GH-3356: Add the way to not load mail message

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

@@ -502,8 +502,8 @@ private void postProcessFilteredMessages(Message[] filteredMessages) throws Mess
 		if (shouldDeleteMessages()) {
 			deleteMessages(filteredMessages);
 		}
-		if (this.headerMapper == null) {
-			// Copy messages to cause an eager fetch
+		// Copy messages to cause an eager fetch
+		if (this.headerMapper == null && (this.autoCloseFolder || this.simpleContent)) {
 			for (int i = 0; i < filteredMessages.length; i++) {
 				MimeMessage mimeMessage = new IntegrationMimeMessage((MimeMessage) filteredMessages[i]);
 				filteredMessages[i] = mimeMessage;

spring-integration-mail/src/test/java/org/springframework/integration/mail/dsl/MailTests.java

@@ -33,9 +33,8 @@
 import javax.mail.search.FromTerm;
 import javax.mail.search.SearchTerm;
 
-import org.junit.BeforeClass;
-import org.junit.Test;
-import org.junit.runner.RunWith;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Qualifier;
@@ -63,13 +62,13 @@
 import org.springframework.messaging.MessageHeaders;
 import org.springframework.messaging.PollableChannel;
 import org.springframework.test.annotation.DirtiesContext;
-import org.springframework.test.context.junit4.SpringRunner;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
 
 /**
  * @author Gary Russell
  * @author Artem Bilan
  */
-@RunWith(SpringRunner.class)
+@SpringJUnitConfig
 @DirtiesContext
 public class MailTests {
 
@@ -82,7 +81,7 @@ public class MailTests {
 	private static final ImapServer imapIdleServer = TestMailServer.imap(0);
 
 
-	@BeforeClass
+	@BeforeAll
 	public static void setup() throws InterruptedException {
 		int n = 0;
 		while (n++ < 100 && (!smtpServer.isListening() || !pop3Server.isListening()
@@ -216,7 +215,7 @@ public IntegrationFlow sendMailFlow() {
 					.enrichHeaders(Mail.headers()
 							.subjectFunction(m -> "foo")
 							.from("foo@bar")
-							.toFunction(m -> new String[] { "bar@baz" }))
+							.toFunction(m -> new String[]{ "bar@baz" }))
 					.handle(Mail.outboundAdapter("localhost")
 									.port(smtpServer.getPort())
 									.credentials("user", "pw")

src/reference/asciidoc/mail.adoc

@@ -128,13 +128,17 @@ Closeable closeableResource = StaticMessageHeaderAccessor.getCloseableResource(m
 if (closeableResource != null) {
     closeableResource.close();
 }
-
 ----
 ====
 
 Keeping the folder open is useful in cases where communication with the server is needed during parsing multipart content of the email with attachments.
 The `close()` on the `IntegrationMessageHeaderAccessor.CLOSEABLE_RESOURCE` header delegates to the `AbstractMailReceiver` to close the folder with `expunge` option if `shouldDeleteMessages` is configured respectively on the `AbstractMailReceiver`.
 
+Starting with version 5.4, it is possible now to return a `MimeMessage` as is without any conversion or eager content loading.
+This functionality is enabled with this combination of options: no `headerMapper` provided, the `simpleContent` property is `false` and the `autoCloseFolder` property is `false`.
+The `MimeMessage` is present as the payload of the Spring message produced.
+In this case, the only header populated is the above mentioned `IntegrationMessageHeaderAccessor.CLOSEABLE_RESOURCE` for the folder which must be closed when processing of the the `MimeMessage` is complete.
+
 [[mail-mapping]]
 === Inbound Mail Message Mapping
 
@@ -210,6 +214,10 @@ Starting with version 4.3, the transformer handles embedded `Part` instances (as
 The transformer is a subclass of `AbstractMailTransformer` that maps the address and subject headers from the preceding list.
 If you wish to perform some other transformation on the message, consider subclassing `AbstractMailTransformer`.
 
+Starting with version 5.4, when no `headerMapper` is provided, `autoCloseFolder` is `false` and `simpleContent` is `false`, the `MimeMessage` is returned as-is in the payload of the Spring message produced.
+This way, the content of the `MimeMessage` is loaded on demand when referenced, later in the flow.
+All of the mentioned above transformations are still valid.
+
 [[mail-namespace]]
 === Mail Namespace Support
 

src/reference/asciidoc/whats-new.adoc

@@ -78,6 +78,7 @@ See <<./ip.adoc#ip-collaborating-adapters,Collaborating Channel Adapters>> and <
 The `spring-integration-rmi` module is deprecated with no replacement and is going to be removed in the next major version.
 See <<./rmi.adoc#rmi, RMI Support>> for more information.
 
+
 [[x5.4-amqp]]
 === AMQP Changes
 
@@ -86,3 +87,9 @@ See <<./amqp.adoc#alternative-confirms-returns,Alternative Mechanism for Publish
 
 A new `BatchMode.EXTRACT_PAYLOAD_WITH_HEADERS` is supported by the `AmqpInboundChannelAdapter`.
 See <<./amqp.adoc#amqp-inbound-channel-adapter,Inbound Channel Adapter>> for more information.
+
+[[x5.4-mail]]
+=== Mail Changes
+
+The `AbstractMailReceiver` can now produce the `MimeMessage` as-is without eager fetching its content.
+See <<./mail.adoc#mail-inbound, Mail-receiving Channel Adapter>> for more information.