一步一步学习hadoop（八）

Map任务执行类的实现

     Hadoop MapReduce框架将要处理数据切分成片，将每个分片让一个map任务去完成，每个map任务都将分片数据解析为键值对数据，调用Mapper类的map函数，将输入键值对转化为输出键值对，输出键值对的类型和输入键值对的类型可以没有任何关系。一个输入键值键值对可以得到0个或者多个输出键值对。
    Hadoop中的MapReduce作业调用Mapper类来完成map任务，Mapper有两个版本，一个是老版的在org.apache.hadoop.mapred里，新版的在org.apache.hadoop.mapreduce里，新版的将原来的OutputCollector和Reporter整合为一个新的Context对象，使用起来更加简单，功能要更加强大。下面的例子实现新版的Mapper。一般来讲只需对Mapper类的map函数进行重载就可以满足大部分的需求了，有时需要在执行map任务前获取一些自定义的数据，比如传入的参数（如例子中的fieldSeparator），从DisturbuteCache中获取共享数据等特殊要求，需要实现setup函数，该函数在每次执行map任务前调用一次，同理，需要在执行完map任务后清理map任务，需要实现cleanup函数。
    下面以一个类似linux的cut工具的map来讲解mapper类的构造过程。
    该mapper将输入的键值对看成以分隔符分离的字段集，分隔符通过设置mapreduce.fieldsel.data.field.separator指定，默认为\t。输出的key/value对，以mapreduce.fieldsel.map.output.key.value.fields.spec指定，格式为2,3,5:6-9,12-表示输出键为字段2,3,5,输出值为字段6,7,8,9,12和大于12的所有字段。

import java.io.IOException;import java.util.ArrayList;import java.util.List;import org.apache.commons.logging.Log;import org.apache.commons.logging.LogFactory;import org.apache.hadoop.classification.InterfaceAudience;import org.apache.hadoop.classification.InterfaceStability;import org.apache.hadoop.conf.Configuration;import org.apache.hadoop.io.Text;import org.apache.hadoop.mapreduce.Mapper;import org.apache.hadoop.mapreduce.lib.input.TextInputFormat;public class FieldSelectionMapper<K, V>    extends Mapper<K, V, Text, Text> {  private String mapOutputKeyValueSpec;  private boolean ignoreInputKey;  private String fieldSeparator = "\t";  private List<Integer> mapOutputKeyFieldList = new ArrayList<Integer>();  private List<Integer> mapOutputValueFieldList = new ArrayList<Integer>();  private int allMapValueFieldsFrom = -1;  public void setup(Context context)      throws IOException, InterruptedException {    Configuration conf = context.getConfiguration();    this.fieldSeparator =      conf.get(FieldSelectionHelper.DATA_FIELD_SEPERATOR, "\t");    this.mapOutputKeyValueSpec =      conf.get(FieldSelectionHelper.MAP_OUTPUT_KEY_VALUE_SPEC, "0-:");    try {      this.ignoreInputKey = TextInputFormat.class.getCanonicalName().equals(        context.getInputFormatClass().getCanonicalName());    } catch (ClassNotFoundException e) {      throw new IOException("Input format class not found", e);    }    allMapValueFieldsFrom = FieldSelectionHelper.parseOutputKeyValueSpec(      mapOutputKeyValueSpec, mapOutputKeyFieldList, mapOutputValueFieldList);  }  public void map(K key, V val, Context context)      throws IOException, InterruptedException {    FieldSelectionHelper helper = new FieldSelectionHelper(      FieldSelectionHelper.emptyText, FieldSelectionHelper.emptyText);    helper.extractOutputKeyValue(key.toString(), val.toString(),      fieldSeparator, mapOutputKeyFieldList, mapOutputValueFieldList,      allMapValueFieldsFrom, ignoreInputKey, true);    context.write(helper.getKey(), helper.getValue());  }}

