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.library.module;
  
  import java.util.ArrayList;
  import java.util.List;
  import java.util.Map;
  import lombok.extern.slf4j.Slf4j;
  import org.apache.skywalking.oap.server.library.util.CollectionUtils;
  
  @Slf4j
  class BootstrapFlow {
      private Map<String, ModuleDefine> loadedModules;
      private List<ModuleProvider> startupSequence;
  
      BootstrapFlow(Map<String, ModuleDefine> loadedModules) throws CycleDependencyException, ModuleNotFoundException {
          this.loadedModules = loadedModules;
          startupSequence = new ArrayList<>();
  
          makeSequence();
      }
  
      @SuppressWarnings("unchecked")
      void start(
          ModuleManager moduleManager) throws ModuleNotFoundException, ServiceNotProvidedException, ModuleStartException {
          for (ModuleProvider provider : startupSequence) {
              log.info("start the provider {} in {} module.", provider.name(), provider.getModuleName());
              provider.requiredCheck(provider.getModule().services());
  
              provider.start();
          }
      }
  
      void notifyAfterCompleted() throws ServiceNotProvidedException, ModuleStartException {
          for (ModuleProvider provider : startupSequence) {
              provider.notifyAfterCompleted();
          }
      }
  
      private void makeSequence() throws CycleDependencyException, ModuleNotFoundException {
          List<ModuleProvider> allProviders = new ArrayList<>();
          for (final ModuleDefine module : loadedModules.values()) {
              String[] requiredModules = module.provider().requiredModules();
              if (requiredModules != null) {
                  for (String requiredModule : requiredModules) {
                      if (!loadedModules.containsKey(requiredModule)) {
                          throw new ModuleNotFoundException(
                              requiredModule + " module is required by "
                                  + module.provider().getModuleName() + "."
                                  + module.provider().name() + ", but not found.");
                      }
                  }
              }
  
              allProviders.add(module.provider());
          }
  
          do {
              int numOfToBeSequenced = allProviders.size();
              for (int i = 0; i < allProviders.size(); i++) {
                  ModuleProvider provider = allProviders.get(i);
                  String[] requiredModules = provider.requiredModules();
                  if (CollectionUtils.isNotEmpty(requiredModules)) {
                      boolean isAllRequiredModuleStarted = true;
                      for (String module : requiredModules) {
                          // find module in all ready existed startupSequence
                          boolean exist = false;
                          for (ModuleProvider moduleProvider : startupSequence) {
                              if (moduleProvider.getModuleName().equals(module)) {
                                  exist = true;
                                  break;
                              }
                          }
                          if (!exist) {
                              isAllRequiredModuleStarted = false;
                              break;
                          }
                      }
  
                      if (isAllRequiredModuleStarted) {
                          startupSequence.add(provider);
                          allProviders.remove(i);
                          i--;
                      }
                  } else {
                      startupSequence.add(provider);
                      allProviders.remove(i);
                      i--;
                  }
              }
  
              if (numOfToBeSequenced == allProviders.size()) {
                  StringBuilder unSequencedProviders = new StringBuilder();
                  allProviders.forEach(provider -> unSequencedProviders.append(provider.getModuleName())
                                                                       .append("[provider=")
                                                                       .append(provider.getClass().getName())
                                                                       .append("]\n"));
                  throw new CycleDependencyException(
                      "Exist cycle module dependencies in \n" + unSequencedProviders.substring(0, unSequencedProviders
                          .length() - 1));
              }
          }
          while (allProviders.size() != 0);
      }
  }
  