Spring Boot 2 Meets Kotlin

Hello Guys.

Today we will talk about the new feature added in Spring 5.0 and Spring Boot 2. We will understand the Kotlin support for Spring Boot Applications.

Kotlin is a new language created by JetBrains Team. The language is JVM language. It means the language creates a bytecode to run on Java Virtual Machine.

As we can see, the primary inspiration is Scala language. There are many constructions similar in both languages, the data classes concepts for instance.

There are some interesting advantages when we adopt Kotlin to code. The most exciting is reduce boilerplate code and make our code more concise, it brings more maintainability and transforms our code more readable.

We will understand these topics on next examples, then is Time to Code!!!

Create the Tasks Project with Spring Initializr

We will create a simple project to manage Tasks. The main idea here is to explain some Kotlin interesting points on this project.

Let’s go to Spring Initiliazr page. The project should be created with these configurations below:

The interesting points here are:

  • Maven Project
  • Kotlin Language
  • Spring Boot 2
  • Dependencies: Reactive Web, Actuator and Reactive MongoDB

Creating our TaskEntity

We need to create our main entity. The entity is pretty simple and so easy to implement the Task class should be like this:

As we can see, there are some Kotlin interesting points here. There is a data class keyword it means the class has the purpose of holding data. Kotlin will add some important behaviors automatically like:

  • equals() and hashCode()
  • toString()
  • copy()

there are some restrictions the data classes cannot be abstract, open, sealed or inner.

The full data classes documentation can be found here.

Creating the Reactive Task Repository

Now, we will use the Spring Data MongoDB Reactive implementation. The behaviors are similar in the blocking version, but it will not blocking because is the reactive version. The way to thinking is similar there is DSL to use objects properties to create queries automatically.

The TaskRepository should be like this:

The keyword interface is the same in Java language, but the way to extends is Kotlin is slightly different, in Kotlin we will use “:” instead of extends.

Creating the TaskService

Let’s create our TaskService class it will invoke the repository implementations. The code of TaskService should be like this:

There are a couple of interesting things here. Let’s start by injection, there is no necessity to use @Autowired in class constructor since Spring Framework 4 version, as we can see it will work as expected here as well. We use val in favor immutability.

Let’s understand the tasks() function. The function has no body because of the implementation has one line only. In this case, the function return can be omitted as well. It makes the code more concise and easy to understand.

We have used the same features in our other functions.

The full documentation about functions can be found here.

The REST Layer

Our REST Layer should be reactive. Then we need to return Flux or Mono in our methods. We will use the one line function and we can omit these declarations, keep in mind to achieve reactive functionalities we need Flux or Mono in our methods.

The TaskResource class should be like this:

As we can see we are using the same as we did before, we have not used return methods declaration, the compiler can infer it for us, also it prevents developers errors as well.

We are using the @GetMapping and @PostMapping instead of @RequestMapping annotations, it makes our code more readable.

Configuring the MongoDB connections

We will use the yaml file, it makes our configuration file more readable and introduces semantics in our file. The configuration file should be like this:

There is nothing special here, a couple of configurations for mongoDB and tomcat server.

The Final Project Structure

Let’s analyze the final project structure, you can put in your preferred structure. I suggest the following one:


Excellent, now we can run it.

Run it and try your pretty new API using Kotlin Language!!!

Awesome Job, well done!!!

The full source code can be found here.


I recommend docker to run a mongoDB instance it makes your life extremely easy.


You can find more detailed implementations and different Use Case in my book ( Spring 5.o By Example), published by Packt.


Thank you, for next Post, I will write about Spring Cloud Gateway and how it can help developers to work with Routes.


Continuous Query With Spring Data Reactive MongoDB

Continuous Query With Spring Data Reactive MongoDB

On this blog post, we will take a look how to implement Continuous Queries in MongoDB. Also, we will have a dash of Spring WebFlux in Action.

Let’s do it, right now!!!

What is Continuous Query???

Is a kind of active query, when the data were arriving in the database, if this piece of data matches accordingly with our query an event will be emitted to our application, with the piece of data. We can think a kind of Event-Driven Programming using the database as an event trigger.

It is a powerful feature and enable us to add interactive behaviors in our application.

Capped Collections and Tailable Cursors

In MongoDB, there is a feature called Capped Collections. This kind of collection has a fixed size and support high-throughput operations and retrieve the documents based on insert order. We will use this kind of collection to store our data to simulate the “continuous query.”

Also, MongoDB has an exciting feature called Tailable Cursors which we are able to use on Capped Collections. This cursor is similar to

in Unix command system, which means we will retrieve the documents in natural order.

Spring Data Reactive MongoDB

Now we know how Capped Collections and Tailable Cursor works. Then is time to use Spring Data Reactive Mongo DB stuff.

Use Case

We will simulate a kind of data collector for IoT components. The main feature of our project is collect components data and provides an API to show max and min temperatures when the temperature reaches the limits configured for a specific device. Then the Frontend Team can build a fantastic reactive page to display the device information.

Our API will have five principal operations:

  • Create Device
  • Input Temperature
  • Device Temperature Stream
  • Device Min Temperature Stream
  • Device Max Temperature Stream

Let’s start with our primary Entity Temperature, the temperature class should be:

As we can see there is nothing special, we have used the Lombok Annotations to remove boilerplate code.

We have the Temperature class created, then we can create our TemperatureRepository, the interface can be declared like this:

Awesome, there is some interesting stuff here. The first one is the @Tailable annotation, which instructs the Spring Frameworks to await the MongoDB Tailable Cursors. When the data arrives in MongoDb with de data matches with a specific query one event will be emitted. In this case, we will watch events from a specific device. Take a look at our return type we always should use the Flux<T> to make application reactive.

Time to create our service class, the implementation should be like this:

On our method init(), we have declared a capped collection. First, we have removed the collection created and then reactively we have created and configured the temperatures collection. Remember this method will call one time. The others methods are so simple the main idea here is to call the Repository Layer to retrieve the documents. The important part here is the return types of methods, always Flux<T> when we expect the events.

In our RestController we will use the Server-Sent Event (SSE), to add more interactive behaviors for our API. The main idea here is to push data to the client using the Half-Duplex HTTP connection.

Our DeviceResource should be like this:

As we can see take a look at the methods declarations we have used the MediaType.APPLICATION_STREAM_JSON_VALUE. It is much important to add Stream Behavior for our application.

Simple like that we have created a couple of classes and lines, to add a powerful feature like “continuous query” in our application. Spring Data Reactive reactive brings reactive characteristics for our application and make them more resilient and cost-effective.

The Full source code can be found at GitHub. There is a client to add some data and make the test easier, the client will send data to interval pre-configured.

The Spring 5.0 By Example Book

Recently I launched the Spring 5.0 By Example Book. This book contains a lot of Spring Concepts and Examples, using the Microservices Architecture and much codes using the Spring Boot 2.

The key features of the book are:

  • Learn reactive programming by implementing a reactive application with Spring Webflux
  • Create a robust and scalable messaging application with Spring messaging support
  • Apply your knowledge to build three real-world projects in Spring

You can find the book on Amazon.


MongoDb instance up and running. I recommend Docker to spin up a MongoDB container.

The MongoDb should be listening at “localhost”, otherwise you can configure using the application.yaml or application.properties.






See you. BYE.