MINOR: Disallow unused local variables (#18963)

Recently, we found a regression that could have been detected by static
analysis, since a local variable wasn't being passed to a method during
a refactoring, and was left unused. It was fixed in
[7a749b5](7a749b589f),
but almost slipped into 4.0. Unused variables are typically detected by
IDEs, but this is insufficient to prevent these kinds of bugs. This
change enables unused local variable detection in checkstyle for Kafka.

A few notes on the usage:
- There are two situations in which people actually want to have a local
variable but not use it. First, there are `for (Type ignored:
collection)` loops which have to loop `collection.length` number of
times, but that do not use `ignored` in the loop body. These are
typically still easier to read than a classical `for` loop. Second, some
IDEs detect it if a return value of a function such as `File.delete` is
not being used. In this case, people sometimes store the result in an
unused local variable to make ignoring the return value explicit and to
avoid the squiggly lines.
- In Java 22, unsued local variables can be omitted by using a single
underscore `_`. This is supported by checkstyle. In pre-22 versions,
IntelliJ allows such variables to be named `ignored` to suppress the
unused local variable warning. This pattern is often (but not
consistently) used in the Kafka codebase. This is, however, not
supported by checkstyle.

Since we cannot switch to Java 22, yet, and we want to use automated
detection using checkstyle, we have to resort to prefixing the unused
local variables with `@SuppressWarnings("UnusedLocalVariable")`. We have
to apply this in 11 cases across the Kafka codebase. While not being
pretty, I'd argue it's worth it to prevent bugs like the one fixed in
[7a749b5](7a749b589f).

Reviewers: Andrew Schofield <aschofield@confluent.io>, David Arthur
<mumrah@gmail.com>, Matthias J. Sax <matthias@confluent.io>, Bruno
Cadonna <cadonna@apache.org>, Kirk True <ktrue@confluent.io>

checkstyle/checkstyle.xml

@@ -43,6 +43,7 @@
       <property name="tokens" value="IDENT, NUM_DOUBLE, LAMBDA, TEXT_BLOCK_LITERAL_BEGIN, UNARY_MINUS, UNARY_PLUS, INC, DEC, POST_INC, POST_DEC" />
     </module>
     <module name="SimplifyBooleanReturn"/>
+    <module name="UnusedLocalVariable"/>
 
     <!-- style -->
     <module name="DefaultComesLast"/>

clients/src/main/java/org/apache/kafka/common/requests/DescribeUserScramCredentialsRequest.java

@@ -68,11 +68,12 @@ public class DescribeUserScramCredentialsRequest extends AbstractRequest {
                 .setThrottleTimeMs(throttleTimeMs)
                 .setErrorCode(apiError.error().code())
                 .setErrorMessage(apiError.message());
-        for (DescribeUserScramCredentialsRequestData.UserName user : data.users()) {
+
+        data.users().forEach(__ ->
             response.results().add(new DescribeUserScramCredentialsResponseData.DescribeUserScramCredentialsResult()
                 .setErrorCode(apiError.error().code())
-                    .setErrorMessage(apiError.message()));
+                .setErrorMessage(apiError.message()))
-        }
+        );
         return new DescribeUserScramCredentialsResponse(response);
     }
 }

clients/src/test/java/org/apache/kafka/clients/consumer/internals/ConsumerHeartbeatRequestManagerTest.java

@@ -444,7 +444,7 @@ public class ConsumerHeartbeatRequestManagerTest {
         time.sleep(DEFAULT_HEARTBEAT_INTERVAL_MS);
         NetworkClientDelegate.PollResult result = heartbeatRequestManager.poll(time.milliseconds());
         assertEquals(1, result.unsentRequests.size());
-        ClientResponse response = createHeartbeatResponse(result.unsentRequests.get(0), Errors.GROUP_AUTHORIZATION_FAILED);
+        createHeartbeatResponse(result.unsentRequests.get(0), Errors.GROUP_AUTHORIZATION_FAILED);
         result.unsentRequests.get(0).handler().onFailure(time.milliseconds(), new AuthenticationException("Fatal error in HB"));
 
         // The error should be propagated before notifying the group manager. This ensures that the app thread is aware

clients/src/test/java/org/apache/kafka/clients/consumer/internals/FetcherTest.java

