org.apache.hadoop.io

Class GenericWritable

java.lang.Object

org.apache.hadoop.io.GenericWritable

All Implemented Interfaces:

Configurable, Writable

@InterfaceAudience.Public
 @InterfaceStability.Stable
public abstract class GenericWritable
extends Object
implements Writable, Configurable

A wrapper for Writable instances.

When two sequence files, which have same Key type but different Value types, are mapped out to reduce, multiple Value types is not allowed. In this case, this class can help you wrap instances with different types.

Compared with ObjectWritable, this class is much more effective, because ObjectWritable will append the class declaration as a String into the output file in every Key-Value pair.

Generic Writable implements Configurable interface, so that it will be configured by the framework. The configuration is passed to the wrapped objects implementing Configurable interface before deserialization.

how to use it:
1. Write your own class, such as GenericObject, which extends GenericWritable.
2. Implements the abstract method getTypes(), defines the classes which will be wrapped in GenericObject in application. Attention: this classes defined in getTypes() method, must implement Writable interface.

The code looks like this:

 public class GenericObject extends GenericWritable {
 
   private static Class[] CLASSES = {
               ClassType1.class, 
               ClassType2.class,
               ClassType3.class,
               };

   protected Class[] getTypes() {
       return CLASSES;
   }

 }
 

Since:

Nov 8, 2006

Constructor Summary

Constructors

Constructor and Description
GenericWritable()

Method Summary

Modifier and Type	Method and Description
Writable	get() Return the wrapped instance.
Configuration	getConf() Return the configuration used by this object.
protected abstract Class<? extends Writable>[]	getTypes() Return all classes that may be wrapped.
void	readFields(DataInput in) Deserialize the fields of this object from in.
void	set(Writable obj) Set the instance that is wrapped.
void	setConf(Configuration conf) Set the configuration to be used by this object.
String	toString()
void	write(DataOutput out) Serialize the fields of this object to out.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

GenericWritable

public GenericWritable()

Method Detail

set

public void set(Writable obj)

Set the instance that is wrapped.

Parameters:

obj -

get

public Writable get()

Return the wrapped instance.

toString

public String toString()

Overrides:

toString in class Object

readFields

public void readFields(DataInput in)
                throws IOException

Description copied from interface: Writable

Deserialize the fields of this object from in.

For efficiency, implementations should attempt to re-use storage in the existing object where possible.

Specified by:

readFields in interface Writable

Parameters:

in - DataInput to deseriablize this object from.

Throws:

IOException

write

public void write(DataOutput out)
           throws IOException

Description copied from interface: Writable

Serialize the fields of this object to out.

Specified by:

write in interface Writable

Parameters:

out - DataOuput to serialize this object into.

Throws:

IOException

getTypes

protected abstract Class<? extends Writable>[] getTypes()

Return all classes that may be wrapped. Subclasses should implement this to return a constant array of classes.

getConf

public Configuration getConf()

Description copied from interface: Configurable

Return the configuration used by this object.

Specified by:

getConf in interface Configurable

Returns:

Configuration

setConf

public void setConf(Configuration conf)

Description copied from interface: Configurable

Set the configuration to be used by this object.

Specified by:

setConf in interface Configurable

Parameters:

conf - configuration to be used