GenericWritable.java
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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.io;
- import java.io.DataInput;
- import java.io.DataOutput;
- import java.io.IOException;
- import org.apache.hadoop.conf.Configurable;
- import org.apache.hadoop.conf.Configuration;
- import org.apache.hadoop.util.ReflectionUtils;
- /**
- * A wrapper for Writable instances.
- * <p>
- * When two sequence files, which have same Key type but different Value
- * types, are mapped out to reduce, multiple Value types is not allowed.
- * In this case, this class can help you wrap instances with different types.
- * </p>
- *
- * <p>
- * Compared with <code>ObjectWritable</code>, this class is much more effective,
- * because <code>ObjectWritable</code> will append the class declaration as a String
- * into the output file in every Key-Value pair.
- * </p>
- *
- * <p>
- * Generic Writable implements {@link Configurable} interface, so that it will be
- * configured by the framework. The configuration is passed to the wrapped objects
- * implementing {@link Configurable} interface <i>before deserialization</i>.
- * </p>
- *
- * how to use it: <br>
- * 1. Write your own class, such as GenericObject, which extends GenericWritable.<br>
- * 2. Implements the abstract method <code>getTypes()</code>, defines
- * the classes which will be wrapped in GenericObject in application.
- * Attention: this classes defined in <code>getTypes()</code> method, must
- * implement <code>Writable</code> interface.
- * <br><br>
- *
- * The code looks like this:
- * <blockquote><pre>
- * public class GenericObject extends GenericWritable {
- *
- * private static Class[] CLASSES = {
- * ClassType1.class,
- * ClassType2.class,
- * ClassType3.class,
- * };
- *
- * protected Class[] getTypes() {
- * return CLASSES;
- * }
- *
- * }
- * </pre></blockquote>
- *
- * @since Nov 8, 2006
- */
- public abstract class GenericWritable implements Writable, Configurable {
- private static final byte NOT_SET = -1;
- private byte type = NOT_SET;
- private Writable instance;
- private Configuration conf = null;
-
- /**
- * Set the instance that is wrapped.
- *
- * @param obj
- */
- public void set(Writable obj) {
- instance = obj;
- Class<? extends Writable> instanceClazz = instance.getClass();
- Class<? extends Writable>[] clazzes = getTypes();
- for (int i = 0; i < clazzes.length; i++) {
- Class<? extends Writable> clazz = clazzes[i];
- if (clazz.equals(instanceClazz)) {
- type = (byte) i;
- return;
- }
- }
- throw new RuntimeException("The type of instance is: "
- + instance.getClass() + ", which is NOT registered.");
- }
- /**
- * Return the wrapped instance.
- */
- public Writable get() {
- return instance;
- }
-
- public String toString() {
- return "GW[" + (instance != null ? ("class=" + instance.getClass().getName() +
- ",value=" + instance.toString()) : "(null)") + "]";
- }
- public void readFields(DataInput in) throws IOException {
- type = in.readByte();
- Class<? extends Writable> clazz = getTypes()[type & 0xff];
- try {
- instance = ReflectionUtils.newInstance(clazz, conf);
- } catch (Exception e) {
- e.printStackTrace();
- throw new IOException("Cannot initialize the class: " + clazz);
- }
- instance.readFields(in);
- }
- public void write(DataOutput out) throws IOException {
- if (type == NOT_SET || instance == null)
- throw new IOException("The GenericWritable has NOT been set correctly. type="
- + type + ", instance=" + instance);
- out.writeByte(type);
- instance.write(out);
- }
- /**
- * Return all classes that may be wrapped. Subclasses should implement this
- * to return a constant array of classes.
- */
- abstract protected Class<? extends Writable>[] getTypes();
- public Configuration getConf() {
- return conf;
- }
- public void setConf(Configuration conf) {
- this.conf = conf;
- }
-
- }