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Hadoop is the distributed computing framework of Apache; hadoop-core contains
the filesystem, job tracker and map/reduce modules
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/**
* 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.hadoop.mapred.lib.aggregate;
import java.lang.reflect.Constructor;
import java.util.ArrayList;
import java.util.Map.Entry;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapred.JobConf;
/**
* This class implements a wrapper for a user defined value aggregator descriptor.
* It servs two functions: One is to create an object of ValueAggregatorDescriptor from the
* name of a user defined class that may be dynamically loaded. The other is to
* deligate inviokations of generateKeyValPairs function to the created object.
*
*/
public class UserDefinedValueAggregatorDescriptor implements
ValueAggregatorDescriptor {
private String className;
private ValueAggregatorDescriptor theAggregatorDescriptor = null;
private static final Class[] argArray = new Class[] {};
/**
* Create an instance of the given class
* @param className the name of the class
* @return a dynamically created instance of the given class
*/
public static Object createInstance(String className) {
Object retv = null;
try {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
Class theFilterClass = Class.forName(className, true, classLoader);
Constructor meth = theFilterClass.getDeclaredConstructor(argArray);
meth.setAccessible(true);
retv = meth.newInstance();
} catch (Exception e) {
throw new RuntimeException(e);
}
return retv;
}
private void createAggregator(JobConf job) {
if (theAggregatorDescriptor == null) {
theAggregatorDescriptor = (ValueAggregatorDescriptor) createInstance(this.className);
theAggregatorDescriptor.configure(job);
}
}
/**
*
* @param className the class name of the user defined descriptor class
* @param job a configure object used for decriptor configuration
*/
public UserDefinedValueAggregatorDescriptor(String className, JobConf job) {
this.className = className;
this.createAggregator(job);
}
/**
* Generate a list of aggregation-id/value pairs for the given key/value pairs
* by delegating the invocation to the real object.
*
* @param key
* input key
* @param val
* input value
* @return a list of aggregation id/value pairs. An aggregation id encodes an
* aggregation type which is used to guide the way to aggregate the
* value in the reduce/combiner phrase of an Aggregate based job.
*/
public ArrayList> generateKeyValPairs(Object key,
Object val) {
ArrayList> retv = new ArrayList>();
if (this.theAggregatorDescriptor != null) {
retv = this.theAggregatorDescriptor.generateKeyValPairs(key, val);
}
return retv;
}
/**
* @return the string representation of this object.
*/
public String toString() {
return "UserDefinedValueAggregatorDescriptor with class name:" + "\t"
+ this.className;
}
/**
* Do nothing.
*/
public void configure(JobConf job) {
}
}