@@ -2881,11 +2881,11 @@ public class FetcherTest {
                                 field.setAccessible(true);
                                 LinkedHashMap<?, ?> sessionPartitions =
                                         (LinkedHashMap<?, ?>) field.get(handler);
-                                for (Map.Entry<?, ?> entry : sessionPartitions.entrySet()) {
                                 // If `sessionPartitions` are modified on another thread, Thread.yield will increase the
                                 // possibility of ConcurrentModificationException if appropriate synchronization is not used.
-                                    Thread.yield();
+                                sessionPartitions.forEach(
-                                }
+                                    (key, value) -> Thread.yield()
+                                );
                             } catch (Exception e) {
                                 throw new RuntimeException(e);
                             }

clients/src/test/java/org/apache/kafka/clients/producer/internals/RecordAccumulatorTest.java

@@ -397,7 +397,7 @@ public class RecordAccumulatorTest {
             List<ProducerBatch> batches = accum.drain(metadataCache, nodes, 5 * 1024, 0).get(node1.id());
             if (batches != null) {
                 for (ProducerBatch batch : batches) {
-                    for (Record record : batch.records().records())
+                    for (@SuppressWarnings("UnusedLocalVariable") Record ignored : batch.records().records())
                         read++;
                     accum.deallocate(batch);
                 }

clients/src/test/java/org/apache/kafka/clients/producer/internals/SenderTest.java

@@ -1674,7 +1674,7 @@ public class SenderTest {
         Sender sender = new Sender(logContext, client, metadata, this.accumulator, true, MAX_REQUEST_SIZE, ACKS_ALL, 10,
             senderMetrics, time, REQUEST_TIMEOUT, RETRY_BACKOFF_MS, transactionManager, apiVersions);
 
-        Future<RecordMetadata> failedResponse = appendToAccumulator(tp0);
+        appendToAccumulator(tp0);
         Future<RecordMetadata> successfulResponse = appendToAccumulator(tp1);
         sender.runOnce();  // connect and send.
 

clients/src/test/java/org/apache/kafka/common/requests/RequestResponseTest.java

@@ -527,11 +527,6 @@ public class RequestResponseTest {
     public void fetchResponseVersionTest() {
         Uuid id = Uuid.randomUuid();
         MemoryRecords records = MemoryRecords.readableRecords(ByteBuffer.allocate(10));
-        FetchResponseData.PartitionData partitionData = new FetchResponseData.PartitionData()
-                .setPartitionIndex(0)
-                .setHighWatermark(1000000)
-                .setLogStartOffset(-1)
-                .setRecords(records);
         LinkedHashMap<TopicIdPartition, FetchResponseData.PartitionData> idResponseData = new LinkedHashMap<>();
         idResponseData.put(new TopicIdPartition(id, new TopicPartition("test", 0)),
                 new FetchResponseData.PartitionData()

connect/mirror/src/test/java/org/apache/kafka/connect/mirror/MirrorSourceTaskTest.java

@@ -20,7 +20,6 @@ import org.apache.kafka.clients.consumer.ConsumerRecord;
 import org.apache.kafka.clients.consumer.ConsumerRecords;
 import org.apache.kafka.clients.consumer.KafkaConsumer;
 import org.apache.kafka.clients.consumer.OffsetAndMetadata;
-import org.apache.kafka.clients.producer.KafkaProducer;
 import org.apache.kafka.clients.producer.RecordMetadata;
 import org.apache.kafka.common.TopicPartition;
 import org.apache.kafka.common.header.Header;
@@ -283,8 +282,6 @@ public class MirrorSourceTaskTest {
 
         @SuppressWarnings("unchecked")
         KafkaConsumer<byte[], byte[]> consumer = mock(KafkaConsumer.class);
-        @SuppressWarnings("unchecked")
-        KafkaProducer<byte[], byte[]> producer = mock(KafkaProducer.class);
         MirrorSourceMetrics metrics = mock(MirrorSourceMetrics.class);
 
         String sourceClusterName = "cluster1";

connect/runtime/src/test/java/org/apache/kafka/connect/integration/MonitorableSinkConnector.java

@@ -60,9 +60,7 @@ public class MonitorableSinkConnector extends TestableSinkConnector {
         @Override
         public void put(Collection<SinkRecord> records) {
             super.put(records);
-            for (SinkRecord ignore : records) {
+            count += records.size();
-                count++;
-            }
         }
 
     }

connect/runtime/src/test/java/org/apache/kafka/connect/runtime/AbstractWorkerSourceTaskTest.java

@@ -752,9 +752,6 @@ public class AbstractWorkerSourceTaskTest {
 
         expectConvertHeadersAndKeyValue(emptyHeaders(), TOPIC);
 
