Register Expression serializer on binder ObjectMapper

Restore the intended Expression serializer registration so bindings actuator output can render SpEL expressions as strings instead of traversing Expression internals.

Author: wadhwaroh-lang

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

@@ -157,7 +157,7 @@ protected void onInit() throws Exception {
 //		}
 		SimpleModule module = new SimpleModule();
 		module.addSerializer(Expression.class, new ExpressionSerializer(Expression.class));
-		this.objectMapper = this.objectMapper.rebuild().build();
+		this.objectMapper = this.objectMapper.rebuild().addModule(module).build();
 	}
 
 	public AbstractMessageChannelBinder(String[] headersToEmbed, PP provisioningProvider,

core/spring-cloud-stream/src/test/java/org/springframework/cloud/stream/binder/AbstractMessageChannelBinderTests.java

@@ -27,6 +27,8 @@
 import org.springframework.cloud.stream.provisioning.ConsumerDestination;
 import org.springframework.cloud.stream.provisioning.ProducerDestination;
 import org.springframework.context.support.GenericApplicationContext;
+import org.springframework.expression.Expression;
+import org.springframework.expression.spel.standard.SpelExpressionParser;
 import org.springframework.integration.core.MessageProducer;
 import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.MessageHandler;
@@ -60,6 +62,24 @@ void serializeDurationOnObjectMapperInAMCB() throws Exception {
 		assertThat(convertedMap).isNotEmpty();
 	}
 
+	@Test
+	@SuppressWarnings("unchecked")
+	void serializeExpressionOnObjectMapperInAMCB() throws Exception {
+		AbstractMessageChannelBinder<?, ?, ?> binder = createBinderInstance();
+
+		Field objectMapperField = ReflectionUtils.findField(AbstractMessageChannelBinder.class, "objectMapper");
+		assertThat(objectMapperField).isNotNull();
+		ReflectionUtils.makeAccessible(objectMapperField);
+		ObjectMapper objectMapper = (ObjectMapper) ReflectionUtils.getField(objectMapperField, binder);
+		assertThat(objectMapper).isNotNull();
+
+		Expression expression = new SpelExpressionParser().parseExpression("'routing.key'");
+		Map<String, Object> properties = Map.of("routingKeyExpression", expression);
+		Map<String, Object> convertedMap = objectMapper.convertValue(properties, Map.class);
+
+		assertThat(convertedMap).containsEntry("routingKeyExpression", "'routing.key'");
+	}
+
 	@NotNull
 	private static AbstractMessageChannelBinder<?, ?, ?> createBinderInstance() throws Exception {
 		AbstractMessageChannelBinder<?, ?, ?> binder = new AbstractMessageChannelBinder<>(null, null) {