自定义Writable实现类
public class ContentWritable implements Writable{private Text before;private IntWritable num;//默认的构造函数必须写，不然会出NullPointException异常，而且必须默认初始化成员变量，不知道为何，看底层是反射实例化的，估计跟这个有关。public ContentWritable(){this.before = new Text();this.num = new IntWritable();}public ContentWritable(Text before,IntWritable num){this.before = before;this.num = num;}@Overridepublic void readFields(DataInput in) throws IOException {before.readFields(in);num.readFields(in);}@Overridepublic void write(DataOutput out) throws IOException {before.write(out);num.write(out);}public Text getBefore() {return before;}public IntWritable getNum() {return num;}public void setBefore(Text before) {this.before = before;}public void setNum(IntWritable num) {this.num = num;}}