-        TopicPartitionInfo topicPartitionInfo = new TopicPartitionInfo(0, null, Collections.emptyList(), Collections.emptyList());
-        TopicDescription topicDesc = new TopicDescription(TOPIC, false, Collections.singletonList(topicPartitionInfo));
-
         workerTask.toSend = Arrays.asList(record1);
 
         // The transformation errored out so the error should be ignored & the record skipped with error tolerance all

connect/runtime/src/test/java/org/apache/kafka/connect/runtime/WorkerConnectorTest.java

@@ -621,6 +621,7 @@ public class WorkerConnectorTest {
         String type = metrics.currentMetricValueAsString(metricGroup, "connector-type");
         String clazz = metrics.currentMetricValueAsString(metricGroup, "connector-class");
         String version = metrics.currentMetricValueAsString(metricGroup, "connector-version");
+        assertEquals("unassigned", status);
         assertEquals(expectedType, type);
         assertNotNull(clazz);
         assertEquals(VERSION, version);

core/src/main/java/kafka/log/remote/RemoteLogManager.java

@@ -960,6 +960,7 @@ public class RemoteLogManager implements Closeable, AsyncOffsetReader {
                                     // back to the caller. It's important to note that the task being executed is already
                                     // cancelled before the executing thread is interrupted. The caller is responsible
                                     // for handling the exception gracefully by checking if the task is already cancelled.
+                                    @SuppressWarnings("UnusedLocalVariable")
                                     boolean ignored = copyQuotaManagerLockCondition.await(quotaTimeout().toMillis(), TimeUnit.MILLISECONDS);
                                     throttleTimeMs = rlmCopyQuotaManager.getThrottleTimeMs();
                                 }

generator/src/main/java/org/apache/kafka/message/checker/EvolutionVerifier.java

@@ -74,7 +74,7 @@ public class EvolutionVerifier {
             verifyVersionsMatchTopLevelMessage(what, topLevelMessage, field);
         }
         for (StructSpec struct : topLevelMessage.commonStructs()) {
-            for (FieldSpec field : topLevelMessage.fields()) {
+            for (FieldSpec field : struct.fields()) {
                 verifyVersionsMatchTopLevelMessage(what, topLevelMessage, field);
             }
         }

group-coordinator/src/test/java/org/apache/kafka/coordinator/group/OffsetMetadataManagerTest.java

@@ -2967,7 +2967,7 @@ public class OffsetMetadataManagerTest {
         assertEquals(1, group.generationId());
         group.transitionTo(ClassicGroupState.STABLE);
 
-        CoordinatorResult<OffsetCommitResponseData, CoordinatorRecord> result = context.commitOffset(
+        context.commitOffset(
             new OffsetCommitRequestData()
                 .setGroupId("foo")
                 .setMemberId("member")

group-coordinator/src/test/java/org/apache/kafka/coordinator/group/modern/TopicMetadataTest.java

@@ -21,10 +21,6 @@ import org.apache.kafka.coordinator.group.generated.ConsumerGroupPartitionMetada
 
 import org.junit.jupiter.api.Test;
 
-import java.util.Map;
-import java.util.Set;
-
-import static org.apache.kafka.coordinator.group.GroupCoordinatorRecordHelpersTest.mkMapOfPartitionRacks;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
@@ -49,7 +45,6 @@ public class TopicMetadataTest {
     @Test
     public void testEquals() {
         Uuid topicId = Uuid.randomUuid();
-        Map<Integer, Set<String>> partitionRacks = mkMapOfPartitionRacks(15);
         TopicMetadata topicMetadata = new TopicMetadata(topicId, "foo", 15);
 
         assertEquals(new TopicMetadata(topicId, "foo", 15), topicMetadata);

metadata/src/test/java/org/apache/kafka/controller/QuorumControllerMetricsIntegrationTest.java

@@ -198,7 +198,7 @@ public class QuorumControllerMetricsIntegrationTest {
                                                      build()
         ) {
             QuorumController active = controlEnv.activeController();
-            Map<Integer, Long> brokerEpochs = registerBrokersAndUnfence(active, 3);
+            registerBrokersAndUnfence(active, 3);
 
             // Test that a new operation increments operationsStarted. We retry this if needed
             // to handle the case where another operation is performed in between loading

metadata/src/test/java/org/apache/kafka/controller/ReplicationControlManagerTest.java

