Hadoop-common之ObjectWritable

        ObjectWritable是一个封装类，适用于字段需要使用多种类型。

         ObjectWritable有三个主要的成员变量，被封装的对象实例instance、该对象运行时类的Class 对象和Configuration 对象

 

private Class declaredClass;
private Object instance;
private Configuration conf;

          ObjectWritable中有个内部类NullInstance，从类名就可以看出，这是用来处理instance为空的。NullInstance有两个构造参数，一个默认构造参数，返回一个null，另一个构造参数有两个参数：declaredClass、conf。

     

private static class NullInstance extends Configured implements Writable {
    private Class<?> declaredClass;
    public NullInstance() { super(null); }
    public NullInstance(Class declaredClass, Configuration conf) {
      super(conf);
      this.declaredClass = declaredClass;
    }

            NullInstance有两个主要方法，write和readFields。write方法仅仅将类名称进行序列化，readFields方法，首先从流中读取一个String，然后在基本类型映射PRIMITIVE_NAMES中找到对应的类型，如果找不到，则去conf里

 

     

@Override
    public void readFields(DataInput in) throws IOException {
      String className = UTF8.readString(in);
      declaredClass = PRIMITIVE_NAMES.get(className);
      if (declaredClass == null) {
        try {
          declaredClass = getConf().getClassByName(className);
        } catch (ClassNotFoundException e) {
          throw new RuntimeException(e.toString());
        }
      }
    }

 

 

           继续回到ObjectWritable，下面介绍ObjectWritable的两个核心方法，writeObject和readObject

           writeObject顾名思义就是将对象序列化，writeObject有2个重载方法，不过最后都有调用writeObject(DataOutput out, Object instance, Class declaredClass, Configuration conf, boolean allowCompactArrays)。本方法主要分为两部分，第一部分将类名写入序列化，第二部分根据不同的类型，将数据进行序列化

      

  /** Write a {@link Writable}, {@link String}, primitive type, or an array of
   * the preceding. */
  public static void writeObject(DataOutput out, Object instance,
                                 Class declaredClass, 
                                 Configuration conf) throws IOException {
    writeObject(out, instance, declaredClass, conf, false);
  }
  
    /** 
     * Write a {@link Writable}, {@link String}, primitive type, or an array of
     * the preceding.  
     * 
     * @param allowCompactArrays - set true for RPC and internal or intra-cluster
     * usages.  Set false for inter-cluster, File, and other persisted output 
     * usages, to preserve the ability to interchange files with other clusters 
     * that may not be running the same version of software.  Sometime in ~2013 
     * we can consider removing this parameter and always using the compact format.
     */
    public static void writeObject(DataOutput out, Object instance,
        Class declaredClass, Configuration conf, boolean allowCompactArrays) 
    throws IOException {

    if (instance == null) {
    	// 如果实例为空，则new NullInstance作为实例
      instance = new NullInstance(declaredClass, conf);
      declaredClass = Writable.class;
    }
    
    // Special case: must come before writing out the declaredClass.
    // If this is an eligible array of primitives,
    // wrap it in an ArrayPrimitiveWritable$Internal wrapper class.
    //如果declaredClass是个数组，且允许压缩数组（allowCompactArrays=true）
    if (allowCompactArrays && declaredClass.isArray()
        && instance.getClass().getName().equals(declaredClass.getName())
        && instance.getClass().getComponentType().isPrimitive()) {
      instance = new ArrayPrimitiveWritable.Internal(instance);
      declaredClass = ArrayPrimitiveWritable.Internal.class;
    }
    //序列化类名
    UTF8.writeString(out, declaredClass.getName()); // always write declared

    //allowCompactArrays=false，declaredClass是个数组
    if (declaredClass.isArray()) {     // non-primitive or non-compact array
      int length = Array.getLength(instance);
      out.writeInt(length);
      for (int i = 0; i < length; i++) {
        writeObject(out, Array.get(instance, i),
            declaredClass.getComponentType(), conf, allowCompactArrays);
      }
      
    } else if (declaredClass == ArrayPrimitiveWritable.Internal.class) {
      ((ArrayPrimitiveWritable.Internal) instance).write(out);
      
    } else if (declaredClass == String.class) {   // String
      UTF8.writeString(out, (String)instance);
      
