Introduce ContextRestartedEvent as a subtype of ContextStartedEvent

Prior to this commit, there was no way to differentiate between an
ApplicationContext "start" and "restart" in terms of events. This was
due to the fact that a ContextStartedEvent was fired for both
AbstractApplicationContext.start() and
AbstractApplicationContext.restart().

To assist developers who may wish to differentiate between "start" and
"restart" events, this commit introduces a new ContextRestartedEvent as
a subtype of ContextStartedEvent. In addition,
AbstractApplicationContext.restart() now publishes a
ContextRestartedEvent instead of a ContextStartedEvent.

By making ContextRestartedEvent a subtype of ContextStartedEvent,
applications can still expect ContextStoppedEvent/ContextStartedEvent
event pairs for consistent lifecycle semantics, and they can optionally
check if the ContextStartedEvent is a ContextRestartedEvent.
Alternatively, applications can explicitly react to a
ContextRestartedEvent.

See gh-35168
See gh-35171
Closes gh-35194

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

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2002-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.context.event;
+
+import org.springframework.context.ApplicationContext;
+
+/**
+ * Event raised when an {@code ApplicationContext} gets restarted.
+ *
+ * <p>Note that {@code ContextRestartedEvent} is a specialization of
+ * {@link ContextStartedEvent}.
+ *
+ * @author Sam Brannen
+ * @since 7.0
+ * @see ContextStartedEvent
+ * @see ContextStoppedEvent
+ */
+@SuppressWarnings("serial")
+public class ContextRestartedEvent extends ContextStartedEvent {
+
+	/**
+	 * Create a new {@code ContextRestartedEvent}.
+	 * @param source the {@code ApplicationContext} that has been restarted
+	 * (must not be {@code null})
+	 */
+	public ContextRestartedEvent(ApplicationContext source) {
+		super(source);
+	}
+
+}

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

@@ -24,6 +24,7 @@ import org.springframework.context.ApplicationContext;
  * @author Mark Fisher
  * @author Juergen Hoeller
  * @since 2.5
+ * @see ContextRestartedEvent
  * @see ContextStoppedEvent
  */
 @SuppressWarnings("serial")

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

@@ -25,6 +25,7 @@ import org.springframework.context.ApplicationContext;
  * @author Juergen Hoeller
  * @since 2.5
  * @see ContextStartedEvent
+ * @see ContextRestartedEvent
  */
 @SuppressWarnings("serial")
 public class ContextStoppedEvent extends ApplicationContextEvent {

spring-context/src/main/java/org/springframework/context/support/AbstractApplicationContext.java

@@ -67,6 +67,7 @@ import org.springframework.context.ResourceLoaderAware;
 import org.springframework.context.event.ApplicationEventMulticaster;
 import org.springframework.context.event.ContextClosedEvent;
 import org.springframework.context.event.ContextRefreshedEvent;
+import org.springframework.context.event.ContextRestartedEvent;
 import org.springframework.context.event.ContextStartedEvent;
 import org.springframework.context.event.ContextStoppedEvent;
 import org.springframework.context.event.SimpleApplicationEventMulticaster;
@@ -1551,7 +1552,7 @@ public abstract class AbstractApplicationContext extends DefaultResourceLoader
 	@Override
 	public void restart() {
 		getLifecycleProcessor().onRestart();
-		publishEvent(new ContextStartedEvent(this));
+		publishEvent(new ContextRestartedEvent(this));
 	}
 
 	@Override