@@ -1535,7 +1535,6 @@ public class ReplicationControlManagerTest {
             anonymousContextFor(ApiKeys.CREATE_TOPICS);
         ControllerResult<CreateTopicsResponseData> createResult =
             replicationControl.createTopics(createTopicsRequestContext, request, Collections.singleton("foo"));
-        CreateTopicsResponseData expectedResponse = new CreateTopicsResponseData();
         CreatableTopicResult createdTopic = createResult.response().topics().find("foo");
         assertEquals(NONE.code(), createdTopic.errorCode());
         ctx.replay(createResult.records());

raft/src/test/java/org/apache/kafka/raft/KafkaRaftClientReconfigTest.java

@@ -189,24 +189,6 @@ public class KafkaRaftClientReconfigTest {
             .withUnknownLeader(0)
             .build();
 
-        List<List<ControlRecord>> expectedBootstrapRecords = Arrays.asList(
-            Arrays.asList(
-                new ControlRecord(
-                    ControlRecordType.SNAPSHOT_HEADER,
-                    new SnapshotHeaderRecord()
-                        .setVersion((short) 0)
-                        .setLastContainedLogTimestamp(0)
-                )
-            ),
-            Arrays.asList(
-                new ControlRecord(
-                    ControlRecordType.SNAPSHOT_FOOTER,
-                    new SnapshotFooterRecord()
-                        .setVersion((short) 0)
-                )
-            )
-        );
-
         // check leader does not write bootstrap records to log
         context.unattachedToLeader();
 

server-common/src/test/java/org/apache/kafka/server/share/persister/PersisterStateManagerTest.java

@@ -653,9 +653,8 @@ class PersisterStateManagerTest {
 
         CompletableFuture<ReadShareGroupStateResponse> resultFuture = handler.result();
 
-        ReadShareGroupStateResponse result = null;
         try {
-            result = resultFuture.get();
+            resultFuture.get();
         } catch (Exception e) {
             fail("Failed to get result from future", e);
         }

share-coordinator/src/test/java/org/apache/kafka/coordinator/share/ShareCoordinatorShardTest.java

@@ -1237,7 +1237,6 @@ class ShareCoordinatorShardTest {
         assertEquals(expectedRecords, result.records());
 
         // invalid state epoch
-        int stateEpoch = 1;
         partition = 0;
         shard.replay(0L, 0L, (short) 0, ShareCoordinatorRecordHelpers.newShareSnapshotRecord(
             GROUP_ID, TOPIC_ID, partition, new ShareGroupOffset.Builder()

storage/src/main/java/org/apache/kafka/storage/internals/log/LogLoader.java

@@ -169,6 +169,7 @@ public class LogLoader {
                     long offset = LogFileUtils.offsetFromFile(file);
                     if (offset >= minSwapFileOffset && offset < maxSwapFileOffset) {
                         logger.info("Deleting segment files {} that is compacted but has not been deleted yet.", file.getName());
+                        @SuppressWarnings("UnusedLocalVariable")
                         boolean ignore = file.delete();
                     }
                 }
@@ -186,6 +187,7 @@ public class LogLoader {
             }
             if (file.getName().endsWith(LogFileUtils.SWAP_FILE_SUFFIX)) {
                 logger.info("Recovering file {} by renaming from {} files.", file.getName(), LogFileUtils.SWAP_FILE_SUFFIX);
+                @SuppressWarnings("UnusedLocalVariable")
                 boolean ignore = file.renameTo(new File(Utils.replaceSuffix(file.getPath(), LogFileUtils.SWAP_FILE_SUFFIX, "")));
             }
         }

storage/src/test/java/org/apache/kafka/storage/internals/log/LogSegmentTest.java

@@ -144,7 +144,6 @@ public class LogSegmentTest {
     })
     public void testAppendForLogSegmentOffsetOverflowException(long baseOffset, long largestOffset) throws IOException {
         try (LogSegment seg = createSegment(baseOffset, 10, Time.SYSTEM)) {
-            long currentTime = Time.SYSTEM.milliseconds();
             MemoryRecords memoryRecords = v1Records(0, "hello");
             assertThrows(LogSegmentOffsetOverflowException.class, () -> seg.append(largestOffset, memoryRecords));
         }

streams/integration-tests/src/test/java/org/apache/kafka/streams/integration/KTableEfficientRangeQueryTest.java

