Are template engines (i.e. Thymeleaf) necessary for Spring Boot applications? - spring-boot

Question

I am currently working on a Spring Boot project and I am fairly new to template engines. This will also be my first own private project with Spring Boot.
I would like to know whether it is necessary to include a template engine, such as Thymeleaf, while developing a web application with Spring Boot. I'm using PostegreSQL for the database.
I read under another post that a template engine is not needed, if the backend framework uses JSON for data exchange, because template engines are for rendering retrieved data to HTML for the client. I retrieve JSON objects from the database, can I leave template engines out of my project then?
If any more details are needed, leave a comment below.

Answer 1

No they aren't necessary, in fact most new projects that require web-pages are using single page applications now like Angular, React, Vue, ... over thymeleaf or jsp.
Aside from that a Spring project doesn't always need web pages, for instance when you are just creating a REST API for other applications to call on, or when you are automating things like: a mail service / printing / ... You name it.
However, when you DO want a simple solution with some pages that aren't all that dynamic or complex, pivotal / VMware does recommend to use thymeleaf (over jsp and other solutions) because it integrates easily.

Answer 2

I read under another post that a template engine is not needed, if the backend framework uses JSON for data exchange, because template engines are for rendering retrieved data to HTML for the client. I retrieve JSON objects from the database, can I leave template engines out of my project then?
This is partly true. Yes, Thymeleaf and alike are mostly intended to render data to HTML. They can render any text data, including JSON, but there are tools better suited for the job. On other hand it does not matter how you store the data in your database or what database you are using. You can't just skip rendering (serializing) the response so it does not matter how you store it. What matters is what you want to return as response. For HTML Thymeleaf or even JSP are suitable, but for JSON you may want to use Jackson or Gson instead.
You didn't mentioned the technology you are going to use, so for my examples I'll assume you intend to use Spring Web MVC. Lets take a look at "traditional" controller:
#Controller
public class GreetingController {
#GetMapping("/greeting")
public ModelAndView greeting(#RequestParam(value = "name", defaultValue = "World") String name) {
return new ModelAndView("greeting", "greeting", new Greeting(name));
}
}
When you make GET request to "/greeting", Spring will call greeting and get the object it returns. In this case it contains the model (the data we want to render) and the view (the template file to use). Then it will try to find a view (something like greeting.html or greeting.jsp) and use template engine like Thymeleaf (or whatever else is configured) to render it (typically to HTML).
What if we want to return JSON instead? In this case we need to:
modify greeting to return Greeting instance instead of ModelAndView
Use RestController instead of Controller. This will tell Spring MVC that we want to directly serialize the object returned to JSON (or similar format) instead of using template to do that.
Here is the modified example:
#RestController
public class GreetingController {
#GetMapping("/greeting")
public Greeting greeting(#RequestParam(value = "name", defaultValue = "World") String name) {
return new Greeting(name);
}
}
Spring MVC still needs some help to serialize the Greeting instance to JSON. But if you use Spring Boot and the web starter, you don't have to worry about it. It will include Jackson for you.
This is in no way exhaustive answer. As a matter of fact it skips a lot of details, but I hope nevertheless it is useful one. If you want to create REST API using JSON, this Building a RESTful Web Service guide is a good place to start. If you follow the Starting with Spring Initializr steps you'll get a project setup with only what is needed (well maybe with a bit extra but I would not worry too much about it).