org.apache.hadoop.io

类 GenericWritable

java.lang.Object

org.apache.hadoop.io.GenericWritable

所有已实现的接口:

Configurable, Writable

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;
   }

 }
 

从以下版本开始:

Nov 8, 2006

构造器概要

构造器

构造器和说明
GenericWritable()

方法概要

方法

限定符和类型	方法和说明
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.

从类继承的方法 java.lang.Object

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

构造器详细资料

GenericWritable

public GenericWritable()

方法详细资料

set

public void set(Writable obj)

Set the instance that is wrapped.

参数:

obj -

get

public Writable get()

Return the wrapped instance.

toString

public String toString()

覆盖:

toString 在类中 Object

readFields

public void readFields(DataInput in)
                throws IOException

从接口复制的说明: Writable

Deserialize the fields of this object from in.

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

指定者:

readFields 在接口中 Writable

参数:

in - DataInput to deseriablize this object from.

抛出:

IOException

write

public void write(DataOutput out)
           throws IOException

从接口复制的说明: Writable

Serialize the fields of this object to out.

指定者:

write 在接口中 Writable

参数:

out - DataOuput to serialize this object into.

抛出:

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()

从接口复制的说明: Configurable

Return the configuration used by this object.

指定者:

getConf 在接口中 Configurable

setConf

public void setConf(Configuration conf)

从接口复制的说明: Configurable

Set the configuration to be used by this object.

指定者:

setConf 在接口中 Configurable