@@ -26,7 +26,6 @@ import org.apache.kafka.streams.StreamsBuilder;
 import org.apache.kafka.streams.TestInputTopic;
 import org.apache.kafka.streams.Topology;
 import org.apache.kafka.streams.TopologyTestDriver;
-import org.apache.kafka.streams.kstream.KTable;
 import org.apache.kafka.streams.kstream.Materialized;
 import org.apache.kafka.streams.state.KeyValueBytesStoreSupplier;
 import org.apache.kafka.streams.state.KeyValueIterator;
@@ -117,7 +116,6 @@ public class KTableEfficientRangeQueryTest {
         final Materialized<String, String, KeyValueStore<Bytes, byte[]>> stateStoreConfig = getStoreConfig(storeType, TABLE_NAME, enableLogging, enableCaching);
         //Create topology: table from input topic
         final StreamsBuilder builder = new StreamsBuilder();
-        final KTable<String, String> table =
         builder.table("input", stateStoreConfig);
         final Topology topology = builder.build();
 

streams/integration-tests/src/test/java/org/apache/kafka/streams/integration/RegexSourceIntegrationTest.java

@@ -212,7 +212,7 @@ public class RegexSourceIntegrationTest {
 
             final StreamsBuilder builder = new StreamsBuilder();
             final KStream<String, String> pattern1Stream = builder.stream(Pattern.compile("TEST-TOPIC-\\d"));
-            final KStream<String, String> otherStream = builder.stream(Pattern.compile("not-a-match"));
+            builder.stream(Pattern.compile("not-a-match"));
 
             pattern1Stream
                 .selectKey((k, v) -> k)

streams/src/main/java/org/apache/kafka/streams/processor/internals/DefaultStateUpdater.java

@@ -885,7 +885,10 @@ public class DefaultStateUpdater implements StateUpdater {
                 restoredActiveTasksLock.lock();
                 try {
                     while (restoredActiveTasks.isEmpty() && now <= deadline) {
-                        final boolean elapsed = restoredActiveTasksCondition.await(deadline - now, TimeUnit.MILLISECONDS);
+                        // We can ignore whether the deadline expired during await, as the while loop condition will
+                        // check again for deadline expiration.
+                        @SuppressWarnings("UnusedLocalVariable")
+                        final boolean ignored = restoredActiveTasksCondition.await(deadline - now, TimeUnit.MILLISECONDS);
                         now = time.milliseconds();
                     }
                     result.addAll(restoredActiveTasks);

streams/src/test/java/org/apache/kafka/streams/kstream/internals/InternalStreamsBuilderTest.java

@@ -472,7 +472,6 @@ public class InternalStreamsBuilderTest {
         final GraphNode join = getNodeByType(builder.root, StreamStreamJoinNode.class, new HashSet<>());
         assertNotNull(join);
         assertTrue(((StreamStreamJoinNode) join).getSelfJoin());
-        final GraphNode parent = join.parentNodes().stream().findFirst().get();
         final AtomicInteger count = new AtomicInteger();
         countJoinWindowNodes(count, builder.root, new HashSet<>());
         assertEquals(count.get(), 1);

streams/src/test/java/org/apache/kafka/streams/kstream/internals/SuppressScenarioTest.java

@@ -34,7 +34,6 @@ import org.apache.kafka.streams.TopologyTestDriver;
 import org.apache.kafka.streams.kstream.Consumed;
 import org.apache.kafka.streams.kstream.Grouped;
 import org.apache.kafka.streams.kstream.KGroupedStream;
-import org.apache.kafka.streams.kstream.KStream;
 import org.apache.kafka.streams.kstream.KTable;
 import org.apache.kafka.streams.kstream.Materialized;
 import org.apache.kafka.streams.kstream.Named;
@@ -816,7 +815,7 @@ public class SuppressScenarioTest {
 
         final KGroupedStream<String, String> stream1 = builder.stream("one", Consumed.with(Serdes.String(), Serdes.String())).groupByKey(Grouped.with(Serdes.String(), Serdes.String()));
         final KGroupedStream<String, String> stream2 = builder.stream("two", Consumed.with(Serdes.String(), Serdes.String())).groupByKey(Grouped.with(Serdes.String(), Serdes.String()));
-        final KStream<Windowed<String>, Object> cogrouped = stream1.cogroup((key, value, aggregate) -> aggregate + value).cogroup(stream2, (key, value, aggregate) -> aggregate + value)
+        stream1.cogroup((key, value, aggregate) -> aggregate + value).cogroup(stream2, (key, value, aggregate) -> aggregate + value)
             .windowedBy(TimeWindows.ofSizeWithNoGrace(Duration.ofMinutes(15)))
             .aggregate(() -> "", Named.as("test"), Materialized.as("store"))
             .suppress(Suppressed.untilWindowCloses(unbounded()))
@@ -829,7 +828,7 @@ public class SuppressScenarioTest {
 
