An Introduction to Spring Boot

This article gives an overview of various Spring configuration styles and helps you understand the complexity of configuring Spring applications.

This article gives an overview of various Spring configuration styles and helps you understand the complexity of configuring Spring applications.

Spring is a very popular Java-based framework for building web and enterprise applications. Unlike many other frameworks, which focus on only one area, Spring framework provides a wide verity of features addressing the modern business needs via its portfolio projects.

Spring framework provides flexibility to configure beans in multiple ways such as XML, Annotations, and JavaConfig. With the number of features increased the complexity also gets increased and configuring Spring applications becomes tedious and error-prone.

The Spring team created Spring Boot to address the complexity of configuration.

But before diving into SpringBoot, we will take a quick look at the Spring framework and see what kind of problems SpringBoot is trying to address.

In this article we will cover:

• Overview of Spring framework
• A web application using Spring MVC and JPA (Hibernate)
• A quick taste of Spring Boot

  Overview of Spring Framework

If you are a Java developer then there is a high chance that you might have heard about Spring framework and probably have used it in your projects. Spring framework was created primarily as a Dependency Injection container but it is much more than that.

Spring is very popular for several reasons:

• Overview of Spring framework
• A web application using Spring MVC and JPA (Hibernate)
• A quick taste of Spring Boot

Along with the Spring framework, there are many other Spring sister projects that help to build applications addressing modern business needs:

• Overview of Spring framework
• A web application using Spring MVC and JPA (Hibernate)
• A quick taste of Spring Boot

There are many other interesting projects addressing various other modern application development needs. For more information, take a look at http://spring.io/projects.

In the initial days, the Spring framework provided an XML-based approach for configuring beans. Later Spring introduced XML-based DSLs, Annotations, and JavaConfig-based approaches for configuring beans.

Let us take a quick look at how each of those configuration styles looks.

XML-Based Configuration

<bean id="userService" class="com.sivalabs.myapp.service.UserService">
    <property name="userDao" ref="userDao"/>
</bean>

<bean id="userDao" class="com.sivalabs.myapp.dao.JdbcUserDao">
    <property name="dataSource" ref="dataSource"/>
</bean>

<bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
    <property name="driverClassName" value="com.mysql.jdbc.Driver"/>
    <property name="url" value="jdbc:mysql://localhost:3306/test"/>
    <property name="username" value="root"/>
    <property name="password" value="secret"/>
</bean>

Annotation-Based Configuration

@Service
public class UserService
{
    private UserDao userDao;

    @Autowired
    public UserService(UserDao dao){
        this.userDao = dao;
    }
    ...
    ...
}
@Repository
public class JdbcUserDao
{
    private DataSource dataSource;

    @Autowired
    public JdbcUserDao(DataSource dataSource){
        this.dataSource = dataSource;
    }
    ...
    ...
}

JavaConfig-Based Configuration

@Configuration
public class AppConfig
{
    @Bean
    public UserService userService(UserDao dao){
        return new UserService(dao);
    }

    @Bean
    public UserDao userDao(DataSource dataSource){
        return new JdbcUserDao(dataSource);
    }

    @Bean
    public DataSource dataSource(){
        BasicDataSource dataSource = new BasicDataSource();
        dataSource.setDriverClassName("com.mysql.jdbc.Driver");
        dataSource.setUrl("jdbc:mysql://localhost:3306/test");
        dataSource.setUsername("root");
        dataSource.setPassword("secret");
        return dataSource;
    }
}

Wow… Spring provides many approaches for doing the same thing, and we can even mix the approaches and use both JavaConfig- and Annotation-based configuration styles in the same application.

That is a lot of flexibility and it is both good and bad. People new to the Spring framework may get confused about which approach to follow. As of now, the Spring team suggests following a JavaConfig-based approach as it gives more flexibility.

But there is no one-size fits all kind of solution. One has to choose the approach based on their own application needs.

OK, now that you had a glimpse of how various styles of Spring bean configurations look.

Let us take a quick look at what the configuration of a typical SpringMVC + JPA/Hibernate web application configuration looks like.

A Web Application Using Spring MVC and JPA (Hibernate)

Before getting to know what Spring Boot is and what kind of features it provides, let us take a look at how a typical Spring web application configuration looks, the pain points, and how Spring Boot addresses those problems.

