code: public boolean isExistsIndex(String indexName) {
          IndicesExistsResponse response =
                  getClient().admin().indices().exists(
                          new IndicesExistsRequest().indices(new String[]{indexName})).actionGet();
          return response.isExists();
      }
docs: 判断指定的索引是否存在
  
  @param indexName 索引名
  @return true/false