其中引用的FieldSelectionHelper类的实现如下：

import java.util.List;import org.apache.hadoop.classification.InterfaceAudience;import org.apache.hadoop.classification.InterfaceStability;import org.apache.hadoop.io.Text;public class FieldSelectionHelper {  public static Text emptyText = new Text("");  public static final String DATA_FIELD_SEPERATOR =    "mapreduce.fieldsel.data.field.separator";  public static final String MAP_OUTPUT_KEY_VALUE_SPEC =    "mapreduce.fieldsel.map.output.key.value.fields.spec";  public static final String REDUCE_OUTPUT_KEY_VALUE_SPEC =    "mapreduce.fieldsel.reduce.output.key.value.fields.spec";  private static int extractFields(String[] fieldListSpec,      List<Integer> fieldList) {    int allFieldsFrom = -1;    int i = 0;    int j = 0;    int pos = -1;    String fieldSpec = null;    for (i = 0; i < fieldListSpec.length; i++) {      fieldSpec = fieldListSpec[i];      if (fieldSpec.length() == 0) {        continue;      }      pos = fieldSpec.indexOf('-');      if (pos < 0) {        Integer fn = new Integer(fieldSpec);        fieldList.add(fn);      } else {        String start = fieldSpec.substring(0, pos);        String end = fieldSpec.substring(pos + 1);        if (start.length() == 0) {          start = "0";        }        if (end.length() == 0) {          allFieldsFrom = Integer.parseInt(start);          continue;        }        int startPos = Integer.parseInt(start);        int endPos = Integer.parseInt(end);        for (j = startPos; j <= endPos; j++) {          fieldList.add(j);        }      }    }    return allFieldsFrom;  }  private static String selectFields(String[] fields, List<Integer> fieldList,      int allFieldsFrom, String separator) {    String retv = null;    int i = 0;    StringBuffer sb = null;    if (fieldList != null && fieldList.size() > 0) {      if (sb == null) {        sb = new StringBuffer();      }      for (Integer index : fieldList) {        if (index < fields.length) {          sb.append(fields[index]);        }        sb.append(separator);      }    }    if (allFieldsFrom >= 0) {      if (sb == null) {        sb = new StringBuffer();      }      for (i = allFieldsFrom; i < fields.length; i++) {        sb.append(fields[i]).append(separator);      }    }    if (sb != null) {      retv = sb.toString();      if (retv.length() > 0) {        retv = retv.substring(0, retv.length() - 1);      }    }    return retv;  }   public static int parseOutputKeyValueSpec(String keyValueSpec,      List<Integer> keyFieldList, List<Integer> valueFieldList) {    String[] keyValSpecs = keyValueSpec.split(":", -1);        String[] keySpec = keyValSpecs[0].split(",");        String[] valSpec = new String[0];    if (keyValSpecs.length > 1) {      valSpec = keyValSpecs[1].split(",");    }    FieldSelectionHelper.extractFields(keySpec, keyFieldList);    return FieldSelectionHelper.extractFields(valSpec, valueFieldList);  }  public static String specToString(String fieldSeparator, String keyValueSpec,      int allValueFieldsFrom, List<Integer> keyFieldList,      List<Integer> valueFieldList) {    StringBuffer sb = new StringBuffer();    sb.append("fieldSeparator: ").append(fieldSeparator).append("\n");    sb.append("keyValueSpec: ").append(keyValueSpec).append("\n");    sb.append("allValueFieldsFrom: ").append(allValueFieldsFrom);    sb.append("\n");    sb.append("keyFieldList.length: ").append(keyFieldList.size());    sb.append("\n");    for (Integer field : keyFieldList) {      sb.append("\t").append(field).append("\n");    }    sb.append("valueFieldList.length: ").append(valueFieldList.size());    sb.append("\n");    for (Integer field : valueFieldList) {      sb.append("\t").append(field).append("\n");    }    return sb.toString();  }  private Text key = null;  private Text value = null;   public FieldSelectionHelper() {  }  public FieldSelectionHelper(Text key, Text val) {    this.key = key;    this.value = val;  }   public Text getKey() {    return key;  }   public Text getValue() {    return value;  }  public void extractOutputKeyValue(String key, String val,      String fieldSep, List<Integer> keyFieldList, List<Integer> valFieldList,      int allValueFieldsFrom, boolean ignoreKey, boolean isMap) {    if (!ignoreKey) {      val = key + val;    }    String[] fields = val.split(fieldSep);        String newKey = selectFields(fields, keyFieldList, -1, fieldSep);    String newVal = selectFields(fields, valFieldList, allValueFieldsFrom,      fieldSep);    if (isMap && newKey == null) {      newKey = newVal;      newVal = null;    }        if (newKey != null) {      this.key = new Text(newKey);    }    if (newVal != null) {      this.value = new Text(newVal);    }  }}