Step 1: Configure Maven Dependencies

The first thing we need to do is configure all the dependencies required in our 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/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.sivalabs</groupId>
    <artifactId>springmvc-jpa-demo</artifactId>
    <packaging>war</packaging>
    <version>1.0-SNAPSHOT</version>
    <name>springmvc-jpa-demo</name>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>      
        <failOnMissingWebXml>false</failOnMissingWebXml>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webmvc</artifactId>
            <version>4.2.4.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.data</groupId>
            <artifactId>spring-data-jpa</artifactId>
            <version>1.9.2.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>jcl-over-slf4j</artifactId>
            <version>1.7.13</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>1.7.13</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-log4j12</artifactId>
            <version>1.7.13</version>
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
            <version>1.4.190</version>
        </dependency>
        <dependency>
            <groupId>commons-dbcp</groupId>
            <artifactId>commons-dbcp</artifactId>
            <version>1.4</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.38</version>
        </dependency>
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-entitymanager</artifactId>
            <version>4.3.11.Final</version>
        </dependency>
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>3.1.0</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>org.thymeleaf</groupId>
            <artifactId>thymeleaf-spring4</artifactId>
            <version>2.1.4.RELEASE</version>
        </dependency>
    </dependencies>
</project>

We have configured all our Maven jar dependencies to include Spring MVC, Spring Data JPA, JPA/Hibernate, Thymeleaf, and Log4j.

Step 2: Configure Service/DAO Layer Beans Using JavaConfig

@Configuration
@EnableTransactionManagement
@EnableJpaRepositories(basePackages="com.sivalabs.demo")
@PropertySource(value = { "classpath:application.properties" })
public class AppConfig 
{
    @Autowired
    private Environment env;

    @Bean
    public static PropertySourcesPlaceholderConfigurer placeHolderConfigurer()
    {
        return new PropertySourcesPlaceholderConfigurer();
    }

    @Value("${init-db:false}")
    private String initDatabase;

    @Bean
    public PlatformTransactionManager transactionManager()
    {
        EntityManagerFactory factory = entityManagerFactory().getObject();
        return new JpaTransactionManager(factory);
    }

    @Bean
    public LocalContainerEntityManagerFactoryBean entityManagerFactory()
    {
        LocalContainerEntityManagerFactoryBean factory = new LocalContainerEntityManagerFactoryBean();

        HibernateJpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
        vendorAdapter.setGenerateDdl(Boolean.TRUE);
        vendorAdapter.setShowSql(Boolean.TRUE);

        factory.setDataSource(dataSource());
        factory.setJpaVendorAdapter(vendorAdapter);
        factory.setPackagesToScan("com.sivalabs.demo");

        Properties jpaProperties = new Properties();
        jpaProperties.put("hibernate.hbm2ddl.auto", env.getProperty("hibernate.hbm2ddl.auto"));
        factory.setJpaProperties(jpaProperties);

        factory.afterPropertiesSet();
        factory.setLoadTimeWeaver(new InstrumentationLoadTimeWeaver());
        return factory;
    }

    @Bean
    public HibernateExceptionTranslator hibernateExceptionTranslator()
    {
        return new HibernateExceptionTranslator();
    }

    @Bean
    public DataSource dataSource()
    {
        BasicDataSource dataSource = new BasicDataSource();
        dataSource.setDriverClassName(env.getProperty("jdbc.driverClassName"));
        dataSource.setUrl(env.getProperty("jdbc.url"));
        dataSource.setUsername(env.getProperty("jdbc.username"));
        dataSource.setPassword(env.getProperty("jdbc.password"));
        return dataSource;
    }

    @Bean
    public DataSourceInitializer dataSourceInitializer(DataSource dataSource) 
    {
        DataSourceInitializer dataSourceInitializer = new DataSourceInitializer();
        dataSourceInitializer.setDataSource(dataSource);
        ResourceDatabasePopulator databasePopulator = new ResourceDatabasePopulator();
        databasePopulator.addScript(new ClassPathResource("data.sql"));
        dataSourceInitializer.setDatabasePopulator(databasePopulator);
        dataSourceInitializer.setEnabled(Boolean.parseBoolean(initDatabase));
        return dataSourceInitializer;
    }   
}

In our AppConfig.java configuration class we have done the following:

• Overview of Spring framework
• A web application using Spring MVC and JPA (Hibernate)
• A quick taste of Spring Boot

We need to configure property placeholder values in application.properties as follows:

jdbc.driverClassName=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/test
jdbc.username=root
jdbc.password=admin
init-db=true
hibernate.dialect=org.hibernate.dialect.MySQLDialect
hibernate.show_sql=true
hibernate.hbm2ddl.auto=update

We can create a simple SQL script data.sql to populate sample data into USER table:

delete from user;
insert into user(id, name) values(1,'Siva');
insert into user(id, name) values(2,'Prasad');
insert into user(id, name) values(3,'Reddy');

We can create log4j.properties file with basic configuration as follows:

log4j.rootCategory=INFO, stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p %t %c{2}:%L - %m%n

log4j.category.org.springframework=INFO
log4j.category.com.sivalabs=DEBUG

Step 3: Configure Spring MVC Web Layer Beans

We will have to configure Thymeleaf ViewResolver, static ResourceHandlers, MessageSource for i18n etc.

