Refactor AbstractMailReceiver for proper locking

According Sonar we can't just leave lock unlocked in th method, so
apply refactoring for the `AbstractMailReceiver.receive()` where we don't
have uncontrolled `lock()` in the separate method

Author: artembilan

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

@@ -343,25 +343,19 @@ public Object[] receive() throws javax.mail.MessagingException {
 		this.folderReadLock.lock();
 		try {
 			try {
-				obtainFolder();
-				MimeMessage[] filteredMessages = searchAndFilterMessages();
-				if (this.headerMapper != null) {
-					org.springframework.messaging.Message<?>[] converted =
-							new org.springframework.messaging.Message<?>[filteredMessages.length];
-					int n = 0;
-					for (MimeMessage message : filteredMessages) {
-						Map<String, Object> headers = this.headerMapper.toHeaders(message);
-						converted[n++] =
-								getMessageBuilderFactory()
-										.withPayload(extractContent(message, headers))
-										.copyHeaders(headers)
-										.build();
+				Folder folderToCheck = getFolder();
+				if (folderToCheck == null || !folderToCheck.isOpen()) {
+					this.folderReadLock.unlock();
+					this.folderWriteLock.lock();
+					try {
+						openFolder();
+						this.folderReadLock.lock();
+					}
+					finally {
+						this.folderWriteLock.unlock();
 					}
-					return converted;
-				}
-				else {
-					return filteredMessages;
 				}
+				return convertMessagesIfNecessary(searchAndFilterMessages());
 			}
 			finally {
 				MailTransportUtils.closeFolder(this.folder, this.shouldDeleteMessages);
@@ -372,21 +366,6 @@ public Object[] receive() throws javax.mail.MessagingException {
 		}
 	}
 
-	private void obtainFolder() throws MessagingException {
-		Folder folder = getFolder();
-		if (folder == null || !folder.isOpen()) {
-			this.folderReadLock.unlock();
-			this.folderWriteLock.lock();
-			try {
-				openFolder();
-				this.folderReadLock.lock();
-			}
-			finally {
-				this.folderWriteLock.unlock();
-			}
-		}
-	}
-
 	private MimeMessage[] searchAndFilterMessages() throws MessagingException {
 		if (this.logger.isInfoEnabled()) {
 			this.logger.info("attempting to receive mail from folder [" + this.folder.getFullName() + "]");
@@ -414,6 +393,26 @@ private MimeMessage[] searchAndFilterMessages() throws MessagingException {
 		return filteredMessages;
 	}
 
+	private Object[] convertMessagesIfNecessary(MimeMessage[] filteredMessages) {
+		if (this.headerMapper != null) {
+			org.springframework.messaging.Message<?>[] converted =
+					new org.springframework.messaging.Message<?>[filteredMessages.length];
+			int n = 0;
+			for (MimeMessage message : filteredMessages) {
+				Map<String, Object> headers = this.headerMapper.toHeaders(message);
+				converted[n++] =
+						getMessageBuilderFactory()
+								.withPayload(extractContent(message, headers))
+								.copyHeaders(headers)
+								.build();
+			}
+			return converted;
+		}
+		else {
+			return filteredMessages;
+		}
+	}
+
 	private Object extractContent(MimeMessage message, Map<String, Object> headers) {
 		Object content;
 		try {