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  package org.apache.cayenne.query;
  
  import org.apache.cayenne.access.QueryEngine;
  import org.apache.cayenne.map.DataMap;
  
  /**
   * An interface used by Queries to route themselves to an appropriate QueryEngine. As of
   * 1.2 QueryRouter only supports routing by DataMap.
   * 
   * @since 1.2
   */
  public interface QueryRouter {
  
      /**
       * A callback method that allows a query to set its preferred engine during the
       * routing phase. It allows query to further customize its routing, e.g. it is
       * possible to implement query chains that pass multiple queries for execution.
       * 
       * @param engine engine to use for query execution
       * @param query A query to execute.
       * @param substitutedQuery a query that was substituted for "query". Results must be
       *            mapped back to substituted query.
       */
      void route(QueryEngine engine, Query query, Query substitutedQuery);
      
      /**
       * Returns a QueryEngine for a given name. If the name is null, a default
       * QueryEngine is returned. If there's no default engine, an exception is
       * thrown.
       * 
       * @since 4.0
       */
      QueryEngine engineForName(String name);
  
      /**
       * Returns a QueryEngine that is configured to handle a given DataMap.
       * 
       * @throws org.apache.cayenne.CayenneRuntimeException if an engine can't be found.
       * @throws NullPointerException if a map parameter is null.
       */
      QueryEngine engineForDataMap(DataMap map);
  }
  