         final KGroupedStream<String, String> stream1 = builder.stream("one", Consumed.with(Serdes.String(), Serdes.String())).groupByKey(Grouped.with(Serdes.String(), Serdes.String()));
         final KGroupedStream<String, String> stream2 = builder.stream("two", Consumed.with(Serdes.String(), Serdes.String())).groupByKey(Grouped.with(Serdes.String(), Serdes.String()));
-        final KStream<Windowed<String>, Object> cogrouped = stream1.cogroup((key, value, aggregate) -> aggregate + value).cogroup(stream2, (key, value, aggregate) -> aggregate + value)
+        stream1.cogroup((key, value, aggregate) -> aggregate + value).cogroup(stream2, (key, value, aggregate) -> aggregate + value)
             .windowedBy(SlidingWindows.ofTimeDifferenceWithNoGrace(Duration.ofMinutes(15)))
             .aggregate(() -> "", Named.as("test"), Materialized.as("store"))
             .suppress(Suppressed.untilWindowCloses(unbounded()))
@@ -842,7 +841,7 @@ public class SuppressScenarioTest {
 
         final KGroupedStream<String, String> stream1 = builder.stream("one", Consumed.with(Serdes.String(), Serdes.String())).groupByKey(Grouped.with(Serdes.String(), Serdes.String()));
         final KGroupedStream<String, String> stream2 = builder.stream("two", Consumed.with(Serdes.String(), Serdes.String())).groupByKey(Grouped.with(Serdes.String(), Serdes.String()));
-        final KStream<Windowed<String>, Object> cogrouped = stream1.cogroup((key, value, aggregate) -> aggregate + value).cogroup(stream2, (key, value, aggregate) -> aggregate + value)
+        stream1.cogroup((key, value, aggregate) -> aggregate + value).cogroup(stream2, (key, value, aggregate) -> aggregate + value)
             .windowedBy(SessionWindows.ofInactivityGapAndGrace(Duration.ofMinutes(15), Duration.ofMinutes(5)))
             .aggregate(() -> "", (k, v1, v2) -> "", Named.as("test"), Materialized.as("store"))
             .suppress(Suppressed.untilWindowCloses(unbounded()))

streams/src/test/java/org/apache/kafka/streams/processor/internals/ProcessorRecordContextTest.java

@@ -44,7 +44,6 @@ public class ProcessorRecordContextTest {
 
     @Test
     public void shouldEstimateNullTopicAndEmptyHeadersAsZeroLength() {
-        final Headers headers = new RecordHeaders();
         final ProcessorRecordContext context = new ProcessorRecordContext(
             42L,
             73L,

streams/src/test/java/org/apache/kafka/streams/processor/internals/StreamThreadTest.java

@@ -1370,8 +1370,6 @@ public class StreamThreadTest {
         final TaskManager taskManager = mock(TaskManager.class);
 
         final StreamsConfig config = new StreamsConfig(configProps(false, stateUpdaterEnabled, processingThreadsEnabled));
-        final StreamsMetricsImpl streamsMetrics =
-            new StreamsMetricsImpl(metrics, CLIENT_ID, PROCESS_ID.toString(), mockTime);
         final TopologyMetadata topologyMetadata = new TopologyMetadata(internalTopologyBuilder, config);
         topologyMetadata.buildAndRewriteTopology();
         thread = buildStreamThread(consumer, taskManager, config, topologyMetadata)
@@ -1471,8 +1469,6 @@ public class StreamThreadTest {
         final TaskManager taskManager = mock(TaskManager.class);
 
         final StreamsConfig config = new StreamsConfig(configProps(false, stateUpdaterEnabled, processingThreadsEnabled));
-        final StreamsMetricsImpl streamsMetrics =
-            new StreamsMetricsImpl(metrics, CLIENT_ID, PROCESS_ID.toString(), mockTime);
         final TopologyMetadata topologyMetadata = new TopologyMetadata(internalTopologyBuilder, config);
         topologyMetadata.buildAndRewriteTopology();
         thread = buildStreamThread(consumer, taskManager, config, topologyMetadata)
@@ -1491,8 +1487,6 @@ public class StreamThreadTest {
         final TaskManager taskManager = mock(TaskManager.class);
 
         final StreamsConfig config = new StreamsConfig(configProps(false, stateUpdaterEnabled, processingThreadsEnabled));
-        final StreamsMetricsImpl streamsMetrics =
-            new StreamsMetricsImpl(metrics, CLIENT_ID, PROCESS_ID.toString(), mockTime);
         final TopologyMetadata topologyMetadata = new TopologyMetadata(internalTopologyBuilder, config);
         topologyMetadata.buildAndRewriteTopology();
         thread = buildStreamThread(consumer, taskManager, config, topologyMetadata)
@@ -2590,8 +2584,6 @@ public class StreamThreadTest {
 
         doThrow(new TaskMigratedException("Task lost exception", new RuntimeException())).when(taskManager).handleLostAll();
 