@Configuration
@ComponentScan(basePackages = { "com.sivalabs.demo"}) 
@EnableWebMvc
public class WebMvcConfig extends WebMvcConfigurerAdapter
{
    @Bean
    public TemplateResolver templateResolver() {
        TemplateResolver templateResolver = new ServletContextTemplateResolver();
        templateResolver.setPrefix("/WEB-INF/views/");
        templateResolver.setSuffix(".html");
        templateResolver.setTemplateMode("HTML5");
        templateResolver.setCacheable(false);
        return templateResolver;
    }

    @Bean
    public SpringTemplateEngine templateEngine() {
        SpringTemplateEngine templateEngine = new SpringTemplateEngine();
        templateEngine.setTemplateResolver(templateResolver());
        return templateEngine;
    }

    @Bean
    public ThymeleafViewResolver viewResolver() {
        ThymeleafViewResolver thymeleafViewResolver = new ThymeleafViewResolver();
        thymeleafViewResolver.setTemplateEngine(templateEngine());
        thymeleafViewResolver.setCharacterEncoding("UTF-8");
        return thymeleafViewResolver;
    }

    @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry)
    {
        registry.addResourceHandler("/resources/**").addResourceLocations("/resources/");
    }

    @Override
    public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer)
    {
        configurer.enable();
    }

    @Bean(name = "messageSource")
    public MessageSource configureMessageSource()
    {
        ReloadableResourceBundleMessageSource messageSource = new ReloadableResourceBundleMessageSource();
        messageSource.setBasename("classpath:messages");
        messageSource.setCacheSeconds(5);
        messageSource.setDefaultEncoding("UTF-8");
        return messageSource;
    }
}

In our WebMvcConfig.java configuration class we have done the following:

• Overview of Spring framework
• A web application using Spring MVC and JPA (Hibernate)
• A quick taste of Spring Boot

For now we do not have any messages to be configured, so create an empty messages.properties file in src/main/resources folder.

Step 4: Register Spring MVC FrontController Servlet DispatcherServlet

Prior to Servlet 3.x specification we have to register Servlets/Filters in web.xml. Since Servlet 3.x specification we can register Servlets/Filters programatically using ServletContainerInitializer.

Spring MVC provides a convenient class AbstractAnnotationConfigDispatcherServletInitializer to register DispatcherServlet.

public class SpringWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer
{

    @Override
    protected Class<?>[] getRootConfigClasses()
    {
        return new Class<?>[] { AppConfig.class};
    }

    @Override
    protected Class<?>[] getServletConfigClasses()
    {
        return new Class<?>[] { WebMvcConfig.class };
    }

    @Override
    protected String[] getServletMappings()
    {
        return new String[] { "/" };
    }

    @Override
    protected Filter[] getServletFilters() {
       return new Filter[]{ new OpenEntityManagerInViewFilter() };
    }
}

In our SpringWebAppInitializer.java configuration class we have done the following:

• Overview of Spring framework
• A web application using Spring MVC and JPA (Hibernate)
• A quick taste of Spring Boot

  Step 5: Create a JPA Entity and Spring Data JPA Repository

Create a JPA entity User.java and a Spring Data JPA repository for User entity.

@Entity
public class User
{
    @Id @GeneratedValue(strategy=GenerationType.AUTO)
    private Integer id;
    private String name;

    //setters and getters
}
public interface UserRepository extends JpaRepository<User, Integer>
{
}

Step 6: Create a SpringMVC Controller

Create a SpringMVC controller to handle URL “/” and render a list of users.

@Controller
public class HomeController
{
    @Autowired UserRepository userRepo;

    @RequestMapping("/")
    public String home(Model model)
    {
        model.addAttribute("users", userRepo.findAll());
        return "index";
    }
}

Step 7: Create a Thymeleaf View /WEB-INF/views/index.html to Render List of Users

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml"
      xmlns:th="http://www.thymeleaf.org">
<head>
<meta charset="utf-8"/>
<title>Home</title>
</head>
<body>
    <table>
        <thead>
            <tr>
                <th>Id</th>
                <th>Name</th>
            </tr>
        </thead>
        <tbody>
            <tr th:each="user : ${users}">
                <td th:text="${user.id}">Id</td>
                <td th:text="${user.name}">Name</td>
            </tr>
        </tbody>
    </table>
</body>
</html>

We are all set now to run the application. But before that we need to download and configure the server like Tomcat or Jetty or Wildfly etc in your IDE.

