Add ExceptionMapper for validation errors.

pom.xml

@@ -122,6 +122,11 @@
 				<artifactId>jersey-container-servlet</artifactId>
 				<version>${jersey3-version}</version>
 			</dependency>
+			<dependency>
+			    <groupId>org.glassfish.jersey.ext</groupId>
+			    <artifactId>jersey-bean-validation</artifactId>
+			    <version>${jersey3-version}</version>
+			</dependency>
 
 			<dependency>
 				<groupId>com.fasterxml.jackson.datatype</groupId>

smart-connector-rest-server/pom.xml

@@ -70,6 +70,10 @@
 			<groupId>org.glassfish.jersey.containers</groupId>
 			<artifactId>jersey-container-servlet</artifactId>
 		</dependency>
+		<dependency>
+    		<groupId>org.glassfish.jersey.ext</groupId>
+    		<artifactId>jersey-bean-validation</artifactId>
+		</dependency>
 
 		<!-- Add XML suport -->
 		<dependency>

smart-connector-rest-server/src/main/java/eu/knowledge/engine/rest/api/JsonExceptionMapper.java

@@ -1,12 +1,13 @@
 package eu.knowledge.engine.rest.api;
 
+import com.fasterxml.jackson.core.JsonProcessingException;
+
+import eu.knowledge.engine.rest.model.ResponseMessage;
 import jakarta.ws.rs.core.MediaType;
 import jakarta.ws.rs.core.Response;
 import jakarta.ws.rs.ext.ExceptionMapper;
 import jakarta.ws.rs.ext.Provider;
 
-import com.fasterxml.jackson.core.JsonProcessingException;
-
 /**
  * Since apparently Jersey gives a status 500 response when it encounters
  * unexpected input in request bodies, this exception mapper maps those
@@ -16,10 +17,11 @@
 public class JsonExceptionMapper implements ExceptionMapper<JsonProcessingException> {
 	@Override
 	public Response toResponse(JsonProcessingException exception) {
-		return Response
-			.status(Response.Status.BAD_REQUEST)
-			.entity(exception.getOriginalMessage())
-			.type(MediaType.TEXT_PLAIN)
-			.build();
+
+		var response = new ResponseMessage();
+		response.setMessageType("error");
+		response.setMessage(exception.getClass().getSimpleName() + ": " + exception.getOriginalMessage());
+
+		return Response.status(Response.Status.BAD_REQUEST).entity(response).type(MediaType.APPLICATION_JSON).build();
 	}
 }

smart-connector-rest-server/src/main/java/eu/knowledge/engine/rest/api/ValidationExceptionMapper.java

@@ -0,0 +1,34 @@
+package eu.knowledge.engine.rest.api;
+
+import eu.knowledge.engine.rest.model.ResponseMessage;
+import jakarta.validation.ConstraintViolation;
+import jakarta.validation.ConstraintViolationException;
+import jakarta.ws.rs.core.MediaType;
+import jakarta.ws.rs.core.Response;
+import jakarta.ws.rs.ext.ExceptionMapper;
+import jakarta.ws.rs.ext.Provider;
+
+/**
+ * Validation errors by default are returned as plain text and this class maps
+ * those errors to our {@link ResponseMessage}.
+ */
+@Provider
+public class ValidationExceptionMapper implements ExceptionMapper<ConstraintViolationException> {
+	@Override
+	public Response toResponse(ConstraintViolationException exception) {
+
+		var response = new ResponseMessage();
+		response.setMessageType("error");
+		response.setMessage(exception.getClass().getSimpleName() + ": " + prepareMessage(exception));
+
+		return Response.status(Response.Status.BAD_REQUEST).entity(response).type(MediaType.APPLICATION_JSON).build();
+	}
+
+	private String prepareMessage(ConstraintViolationException exception) {
+		StringBuilder message = new StringBuilder();
+		for (ConstraintViolation<?> cv : exception.getConstraintViolations()) {
+			message.append(cv.getPropertyPath() + " " + cv.getMessage() + "\n ");
+		}
+		return message.toString();
+	}
+}