public class VIntWritable implements WritableComparable {
   private int value;
   ……
   // 设置VIntWritable的值
   public void set(int value) { this.value = value; }

   // 获取VIntWritable的值
   public int get() { return value; }

   public void readFields(DataInput in) throws IOException {
      value = WritableUtils.readVInt(in);
   }

   public void write(DataOutput out) throws IOException {
      WritableUtils.writeVInt(out, value);
   }
   ……
}

public final class WritableUtils  {
   public stati cvoid writeVInt(DataOutput stream, int i) throws IOException
   {
      writeVLong(stream, i);
   }
   /**
    * @param stream保存系列化结果输出流
    * @param i 被序列化的整数
    * @throws java.io.IOException
    */
   public static void writeVLong(DataOutput stream, long i) throws……
   {
      //处于[-112, 127]的整数
      if (i >= -112 && i <= 127) {
         stream.writeByte((byte)i);
         return;
      }
      //计算情况2的第一个字节
      int len = -112;
      if (i < 0) {
         i ^= -1L;
         len = -120;
      }
      long tmp = i;
      while (tmp != 0) {
         tmp = tmp >> 8;
         len--;
      }
      stream.writeByte((byte)len);
      len = (len < -120) ? -(len + 120) : -(len + 112);
      //输出后续字节
      for (int idx = len; idx != 0; idx--) {
         int shiftbits = (idx - 1) * 8;
         long mask = 0xFFL << shiftbits;
         stream.writeByte((byte)((i & mask) >> shiftbits));
      }
   }
}

public class ObjectWritable implements Writable, Configurable {
   private Class declaredClass;//保存于ObjectWritable的对象对应的类对象
   private Object instance;//被保留的对象
   private Configuration conf;

   public ObjectWritable() {}

   public ObjectWritable(Object instance) {
      set(instance);
   }

   public ObjectWritable(Class declaredClass, Object instance) {
      this.declaredClass = declaredClass;
      this.instance = instance;
   }           
   ……
   public void readFields(DataInput in) throws IOException {
      readObject(in, this, this.conf);
   }

   public void write(DataOutput out) throws IOException {
      writeObject(out, instance, declaredClass, conf);
   }
   ……
   public static void writeObject(DataOutput out, Object instance,
         Class declaredClass,Configuration conf) throws……{

      if (instance == null) {//空
         instance = new NullInstance(declaredClass, conf);
         declaredClass = Writable.class;
      }

      // 写出declaredClass的规范名
      UTF8.writeString(out, declaredClass.getName());

      if (declaredClass.isArray()) {//数组
         ……
      } else if (declaredClass == String.class) {//字符串
         ……
      } else if (declaredClass.isPrimitive()) {//基本类型
         if (declaredClass == Boolean.TYPE) {        //boolean
            out.writeBoolean(((Boolean)instance).booleanValue());
         } else if (declaredClass == Character.TYPE) { //char
            ……
         }
      } else if (declaredClass.isEnum()) {//枚举类型
         ……
      } else if (Writable.class.isAssignableFrom(declaredClass)) {
         //Writable的子类
         UTF8.writeString(out, instance.getClass().getName());
         ((Writable)instance).write(out);
      } else {
         ……
   }

   public static Object readObject(DataInput in,
                  ObjectWritable objectWritable, Configuration conf){
      ……
      Class instanceClass = null;
      ……
      Writable writable = WritableFactories.newInstance(instanceClass,
            conf);
      writable.readFields(in);
      instance = writable;
      ……
   }
}

public class WritableFactories {
   //保存了类型和WritableFactory工厂的对应关系
   private static final HashMap<Class, WritableFactory>CLASS_TO_FACTORY
      = new HashMap<Class, WritableFactory>();
   ……
   public static Writable newInstance(Class<? extends Writable> c,
                                      Configuration conf) {
      WritableFactory factory = WritableFactories.getFactory(c);
      if (factory != null) {
         Writable result = factory.newInstance();
         if (result instanceof Configurable) {
            ((Configurable) result).setConf(conf);
         }
         return result;
      } else {
         //采用传统的反射工具ReflectionUtils，创建对象
         return ReflectionUtils.newInstance(c, conf);
      }
   }
}

public class Block implements Writable, Comparable<Block> {
   static {
      WritableFactories.setFactory
         (Block.class,//类对象
         new WritableFactory() {//对应类的WritableFactory实现
            public Writable newInstance() { return new Block(); }
         });
   }           
   ……
}