorial

If you want to “ monitor ” event , We can write... At the source of the event “ Monitor ” To listen for events , But it tightly couples the event source with the logic of the listener . We can dynamically register and unregister listeners for certain events as needed . For the same event , We can also have multiple listeners . This tutorial provides an overview of how to publish and listen to custom events , And explained Spring Boot Built in events .

Events and direct method calls

Events and direct method calls are suitable for different situations . For method calls , It's like an assertion , Regardless of the status of the sending and receiving modules , They all need to know what happened .

On the other hand , For events , We're just saying that an event happened , And inform which modules are not our concern . When we want to pass processing to another thread , It's best to use events ( for example : Send e-mail on completion of certain tasks ). Again , Events are very useful for Test Driven Development .

Events are used to exchange information between loosely coupled components . Because there is no direct coupling between publishers and subscribers , So we can modify the subscriber without affecting the publisher , vice versa . Let's see how Spring Boot Created in the application , Publish and listen to custom events .

1.  Create a  ApplicationEvent

We can use Spring Framework Event publishing mechanism to publish application events .

Let's create a UserCreatedEvent Custom events called by extension ApplicationEvent:

class UserCreatedEvent extends ApplicationEvent {
  private String name;
  UserCreatedEvent(Object source, String name) {
    this.name = name;

source Objects are parameters that can be initialized and passed when an event occurs , Transmission path super() Method .

from Spring 4.2 Start , We can also publish objects directly as events , And there's no need to expand ApplicationEvent:

class UserRemovedEvent {
  private String name;
  UserRemovedEvent(String name) {
    this.name = name;

2. Publish a  ApplicationEvent

We use ApplicationEventPublisher Interface to publish Events :

class Publisher {
  private final ApplicationEventPublisher publisher;
    Publisher(ApplicationEventPublisher publisher) {
      this.publisher = publisher;
  void publishEvent(final String name) {
    // Publishing event created by extending ApplicationEvent
    publisher.publishEvent(new UserCreatedEvent(this, name));
    // Publishing an object as an event
    publisher.publishEvent(new UserRemovedEvent(name));

When the object we publish is not ApplicationEvent when ,Spring Will use automatically PayloadApplicationEvent Pack it

3. Monitoring events

Now we know how to create and publish custom events , Let's see how to listen to events . An event can have multiple listeners that perform different tasks according to the requirements of the application .

There are two ways to define listeners . We can use @EventListener Comments or implementations ApplicationListener Interface . In either case , The listener class must be Spring management .

from Spring 4.1 Start , Now you can simply annotate hosting bean Methods ,@EventListener To automatically register ApplicationListener The method that matches the signature of the method :

class UserRemovedListener {
  ReturnedEvent handleUserRemovedEvent(UserRemovedEvent event) {
    // handle UserRemovedEvent ...
    return new ReturnedEvent();
  void handleReturnedEvent(ReturnedEvent event) {
        // handle ReturnedEvent ...

When enabling annotation driven configuration , No other configuration required . Our method can listen to multiple events , Or if we want to define it without any parameters at all , Then the event type can also be specified on the annotation itself . Example :@EventListener({ContextStartedEvent.class, ContextRefreshedEvent.class}).

For those with comments @EventListener The return type of the method is defined as not void,Spring The results will be released to us as new events . In the example above ,ReturnedEvent The results returned by the first method will be published , And then it's dealt with in the second way .

If specified SpEL,Spring Only in some cases is it allowed to trigger our listener condition:

class UserRemovedListener {
  @EventListener(condition = "#event.name eq 'reflectoring'")
  void handleConditionalListener(UserRemovedEvent event) {
    // handle UserRemovedEvent

Only if the expression evaluates to true, Or contains one of the following strings :“true”, “on”, “yes”, or “1”. Method parameters are exposed by their names . The conditional expression also exposes a reference to raw ApplicationEvent(#root.event) And the actual method parameters “ root ” Variable (#root.args)

In the example above ,UserRemovedEvent Only when the #event.name The value of is 'reflectoring', Will trigger the listener .

Another way to listen for events is to implement ApplicationListener Interface :

class UserCreatedListener implements ApplicationListener<UserCreatedEvent> {
  public void onApplicationEvent(UserCreatedEvent event) {
    // handle UserCreatedEvent

As long as the listener object is in Spring Register in the context of the application , It will receive events . When Spring When routing an event , It uses the signature of the listener to determine if it matches the event .

Asynchronous event listener

By default ,spring Events are synchronized , This means that the publisher thread will block , Until all listeners have finished processing the event .

