Kotlin is a powerful language, and its power increases, even more, when the Spring Framework is used with it. Up until this point, you've learned the concepts of reactive programming and how to apply these concepts to Kotlin. So far, we developed and wrote code that interacts with the console, but that's not what we will do while developing professional apps. We will either build apps that will run on mobile devices or we will build web applications or REST APIs. At least those are the most commonly built professional software solutions.

So, how to build them? How to create RESTful web APIs and Android apps? Let's discover. The last three chapters of this book are dedicated to building REST APIs and Android apps and, most importantly, making them reactive. Spring is such a vast topic that covering it in a single chapter is simply not possible, so we will have two chapters on Spring.

This chapter will start by introducing you to Spring, and, by the end of this chapter, you should be proficient enough to write REST APIs in Kotlin with Spring. We will not add reactive features in this chapter because we don't want to distract you from the concepts and ideas of Spring. We want you to grasp the concepts and knowledge of Spring itself well enough before moving ahead with making them reactive.

In this chapter, we will cover the following topics:

• Introduction to Spring, history, and origin of Spring
• Spring IoC and dependency injection
• Aspect-oriented programming in Spring
• Introduction to Spring Boot
• Building REST APIs with Spring Boot

So, what are we waiting for? Let's get started and get familiar with Spring.