    } else if (declaredClass.isPrimitive()) {     // primitive type

      if (declaredClass == Boolean.TYPE) {        // boolean
        out.writeBoolean(((Boolean)instance).booleanValue());
      } else if (declaredClass == Character.TYPE) { // char
        out.writeChar(((Character)instance).charValue());
      } else if (declaredClass == Byte.TYPE) {    // byte
        out.writeByte(((Byte)instance).byteValue());
      } else if (declaredClass == Short.TYPE) {   // short
        out.writeShort(((Short)instance).shortValue());
      } else if (declaredClass == Integer.TYPE) { // int
        out.writeInt(((Integer)instance).intValue());
      } else if (declaredClass == Long.TYPE) {    // long
        out.writeLong(((Long)instance).longValue());
      } else if (declaredClass == Float.TYPE) {   // float
        out.writeFloat(((Float)instance).floatValue());
      } else if (declaredClass == Double.TYPE) {  // double
        out.writeDouble(((Double)instance).doubleValue());
      } else if (declaredClass == Void.TYPE) {    // void
      } else {
        throw new IllegalArgumentException("Not a primitive: "+declaredClass);
      }
    } else if (declaredClass.isEnum()) {         // enum
      UTF8.writeString(out, ((Enum)instance).name());
    } else if (Writable.class.isAssignableFrom(declaredClass)) { // Writable
      UTF8.writeString(out, instance.getClass().getName());
      ((Writable)instance).write(out);

    } else if (Message.class.isAssignableFrom(declaredClass)) {
      ((Message)instance).writeDelimitedTo(
          DataOutputOutputStream.constructOutputStream(out));
    } else {
      throw new IOException("Can't write: "+instance+" as "+declaredClass);
    }
  }

   readObject反序列化。和writeObject一样，有2个重载方法，最后实现方法是readObject(DataInput in, ObjectWritable objectWritable, Configuration conf)。一样分为两部分，首先读取类名，接着读取数据，并根据不同的类型调用不同的读取方法

  

/** Read a {@link Writable}, {@link String}, primitive type, or an array of
   * the preceding. */
  @SuppressWarnings("unchecked")
  public static Object readObject(DataInput in, ObjectWritable objectWritable, Configuration conf)
    throws IOException {
	//读取类型
    String className = UTF8.readString(in);
    //从PRIMITIVE_NAMES匹配基本数据类型
    Class<?> declaredClass = PRIMITIVE_NAMES.get(className);
    if (declaredClass == null) {
      //从conf中找类型
      declaredClass = loadClass(conf, className);
    }
    
    Object instance;
    //如果是基础类型，根据不同类型进行不同的读取
    if (declaredClass.isPrimitive()) {            // primitive types

      if (declaredClass == Boolean.TYPE) {             // boolean
        instance = Boolean.valueOf(in.readBoolean());
      } else if (declaredClass == Character.TYPE) {    // char
        instance = Character.valueOf(in.readChar());
      } else if (declaredClass == Byte.TYPE) {         // byte
        instance = Byte.valueOf(in.readByte());
      } else if (declaredClass == Short.TYPE) {        // short
        instance = Short.valueOf(in.readShort());
      } else if (declaredClass == Integer.TYPE) {      // int
        instance = Integer.valueOf(in.readInt());
      } else if (declaredClass == Long.TYPE) {         // long
        instance = Long.valueOf(in.readLong());
      } else if (declaredClass == Float.TYPE) {        // float
        instance = Float.valueOf(in.readFloat());
      } else if (declaredClass == Double.TYPE) {       // double
        instance = Double.valueOf(in.readDouble());
      } else if (declaredClass == Void.TYPE) {         // void
        instance = null;
      } else {
        throw new IllegalArgumentException("Not a primitive: "+declaredClass);
      }

    } else if (declaredClass.isArray()) {              // array
      int length = in.readInt();
      instance = Array.newInstance(declaredClass.getComponentType(), length);
      for (int i = 0; i < length; i++) {
        Array.set(instance, i, readObject(in, conf));
      }
      
    } else if (declaredClass == ArrayPrimitiveWritable.Internal.class) {
      // Read and unwrap ArrayPrimitiveWritable$Internal array.
      // Always allow the read, even if write is disabled by allowCompactArrays.
      ArrayPrimitiveWritable.Internal temp = 
          new ArrayPrimitiveWritable.Internal();
      temp.readFields(in);
      instance = temp.get();
      declaredClass = instance.getClass();

    } else if (declaredClass == String.class) {        // String
      instance = UTF8.readString(in);
    } else if (declaredClass.isEnum()) {         // enum
      instance = Enum.valueOf((Class<? extends Enum>) declaredClass, UTF8.readString(in));
    } else if (Message.class.isAssignableFrom(declaredClass)) {
      instance = tryInstantiateProtobuf(declaredClass, in);
    } else {                                      // Writable
      Class instanceClass = null;
      String str = UTF8.readString(in);
      instanceClass = loadClass(conf, str);
      
      Writable writable = WritableFactories.newInstance(instanceClass, conf);
      writable.readFields(in);
      instance = writable;

      if (instanceClass == NullInstance.class) {  // null
        declaredClass = ((NullInstance)instance).declaredClass;
        instance = null;
      }
    }

    if (objectWritable != null) {                 // store values
      objectWritable.declaredClass = declaredClass;
      objectWritable.instance = instance;
    }

    return instance;
      
  }