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.beam.sdk.transforms;
  
  import java.util.List;
  import java.util.Optional;
  import org.apache.beam.sdk.expansion.ExternalTransformRegistrar;
  import org.apache.beam.sdk.options.PipelineOptions;
  import org.apache.beam.sdk.values.PInput;
  import org.apache.beam.sdk.values.POutput;
  
  /**
   * An interface for building a transform from an externally provided configuration.
   *
   * <p>Classes which implement this interface will be instantiated externally and require a zero-args
   * constructor. The {@code buildExternal} method will be called with the configuration object as a
   * parameter.
   *
   * <p>This builder needs to be registered alongside with a URN through {@link
   * ExternalTransformRegistrar}. Note that the configuration requires setters for all configuration
   * parameters, e.g. if there is a parameter "start", there should be a corresponding setter
   * "setStart".
   *
   * @param <ConfigT> A configuration object which will be populated with the external configuration.
   * @param <InputT> The input type of the externally configured PTransform.
   * @param <OutputT> The output type of the externally configured PTransform.
   */
  public interface ExternalTransformBuilder<ConfigT, InputT extends PInput, OutputT extends POutput> {
  
    /** Builds the transform after it has been configured. */
    PTransform<InputT, OutputT> buildExternal(ConfigT configuration);
  
    /**
     * List the dependencies needed for this transform. Jars from classpath are used by default when
     * Optional.empty() is returned.
     */
    default Optional<List<String>> getDependencies(ConfigT configuration, PipelineOptions options) {
      return Optional.empty();
    }
  }
  