dpkg

Manipulating MongoDB with Java

Initially, i was thinking of writing about why i was using MongoDB and how it was different from other databases. However, you can easily find all of these information on the internet.

To sum up, MongoDB (from “humongous”) is a scalable, high-performance, open source NoSQL database. It is a document-oriented database. This means, data will be stored in the form of documents. Documents are stored within Collections (Table, if you would like to make a reference to a ‘normal’ database). Graphically, here is how data is stored.

Source: johanlouwers.blogspot.com

 Getting started with MongoDB on Windows.

1. First, you will need to download MongoDB for Windows, either 32-bit or 64-bit depending on your respective operating system. You can download the files here.
2. Create a folder in your root folder and name it ‘data’. Create a subfolder within data and name it db. By default, MongoDB will store data there but you can change that by using mongod –dbpath.
3. Extract your downloaded file. Copy it to your root folder. (For e.g C:\Mongo)
4. Open mongod.exe from a CMD window.
5. Open mongo.exe from another CMD window.

You can find a more comprehensive guide for Windows,UNIX or Mac OSX here.

Java & MongoDB.

I personally use Eclipse IDE to code in Java. However, adding drivers should be possible with any other IDE You should add the MongoDB drivers to your new project, under the ‘libraries’ tab. Click on External JARS and browse to the location of the driver. You can download the Java Driver here (Latest version is 2.7.3) and any other drivers can be found on MongoDB’s GitHub.

After you are done with all these,start coding. (I am assuming that MongoDB is already running on your computer)

Import the following. These are self-explanatory.

import java.net.*;
import com.mongodb.*;

Mongo dpkg = new Mongo();
DB db = dpkg.getDB( "dpkg" );
DBCollection coll = db.getCollection("test") ;

Here a new instance of MongoDB is created, called dpkg. We are also choosing the database dpkg. Make sure the database exists. If it does not exist, MongoDB will not create it for you. Finally we are getting the Collection test and assigning it to a variable coll. If test does not exist, it will be automatically created.

Adding documents:

// Adding a document to the Collection "test"
BasicDBObject data = new BasicDBObject();
data.put("X", 1);
data.put("Y", 2);
coll.insert(data);

The above code adds the data {X : 1} and {Y : 2} in a document to the collection test. Here is a screenshot of the document in the collection.

Displaying all documents in a collection:

//Displaying all the documents in the Collection "test"
DBCursor curs = coll.find();
while(curs.hasNext()) {
DBObject a = curs.next();
System.out.println(a);
}

The above code will print all the documents in the Collection test.

Searching for a particular document:

Additionally, you can specify criteria and retrieve documents matching those criteria only.  To do that, let us add a new document within our collection test.

BasicDBObject data1 = new BasicDBObject();
data1.put("X", 3);
data1.put("Y", 2);
coll.insert(data1);

X now has a value of 3 in this document. To retrieve the document(s) where X = 3 , do the following.

BasicDBObject query = new BasicDBObject("X", 3);
DBCursor curs1 = coll.find(query);
while(curs1.hasNext()) {
DBObject b = curs1.next();
System.out.println(b);
}

This will retrieve the document where the criteria are matched. The screenshot shows this.

You can also specify which fields you want to display.

BasicDBObject query1 = new BasicDBObject("X", 3);

BasicDBObject fields = new BasicDBObject("X",true).append("_id",false) ;

DBCursor curs2 = coll.find(query1,fields);
while(curs2.hasNext()) {
DBObject c = curs2.next();
System.out.println(c);
}

query1 is the condition. All documents where X = 3 will be displayed. fields is the field that you want to display.  In this case, we want to display X only. You MUST explicitly declare “_id” to false  to remove the fields “_id” and “$oid” from the document. The output is shown below.

You can also retrieve only the value associated with X. To illustrate this example, consider the following database.

If i want to retrieve the first record, i.e, Mozammil, my code would look like this.

BasicDBObject query = new BasicDBObject("StudentID", "1010101" );
DBCursor curs = coll.find(query);
while(curs.hasNext()) {
DBObject o = curs.next();
System.out.println(o.get("Firstname"));
}

Notice the o.get(“Firstname”). This will only print Mozammil to the console.

Deleting documents:

To delete a document, you can simply use, collection.remove(query).

coll.remove(new BasicDBObject());

The above code will delete everything in the collection test. No query means it will delete all documents.

You can also specify criteria to delete particular documents. For e.g, if you want to delete all documents where X = 3. You can do this.

BasicDBObject query2 = new BasicDBObject("X", 3);
DBCursor curs3 = coll.find(query);
while(curs3.hasNext()) {
DBObject b = curs3.next();
coll.remove(b) ;
}

Eventually, all documents where X =3 will be removed.

Finally, make sure you use try & catch to cater for any errors that might occur during manipulating the database. Here is the full code.

import java.net.*;
import com.mongodb.*;

public class MongoManip {

public static void main(String[] args) {

try {

Mongo dpkg = new Mongo();
DB db = dpkg.getDB( "dpkg" );
DBCollection coll = db.getCollection("test") ;

// Adding data to the Collection "test"
BasicDBObject data = new BasicDBObject();
data.put("X", 1);
data.put("Y", 2);

coll.insert(data);

BasicDBObject data1 = new BasicDBObject();
data1.put("X", 3);
data1.put("Y", 2);
coll.insert(data1);

//Displaying all the documents in the Collection "test"
DBCursor curs = coll.find();
while(curs.hasNext()) {
DBObject a = curs.next();
System.out.println(a);
}

//Displaying documents based on a certain condition (Searching)
BasicDBObject query = new BasicDBObject("X", 3);
DBCursor curs1 = coll.find(query);
while(curs1.hasNext()) {
DBObject b = curs1.next();
System.out.println(b);
}

//Displaying documents (& specific fields) based on a certain condition
BasicDBObject query1 = new BasicDBObject("X", 3);
BasicDBObject fields = new BasicDBObject("X",true).append("_id",false) ;
DBCursor curs2 = coll.find(query1,fields);
while(curs2.hasNext()) {
DBObject c = curs2.next();
System.out.println(c);
}

//Deleting every documents in the Collection "test"
coll.remove(new BasicDBObject());

//Deleting documents based on a certain condition.
BasicDBObject query2 = new BasicDBObject("X", 3);
DBCursor curs3 = coll.find(query);
while(curs3.hasNext()) {
DBObject b = curs3.next();
coll.remove(b) ;
}

} catch (UnknownHostException e) {
e.printStackTrace();
} catch (MongoException e) {
e.printStackTrace();
}

}

}

This is only a beginner tutorial to show the basic manipulations of MongoDB with Java. In my next blog post, i will focus more on Advanced Querying with MongoDB.

Stay tuned.

10. March 2012 by Mozammil
Categories: Programming | Tags: eclipse, java, mongodb | Leave a comment