Create a new Maven Project and we have two class under the package com.example.demo like below screen shot.

This page mainly focus on how to make the change you make ready to show the result without restarting spring boot application with Maven.

package com.example.demo;

import org.springframework.boot.SpringApplication;
 
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication {

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

package com.example.demo;

import org.springframework.web.bind.annotation.RequestMapping;
 
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HelloController {
    @RequestMapping(value = "hi" , method = RequestMethod.GET)
    public String sayHi() {
        return "Hello World!";
    }

}
 

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>
    <artifactId>demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>demo</name>
    <description>Demo project for Spring Boot</description>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.5.6.RELEASE</version>
        <relativePath/>
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.8</java.version>
    </properties>

    <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>


</project>

Then,run the DemoApplication as Java Application or Spring Boot App (if you are using STS),after the application starts up,input the link localhost:8080/hi ,you will get the Hello World! as the output.

But when you want to change the RequestMapping from "hi" to "hello" ,to make it ready to use you need to change it and restart the application.

with spring-boot-devtools we just change the code and save it without restart the application manually,spring boot help us restart the application .

To achieve this , all you need to do are adding the spring-boot-devtools dependency in the pom.xml and make sure Project>build automatically is chosen in the IDE

<dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-devtools</artifactId>
        <optional>true</optional>
    </dependency>

On the condition that you add the dependency in the pom.xml file and you don‘t choose the build automatically within the IDE(Eclipse or STS here),then when you change the the RequestMapping from "hi" to "hello" and save it,spring boot will not restart the application for you ,you have to bulid the pom file and restart the application manualy.Because the spring-boot-devtools monitor the code under the target to restart the application to make the change you made ready to use ,so you choose the build atuomatically and change RequestMapping and save it ,the IDE build and the code under the target is updated to trigger the restart of the spring boot application automatically.

For more details,please refer the spring boot devtools reference guide : http://docs.spring.io/spring-boot/docs/current/reference/html/using-boot-devtools.html

spring-boot-devtools