Throw identifiable exceptions on input/output conversion failure.

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/catalog/BeanFactoryAwareFunctionRegistry.java

@@ -71,6 +71,7 @@ import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageHeaders;
 import org.springframework.messaging.converter.CompositeMessageConverter;
+import org.springframework.messaging.converter.MessageConversionException;
 import org.springframework.messaging.support.GenericMessage;
 import org.springframework.messaging.support.MessageBuilder;
 import org.springframework.util.Assert;
@@ -94,6 +95,16 @@ public class BeanFactoryAwareFunctionRegistry
 
 	private static Log logger = LogFactory.getLog(BeanFactoryAwareFunctionRegistry.class);
 
+	/**
+	 * Identifies MessageConversionExceptions that happen when input can't be converted.
+	 */
+	public static final String COULD_NOT_CONVERT_INPUT = "Could Not Convert Input";
+
+	/**
+	 * Identifies MessageConversionExceptions that happen when output can't be converted.
+	 */
+	public static final String COULD_NOT_CONVERT_OUTPUT = "Could Not Convert Output";
+
 	private ConfigurableApplicationContext applicationContext;
 
 	private final Map<Object, FunctionRegistration<Object>> registrationsByFunction = new HashMap<>();
@@ -655,7 +666,10 @@ public class BeanFactoryAwareFunctionRegistry
 					}
 				}
 			}
-			return convertedValue != null ? convertedValue : value;
+			if (convertedValue == null) {
+				throw new MessageConversionException(COULD_NOT_CONVERT_OUTPUT);
+			}
+			return convertedValue;
 
 		}
 
@@ -764,6 +778,9 @@ public class BeanFactoryAwareFunctionRegistry
 			if (logger.isDebugEnabled()) {
 				logger.debug("Converted input value " + convertedValue);
 			}
+			if (convertedValue == null) {
+				throw new MessageConversionException(COULD_NOT_CONVERT_INPUT);
+			}
 			return convertedValue;
 		}