33

So i wrote a small application, In order to get familiar with basics i made it as simple as possible. I made a simple mvc application with Config.java file and when i thought that now the application should throw an error it actually works.

Here's my pom.xml

<?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>2.0.3.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </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</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
         <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-thymeleaf</artifactId>
        </dependency> 
        <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

My Config file which only has a view resolver:

package com.example.demo;

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.ViewResolver;

import org.springframework.web.servlet.view.InternalResourceViewResolver;
import org.springframework.web.servlet.view.JstlView;


@Configuration
public class DemoConfig {

    @Bean
    public ViewResolver internalResourceViewResolver() {
        InternalResourceViewResolver bean = new InternalResourceViewResolver();
        bean.setViewClass(JstlView.class);
        bean.setPrefix("/templates/");
        bean.setSuffix(".html");
        return bean;
    }
}

Main file

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);
    }
}

And finally the controller class : package com.example.demo.controller;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;

@Controller
public class TestController {
    @GetMapping(value="home")
    public String home() {
        return "home";
    }
}

Application.properties

server.servlet.context-path=/demo

So this is the entire application , as i can recall i require mvc:annotation- driven in web.xml or @enablewebmvc for getting @getmapping and @controller to work but my application works completely . How is it not throwing an error ?

davidxxx
  • 125,838
  • 23
  • 214
  • 215
Gagan Singh
  • 503
  • 1
  • 4
  • 12

3 Answers3

49

@SpringBootApplication is a convenience annotation that adds all of the following:

  • @Configuration tags the class as a source of bean definitions for the application context.
  • @EnableAutoConfiguration tells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings.
  • Normally you would add @EnableWebMvc for a Spring MVC app, but Spring Boot adds it automatically when it sees spring-webmvc on the classpath. This flags the application as a web application and activates key behaviors such as setting up a DispatcherServlet.
  • @ComponentScan tells Spring to look for other components, configurations, and services in the hello package, allowing it to find the controllers.
Gaurav Srivastav
  • 2,381
  • 1
  • 15
  • 18
  • 21
    It is important to remember that 3rd point. Do not annotate any `@Configuration` classes with `@EnableWebMvc`. Otherwise, Spring MVC will load and use its own serialization/deserialization configuration, ignoring your Spring Boot config. – Jack Straw Mar 21 '19 at 23:19
  • 2
    If you don't use @EnableWebMvc annotation you might not initially notice any difference but things like content-type and accept header, generally content negotiation won't work. – Bhargav Patel Jun 26 '19 at 04:09
  • ~5 hours to understand that also if the web application was working, I didn't had `spring-autoconfigure-web` in the pom.xml, so no @EnableWebMvc, and so just the MultipartFile wasn't working of the entire web related stuffs, thank you autoconfiguration to implement this easy configuration hard debugging workflow. – rascio Mar 29 '21 at 15:55
7

The behavior you get : "all is working" is expected with Spring Boot.
Spring Boot is not Spring : this goes further than Spring.
Indeed, Spring Boot reduces as much as possible the required configuration to allow your application to work.
The @SpringBootApplication annotation that was introduce to make your application a Spring powered application is a good example.
Besides, Spring Boot proposes some starters to package dependencies but also Spring configurations.

In your case, as you declared spring-boot-starter-web as a dependency, the Spring MVC configuration and other things related to web applications with Spring are set.
The documentation states indeed :

11.3.2 The @EnableAutoConfiguration Annotation

Since spring-boot-starter-web added Tomcat and Spring MVC, the auto-configuration assumes that you are developing a web application and sets up Spring accordingly.

davidxxx
  • 125,838
  • 23
  • 214
  • 215
0

Because your using boot application , @SpringBootApplication this annotation by default enable annotation driven application ( mvc:annotation- driven). You do not need to provide the configuration. Read about @SpringBootApplication https://docs.spring.io/spring-boot/docs/current/reference/html/using-boot-using-springbootapplication-annotation.html.

Ashwani Tiwari
  • 1,497
  • 18
  • 28