This document summarizes Java development options for MongoDB, including simple Java usage, the Morphia ORM, concurrency patterns, write concerns, data types, custom encoding/decoding, GridFS for file storage, and running MapReduce jobs on MongoDB data using Hadoop. Code examples are provided for common operations like inserting documents, querying with Morphia, and running a word count MapReduce job.

1. Java Development
Eliot Horowitz
@eliothorowitz
MongoBoston
September 20, 2010

2. Java Choices
• Straight driver - map like interface
• Reflection - slow but easy
• Morphia - annotation based

3. Simple Java
Mongo m = new Mongo();
DB db = m.getDB( “myapp” );
DBCollection users = db.getCollection( “users” );
BasicDBObject person = new BasicDBObject();
person.put( “name” , “Eliot” );
person.put( “creation_date” , new java.util.Date()
users.insert( person );

4. Morphia
public class Person {
@Id
String username;
Date birthDate;
@Indexed
String state;
}

5. Concurrency
• Ideally 1 Mongo instance per JVM
• Connection pooling built in
• Connections sticky to threads
• Force with requestStart/End

6. WriteConcern
• By default writes are fire and forget
• Can set WriteConcern on
Mongo,DB,DBCollection or per write call
• Configurable fsync, w=, wtimeout=
• Will throw exceptions for unique key
exceptions, etc...
• WriteResult will either cache result or call
getLastError if possible.

7. Types
• double,int,long,string, boolean map natively
• arrays <=> List
• sub-objects <=> Map
• date <=> java.util.Date

8. Custom Type Encoding/Decoding
• BSON.addEncodingHook( Class c ,
Transformer t );
• BSON.addDecodingHook( Class c ,
Transformer t );

9. GridFS
Mongo m = new Mongo();
DB db = m.getDB( “myapp” );
GridFS fs = new GridFS( db );
GridFSInputFile inFile = fs.createFile( new File( “foo.txt” ) );
in.save();
GridFSDBFile outFile = fs.findOne( “foo.txt” );
outFile.writeTo( “foo2.txt” );

10. Hadoop
• Coming Soon...
• Support for input/output
• Suggestions welcome

11. Mapper
public static class TokenizerMapper extends Mapper<Object, BSONObject, Text, IntWritable>{
private final static IntWritable one = new IntWritable(1);
private Text word = new Text();
public void map(Object key, BSONObject value, Context context )
throws IOException, InterruptedException {
StringTokenizer itr = new StringTokenizer(value.get( "x" ).toString());
while (itr.hasMoreTokens()) {
word.set(itr.nextToken());
context.write(word, one);
}
}
}

12. Reducer
public static class IntSumReducer extends Reducer<Text,IntWritable,Text,IntWritable> {
private IntWritable result = new IntWritable();
public void reduce(Text key, Iterable<IntWritable> values, Context context )
throws IOException, InterruptedException {
int sum = 0;
for (IntWritable val : values) {
sum += val.get();
}
result.set(sum);
context.write(key, result);
}
}

13. Running
Configuration conf = new Configuration();
MongoConfig.setInput( conf , "localhost" , "test" , "in" );
MongoConfig.setOutput( conf , "localhost" , "test" , "out" );
Job job = new Job(conf, "word count");
job.setJarByClass(WordCount.class);
job.setMapperClass(TokenizerMapper.class);
job.setCombinerClass(IntSumReducer.class);
job.setReducerClass(IntSumReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
job.setInputFormatClass( MongoInputFormat.class );
job.setOutputFormatClass( MongoOutputFormat.class );
System.exit(job.waitForCompletion(true) ? 0 : 1);

14. Download MongoDB
http://www.mongodb.org
and let us know what you think
@eliothorowitz @mongodb
10gen is hiring!
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