[kafka][metrics] Fix bean conditional resolution order to prioritize user-defined beans over binder's conditional beans in a multi-binder scenario

This change modifies the loading order in DefaultBinderFactory.initializeBinderContextSimple
to ensure that user configuration classes specified via spring.main.sources are loaded before
binder configuration classes. This allows @ConditionalOnMissingBean annotations in the binder
configurations to properly detect user-provided beans.

Previously, when using KafkaBinderConfiguration with @ConditionalOnMissingBean(KafkaBinderMetrics.class),
even if the user had configured a custom KafkaBinderMetrics bean via spring.main.sources,
the condition would not work correctly because binder classes were loaded first.

Fixes gh-3114

Signed-off-by: ferblaca <[email protected]>

Author: ferblaca

core/spring-cloud-stream/src/main/java/org/springframework/cloud/stream/binder/DefaultBinderFactory.java

@@ -510,8 +510,10 @@ public void onApplicationEvent(ApplicationEvent event) {
 			}
 		}
 
-		// Register the sources classes to the specific binder context after configuring the environment property sources
-		List<Class> sourceClasses = new ArrayList<>(Arrays.asList(binderType.getConfigurationClasses()));
+		// Modified registration: first user classes (spring.main.sources) and then binder classes
+		List<Class> sourceClasses = new ArrayList<>();
+
+		// First register user classes if defined in spring.main.sources
 		if (binderProperties.containsKey("spring.main.sources")) {
 			String sources = (String) binderProperties.get("spring.main.sources");
 			if (StringUtils.hasText(sources)) {
@@ -525,6 +527,10 @@ public void onApplicationEvent(ApplicationEvent event) {
 				});
 			}
 		}
+
+		// Then add binder configuration classes
+		sourceClasses.addAll(Arrays.asList(binderType.getConfigurationClasses()));
+
 		binderProducingContext.register(sourceClasses.toArray(new Class[] {}));
 
 		if (refresh) {