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   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF licenses this file
   * to you 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 org.apache.bifromq.basekv.localengine.metrics;
  
  import static org.awaitility.Awaitility.await;
  import static org.testng.Assert.assertFalse;
  
  import org.apache.bifromq.basekv.localengine.MockableTest;
  import io.micrometer.core.instrument.DistributionSummary;
  import io.micrometer.core.instrument.Gauge;
  import io.micrometer.core.instrument.Metrics;
  import io.micrometer.core.instrument.Tags;
  import java.lang.ref.WeakReference;
  import java.util.concurrent.atomic.AtomicInteger;
  import lombok.extern.slf4j.Slf4j;
  import org.testng.annotations.Test;
  
  @Slf4j
  public class KVSpaceMetersTest extends MockableTest {
      @Test
      public void removeGaugeWhenNoRef() {
          AtomicInteger gaugeCounter = new AtomicInteger();
          Gauge gauge = KVSpaceMeters.getGauge("testSpace", GeneralKVSpaceMetric.CheckpointNumGauge, gaugeCounter::get,
              Tags.of(new String[0]));
          String gaugeName = gauge.getId().getName();
          WeakReference<Gauge> weakRef = new WeakReference<>(gauge);
          assertFalse(Metrics.globalRegistry.find(gaugeName).gauges().isEmpty());
          gauge = null;
          await().forever().until(() -> {
              System.gc();
              return weakRef.get() == null && Metrics.globalRegistry.find(gaugeName).gauges().stream()
                  .noneMatch(g -> g.getId().getName().equals("testSpace"));
          });
      }
  
  
      @Test
      public void removeSummaryWhenNoRef() {
          DistributionSummary summary = KVSpaceMeters.getSummary("testSpace", GeneralKVSpaceMetric.ReadBytesDistribution,
              Tags.of(new String[0]));
          String timerName = summary.getId().getName();
          WeakReference<DistributionSummary> weakRef = new WeakReference<>(summary);
          assertFalse(Metrics.globalRegistry.find(timerName).summaries().isEmpty());
          summary = null;
          await().forever().until(() -> {
              System.gc();
              return weakRef.get() == null && Metrics.globalRegistry.find(timerName).counters().isEmpty();
          });
  
      }
  }
  