| /** |
| * 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.crunch.materialize.pobject; |
| |
| import org.apache.crunch.PCollection; |
| import org.apache.crunch.PObject; |
| |
| /** |
| * An abstract implementation of {@link PObject} that is backed by a {@link PCollection}. |
| * Clients creating a concrete implementation should override the method |
| * {@link PObjectImpl#process(Iterable)}, which transforms the backing PCollection into the |
| * singleton value encapsulated by the PObject. Once this {code PObject}'s value has been |
| * calculated, the value is cached to prevent subsequent materializations of the backing |
| * {@code PCollection}. |
| * |
| * @param <S> The type contained in the underlying PCollection. |
| * @param <T> The type encapsulated by this PObject. |
| */ |
| public abstract class PObjectImpl<S, T> implements PObject<T> { |
| |
| // The name of the collection, used as the name for this instance as well. |
| private String name; |
| |
| // A referenced to the materialized contents of a PCollection. |
| private Iterable<S> iterable; |
| |
| // A variable to hold a cached copy of the value of this {@code PObject}, |
| // to prevent unnecessary materializations of the backing {@code PCollection}. |
| private T cachedValue; |
| |
| // A flag indicating if a value for this {@code PObject} has been cached. |
| private boolean isCached; |
| |
| /** |
| * Constructs a new instance of this {@code PObject} implementation. |
| * |
| * @param collect The backing {@code PCollection} for this {@code PObject}. |
| */ |
| public PObjectImpl(PCollection<S> collect) { |
| this.name = collect.toString(); |
| this.iterable = collect.materialize(); |
| this.cachedValue = null; |
| this.isCached = false; |
| } |
| |
| /** {@inheritDoc} */ |
| @Override |
| public String toString() { |
| return name; |
| } |
| |
| /** {@inheritDoc} */ |
| @Override |
| public final T getValue() { |
| if (!isCached) { |
| cachedValue = process(iterable); |
| isCached = true; |
| } |
| return cachedValue; |
| } |
| |
| /** |
| * Transforms the provided Iterable, obtained from the backing {@link PCollection}, |
| * into the value encapsulated by this {@code PObject}. |
| * |
| * @param input An Iterable whose elements correspond to those of the backing {@code |
| * PCollection}. |
| * @return The value of this {@code PObject}. |
| */ |
| protected abstract T process(Iterable<S> input); |
| } |