To make the event listener run in asynchronous mode , So here's what we're going to do @Async Use comments on this listener :

class AsyncListener {
  void handleAsyncEvent(String event) {
    // handle event

In order to make @Async The notes take effect , We have to annotate one more @Configuration class , Use @EnableAsync notes SpringBootApplication class .

The code example above also shows , We can String As an event . Use at your own risk . It's best to use data types specific to our use cases , To avoid conflict with other events .

Transaction binding events

Spring Allows us to bind the event listener to a stage of the current transaction . When the result of the current transaction is important to the listener , This allows events to be used more flexibly .

When we annotate our methods @TransactionalEventListener, We get an extended event listener , The listener knows the transaction :

class UserRemovedListener {
  void handleAfterUserRemoved(UserRemovedEvent event) {
    // handle UserRemovedEvent

UserRemovedListener  Called only when the current transaction is complete .

We can bind listeners to the following stages of a transaction :

  • AFTER_COMMIT: After the transaction is successfully committed , The event will be handled . If the event listener runs only if the current transaction is successful , You can use this method .
  • AFTER_COMPLETION: This event is handled when the transaction is committed or rolled back . for example , We can use it to perform cleanup after the transaction is completed .
  • AFTER_ROLLBACK: After the transaction is rolled back , The event will be handled .
  • BEFORE_COMMIT: The event will be processed before the transaction is committed . for example , We can use it to make transactional O / R The mapping session refreshes to the database .

Spring Boot Application events for

Above is Spring event ,Spring Boot Provides several predefined ApplicationEvent Of , These predefined bindings are bound to SpringApplication Life cycle .

stay ApplicationContext Some events are triggered before creation , So we can't register these events as @Bean. We can register listeners for these events by manually adding listeners :

public class EventsDemoApplication {
  public static void main(String[] args) {
    SpringApplication springApplication = 
        new SpringApplication(EventsDemoApplication.class);
    springApplication.addListeners(new SpringBuiltInEventsListener());

By way of META-INF/spring.factories File added to our project , We can also register listeners , No matter how you create the app . And through the following org.springframework.context.ApplicationListener Key reference listener :

org.springframework.context.ApplicationListener= com.reflectoring.eventdemo.SpringBuiltInEventsListener

class SpringBuiltInEventsListener 
    implements ApplicationListener<SpringApplicationEvent>{
  public void onApplicationEvent(SpringApplicationEvent event) {
    // handle event

Once you make sure the event listener is properly registered , We can monitor all of the Spring Boot Of SpringApplicationEvents. Let's look at them in the order in which they are executed during application startup :


ApplicationStartingEvent At the start of the run, but before any processing , In addition to the registration of listeners and initializers .


When Environment Available in context , One ApplicationEnvironmentPreparedEvent Be triggered , Because of this time Environment Will be ready , So we can do something else bean Check and modify it before using it .


ApplicationContext When you're ready , One ApplicationContextInitializedEvent Trigger ,ApplicationContextInitializers Called not yet loaded bean Definition . stay bean Initialize to Spring Before the container , We can use it to perform tasks .


When ApllicationContext When ready , One ApplicationPreparedEvent Will trigger , But it won't refresh .

At the ready Environment and bean The definition will be loaded .


When ApplicationContext On refresh ,ContextRefreshedEvent Will trigger .

ContextRefreshedEvent It's directly from Spring, instead of Spring Boot, Does not inherit extensions SpringApplicationEvent.


If we use Web The server ,WebServerInitializedEvent It's in Web When the server is ready, it triggers a.ServletWebServerInitializedEvent and ReactiveWebServerInitializedEvent Namely servlet And reactive variables .

WebServerInitializedEvent Not inheritance extension SpringApplicationEvent.


After the context has been refreshed , One ApplicationStartedEvent Trigger , But in any Spring boot Before both the application and the command line run are called .


One ApplicationReadyEvent When triggered, the application is ready for a service request .

It is recommended not to modify the internal state at this time , Because all initialization steps will be completed .


One ApplicationFailedEvent If there is any abnormality , Application failed to start ignition . This can happen at any time during startup . We can use it to perform some tasks , For example, execute a script or issue a notification when a startup fails .


Events are designed to be in the same application context in Spring bean Simple communication between . from Spring 4.2 Start , The infrastructure has been significantly improved , It also provides the annotation based model and the function of publishing any event .

You can stay GitHub On Find the sample code .

本文为[Jiedao jdon]所创,转载请带上原文链接,感谢

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