Github user fhueske commented on a diff in the pull request:
https://github.com/apache/incubator-flink/pull/37#discussion_r15514510
--- Diff:
flink-addons/flink-hadoop-compatibility/src/main/java/org/apache/flink/hadoopcompatibility/mapred/HadoopReduceFunction.java
---
@@ -0,0 +1,199 @@
+/**
+ * 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.flink.hadoopcompatibility.mapred;
+
+import org.apache.flink.api.java.functions.GroupReduceFunction;
+import
org.apache.flink.api.java.operators.translation.TupleUnwrappingIterator;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.api.java.typeutils.ResultTypeQueryable;
+import org.apache.flink.api.java.typeutils.TupleTypeInfo;
+import org.apache.flink.api.java.typeutils.WritableTypeInfo;
+import
org.apache.flink.hadoopcompatibility.mapred.utils.HadoopConfiguration;
+import
org.apache.flink.hadoopcompatibility.mapred.wrapper.HadoopDummyReporter;
+import
org.apache.flink.hadoopcompatibility.mapred.wrapper.HadoopOutputCollector;
+import org.apache.flink.types.TypeInformation;
+import org.apache.flink.util.Collector;
+import org.apache.flink.util.InstantiationUtil;
+import org.apache.hadoop.io.Writable;
+import org.apache.hadoop.io.WritableComparable;
+import org.apache.hadoop.io.WritableUtils;
+import org.apache.hadoop.mapred.JobConf;
+import org.apache.hadoop.mapred.Reducer;
+import org.apache.hadoop.mapred.Reporter;
+
+import java.io.IOException;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.io.Serializable;
+import java.util.Iterator;
+
+/**
+ * The wrapper for a Hadoop Reducer (mapred API).
+ */
+public class HadoopReduceFunction<KEYIN extends WritableComparable,
VALUEIN extends Writable,
+ KEYOUT extends WritableComparable, VALUEOUT extends Writable>
extends GroupReduceFunction<Tuple2<KEYIN,VALUEIN>,
+ Tuple2<KEYOUT,VALUEOUT>> implements Serializable,
ResultTypeQueryable<Tuple2<KEYOUT,VALUEOUT>> {
+
+ private static final long serialVersionUID = 1L;
+
+ private Class<KEYOUT> keyoutClass;
+ private Class<VALUEOUT> valueoutClass;
+ private JobConf jobConf;
+
+ private Reducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT> reducer;
+ private Reducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT> combiner;
+ private HadoopOutputCollector outputCollector;
+ private Reporter reporter;
+ private ReducerTransformingIterator iterator;
+
+ @SuppressWarnings("unchecked")
+ public HadoopReduceFunction(JobConf jobConf) {
+ this.jobConf = jobConf;
+
+ this.reducer =
InstantiationUtil.instantiate(jobConf.getReducerClass());
+ final Class combinerClass = jobConf.getCombinerClass();
+ if (combinerClass != null) {
+ this.combiner = (Reducer<KEYIN, VALUEIN, KEYOUT,
VALUEOUT>) InstantiationUtil.instantiate(combinerClass);
+ }
+
+ this.keyoutClass = (Class<KEYOUT>) jobConf.getOutputKeyClass();
+ this.valueoutClass = (Class<VALUEOUT>)
jobConf.getOutputValueClass();
+ this.outputCollector = new HadoopOutputCollector(keyoutClass,
valueoutClass);
+ this.iterator = new ReducerTransformingIterator();
+ this.reporter = new HadoopDummyReporter();
+ }
+
+ /**
+ * A wrapping iterator for an iterator of key-value tuples that can be
used as an iterator of values.
+ */
+ private final class ReducerTransformingIterator extends
TupleUnwrappingIterator<VALUEIN,KEYIN>
+ implements java.io.Serializable {
+
+ private static final long serialVersionUID = 1L;
+ private Iterator<Tuple2<KEYIN,VALUEIN>> iterator;
+ private KEYIN key;
+ private Tuple2<KEYIN,VALUEIN> first;
+
+ @Override()
+ public void set(Iterator<Tuple2<KEYIN,VALUEIN>> iterator) {
+ this.iterator = iterator;
+ if(this.hasNext()) {
+ this.first = iterator.next();
+ this.key = this.first.f0;
+ }
+ }
+
+ @Override
+ public boolean hasNext() {
+ if(this.first != null) {
+ return true;
+ }
+ return iterator.hasNext();
+ }
+
+ @Override
+ public VALUEIN next() {
+ if(this.first != null) {
+ final VALUEIN val = this.first.f1;
+ this.first = null;
+ return val;
+ }
+ final Tuple2<KEYIN,VALUEIN> tuple = iterator.next();
+ return tuple.f1;
+ }
+
+ private KEYIN getKey() {
+ return WritableUtils.clone(this.key, jobConf);
+ }
+
+ @Override
+ public void remove() {
+ throw new UnsupportedOperationException();
+ }
+ }
+
+ @Override
+ @SuppressWarnings("unchecked")
+ public void reduce(Iterator<Tuple2<KEYIN,VALUEIN>> values,
Collector<Tuple2<KEYOUT,VALUEOUT>> out)
+ throws Exception {
+ outputCollector.set(out);
+ iterator.set(values);
+ reducer.reduce(iterator.getKey(), iterator, outputCollector,
reporter);
+ }
+
+ @Override
+ @SuppressWarnings("unchecked")
+ public void combine(Iterator<Tuple2<KEYIN,VALUEIN>> values,
Collector<Tuple2<KEYIN,VALUEIN>> out) throws Exception {
+ if (this.combiner == null) {
--- End diff --
The constructor initializes `this.combiner` to the combine class registered
in the `JobConf`. If the combiner is the same class as the reducer it is still
registered in the `JobConf`, right?
In that case, the combine method may not be called if no combiner is
registered, i.e., `this.combiner == null`. Instead, this is a invalid system
behavior and an exception should be raised.
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