MongoDB on DAO with Java Language

A Quick Tour

Using the Java driver is very simple. First, be sure to include the driver jar mongo.jar in your classpath. The following code snippets come from the examples/QuickTour.java example code found in the driver.

Making A Connection

To make a connection to a MongoDB, you need to have at the minimum, the name of a database to connect to. The database doesn't have to exist - if it doesn't, MongoDB will create it for you.

Additionally, you can specify the server address and port when connecting. The following example shows three ways to connect to the database mydb on the local machine :

import com.mongodb.Mongo;
import com.mongodb.DB;
import com.mongodb.DBCollection;
import com.mongodb.BasicDBObject;
import com.mongodb.DBObject;
import com.mongodb.DBCursor;

Mongo m = new Mongo();
Mongo m = new Mongo( "localhost" );
Mongo m = new Mongo( "localhost" , 27017 );

DB db = m.getDB( "mydb" );

 

At this point, the db object will be a connection to a MongoDB server for the specified database. With it, you can do further operations. 

Getting A List Of Collections

Each database has zero or more collections. You can retrieve a list of them from the db (and print out any that are there) :

Set<String> colls = db.getCollectionNames();

for (String s : colls) {
    System.out.println(s);
}

 

and assuming that there are two collections, name and address, in the database, you would see

name
address

as the output.

Inserting a Document

Once you have the collection object, you can insert documents into the collection. For example, lets make a little document that in JSON would be represented as

{
   "name" : "MongoDB",
   "type" : "database",
   "count" : 1,
   "info" : {
               x : 203,
               y : 102
             }
}

Notice that the above has an "inner" document embedded within it. To do this, we can use the BasicDBObject class to create the document (including the inner document), and then just simply insert it into the collection using the insert() method.

 

BasicDBObject doc = new BasicDBObject();

        doc.put("name", "MongoDB");
        doc.put("type", "database");
        doc.put("count", 1);

        BasicDBObject info = new BasicDBObject();

        info.put("x", 203);
        info.put("y", 102);

        doc.put("info", info);

        coll.insert(doc);

Finding the First Document In A Collection using findOne()

To show that the document we inserted in the previous step is there, we can do a simple findOne() operation to get the first document in the collection. This method returns a single document (rather than the DBCursor that the find() operation returns), and it's useful for things where there only is one document, or you are only interested in the first. You don't have to deal with the cursor.

DBObject myDoc = coll.findOne();
System.out.println(myDoc);

and you should see

{ "_id" : "49902cde5162504500b45c2c" , "name" : "MongoDB" , "type" : "database" , "count" : 1 , "info" : { "x" : 203 , "y" : 102} , "_ns" : "testCollection"}

 

Note the _id and _ns elements have been added automatically by MongoDB to your document. Remember, MongoDB reserves element names that start with _ for internal use.

 

 

Adding Multiple Documents

In order to do more interesting things with queries, let's add multiple simple documents to the collection. These documents will just be

{
   "i" : value
}

and we can do this fairly efficiently in a loop

for (int i=0; i < 100; i++) {
    coll.insert(new BasicDBObject().append("i", i));
}

Notice that we can insert documents of different "shapes" into the same collection. This aspect is what we mean when we say that MongoDB is "schema-free"

Counting Documents in A Collection

Now that we've inserted 101 documents (the 100 we did in the loop, plus the first one), we can check to see if we have them all using the getCount() method.

System.out.println(coll.getCount());

 

and it should print 101.

 

Using a Cursor to Get All the Documents

In order to get all the documents in the collection, we will use the find() method. The find() method returns a DBCursor object which allows us to iterate over the set of documents that matched our query. So to query all of the documents and print them out :

        DBCursor cur = coll.find();

        while(cur.hasNext()) {
            System.out.println(cur.next());
        }

and that should print all 101 documents in the collection.

 

Getting A Single Document with A Query

We can create a query to pass to the find() method to get a subset of the documents in our collection. For example, if we wanted to find the document for which the value of the "i" field is 71, we would do the following ;

        BasicDBObject query = new BasicDBObject();

        query.put("i", 71);

        cur = coll.find(query);

        while(cur.hasNext()) {
            System.out.println(cur.next());
        }

 

and it should just print just one document

{ "_id" : "49903677516250c1008d624e" , "i" : 71 , "_ns" : "testCollection"}

Getting A Set of Documents With a Query

We can use the query to get a set of documents from our collection. For example, if we wanted to get all documents where "i" > 50, we could write :

        query = new BasicDBObject();

        query.put("i", new BasicDBObject("$gt", 50));  // e.g. find all where i > 50

        cur = coll.find(query);

        while(cur.hasNext()) {
            System.out.println(cur.next());
        }

 

which should print the documents where i > 50. We could also get a range, say 20 < i <= 30 :

        query = new BasicDBObject();

        query.put("i", new BasicDBObject("$gt", 20).append("$lte", 30));  // i.e.   20 < i <= 30

        cur = coll.find(query);

        while(cur.hasNext()) {
            System.out.println(cur.next());
        }

 

Creating An Index

MongoDB supports indexes, and they are very easy to add on a collection. To create an index, you just specify the field that should be indexed, and specify if you want the index to be ascending (1) or descending (-1). The following creates an ascending index on the "i" field :

coll.createIndex(new BasicDBObject("i", 1));  // create index on "i", ascending

 

Getting a List of Indexes on a Collection

You can get a list of the indexes on a collection :

        List<DBObject> list = coll.getIndexInfo();

        for (DBObject o : list) {
            System.out.println(o);
        }

 

and you should see something like

{ "name" : "i_1" , "ns" : "mydb.testCollection" , "key" : { "i" : 1} , "_ns" : "system.indexes"}

 

Quick Tour of the Administrative Functions

Getting A List of Databases

You can get a list of the available databases:

        Mongo m = new Mongo();

        for (String s : m.getDatabaseNames()) {
            System.out.println(s);
        }

Dropping A Database

You can drop a database by name using the Mongo object:

m.dropDatabase("my_new_db");