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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.mapreduce;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.text.NumberFormat;

import org.apache.hadoop.io.Text;

/**
 * JobID represents the immutable and unique identifier for 
 * the job. JobID consists of two parts. First part 
 * represents the jobtracker identifier, so that jobID to jobtracker map 
 * is defined. For cluster setup this string is the jobtracker 
 * start time, for local setting, it is "local".
 * Second part of the JobID is the job number. 
 
 * An example JobID is : 
 * job_200707121733_0003 , which represents the third job 
 * running at the jobtracker started at 200707121733. 
 * 
 * Applications should never construct or parse JobID strings, but rather 
 * use appropriate constructors or {@link #forName(String)} method. 
 * 
 * @see TaskID
 * @see TaskAttemptID
 * @see org.apache.hadoop.mapred.JobTracker#getNewJobId()
 * @see org.apache.hadoop.mapred.JobTracker#getStartTime()
 */
public class JobID extends org.apache.hadoop.mapred.ID 
                   implements Comparable {
  protected static final String JOB = "job";
  private final Text jtIdentifier;
  
  protected static final NumberFormat idFormat = NumberFormat.getInstance();
  static {
    idFormat.setGroupingUsed(false);
    idFormat.setMinimumIntegerDigits(4);
  }
  
  /**
   * Constructs a JobID object 
   * @param jtIdentifier jobTracker identifier
   * @param id job number
   */
  public JobID(String jtIdentifier, int id) {
    super(id);
    this.jtIdentifier = new Text(jtIdentifier);
  }
  
  public JobID() { 
    jtIdentifier = new Text();
  }
  
  public String getJtIdentifier() {
    return jtIdentifier.toString();
  }
  
  @Override
  public boolean equals(Object o) {
    if (!super.equals(o))
      return false;

    JobID that = (JobID)o;
    return this.jtIdentifier.equals(that.jtIdentifier);
  }
  
  /**Compare JobIds by first jtIdentifiers, then by job numbers*/
  @Override
  public int compareTo(ID o) {
    JobID that = (JobID)o;
    int jtComp = this.jtIdentifier.compareTo(that.jtIdentifier);
    if(jtComp == 0) {
      return this.id - that.id;
    }
    else return jtComp;
  }
  
  /**
   * Add the stuff after the "job" prefix to the given builder. This is useful,
   * because the sub-ids use this substring at the start of their string.
   * @param builder the builder to append to
   * @return the builder that was passed in
   */
  public StringBuilder appendTo(StringBuilder builder) {
    builder.append(SEPARATOR);
    builder.append(jtIdentifier);
    builder.append(SEPARATOR);
    builder.append(idFormat.format(id));
    return builder;
  }

  @Override
  public int hashCode() {
    return jtIdentifier.hashCode() + id;
  }

  @Override
  public String toString() {
    return appendTo(new StringBuilder(JOB)).toString();
  }

  @Override
  public void readFields(DataInput in) throws IOException {
    super.readFields(in);
    this.jtIdentifier.readFields(in);
  }

  @Override
  public void write(DataOutput out) throws IOException {
    super.write(out);
    jtIdentifier.write(out);
  }
  
  /** Construct a JobId object from given string 
   * @return constructed JobId object or null if the given String is null
   * @throws IllegalArgumentException if the given string is malformed
   */
  public static JobID forName(String str) throws IllegalArgumentException {
    if(str == null)
      return null;
    try {
      String[] parts = str.split("_");
      if(parts.length == 3) {
        if(parts[0].equals(JOB)) {
          return new org.apache.hadoop.mapred.JobID(parts[1], 
                                                    Integer.parseInt(parts[2]));
        }
      }
    }catch (Exception ex) {//fall below
    }
    throw new IllegalArgumentException("JobId string : " + str 
        + " is not properly formed");
  }
  
}