hadoop自定义类型异常java.lang.RuntimeException: java.lang.NoSuchMethodException: twicesort.TwoKey.<init>()

java.lang.RuntimeException: java.lang.NoSuchMethodException: twicesort.TwoKey.<init>()    at org.apache.hadoop.util.ReflectionUtils.newInstance(ReflectionUtils.java:131)    at org.apache.hadoop.io.WritableComparator.newKey(WritableComparator.java:144)    at org.apache.hadoop.io.WritableComparator.<init>(WritableComparator.java:130)    at org.apache.hadoop.io.WritableComparator.get(WritableComparator.java:65)    at org.apache.hadoop.mapred.JobConf.getOutputKeyComparator(JobConf.java:887)    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.init(MapTask.java:1001)    at org.apache.hadoop.mapred.MapTask.createSortingCollector(MapTask.java:401)    at org.apache.hadoop.mapred.MapTask.access$100(MapTask.java:81)    at org.apache.hadoop.mapred.MapTask$NewOutputCollector.<init>(MapTask.java:695)    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:767)

重点在这里，java.lang.NoSuchMethodException: twicesort.TwoKey，他说没有这个方法，这个方法是TwoKey的无参构造方法
这个细节在hadoop权威指南上也有提到，就在序列化或者自定义类型那章，具体不记得了，我的理解是首先他反序列化的时候先要拿到一个无参的构造对象，然后再进行对象内值的填充。

让我们再来看看hadoop给我提供的类型是不是都有无参的构造方法

public class IntWritable implements WritableComparable<IntWritable> {  private int value;  public IntWritable() {}  public IntWritable(int value) { set(value); }  }

public class LongWritable implements WritableComparable<LongWritable> {  private long value;  public LongWritable() {}  public LongWritable(long value) { set(value); }  }

public class Text extends BinaryComparable    implements WritableComparable<BinaryComparable> {    public Text() {    bytes = EMPTY_BYTES;  }  /** Construct from a string.    */  public Text(String string) {    set(string);  }  /** Construct from another text. */  public Text(Text utf8) {    set(utf8);  }  /** Construct from a byte array.   */  public Text(byte[] utf8)  {    set(utf8);  }  }

public class BooleanWritable implements WritableComparable<BooleanWritable> {  private boolean value;  /**    */  public BooleanWritable() {};  }

public class ByteWritable implements WritableComparable<ByteWritable> {  private byte value;  public ByteWritable() {}  public ByteWritable(byte value) { set(value); }  }

public class ShortWritable implements WritableComparable<ShortWritable> {  private short value;  public ShortWritable() {  }  public ShortWritable(short value) {    set(value);  }}