BE: KC: Expose Kafka Connect validation errors in the UI

api/src/main/java/io/kafbat/ui/client/RetryingKafkaConnectClient.java

@@ -1,5 +1,6 @@
 package io.kafbat.ui.client;
 
+import com.fasterxml.jackson.annotation.JsonProperty;
 import io.kafbat.ui.config.ClustersProperties;
 import io.kafbat.ui.connect.ApiClient;
 import io.kafbat.ui.connect.api.KafkaConnectClientApi;
@@ -14,9 +15,11 @@
 import io.kafbat.ui.exception.KafkaConnectConflictReponseException;
 import io.kafbat.ui.exception.ValidationException;
 import io.kafbat.ui.util.WebClientConfigurator;
+import jakarta.validation.constraints.NotNull;
 import java.time.Duration;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import javax.annotation.Nullable;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.http.ResponseEntity;
@@ -58,10 +61,24 @@ private static <T> Flux<T> withRetryOnConflict(Flux<T> publisher) {
 
   private static <T> Mono<T> withBadRequestErrorHandling(Mono<T> publisher) {
     return publisher
-        .onErrorResume(WebClientResponseException.BadRequest.class, e ->
-            Mono.error(new ValidationException("Invalid configuration")))
-        .onErrorResume(WebClientResponseException.InternalServerError.class, e ->
-            Mono.error(new ValidationException("Invalid configuration")));
+        .onErrorResume(WebClientResponseException.BadRequest.class,
+            RetryingKafkaConnectClient::parseConnectErrorMessage)
+        .onErrorResume(WebClientResponseException.InternalServerError.class,
+            RetryingKafkaConnectClient::parseConnectErrorMessage);
+  }
+
+  // Adapted from https://github.com/apache/kafka/blob/a0a501952b6d61f6f273bdb8f842346b51e9dfce/connect/runtime/src/main/java/org/apache/kafka/connect/runtime/rest/entities/ErrorMessage.java
+  // Adding the connect runtime dependency for this single class seems excessive
+  private record ErrorMessage(@NotNull @JsonProperty("message") String message) {
+  }
+
+  private static <T> @NotNull Mono<T> parseConnectErrorMessage(WebClientResponseException parseException) {
+    final var errorMessage = parseException.getResponseBodyAs(ErrorMessage.class);
+    return Mono.error(new ValidationException(
+        Objects.requireNonNull(errorMessage,
+                // see https://github.com/apache/kafka/blob/a0a501952b6d61f6f273bdb8f842346b51e9dfce/connect/runtime/src/main/java/org/apache/kafka/connect/runtime/rest/errors/ConnectExceptionMapper.java
+                "This should not happen according to the ConnectExceptionMapper")
+            .message()));
   }
 
   @Override

api/src/test/java/io/kafbat/ui/KafkaConnectServiceTests.java

@@ -4,6 +4,7 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
+import io.kafbat.ui.api.model.ErrorResponse;
 import io.kafbat.ui.model.ConnectorDTO;
 import io.kafbat.ui.model.ConnectorPluginConfigDTO;
 import io.kafbat.ui.model.ConnectorPluginConfigValidationResponseDTO;
@@ -268,19 +269,28 @@ public void shouldReturn400WhenConnectReturns500ForInvalidConfigCreate() {
 
 
   @Test
+  @SuppressWarnings("checkstyle:LineLength")
   public void shouldReturn400WhenConnectReturns400ForInvalidConfigUpdate() {
     webTestClient.put()
         .uri("/api/clusters/{clusterName}/connects/{connectName}/connectors/{connectorName}/config",
             LOCAL, connectName, connectorName)
         .bodyValue(Map.of(
-            "connector.class", "org.apache.kafka.connect.file.FileStreamSinkConnector",
-            "tasks.max", "invalid number",
-            "topics", "another-topic",
-            "file", "/tmp/test"
+                "connector.class", "org.apache.kafka.connect.file.FileStreamSinkConnector",
+                "tasks.max", "invalid number",
+                "topics", "another-topic",
+                "file", "/tmp/test"
             )
         )
         .exchange()
-        .expectStatus().isBadRequest();
+        .expectStatus().isBadRequest()
+        .expectBody(ErrorResponse.class)
+        .value(response -> assertThat(response.getMessage()).isEqualTo(
+            """
+                Connector configuration is invalid and contains the following 2 error(s):
+                Invalid value invalid number for configuration tasks.max: Not a number of type INT
+                Invalid value null for configuration tasks.max: Value must be non-null
+                You can also find the above list of errors at the endpoint `/connector-plugins/{connectorType}/config/validate`"""
+        ));
 
     webTestClient.get()
         .uri("/api/clusters/{clusterName}/connects/{connectName}/connectors/{connectorName}/config",

etc/checkstyle/checkstyle.xml

@@ -46,7 +46,12 @@
         <property name="ignorePattern" value="^package.*|^import.*|a href|href|http://|https://|ftp://"/>
     </module>
 
+    <!-- Needed to use the SuppressWarningsHolder configured below. See https://checkstyle.sourceforge.io/filters/suppresswarningsfilter.html -->
+    <module name="SuppressWarningsFilter" />
+
     <module name="TreeWalker">
+        <!-- Make the @SuppressWarnings annotations available to Checkstyle -->
+        <module name="SuppressWarningsHolder"/>
         <module name="OuterTypeFilename"/>
         <module name="IllegalTokenText">
             <property name="tokens" value="STRING_LITERAL, CHAR_LITERAL"/>