How to Build a Search Service in Java

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http://service_host:8080/search?q=purple%20rain&limit=5&skip=0&project=title,genres,plot,year&search=title,plot&filter=genres:Music

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{
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 "request": {
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   "q": "purple rain",
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   "skip": 0,
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   "limit": 5,
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   "search": "title,plot",
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   "project": "title,genres,plot,year",
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   "filter": [
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     "genres:Music"
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   ]
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 },
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 "docs": [
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   {
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     "plot": "A young musician, tormented by an abusive situation at home, must contend with a rival singer, a burgeoning romance and his own dissatisfied band as his star begins to rise.",
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     "genres": [
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       "Drama",
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       "Music",
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       "Musical"
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     ],
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     "title": "Purple Rain",
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     "year": 1984
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   },
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   {
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     "plot": "Graffiti Bridge is the unofficial sequel to Purple Rain. In this movie, The Kid and Morris Day are still competitors and each runs a club of his own. They make a bet about who writes the ...",
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     "genres": [
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       "Drama",
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       "Music",
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       "Musical"
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     ],
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     "title": "Graffiti Bridge",
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     "year": 1990
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   }
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 ],
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 "meta": [
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   {
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     "count": {
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       "total": 2
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     }
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   }
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 ]
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}

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{
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 "analyzer": "lucene.english",
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 "searchAnalyzer": "lucene.english",
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 "mappings": {
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   "dynamic": true,
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   "fields": {
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     "cast": [
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       {
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         "type": "token"
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       },
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       {
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         "type": "string"
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       }
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     ],
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     "genres": [
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       {
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         "type": "token"
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       },
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       {
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         "type": "string"
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       }
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     ]
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   }
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 }
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}

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public class SearchServlet extends HttpServlet {
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  private MongoCollection<Document> collection;
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  private String indexName;
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  private Logger logger;
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  // ...
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  @Override
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  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
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    String q = request.getParameter("q");
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    String searchFieldsValue = request.getParameter("search");
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    String limitValue = request.getParameter("limit");
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    String skipValue = request.getParameter("skip");
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    String projectFieldsValue = request.getParameter("project");
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    String debugValue = request.getParameter("debug");
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    String[] filters = request.getParameterMap().get("filter");
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    // ...
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  }
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}

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  @Override
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  public void init(ServletConfig config) throws ServletException {
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    super.init(config);
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    logger = Logger.getLogger(config.getServletName());
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    String uri = System.getenv("ATLAS_URI");
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    if (uri == null) {
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      throw new ServletException("ATLAS_URI must be specified");
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    }
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    String databaseName = config.getInitParameter("database");
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    String collectionName = config.getInitParameter("collection");
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    indexName = config.getInitParameter("index");
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    MongoClient mongo_client = MongoClients.create(uri);
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    MongoDatabase database = mongo_client.getDatabase(databaseName);
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    collection = database.getCollection(collectionName);
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    logger.info("Servlet " + config.getServletName() + " initialized: " + databaseName + " / " + collectionName + " / " + indexName);
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  }

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<web-app>
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   <servlet>
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       <servlet-name>SearchServlet</servlet-name>
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       <servlet-class>com.mongodb.atlas.SearchServlet</servlet-class>
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       <load-on-startup>1</load-on-startup>
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       <!--
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         The connection string must be defined in the
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         `ATLAS_URI` environment variable
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       -->
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       <init-param>
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           <param-name>database</param-name>
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           <param-value>sample_mflix</param-value>
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       </init-param>
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       <init-param>
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           <param-name>collection</param-name>
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           <param-value>movies</param-value>
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       </init-param>
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       <init-param>
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           <param-name>index</param-name>
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           <param-value>movies_index</param-value>
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       </init-param>
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   </servlet>
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   <servlet-mapping>
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       <servlet-name>SearchServlet</servlet-name>
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       <url-pattern>/search</url-pattern>
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   </servlet-mapping>
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</web-app>

