Fix JSON parsing of empty OCS results (#112)

Changelog.md

@@ -3,6 +3,9 @@
 ## Version 14.1.6
 - Add optional `expireDate` parameter to `doShare` / `doShareAsync`, so an
   expiration date can be set when creating a share (issue #76)
+- Fix JSON parsing of empty OCS results: the API serializes empty collections
+  as `[]` instead of `{}`, which caused a `MismatchedInputException` when e.g.
+  listing users or groups on an empty result (issue #112)
 - Testing: integration tests can now auto-provision a throw-away Nextcloud
   server via Testcontainers when Docker is available, and are executed in CI
   (GitHub Actions)

src/main/java/org/aarboard/nextcloud/api/utils/JsonAnswerParser.java

@@ -19,6 +19,11 @@ public class JsonAnswerParser<A extends JsonAnswer> implements ConnectorCommon.R
     private JsonAnswerParser(Class<A> answerClass) {
         ObjectMapper objectMapper = new ObjectMapper();
         objectMapper.enable(DeserializationFeature.UNWRAP_ROOT_VALUE);
+        // The OCS API serializes empty PHP associative arrays as a JSON array
+        // "[]" instead of an object "{}". Treat such empty arrays as a null
+        // object so answers with no data (e.g. an empty user/group list)
+        // deserialize instead of throwing a MismatchedInputException.
+        objectMapper.enable(DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT);
 
         objectReader = objectMapper.readerFor(answerClass);
     }

src/test/java/org/aarboard/nextcloud/api/utils/JsonAnswerParserTest.java

@@ -0,0 +1,89 @@
+/*
+ * Copyright (C) 2026 a.schild
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.aarboard.nextcloud.api.utils;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+import java.io.StringReader;
+import java.util.List;
+
+import org.aarboard.nextcloud.api.provisioning.GroupListAnswer;
+import org.aarboard.nextcloud.api.provisioning.UserDetailsListAnswer;
+import org.aarboard.nextcloud.api.provisioning.UserListAnswer;
+import org.junit.Test;
+
+/**
+ * Pure parsing tests (no server required) for the OCS JSON quirk where empty
+ * collections are serialized as an empty array {@code []} instead of an object.
+ *
+ * @author a.schild
+ */
+public class JsonAnswerParserTest {
+
+    /**
+     * An empty group list comes back with {@code "data":[]} instead of an
+     * object; it must deserialize to an empty list rather than throwing.
+     */
+    @Test
+    public void testEmptyGroupListDataAsArray() {
+        String json = "{\"ocs\":{\"meta\":{\"status\":\"ok\",\"statuscode\":100},\"data\":[]}}";
+        GroupListAnswer answer = JsonAnswerParser.getInstance(GroupListAnswer.class)
+                .parseResponse(new StringReader(json));
+        assertTrue(answer.getAllGroups().isEmpty());
+    }
+
+    /**
+     * An empty user-details map comes back with {@code "users":[]}; it must
+     * deserialize to an empty list rather than throwing.
+     */
+    @Test
+    public void testEmptyUserDetailsUsersAsArray() {
+        String json = "{\"ocs\":{\"meta\":{\"status\":\"ok\",\"statuscode\":100},\"data\":{\"users\":[]}}}";
+        UserDetailsListAnswer answer = JsonAnswerParser.getInstance(UserDetailsListAnswer.class)
+                .parseResponse(new StringReader(json));
+        assertTrue(answer.getAllUserDetails().isEmpty());
+    }
+
+    /**
+     * An empty user list comes back with {@code "data":[]}; it must deserialize
+     * to an empty list rather than throwing.
+     */
+    @Test
+    public void testEmptyUserListDataAsArray() {
+        String json = "{\"ocs\":{\"meta\":{\"status\":\"ok\",\"statuscode\":100},\"data\":[]}}";
+        UserListAnswer answer = JsonAnswerParser.getInstance(UserListAnswer.class)
+                .parseResponse(new StringReader(json));
+        assertTrue(answer.getAllUsers().isEmpty());
+    }
+
+    /**
+     * A populated group list must still deserialize correctly (guards against
+     * the empty-array handling breaking the normal case).
+     */
+    @Test
+    public void testPopulatedGroupList() {
+        String json = "{\"ocs\":{\"meta\":{\"status\":\"ok\",\"statuscode\":100},"
+                + "\"data\":{\"groups\":[\"admin\",\"users\"]}}}";
+        GroupListAnswer answer = JsonAnswerParser.getInstance(GroupListAnswer.class)
+                .parseResponse(new StringReader(json));
+        List<String> groups = answer.getAllGroups();
+        assertEquals(2, groups.size());
+        assertEquals("admin", groups.get(0));
+        assertEquals("users", groups.get(1));
+    }
+}