REST APIs with Java, Spring Boot and MongoDB

Maxime Beugnet


If you want to write REST APIs in Java at the speed of light, I have what you need. I wrote this template to get you started. I have tried to solve as many problems as possible in it.

So if you want to start writing REST APIs in Java, clone this project, and you will be up to speed in no time.

git clone

That’s all folks! All you need is in this repository. Below I will explain a few of the features and details about this template, but feel free to skip what is not necessary for your understanding.


All the extra information and commands you need to get this project going are in the file which you can read here.

Spring & MongoDB Configuration

The configuration can be found in the SpringConfiguration class.

package com.mongodb.starter;

import [...]

import static org.bson.codecs.configuration.CodecRegistries.fromProviders;
import static org.bson.codecs.configuration.CodecRegistries.fromRegistries;

public class SpringConfiguration {

    private String connectionString;

    public MongoClient mongoClient() {
        CodecRegistry pojoCodecRegistry = fromProviders(PojoCodecProvider.builder().automatic(true).build());
        CodecRegistry codecRegistry = fromRegistries(MongoClientSettings.getDefaultCodecRegistry(), pojoCodecRegistry);
        return MongoClients.create(MongoClientSettings.builder()
                                                      .applyConnectionString(new ConnectionString(connectionString))


The important section here is the MongoDB configuration of course. Firstly, you will notice the connection string is automatically retrieved from the file, and secondly, you will notice the configuration of the MongoClient bean.

A Codec is the interface that abstracts the processes of decoding a BSON value into a Java object and encoding a Java object into a BSON value.

A CodecRegistry contains a set of Codec instances that are accessed according to the Java classes that they encode from and decode to.

Like I said earlier, the MongoDB driver is capable of encoding and decoding BSON for us so we do not have to take care of this anymore. All the configuration we need for this project to run is here and nowhere else.

You can read the driver documentation if you want to know more about this topic.

Multi-Document ACID Transactions

Just for the sake of it, I also used multi-document ACID transactions in a few methods where it could potentially make sense to use ACID transactions. You can check all the code in the MongoDBPersonRepository class.

Here is an example:

private static final TransactionOptions txnOptions = TransactionOptions.builder()

    public List<Person> saveAll(List<Person> persons) {
        try (ClientSession clientSession = client.startSession()) {
            return clientSession.withTransaction(() -> {
                persons.forEach(p -> p.setId(new ObjectId()));
                personCollection.insertMany(clientSession, persons);
                return persons;
            }, txnOptions);

As you can see, I’m using an auto-closeable try-with-resources which will automatically close the client session at the end. This helps me to keep the code clean and simple.

Some of you may argue that it is actually too simple because transactions (and write operations in general) can throw exceptions and I’m not handling any of them here… You are absolutely right and this is an excellent transition to the next part of this article.

Exception Management

Transactions in MongoDB can raise exceptions for various reasons, and I don’t want to go into the details too much here but since MongoDB 3.6, any write operation that fails can be automatically retried once; and the transactions are no different. See the documentation for retryWrites.

If retryable writes are disabled or if a write operation failed twice then MongoDB will send a MongoException (extends RuntimeException) which should be handled properly.

Luckily, Spring provides the annotation ExceptionHandler to help us do that. See the code in my controller PersonController. Of course you will need to adapt and enhance this in your real project, but you have the main idea here.

    public final ResponseEntity<Exception> handleAllExceptions(RuntimeException e) {
        logger.error("Internal server error.", e);
        return new ResponseEntity<>(e, HttpStatus.INTERNAL_SERVER_ERROR);

Aggregation Pipeline

MongoDB's aggregation pipeline is a very powerful and efficient way to run your complex queries as close as possible to your data for maximum efficiency. Using it can relieve the compute load on your application.

Just to give you a small example, I implemented the /api/persons/averageAge route to show you how I can retrieve the average age of the persons in my collection.

    public double getAverageAge() {
        List<Bson> pipeline = asList(group(new BsonNull(), avg("averageAge", "$age")), project(excludeId()));
        return personCollection.aggregate(pipeline, AverageAgeDTO.class).first().getAverageAge();

Also, you can note here that I’m using the personCollection which was initially instantiated like this:

MongoCollection<Person> personCollection = db.getCollection("persons", Person.class);

Normally my personCollection should encode and decode Person object only, but you can overwrite the type of object your collection is manipulating to return something different. In my case AverageAgeDTO.class as I’m not expecting a Person class here but a POJO that contains only the average age of my people.


Swagger is the tool you need to document your REST APIs and I have already configured it for you. You have nothing to do. Just run the server and navigate to http://localhost:8080/swagger-ui.html, the interface will be waiting for you.

The Swagger UI

You can test the REST APIs from this web page and explore the models. Don’t forget to disable it in production ;-).

Nyan Cat

Yes, there is a Nyan Cat section in this blog post. Nyan Cat is love, and you need some Nyan Cat in your projects :-). Did you know that you can replace the Spring boot logo in the logs by pretty much anything you want?

Nyan Cat

Well now you know. You are welcome. Have a look at the banner.txt file if you want to replace this awesome Nyan Cat with your own custom logo.

In a previous company in my employment history, I was using this website and the “Epic” font for each project name. It was easier to identify which log file I was currently reading.


I hope you like my template, and I hope I will help you be more productive with MongoDB and the Java stack.

If you see something which can be improved, please feel free to open a Github issue or submit directly a pull request, they are very welcome :-).

If you read that far - first of all thank you - and secondly here is a $200 coupon code you can use in MongoDB Atlas to give this project a try in production with a dedicated MongoDB 4.2 cluster: MAXIME200.

If you are new to MongoDB Atlas, give this Quick Start Blog Post a try to get up to speed with MongoDB Atlas in no time.