Checkstyle changes

* Upgrade to Checkstyle 9.0
* apply some JavaDocs rules
* Fix JavaDocs rules violations
* Some other minor clean up in the affected classes

spring-integration-mail/src/main/java/org/springframework/integration/mail/ImapMailReceiver.java

@@ -99,7 +99,7 @@ public class ImapMailReceiver extends AbstractMailReceiver {
 
 	/**
 	 * Provides a way to set custom {@link SearchTermStrategy} to compile a {@link SearchTerm}
-	 * to be applied when retrieving mail
+	 * to be applied when retrieving mail.
 	 * @param searchTermStrategy The search term strategy implementation.
 	 */
 	public void setSearchTermStrategy(SearchTermStrategy searchTermStrategy) {

spring-integration-mail/src/main/java/org/springframework/integration/mail/MailSendingMessageHandler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -43,12 +43,13 @@ import org.springframework.util.StringUtils;
  * header is required. Otherwise, a String type is expected, and its content
  * will be used as the text within a {@link SimpleMailMessage}.
  *
- * @see MailHeaders
  *
  * @author Marius Bogoevici
  * @author Mark Fisher
  * @author Oleg Zhurakousky
  * @author Artem Bilan
+ *
+ * @see MailHeaders
  */
 public class MailSendingMessageHandler extends AbstractMessageHandler {
 

spring-integration-mail/src/main/java/org/springframework/integration/mail/SearchTermStrategy.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -21,20 +21,19 @@ import javax.mail.Folder;
 import javax.mail.search.SearchTerm;
 
 /**
- * Strategy to be used to generate a {@link SearchTerm}
- *
- * See {@link ImapMailReceiver}
+ * Strategy to be used to generate a {@link SearchTerm}.
  *
  * @author Oleg Zhurakousky
+ *
  * @since 2.2
  *
+ * @see ImapMailReceiver
  */
 @FunctionalInterface
 public interface SearchTermStrategy {
 
 	/**
-	 * Will generate an instance of the {@link SearchTerm}
-	 *
+	 * Will generate an instance of the {@link SearchTerm}.
 	 * @param supportedFlags The supported flags.
 	 * @param folder The folder.
 	 * @return The search term.

spring-integration-mail/src/main/java/org/springframework/integration/mail/dsl/ImapIdleChannelAdapterSpec.java

@@ -374,8 +374,8 @@ public class ImapIdleChannelAdapterSpec
 	 * Seconds; default 120 (2 minutes).
 	 * @param interval the interval.
 	 * @return the spec.
-	 * @see ImapMailReceiver#setCancelIdleInterval(long)
 	 * @since 5.2
+	 * @see ImapMailReceiver#setCancelIdleInterval(long)
 	 */
 	public ImapIdleChannelAdapterSpec cancelIdleInterval(long interval) {
 		assertReceiver();
@@ -387,8 +387,8 @@ public class ImapIdleChannelAdapterSpec
 	 * Determine how the content is rendered.
 	 * @param simpleContent true for simple content.
 	 * @return the spec.
-	 * @see ImapMailReceiver#setSimpleContent(boolean)
 	 * @since 5.5
+	 * @see ImapMailReceiver#setSimpleContent(boolean)
 	 */
 	public ImapIdleChannelAdapterSpec simpleContent(boolean simpleContent) {
 		assertReceiver();

spring-integration-mail/src/main/java/org/springframework/integration/mail/transformer/AbstractMailMessageTransformer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -33,6 +33,8 @@ import org.springframework.messaging.Message;
  * Base class for Transformers that convert from a JavaMail Message to a
  * Spring Integration Message.
  *
+ * @param <T> the target payload type.
+ *
  * @author Mark Fisher
  * @author Gary Russell
  * @author Artem Bilan
@@ -88,7 +90,7 @@ public abstract class AbstractMailMessageTransformer<T> implements Transformer,
 			throws Exception; // NOSONAR
 
 
-	private Map<String, Object> extractHeaderMapFromMailMessage(javax.mail.Message mailMessage) {
+	private static Map<String, Object> extractHeaderMapFromMailMessage(javax.mail.Message mailMessage) {
 		return MailUtils.extractStandardHeaders(mailMessage);
 	}