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[
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 {
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   "$search": {
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     "compound": {
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       "must": [
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         {
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           "text": {
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             "query": "purple rain",
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             "path": [
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               "title",
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               "plot"
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             ]
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           }
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         }
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       ],
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       "filter": [
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         {
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           "equals": {
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             "path": "genres",
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             "value": "Music"
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           }
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         }
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       ]
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     },
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     "index": "movies_index",
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     "count": {
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       "type": "total"
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     }
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   }
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 },
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 {
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   "$facet": {
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     "docs": [
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       {
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         "$skip": 0
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       },
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       {
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         "$limit": 5
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       },
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       {
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         "$project": {
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           "title": 1,
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           "genres": 1,
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           "plot": 1,
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           "year": 1,
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           "_id": 0,
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         }
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       }
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     ],
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     "meta": [
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       {
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         "$replaceWith": "$$SEARCH_META"
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       },
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       {
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         "$limit": 1
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       }
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     ]
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   }
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 }
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]

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    // $search
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    List<SearchPath> searchPath = new ArrayList<>();
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    for (String search_field : searchFields) {
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      searchPath.add(SearchPath.fieldPath(search_field));
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    }
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    CompoundSearchOperator operator = SearchOperator.compound()
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        .must(List.of(SearchOperator.text(searchPath, List.of(q))));
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    if (filterOperators.size() > 0)
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      operator = operator.filter(filterOperators);
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    Bson searchStage = Aggregates.search(
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        operator,
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        SearchOptions.searchOptions()
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            .option("scoreDetails", debug)
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            .index(indexName)
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            .count(SearchCount.total())
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    );

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    List<String> projectFields = new ArrayList<>();
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    if (projectFieldsValue != null) {
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      projectFields.addAll(List.of(projectFieldsValue.split(",")));
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    }
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    boolean include_id = false;
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    if (projectFields.contains("_id")) {
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      include_id = true;
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      projectFields.remove("_id");
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    }
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    boolean includeScore = false;
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    if (projectFields.contains("_score")) {
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      includeScore = true;
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      projectFields.remove("_score");
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    }
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    // ...
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    // $project
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    List<Bson> projections = new ArrayList<>();
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    if (projectFieldsValue != null) {
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      // Don't add _id inclusion or exclusion if no `project` parameter specified
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      projections.add(Projections.include(projectFields));
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      if (include_id) {
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        projections.add(Projections.include("_id"));
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      } else {
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        projections.add(Projections.excludeId());
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      }
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    }
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    if (debug) {
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      projections.add(Projections.meta("_scoreDetails", "searchScoreDetails"));
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    }
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    if (includeScore) {
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      projections.add(Projections.metaSearchScore("_score"));
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    }

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    AggregateIterable<Document> aggregationResults = collection.aggregate(List.of(
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        searchStage,
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        facetStage
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    ));
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    Document responseDoc = new Document();
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    responseDoc.put("request", new Document()
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        .append("q", q)
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        .append("skip", skip)
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        .append("limit", limit)
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        .append("search", searchFieldsValue)
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        .append("project", projectFieldsValue)
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        .append("filter", filters==null ? Collections.EMPTY_LIST : List.of(filters)));
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    if (debug) {
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      responseDoc.put("debug", aggregationResults.explain().toBsonDocument());
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    }
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    // When using $facet stage, only one "document" is returned,
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    // containing the keys specified above: "docs" and "meta"
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    Document results = aggregationResults.first();
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    if (results != null) {
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      for (String s : results.keySet()) {
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        responseDoc.put(s,results.get(s));
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      }
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    }
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    response.setContentType("text/json");
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    PrintWriter writer = response.getWriter();
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    writer.println(responseDoc.toJson());
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    writer.close();
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    logger.info(request.getServletPath() + "?" + request.getQueryString());

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./gradlew buildProduct