In this post we will have a look at some of the basics of using Spring Data and MongoDB. Firstly what is MongoDB? It is a NoSQL database that uses JSON-like documents which allows fields to vary between documents and have the data structure change over time. A document model maps to an object that is defined within our code (we will be using Spring Data to do this). That is only a very short and minimalist view of what MongoDB is but for the scope of this post it should be enough information as we will focus on implementation rather than it’s definition.

What is Spring Data? It is a project that encompasses many sub-projects that are implemented to work with specific databases. As mentioned earlier we will be using MongoDB and therefore will want to use the MongoDB specific sub-project.

In this post we will be using Spring Boot to get everything up and running nice and quickly and therefore we will require the following dependency (assuming you are using Maven) which contains the spring-boot-starter and spring-data-mongodb dependencies among other things.

If you do not want to use Spring Boot or spring-boot-starter-data-mongodb then you will require the spring-data-mongodb dependency at the very least.

The full pom.xml file can be found here and have a look at Spring Boot’s guide on setting up a Spring Boot application from scratch.

A little bit more configuration is required if you haven’t used MongoDB before as you will need to setup a local database, which after following the instructions on the MongoDB website did not take long to do and was pretty straight forward. Follow this link for more information.

Now that all the configuration is complete we can actually begin implementing a very basic Spring application so we can focus on how to use Spring Data with MongoDB.

The first thing we will need is a object that will represent some data that we want to keep inside a database. This required very little code and I was very surprised how few annotations were actually required to get this working. Also I used Lombok in this class to save me writing some extra code, see my previous post Being lazy with Lombok for more information.

The only actual Spring Data annotation in this class is the @Id annotation that represents the unique Id of the object which maps to _id and is generated when it is persisted to the database. The annotation can also be left off if the field is named id or _id, therefore in the example above the annotation is not necessary. If the annotation or a correctly named property is not included when persisted an _id field will be created when saved as MongoDB requires the field to be populated. The other properties are left without annotations and when persisting or saving to the database it is assumed that they will map to fields that share the same name within the database.

The next step is creating a repository that will perform all the database operations to do with the Person object. Spring Data injects some magic into this step leaving barely anything for us to actually do.

The interface extends the MongoRepository which provides plenty of CRUD methods and a few extras to get us going quickly. On this interface only one actual definition has been added but obviously more could be added when required. So where does this magic that I mentioned earlier come in? Well normally we would need to create a implementation of the interface that was just created but instead Spring will create this for us when the application is started. Ok, but what about the method that we just defined on the interface surely that needs to know what it’s doing? By using the name of the definition Spring infers the implementation, therefore findBySalary will find Person objects stored in the database by salary, getBySalary could also be used. To execute these queries Spring Data uses the MongoTemplate.

Now to tie all the code together and to show it in action we need to create the main application that has the @SpringBootApplication annotation. In this example the class has implemented CommandLineRunner as a convenient way to quickly show the output of some of the repositories methods.

There isn’t actually very much to explain here as it is really just a way to show some of the repository methods in action. But one important thing to notice is the @EnableMongoRepositories annotation, which is required as the PersonRepository is not found in the same package or sub-package as the @SpringBootApplication class. Therefore the annotation is necessary to specify that the repository should be injected into the application. If the repository was moved into the same package as the application then the basePackageClasses property could be removed from the @EnableMongoRepositories annotation or it could even be taken out all together.

When the application is ran the following output is produced (I ignored the id and dateOfBirth properties as they were long and messy).

Find all
Person(firstName=John, secondName=Doe, profession=Winner, salary=100)
Person(firstName=Joe, secondName=Blogs, profession=Loser, salary=50)
Find by getBySalary
Person(firstName=John, secondName=Doe, profession=Winner, salary=100)
Making John a loser
Find all
Person(firstName=John, secondName=Doe, profession=Loser, salary=100)
Person(firstName=Joe, secondName=Blogs, profession=Loser, salary=50)

Which leaves the following data persisted in the database.

{
 "_id" : ObjectId("592071e667c5852be43d20e8"),
 "_class" : "lankydan.tutorial.mongodb.entities.Person",
 "firstName" : "John",
 "secondName" : "Doe",
 "dateOfBirth" : ISODate("2017-05-20T17:42:14.629+01:00"),
 "profession" : "Loser",
 "salary" : 100
}

{
 "_id" : ObjectId("592071e667c5852be43d20e9"),
 "_class" : "lankydan.tutorial.mongodb.entities.Person",
 "firstName" : "Joe",
 "secondName" : "Blogs",
 "dateOfBirth" : ISODate("2017-05-20T17:42:14.632+01:00"),
 "profession" : "Loser",
 "salary" : 50
}

So there we have it, a quick tutorial on using Spring Data and MongoDB which allows us to easily implement some basic functionality that can perform CRUD operations without really writing any proper code.

If you want to play around with the code it can be found on my GitHub.