#mongo java

[nofatclips]

Nicely building a MongoDB query in Java

To avoid the hellish look of queries with infinitely nested DBObjects and strings as operands, the MongoDB Java driver provides an helper class called QueryBuilder which allows to specify the more commonly used search criteria with a simpler API. Here's the same program from the previous post, modified to build the query nicely:

https://gist.github.com/nofatclips/5638337

Better, uh?

[nofatclips]

Running a MongoDB Query from Java

Surely, finding all the documents in a collection is not the most useful thing to do with a database. To specify a search criterion in your query, just pass a DBObject to the find() and count() methods:

https://gist.github.com/nofatclips/5638242

Since Java maps are a little more verbose than JSON documents, the query looks kinda messier than those we write in the Mongo shell. Luckily, the driver has an helper class to ease things out.

[nofatclips]

Reading all Documents from a MongoDB Collection in Java

While the findOne() method returns a DBObject directly, calling the find() method requires a little more work: we need to handle a DBCursor:

https://gist.github.com/nofatclips/5638057

The call to close() is wrapped in the finally block, so that the remote cursor created on the server get closed no matter what happens in the try block.

Apart from that, DBCursor works just like an iterator of DBObjects.

[nofatclips]

Inserting Documents in MongoDB From Java

To insert an object into MongoDB, you create it and then invoke the insert() method on the DBCollection into which it has to be pushed into.

https://gist.github.com/nofatclips/5637783

The output shows that the driver added an ID field upon the call:

{ "site" : "nofatclips.com" , "author" : "DeK"} { "site" : "nofatclips.com" , "author" : "DeK" , "_id" : { "$oid" : "519e4cdb04089e005378ac14"}}

The ID can also be added explicitly, as a new instance of ObjectId:

doc.put("_id", new ObjectId());

[nofatclips]

Representing MongoDB documents in Java

Java is strict about types, so it requires a little bit of work to create the documents to be stored in Mongo. The BasicDBObject is the simpler implementation of the DBObject interface. It's basically an ordered map (it extends LinkedHashMap even though DBObject doesn't extend the Map interface):

BasicDBObject doc = new BasicDBObject();

One inserts fields in the document with the put() method inherited from HashMap:

doc.put("job", "programmer");

or "fluently" using append():

doc.append("this": "and").append("that", "too");

and that's pretty much all there is to it. Ready to insert it into the DB.

[nofatclips]

Java and Mongo and Spark and Freemarker 101

And to finally put the M in MVC, let's do the same but now reading the "world" part of our HTML page from a document stored in a MongoDB collection.

https://gist.github.com/nofatclips/5597919

Since the DBObject implements the Map interface, it can be passed directly to Freemarker.

[nofatclips]

Java and MongoDB 101

Given that we populated the course DB with a hello collection:

> use course switched to db course > db.hello.insert({"name": "MongoDB"}) > db.hello.findOne() { "_id" : ObjectId("51...6f"), "name" : "MongoDB"

Once imported the Java Mongo driver, the following code will print the record we just inserted (or one from the same collection if others were already existing):

https://gist.github.com/nofatclips/5592044