content: /*
   * 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.calcite.rel.metadata;
  
  import org.apache.calcite.rel.RelNode;
  import org.apache.calcite.util.ControlFlowException;
  
  /**
   * Provides {@link MetadataHandler} call sites for
   * {@link RelMetadataQuery}. The handlers provided are responsible for
   * updating the cache stored in {@link RelMetadataQuery}.
   */
  public interface MetadataHandlerProvider {
  
    /**
     * Provide a handler for the requested metadata class.
     *
     * @param handlerClass The handler interface expected
     * @param <MH> The metadata type the handler relates to.
     * @return The handler implementation.
     */
    <MH extends MetadataHandler<?>> MH handler(Class<MH> handlerClass);
  
    /** Re-generates the handler for a given kind of metadata.  */
    /**
     * Revise the handler for a given kind of metadata.
     *
     * <p>Should be invoked if the existing handler throws a {@link NoHandler} exception.
     *
     * @param handlerClass The type of class to revise.
     * @param <MH> The type metadata the handler provides.
     * @return A new handler that should be used instead of any previous handler provided.
     */
    default <MH extends MetadataHandler<?>> MH revise(Class<MH> handlerClass) {
      throw new UnsupportedOperationException("This provider doesn't support handler revision.");
    }
  
    /** Exception that indicates there there should be a handler for
     * this class but there is not. The action is probably to
     * re-generate the handler class. */
    class NoHandler extends ControlFlowException {
      public final Class<? extends RelNode> relClass;
  
      public NoHandler(Class<? extends RelNode> relClass) {
        this.relClass = relClass;
      }
    }
  
  }
  