FAQ
Ah - this was the problem! Now that I have the constructor, I am able
to serialize either way - using Java's serialization or my own. For
this app, I'm happy either way, but I'll think about sticking with my
own serialization in the future.

Thanks for the help!

-steve
On 10/10/07, Christopher Douglas wrote:
It looks like you're missing a *public*, default constructor; the
framework can't create an instance to call readFields() on.

That said: you might want to reconsider using Java serialization within
Writables. I know it seems restrictive, but you're opening yourself up
to a rancid cornucopia of ClassLoader, static, etc. issues. -C
-----Original Message-----
From: Steve Schlosser
Sent: Wednesday, October 10, 2007 7:57 AM
To: hadoop-user@lucene.apache.org
Subject: Using my own classes as keys and values

Hello all

Is there a best practice for using my own classes as keys and values?

My first attempt at doing this was successful - I built a
BigIntegerWritable class using IntWritable as a template. It was easy
because BigInteger has methods converting to and from byte arrays,
which I could then write into the DataOutput or read from the
DataInput.

It seems like I should be able to use object serialization to write
to/read from the DataOutput/Input objects and make my own classes
implement the Writable interface. It seems like I should be able to
do something like this:

import java.io.*;

import org.apache.hadoop.io.*;

public class Sample implements Writable {

Address address;
SampleValue value; // sampled value at this point

public Sample(Address a, SampleValue v) {
address = a;
value = v;
}

public SampleValue getValue() { return value;}
public Address getAddress() { return address; }

public String toString () {
return (address.toString() + " " + value.toString());
}

[...]

public void readFields(DataInput in) throws IOException {
ObjectInputStream oin = new
ObjectInputStream((DataInputBuffer)in);

try {
address = (Address)oin.readObject();
value = (SampleValue)oin.readObject();
} catch (ClassNotFoundException e) {
throw new IOException(e.toString());
}

}

public void write(DataOutput out) throws IOException {
ObjectOutputStream oout = new
ObjectOutputStream((DataOutputBuffer)out);

oout.writeObject(address);
oout.writeObject(value);
}
}

This code compiles, but throws exceptions at runtime, complaining that
WritableComparator can not access a member of class Sample with
modifiers "". Can someone tell me what this exception is talking
about?

Do I need to implement a WritableComparator for each class that I want
to implement Writable?

Thanks again for the help.

-steve

Search Discussions

Discussion Posts

Previous

Related Discussions

Discussion Navigation
viewthread | post
posts ‹ prev | 9 of 9 | next ›
Discussion Overview
groupcommon-user @
categorieshadoop
postedOct 10, '07 at 3:03p
activeOct 11, '07 at 1:29a
posts9
users5
websitehadoop.apache.org...
irc#hadoop

People

Translate

site design / logo © 2022 Grokbase