code: /*
   * 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.skywalking.oap.server.cluster.plugin.kubernetes;
  
  import io.fabric8.kubernetes.api.model.Pod;
  import io.fabric8.kubernetes.client.KubernetesClientBuilder;
  import io.fabric8.kubernetes.client.informers.ResourceEventHandler;
  import io.fabric8.kubernetes.client.informers.cache.Lister;
  import lombok.extern.slf4j.Slf4j;
  
  import java.util.List;
  import java.util.Optional;
  import java.util.stream.Collectors;
  
  import static java.util.Objects.isNull;
  
  @Slf4j
  public enum NamespacedPodListInformer {
  
      /**
       * contains remote collector instances
       */
      INFORMER;
  
      private Lister<Pod> podLister;
  
      public synchronized void init(ClusterModuleKubernetesConfig podConfig, ResourceEventHandler<Pod> eventHandler) {
          try {
              final var kubernetesClient = new KubernetesClientBuilder().build();
              final var informer = kubernetesClient
                  .pods()
                  .inNamespace(podConfig.getNamespace())
                  .withLabelSelector(podConfig.getLabelSelector())
                  .inform(eventHandler);
  
              podLister = new Lister<>(informer.getIndexer());
  
              Runtime.getRuntime().addShutdownHook(new Thread(() -> {
                  informer.close();
                  kubernetesClient.close();
              }));
          } catch (Exception e) {
              log.error("cannot connect with api server in kubernetes", e);
          }
      }
  
      public Optional<List<Pod>> listPods() {
          if (isNull(podLister)) {
              return Optional.empty();
          }
          return Optional.ofNullable(podLister.list().size() != 0
              ? podLister.list()
                         .stream()
                         .filter(pod -> "Running".equalsIgnoreCase(pod.getStatus().getPhase()))
                         .collect(Collectors.toList())
              : null);
      }
  }
  