How to use the Spring Application Listener
To set the application to use Spring Application listener, you can do the following.
1. In the application context file for Spring, declare a bean as follows
<bean id="applicationEventMulticaster" class="org.springframework.context.event.SimpleApplicationEventMulticaster"/>Actually, this may not needed at all. The following method is from Spring. By default, it will create a SimpleApplicationEventMulticaster if none is given.
package org.springframework.context.support;
public abstract class AbstractApplicationContext extends DefaultResourceLoader
implements ConfigurableApplicationContext, DisposableBean {
/**
* Initialize the ApplicationEventMulticaster.
* Uses SimpleApplicationEventMulticaster if none defined in the context.
* @see org.springframework.context.event.SimpleApplicationEventMulticaster
*/
protected void initApplicationEventMulticaster() {
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
this.applicationEventMulticaster =
beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
if (logger.isDebugEnabled()) {
logger.debug("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
}
}
else {
this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
if (logger.isDebugEnabled()) {
logger.debug("Unable to locate ApplicationEventMulticaster with name '" +
APPLICATION_EVENT_MULTICASTER_BEAN_NAME +
"': using default [" + this.applicationEventMulticaster + "]");
}
}
}
...
}
2. Create Application listeners
import org.springframework.context.ApplicationListener; @Component public class MyEventListener implements ApplicationListener{ ... } import org.springframework.context.ApplicationEvent; public class MyEvent extends ApplicationEvent { ... }
3. Just call the multicastEvent method of the bean applicationEventMulticaster when needed: applicationEventMulticaster.multicastEvent(event);
The method will invoke the application listeners to act on the event.
public void multicastEvent(final ApplicationEvent event) {
for (final ApplicationListener listener : getApplicationListeners(event)) {
Executor executor = getTaskExecutor();
if (executor != null) {
executor.execute(new Runnable() {
@SuppressWarnings("unchecked")
public void run() {
listener.onApplicationEvent(event);
}
});
}
else {
listener.onApplicationEvent(event);
}
}
}
How does the Application know what application listeners are listening?
At the application startup, the beans that implement the ApplicationListener interface will be automatically detected and registered.
ContextLoaderListener(ContextLoader).initWebApplicationContext(ServletContext)
->
XmlWebApplicationContext(AbstractApplicationContext).refresh();
->
DefaultListableBeanFactory(AbstractAutowireCapableBeanFactory). applyBeanPostProcessorsAfterIniitalization
->
AbstractApplicationContext$ApplicationListenerDetector.postProcessAfterInitialization
public Object postProcessAfterInitialization(Object bean, String beanName) {
if (bean instanceof ApplicationListener) {
// potentially not detected as a listener by getBeanNamesForType retrieval
Boolean flag = this.singletonNames.get(beanName);
if (Boolean.TRUE.equals(flag)) {
// singleton bean (top-level or inner): register on the fly
addApplicationListener((ApplicationListener) bean);
}
else if (flag == null) {
if (logger.isWarnEnabled() && !containsBean(beanName)) {
// inner bean with other scope - can't reliably process events
logger.warn("Inner bean '" + beanName + "' implements ApplicationListener interface " +
"but is not reachable for event multicasting by its containing ApplicationContext " +
"because it does not have singleton scope. Only top-level listener beans are allowed " +
"to be of non-singleton scope.");
}
this.singletonNames.put(beanName, Boolean.FALSE);
}
}
return bean;
}
public void addApplicationListener(ApplicationListener listener) {
if (this.applicationEventMulticaster != null) {
this.applicationEventMulticaster.addApplicationListener(listener);
}
else {
this.applicationListeners.add(listener);
}
}
