CAMEL-22549: Add inline comments for non-trivial getOut→getMessage changes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gnodet

components/camel-bean/src/main/java/org/apache/camel/component/bean/MethodInfo.java

@@ -395,7 +395,8 @@ public AccessibleObject getStaticPart() {
     private void fillResult(Exchange exchange, Object result) {
         LOG.trace("Setting bean invocation result : {}", result);
 
-        // the bean component forces OUT if the MEP is OUT capable
+        // the bean component forces OUT if the MEP is OUT capable;
+        // in.copy() both creates the OUT and propagates headers in one step
         if (ExchangeHelper.isOutCapable(exchange) && !ExchangeHelper.hasResponse(exchange)) {
             ExchangeHelper.setResponse(exchange, exchange.getIn().copy());
         }

components/camel-xslt/src/main/java/org/apache/camel/component/xslt/XsltBuilder.java

@@ -108,7 +108,8 @@ public void process(Exchange exchange) throws Exception {
 
         ResultHandler resultHandler = resultHandlerFactory.createResult(exchange);
         Result result = resultHandler.getResult();
-        // let's copy the headers before we invoke the transform in case they modify them
+        // copy headers before invoking the transform; getMessage() returns IN here
+        // (no OUT yet), so copyFrom is a no-op for headers but sets up the message state
         Message out = exchange.getMessage();
         out.copyFrom(exchange.getIn());
 
@@ -489,6 +490,7 @@ protected void configureTransformer(Transformer transformer, Exchange exchange)
         addParameters(transformer, getParameters());
         transformer.setParameter("exchange", exchange);
         transformer.setParameter("in", exchange.getIn());
+        // only set "out" param if a response exists; avoids creating an empty OUT as side effect
         Message response = ExchangeHelper.getResponse(exchange);
         if (response != null) {
             transformer.setParameter("out", response);

core/camel-support/src/main/java/org/apache/camel/support/ExchangeHelper.java

@@ -442,6 +442,7 @@ private static void copyFromOutMessage(Exchange result, Exchange source, boolean
      */
     public static Message getResultMessage(Exchange exchange) {
         if (exchange.getPattern().isOutCapable()) {
+            // explicitly create response if none exists (preserves old getOut() lazy-creation semantics)
             if (!hasResponse(exchange)) {
                 setResponse(exchange, new DefaultMessage(exchange.getContext()));
             }

core/camel-support/src/main/java/org/apache/camel/support/processor/UnmarshalProcessor.java

@@ -77,7 +77,8 @@ public boolean process(Exchange exchange, AsyncCallback callback) {
             }
             final Message out;
             if (allowNullBody && body == null) {
-                // The body is null, and it is an allowed value so let's skip the unmarshalling
+                // The body is null, and it is an allowed value so let's skip the unmarshalling;
+                // use empty response (not in.copy()) so the null body is preserved
                 ExchangeHelper.setResponse(exchange, new DefaultMessage(exchange.getContext()));
                 out = exchange.getMessage();
             } else {