-        final StreamsMetricsImpl streamsMetrics =
-            new StreamsMetricsImpl(metrics, CLIENT_ID, PROCESS_ID.toString(), mockTime);
         final TopologyMetadata topologyMetadata = new TopologyMetadata(internalTopologyBuilder, config);
         topologyMetadata.buildAndRewriteTopology();
         thread = buildStreamThread(consumer, taskManager, config, topologyMetadata)
@@ -2620,8 +2612,6 @@ public class StreamThreadTest {
 
         doThrow(new TaskMigratedException("Revocation non fatal exception", new RuntimeException())).when(taskManager).handleRevocation(assignedPartitions);
 
-        final StreamsMetricsImpl streamsMetrics =
-            new StreamsMetricsImpl(metrics, CLIENT_ID, PROCESS_ID.toString(), mockTime);
         final TopologyMetadata topologyMetadata = new TopologyMetadata(internalTopologyBuilder, config);
         topologyMetadata.buildAndRewriteTopology();
         thread = buildStreamThread(consumer, taskManager, config, topologyMetadata)
@@ -3113,8 +3103,6 @@ public class StreamThreadTest {
 
         when(taskManager.producerMetrics()).thenReturn(dummyProducerMetrics);
 
-        final StreamsMetricsImpl streamsMetrics =
-            new StreamsMetricsImpl(metrics, CLIENT_ID, PROCESS_ID.toString(), mockTime);
         final TopologyMetadata topologyMetadata = new TopologyMetadata(internalTopologyBuilder, config);
         topologyMetadata.buildAndRewriteTopology();
         thread = buildStreamThread(consumer, taskManager, config, topologyMetadata);

streams/src/test/java/org/apache/kafka/streams/processor/internals/TaskManagerTest.java

@@ -1825,9 +1825,6 @@ public class TaskManagerTest {
         final StandbyTask standbyTask = standbyTask(taskId02, taskId02ChangelogPartitions)
             .inState(State.RUNNING)
             .withInputPartitions(taskId02Partitions).build();
-        final StandbyTask unassignedStandbyTask = standbyTask(taskId03, taskId03ChangelogPartitions)
-            .inState(State.CREATED)
-            .withInputPartitions(taskId03Partitions).build();
         final TasksRegistry tasks = mock(TasksRegistry.class);
         final TaskManager taskManager = setUpTaskManager(ProcessingMode.AT_LEAST_ONCE, tasks, true);
         when(tasks.allTasksPerId()).thenReturn(mkMap(mkEntry(taskId00, runningStatefulTask)));

streams/src/test/java/org/apache/kafka/streams/processor/internals/assignment/LegacyStickyTaskAssignorTest.java

@@ -25,7 +25,6 @@ import org.apache.kafka.streams.processor.TaskId;
 import org.apache.kafka.streams.processor.assignment.AssignmentConfigs;
 import org.apache.kafka.streams.processor.assignment.ProcessId;
 import org.apache.kafka.streams.processor.internals.InternalTopicManager;
-import org.apache.kafka.streams.processor.internals.TopologyMetadata.Subtopology;
 
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.ValueSource;
@@ -968,7 +967,6 @@ public class LegacyStickyTaskAssignorTest {
         final Cluster cluster = getRandomCluster(nodeSize, topicSize, partitionSize);
         final Map<TaskId, Set<TopicPartition>> partitionsForTask = getTaskTopicPartitionMap(topicSize, partitionSize, false);
         final Map<TaskId, Set<TopicPartition>> changelogPartitionsForTask = getTaskTopicPartitionMap(topicSize, partitionSize, true);
-        final Map<Subtopology, Set<TaskId>> tasksForTopicGroup = new HashMap<>();
         final Map<ProcessId, Map<String, Optional<String>>> racksForProcessConsumer = getRandomProcessRacks(clientSize, nodeSize);
         final InternalTopicManager internalTopicManager = mockInternalTopicManagerForRandomChangelog(nodeSize, topicSize, partitionSize);
 

streams/src/test/java/org/apache/kafka/streams/state/internals/LogicalKeyValueSegmentsTest.java

