Introduce ApplicationContextEvent.getSource() with covariant return type

Prior to this commit, ApplicationContextEvent inherited getSource()
from java.util.EventObject.getSource() which has an Object return type.

This commit introduces a local getSource() implementation in
ApplicationContextEvent with an ApplicationContext covariant return
type, analogous to TestContextEvent in spring-test.

Closes gh-35197

spring-context/src/main/java/org/springframework/context/event/ApplicationContextEvent.java

@@ -20,9 +20,10 @@ import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationEvent;
 
 /**
- * Base class for events raised for an {@code ApplicationContext}.
+ * Base class for events raised for an {@link ApplicationContext}.
  *
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 2.5
  */
 @SuppressWarnings("serial")
@@ -30,7 +31,7 @@ public abstract class ApplicationContextEvent extends ApplicationEvent {
 
 	/**
 	 * Create a new {@code ApplicationContextEvent}.
-	 * @param source the {@code ApplicationContext} that the event is raised for
+	 * @param source the {@link ApplicationContext} that the event is raised for
 	 * (must not be {@code null})
 	 */
 	public ApplicationContextEvent(ApplicationContext source) {
@@ -38,10 +39,22 @@ public abstract class ApplicationContextEvent extends ApplicationEvent {
 	}
 
 	/**
-	 * Get the {@code ApplicationContext} that the event was raised for.
+	 * Get the {@link ApplicationContext} that the event was raised for.
+	 * @return the {@code ApplicationContext} that the event was raised for
+	 * @since 7.0
+	 * @see #getApplicationContext()
+	 */
+	@Override
+	public ApplicationContext getSource() {
+		return getApplicationContext();
+	}
+
+	/**
+	 * Get the {@link ApplicationContext} that the event was raised for.
+	 * @see #getSource()
 	 */
 	public final ApplicationContext getApplicationContext() {
-		return (ApplicationContext) getSource();
+		return (ApplicationContext) super.getSource();
 	}
 
 }