Add validate action for SR maven plugin (confluentinc#1474)

Author: rayokota

maven-plugin/src/main/java/io/confluent/kafka/schemaregistry/maven/UploadSchemaRegistryMojo.java

@@ -129,8 +129,10 @@ private List<SchemaReference> getReferences(String subject, Map<String, Integer>
 
       Integer version = ref.version != null ? ref.version : schemaVersions.get(ref.subject);
       if (version == null) {
-        throw new IllegalArgumentException(
-            "Could not retrieve version for subject " + ref.subject);
+        getLog().warn(
+            String.format("Version not specified for ref with name '%s' and subject '%s', "
+                + "using latest version", ref.name, ref.subject));
+        version = -1;
       }
       result.add(new SchemaReference(ref.name, ref.subject, version));
     }

maven-plugin/src/main/java/io/confluent/kafka/schemaregistry/maven/ValidateSchemaRegistryMojo.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2020 Confluent Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.confluent.kafka.schemaregistry.maven;
+
+import io.confluent.kafka.schemaregistry.ParsedSchema;
+import io.confluent.kafka.schemaregistry.client.rest.exceptions.RestClientException;
+import java.io.IOException;
+import java.util.Map;
+import org.apache.maven.plugins.annotations.Mojo;
+
+@Mojo(name = "validate", configurator = "custom-basic")
+public class ValidateSchemaRegistryMojo extends UploadSchemaRegistryMojo {
+
+  @Override
+  protected boolean processSchema(String subject,
+                                  ParsedSchema schema,
+                                  Map<String, Integer> schemaVersions)
+      throws IOException, RestClientException {
+
+    if (getLog().isDebugEnabled()) {
+      getLog().debug(
+          String.format("Calling validate('%s', '%s')", subject, schema)
+      );
+    }
+
+    schema.validate();
+    return true;
+  }
+}