GH-3421: Resolve throws Exception; in the API

Fixes https://github.com/spring-projects/spring-integration/issues/3421

Remove `throws Exception;` from production code to honor
the rule `Generic exceptions should never be thrown` which is enabled on SonarQube

* Rework affected usages to `try..catch` with throwing respective runtime exception
or just logging
* Some other refactoring for the affected classes

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -39,8 +39,7 @@ import org.springframework.messaging.Message;
  * @author Gary Russell
  * @author Artem Bilan
  */
-public abstract class AbstractMailMessageTransformer<T> implements Transformer,
-		BeanFactoryAware {
+public abstract class AbstractMailMessageTransformer<T> implements Transformer, BeanFactoryAware {
 
 	private BeanFactory beanFactory;
 
@@ -48,7 +47,6 @@ public abstract class AbstractMailMessageTransformer<T> implements Transformer,
 
 	private boolean messageBuilderFactorySet;
 
-
 	@Override
 	public final void setBeanFactory(BeanFactory beanFactory) {
 		this.beanFactory = beanFactory;
@@ -67,28 +65,19 @@ public abstract class AbstractMailMessageTransformer<T> implements Transformer,
 	@Override
 	public Message<?> transform(Message<?> message) {
 		Object payload = message.getPayload();
-		if (!(payload instanceof jakarta.mail.Message)) {
+		if (!(payload instanceof jakarta.mail.Message mailMessage)) {
 			throw new MessageTransformationException(message, getClass().getSimpleName()
 					+ " requires a jakarta.mail.Message payload");
 		}
-		jakarta.mail.Message mailMessage = (jakarta.mail.Message) payload;
 		AbstractIntegrationMessageBuilder<T> builder;
-		try {
-			builder = this.doTransform(mailMessage);
-		}
-		catch (Exception e) {
-			throw new MessageTransformationException(message, "failed to transform mail message", e);
-		}
+		builder = doTransform(mailMessage);
 		if (builder == null) {
 			throw new MessageTransformationException(message, "failed to transform mail message");
 		}
-		builder.copyHeaders(extractHeaderMapFromMailMessage(mailMessage));
-		return builder.build();
+		return builder.copyHeaders(extractHeaderMapFromMailMessage(mailMessage)).build();
 	}
 
-	protected abstract AbstractIntegrationMessageBuilder<T> doTransform(jakarta.mail.Message mailMessage)
-			throws Exception; // NOSONAR
-
+	protected abstract AbstractIntegrationMessageBuilder<T> doTransform(jakarta.mail.Message mailMessage);
 
 	private static Map<String, Object> extractHeaderMapFromMailMessage(jakarta.mail.Message mailMessage) {
 		return MailUtils.extractStandardHeaders(mailMessage);

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -23,6 +23,7 @@ import jakarta.mail.Multipart;
 import jakarta.mail.Part;
 
 import org.springframework.integration.support.AbstractIntegrationMessageBuilder;
+import org.springframework.integration.transformer.MessageTransformationException;
 import org.springframework.util.Assert;
 
 /**
@@ -51,27 +52,34 @@ public class MailToStringTransformer extends AbstractMailMessageTransformer<Stri
 	}
 
 	@Override
-	protected AbstractIntegrationMessageBuilder<String> doTransform(jakarta.mail.Message mailMessage)
-			throws Exception { // NOSONAR
+	protected AbstractIntegrationMessageBuilder<String> doTransform(jakarta.mail.Message mailMessage) {
+		try {
+			String payload;
+			Object content = mailMessage.getContent();
+			if (content instanceof String value) {
+				payload = value;
+			}
+			else if (content instanceof Multipart multipart) {
+				ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+				multipart.writeTo(outputStream);
+				payload = outputStream.toString(this.charset);
+			}
+			else if (content instanceof Part part) {
+				ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+				part.writeTo(outputStream);
+				payload = outputStream.toString(this.charset);
+			}
+			else {
+				throw new IllegalArgumentException("failed to transform contentType ["
+						+ mailMessage.getContentType() + "] to String.");
+			}
 
-		Object content = mailMessage.getContent();
-		if (content instanceof String) {
-			return this.getMessageBuilderFactory().withPayload((String) content);
+			return getMessageBuilderFactory().withPayload(payload);
 		}
-		if (content instanceof Multipart) {
-			ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
-			((Multipart) content).writeTo(outputStream);
-			return this.getMessageBuilderFactory().withPayload(
-					new String(outputStream.toByteArray(), this.charset));
+		catch (Exception ex) {
+			throw new MessageTransformationException("Cannot transform mail message", ex);
 		}
-		else if (content instanceof Part) {
-			ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
-			((Part) content).writeTo(outputStream);
-			return this.getMessageBuilderFactory().withPayload(
-					new String(outputStream.toByteArray(), this.charset));
-		}
-		throw new IllegalArgumentException("failed to transform contentType ["
-				+ mailMessage.getContentType() + "] to String.");
+
 	}
 
 }