@@ -135,8 +135,8 @@ public class LogicalKeyValueSegmentsTest {
 
     @Test
     public void shouldCleanupSegmentsThatHaveExpired() {
-        final LogicalKeyValueSegment segment1 = segments.getOrCreateSegmentIfLive(0, context, 0);
+        segments.getOrCreateSegmentIfLive(0, context, 0);
-        final LogicalKeyValueSegment segment2 = segments.getOrCreateSegmentIfLive(2, context, SEGMENT_INTERVAL * 2L);
+        segments.getOrCreateSegmentIfLive(2, context, SEGMENT_INTERVAL * 2L);
         final LogicalKeyValueSegment segment3 = segments.getOrCreateSegmentIfLive(3, context, SEGMENT_INTERVAL * 3L);
         final LogicalKeyValueSegment segment4 = segments.getOrCreateSegmentIfLive(7, context, SEGMENT_INTERVAL * 7L);
 
@@ -151,7 +151,7 @@ public class LogicalKeyValueSegmentsTest {
     @Test
     public void shouldNotCleanUpReservedSegments() {
         final LogicalKeyValueSegment reservedSegment = segments.createReservedSegment(-1, "reserved");
-        final LogicalKeyValueSegment segment1 = segments.getOrCreateSegmentIfLive(1, context, SEGMENT_INTERVAL);
+        segments.getOrCreateSegmentIfLive(1, context, SEGMENT_INTERVAL);
         final LogicalKeyValueSegment segment2 = segments.getOrCreateSegmentIfLive(2, context, SEGMENT_INTERVAL * 2L);
 
         segments.cleanupExpiredSegments(SEGMENT_INTERVAL * 6L);

streams/src/test/java/org/apache/kafka/streams/tests/RelationalSmokeTest.java

@@ -672,8 +672,6 @@ public class RelationalSmokeTest extends SmokeTestUtil {
         }
 
         public static boolean verifySync(final String broker, final Instant deadline) throws InterruptedException {
-            final Deserializer<Integer> keyDeserializer = intSerde.deserializer();
-
             final Deserializer<Article> articleDeserializer = new Article.ArticleDeserializer();
 
             final Deserializer<AugmentedArticle> augmentedArticleDeserializer =

streams/src/test/java/org/apache/kafka/test/MockApiFixedKeyProcessor.java

@@ -77,7 +77,6 @@ public class MockApiFixedKeyProcessor<KIn, VIn, VOut> implements FixedKeyProcess
     public void process(final FixedKeyRecord<KIn, VIn> record) {
         final KIn key = record.key();
         final VIn value = record.value();
-        final KeyValueTimestamp<KIn, VIn> keyValueTimestamp = new KeyValueTimestamp<>(key, value, record.timestamp());
 
         if (value != null) {
             lastValueAndTimestampPerKey.put(key, ValueAndTimestamp.make(value, record.timestamp()));

streams/src/test/java/org/apache/kafka/test/MockApiProcessor.java

@@ -77,7 +77,6 @@ public class MockApiProcessor<KIn, VIn, KOut, VOut> implements Processor<KIn, VI
     public void process(final Record<KIn, VIn> record) {
         final KIn key = record.key();
         final VIn value = record.value();
-        final KeyValueTimestamp<KIn, VIn> keyValueTimestamp = new KeyValueTimestamp<>(key, value, record.timestamp());
 
         if (value != null) {
             lastValueAndTimestampPerKey.put(key, ValueAndTimestamp.make(value, record.timestamp()));

tools/src/main/java/org/apache/kafka/tools/ShareConsumerPerformance.java

@@ -79,7 +79,8 @@ public class ShareConsumerPerformance {
                 shareConsumers.forEach(shareConsumer -> shareConsumersMetrics.add(shareConsumer.metrics()));
             }
             shareConsumers.forEach(shareConsumer -> {
-                Map<TopicIdPartition, Optional<KafkaException>> val = shareConsumer.commitSync();
+                @SuppressWarnings("UnusedLocalVariable")
+                Map<TopicIdPartition, Optional<KafkaException>> ignored = shareConsumer.commitSync();
                 shareConsumer.close(Duration.ofMillis(500));
             });
 

trogdor/src/main/java/org/apache/kafka/trogdor/workload/ConfigurableProducerWorker.java

@@ -182,10 +182,8 @@ public class ConfigurableProducerWorker implements TaskWorker {
             long startTimeMs = Time.SYSTEM.milliseconds();
             try {
                 try {
-                    long sentMessages = 0;
                     while (true) {
                         sendMessage();
-                        sentMessages++;
                     }
                 } catch (Exception e) {
                     throw e;