Sunday 30 October 2022

What is Spring? How to Create your first Spring application?

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Spring is an application framework for the Java platform. It helps you build modern web applications with support for microservice architecture, cloud systems, reactive processing, and serverless workloads.

This tutorial describes how to create and run a Springapplication in IntelliJ IDEA. This will be a Spring Boot Maven project generated by Spring InitializrThis is the quickest way to create a Spring application and IntelliJ IDEA provides a dedicated project wizard for it. You will learn how to expose an HTTP endpoint and map it to a method that returns a greeting to the user when accessed through a web browser.

Create a new Spring Boot project

1. From the main menu, select File | New | Project.
2. In the left pane of the New Project wizard, select Spring Initializr.
3. Specify a name for the project: spring-boot-tutorial.
4. From the JDK list, select Download JDK and download the latest version of Oracle 5. OpenJDK.
6. Select the latest Java version.

Spring Initializr in the New Project wizard

7. Click Next to continue.

8. Select Spring Web Dependency under Web. This dependency is required for any web application that uses Spring MVC.

Spring Dependencies in the New Project wizard

9. Click Create to generate and set up the project.

Add a method that sends a greeting

Spring Initializr creates a class with the main() method to bootstrap your Spring application. In this tutorial, we'll add the sayHello() method directly to this class.

Open the SpringBootTutorialApplication.java file under src/main/java/com/example/springboottutorial.

IntelliJ IDEA provides the Go to File action to quickly find and open files. From the main menu, select Navigate | File or press ⇧ ⌘ O, start typing the name of the file and select it from the list.

Using Go To File to open SpringBootTutorialApplication.java

Add the sayHello() method with all of the necessary annotations and imports so that the file looks like this:

package com.example.springboottutorial;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
@RestController
public class SpringBootTutorialApplication {

public static void main(String[] args) {
SpringApplication.run(SpringBootTutorialApplication.class, args);
}

@GetMapping("/hello")
public String sayHello(@RequestParam(value = "myName", defaultValue = "World") String name) {
return String.format("Hello %s!", name);
}

}

The sayHello() method takes the name parameter and returns the word Hello combined with the parameter value. Everything else is handled by adding Spring annotations:

The @RestController annotation marks the SpringBootTutorialApplication class as a request handler (a REST controller).
The @GetMapping("/hello") annotation maps the sayHello() method to GET requests for /hello.
The @RequestParam annotation maps the name method parameter to the myName web request parameter. If you don't provide the myName parameter in your web request, it will default to World.

Run your Spring application

IntelliJ IDEA creates a Spring Boot run configuration that you can use to run your new Spring application.

If the run configuration is selected, press ⌃ R.
You can also use the icon in the gutter of the SpringBootTutorialApplication.java file next to the class declaration or the main() method declaration.

By default, IntelliJ IDEA shows your running Spring Boot application in the Run tool window.

The Run tool window with a running Spring Boot application

The Console tab shows the output of Spring log messages. By default, the built-in Apache Tomcat server is listening on port 8080. Open your web browser and go to http://localhost:8080/hello. If you did everything right, you should see your application respond with Hello World!.

Spring Boot Hello World response in the browser

This is the default generic response. You can provide a parameter in your web request to let the application know how to greet you properly. For example, try http://localhost:8080/hello?myName=Human.

Add a home page

The created Spring Boot application has one endpoint available at /hello. However, if you open the root context of your application at http://localhost:8080/, you will get an error because there is no root resource defined. Let's add a static HTML home page with links to your endpoint.

Create the index.html file under /src/main/resources/static/.
In the Project tool window, right-click the /src/main/resources/static/ directory, select New | HTML File, specify the name index.html, and press Enter.

Modify the default template or replace it with the following HTML code:

<!DOCTYPE HTML>
<html>
<head>
<title>Your first Spring application</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
</head>
<body>
<p><a href="/hello">Greet the world!</a></p>

<form action="/hello" method="GET" id="nameForm">
<div>
<label for="nameField">How should the app call you?</label>
<input name="myName" id="nameField">
<button>Greet me!</button>
</div>
</form>
</body>
</html>