content: /*
   * Copyright 1999-$toady.year Alibaba Group Holding Ltd.
   *
   * Licensed 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 com.alibaba.nacos.config.server.service.query;
  
  import com.alibaba.nacos.config.server.service.query.handler.ConfigQueryHandler;
  import com.alibaba.nacos.config.server.service.query.model.ConfigQueryChainRequest;
  import com.alibaba.nacos.config.server.service.query.model.ConfigQueryChainResponse;
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import java.io.IOException;
  import java.util.Objects;
  
  /**
   * ConfigQueryHandlerChain.
   * @author Nacos
   */
  public class ConfigQueryHandlerChain {
      
      private static final Logger LOGGER = LoggerFactory.getLogger(ConfigQueryHandlerChain.class);
      
      private ConfigQueryHandler head;
      
      private ConfigQueryHandler tail;
      
      public ConfigQueryHandlerChain() {
      }
      
      /**
       * Adds a new configuration query handler to the chain.
       *
       * @param handler the configuration query handler to be added
       * @return the current configuration query handler chain object, supporting method chaining
       */
      public ConfigQueryHandlerChain addHandler(ConfigQueryHandler handler) {
          if (Objects.isNull(handler)) {
              LOGGER.warn("Attempted to add a null config query handler");
              return this;
          }
          
          if (head == null) {
              head = handler;
              tail = handler;
          } else {
              tail.setNextHandler(handler);
              tail = handler;
          }
          
          return this;
      }
      
      public ConfigQueryChainResponse handle(ConfigQueryChainRequest request) throws IOException {
          return head.handle(request);
      }
      
  }