Maven SpringBoot App Demo

Everything so far used small demo projects. Real Java backend work today mostly means Spring Boot, and Spring Boot projects are Maven projects too. In this lesson we build a small Spring Boot web application that greets you based on the time of day, and pay attention to where every file goes in the Maven structure. If you do not know Spring yet, that is fine: follow the Maven side and treat the Java code as given.

Step 1: generate the project with Spring Initializr

Open start.spring.io, the official Spring Initializr. IDEs such as IntelliJ IDEA and Spring Tool Suite have the same wizard built in, but the website works for everyone.

The Initializr settings used in the demo.

Click Generate. A ZIP downloads. Spring Web adds spring-boot-starter-web, which brings Spring MVC and an embedded Apache Tomcat server. JUnit and Spring Core come along automatically through the starters, so you do not add them yourself.

Step 2: import it into Eclipse

Extract the ZIP into your Eclipse workspace folder. Then in Eclipse choose File > Import... > Maven > Existing Maven Projects, click Next, browse to the extracted SpringBootMavenApp folder and click Finish. Eclipse reads pom.xml and downloads every dependency. The first import can take a few minutes.

Open Maven Dependencies: there are dozens of JARs (Spring, Jackson, Tomcat, logging) even though the POM lists only two starters. Those are transitive dependencies, managed for you.

Open pom.xml and notice three Spring Boot specific things:

<parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-parent</artifactId>
  <version>3.4.8</version>
</parent>

<dependencies>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
  </dependency>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
    <scope>test</scope>
  </dependency>
</dependencies>

<build>
  <plugins>
    <plugin>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-maven-plugin</artifactId>
    </plugin>
  </plugins>
</build>

The project also contains mvnw and mvnw.cmd: the Maven Wrapper. With it, anyone can build the project even without Maven installed, using exactly the Maven version the project expects.

Step 3: add the code

All Java code goes under src/main/java. The demo uses two extra packages next to the main class.

package com.telusko.SpringBootMavenApp.service;

public interface IGreetingService {
    String generateGreeting();
}

package com.telusko.SpringBootMavenApp.service;

import java.time.LocalTime;

import org.springframework.stereotype.Service;

@Service
public class GreetingService implements IGreetingService {

    @Override
    public String generateGreeting() {
        LocalTime time = LocalTime.now();
        int hour = time.getHour();
        if (hour < 12)
            return "Good Morning";
        else if (hour < 16)
            return "Good Afternoon";
        else if (hour < 20)
            return "Good Evening";
        else
            return "Good Night";
    }
}

package com.telusko.SpringBootMavenApp.web;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;

import com.telusko.SpringBootMavenApp.service.IGreetingService;

@Controller
public class TeluskoController {

    @Autowired
    private IGreetingService service;

    @GetMapping("/greeting")
    public String generateWish(Model model) {
        String res = service.generateGreeting();
        model.addAttribute("wish", res);
        return "greet";   // name of the view: greet.jsp
    }
}

The controller handles GET /greeting, asks the service for a greeting, stores it as wish, and returns the view name greet.

Step 4: add the web page

Web pages for a JSP based app go in src/main/webapp. A WAR archetype creates that folder for you. A Spring Boot JAR project does not, so create it: right click src/main, New > Folder, name it webapp. Then add greet.jsp:

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Telusko Learning</title>
</head>
<body>
  <h2 style="font-weight: bold; color: darkblue; font-family: sans-serif;">Telusko Learning</h2>
  <h1 style="color: #2c3e50; font-size: 36px; font-family: Arial, sans-serif;">Hey! ${wish}</h1>
</body>
</html>

${wish} prints the value the controller put in the model.

Step 5: configure application.properties

Configuration files go in src/main/resources. Open application.properties:

spring.application.name=SpringBootMavenApp
server.port=8484
spring.mvc.view.prefix=/
spring.mvc.view.suffix=.jsp

Step 6: run it, and fix the first problem

Right click SpringBootMavenAppApplication.java and choose Run As > Java Application. The console shows Tomcat starting on port 8484. Open http://localhost:8484/greeting.

Instead of a page, the browser downloads a file. Embedded Tomcat does not include the JSP compiler (Jasper) by default, so it serves the JSP as a plain file. Stop the app and add the JSP engine to pom.xml:

<dependency>
  <groupId>org.apache.tomcat.embed</groupId>
  <artifactId>tomcat-embed-jasper</artifactId>
</dependency>

Save, run Maven > Update Project..., start the application again and refresh the browser:

The finished app. The greeting changes with the time of day.

Running it with Maven instead of the IDE

Run As > Java Application compiles and starts the app inside Eclipse. The same thing from a terminal, using the wrapper in the project folder:

mvnw.cmd clean package
mvnw.cmd spring-boot:run

./mvnw clean package
./mvnw spring-boot:run

clean package compiles, runs the tests and produces target/SpringBootMavenApp-0.0.1-SNAPSHOT.jar, an executable JAR you could start with java -jar. spring-boot:run starts the app directly from the project folder.

Try It Yourself

1. Generate the project from start.spring.io with Maven and Spring Web, import it, and run it.
2. Reproduce the "file downloads instead of page" problem, then fix it with tomcat-embed-jasper.
3. Change server.port to 9090, restart, and open the new URL.
4. Run ./mvnw clean package (or mvnw.cmd) and look at the size of the JAR in target. Run jar tf on it and find BOOT-INF/lib.
5. Run mvn dependency:tree and find which starter brought in tomcat-embed-core.

Official references

• Spring Initializr
• Spring Boot Maven Plugin documentation
• Spring Boot reference: Servlet web applications and JSP limitations
• Apache Maven: Maven Wrapper

Written By: Shramik Masti