You can download Tomcat 8 and configure in your favorite IDE, run the application and point your browser to http://localhost:8080/springmvcjpa-demo. You should see the list of users details in a table.

Yay…We did it.

But wait…Isn’t it too much work to just show a list of user details pulled from a database table?

Let us be honest and fair. All this configuration is not just for this one use-case. This configuration is the basis for rest of the application also.

But again, this is too much of work to do if you want to quickly get up and running.

Another problem with it is, assume you want to develop another SpringMVC application with a similar technical stack?

Well, you copy-paste the configuration and tweak it. Right? But remember one thing: if you have to do the same thing again and again, you should find an automated way to do it.

Apart from writing the same configuration again and again, do you see any other problems here?

Well, let me list what are the problems I am seeing here.

• Overview of Spring framework
• A web application using Spring MVC and JPA (Hibernate)
• A quick taste of Spring Boot

  If Spring can automatically do it for me that would be awesome!!!. Imagine, what if Spring is capable of configuring beans automatically? What if you can customize the automatic configuration using simple customizable properties?

For example, instead of mapping DispatcherServlet url-pattern to “/” you want to map it to “/app/”. Instead of putting Thymeleaf views in “/WEB-INF/views” folder you may want to place them in “/WEB-INF/templates/” folder.

So basically you want Spring to do things automatically but provide the flexibility to override the default configuration in a simpler way?

Well, you are about to enter into the world of SpringBoot where your dreams come true!!!

A Quick Taste of Spring Boot

Welcome to Spring Boot! Spring Boot does what exactly you are looking for. It will do things automatically for you but allows you to override the defaults if you want to.

Instead of explaining in theory I prefer to explain by example.

So let us implement the same application that we built earlier but this time using SpringBoot.

Step 1: Create a Maven-Based Spring Boot Project

Create a Maven project and configure the dependencies as follows:

<?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/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.sivalabs</groupId>
    <artifactId>hello-springboot</artifactId>
    <packaging>jar</packaging>
    <version>1.0-SNAPSHOT</version>
    <name>hello-springboot</name>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.3.2.RELEASE</version>
    </parent>

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

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-thymeleaf</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>
    </dependencies>
</project>

Wow our pom.xml suddenly become so small!

Step 2: Configure Datasource/JPA Properties in application.properties as Follows

spring.datasource.driver-class-name=com.mysql.jdbc.Driver
spring.datasource.url=jdbc:mysql://localhost:3306/test
spring.datasource.username=root
spring.datasource.password=admin
spring.datasource.initialize=true
spring.jpa.hibernate.ddl-auto=update
spring.jpa.show-sql=true

You can copy the same data.sql file into src/main/resources folder.

Step 3: Create a JPA Entity and Spring Data JPA Repository Interface for the Entity

Create User.java, UserRepository.java and HomeController.java same as in springmvc-jpa-demo application.

Step 4: Create Thymeleaf View to Show List of Users

Copy /WEB-INF/views/index.html that we created in springmvc-jpa-demo application intosrc/-main/resources/templates folder in our new project.

Step 5: Create SpringBoot EntryPoint Class

Create a Java class Application.java with main method as follows:

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

Now run Application.java as a Java Application and point your browser to http://localhost:8080/.

You should see the list of users in table format. Coool!!!

Ok ok, I hear you are shouting “What is going on???”.

Let me explain what just happened.

1. Easy dependency Management

• Overview of Spring framework
• A web application using Spring MVC and JPA (Hibernate)
• A quick taste of Spring Boot

2. Auto Configuration

• Overview of Spring framework
• A web application using Spring MVC and JPA (Hibernate)
• A quick taste of Spring Boot

3. Embedded Servlet Container Support

The most important and surprising thing is we have created a simple Java class annotated with some magical annotation @SpringApplication having a main method and by running that main we are able to run the application and access it at http://localhost:8080/.

Where is the servlet container comes from?

We have added spring-boot-starter-web which pulls the spring-boot-starter-tomcat automatically and when we run the main() method it started tomcat as an embedded container so that we don’t have to deploy our application on any externally installed tomcat server.

By the way have you observe that our packaging type in pom.xml is ‘jar’ not ‘war’. Wonderful!

Ok, but what if I want to use Jetty server instead of tomcat?

Simple, exclude spring-bootstarter-tomcat from spring-boot-starter-web and include spring-boot-starter-jetty.

That’s it.

But, this looks all magical!!!

I can imagine what you are thinking. You are thinking like SpringBoot looks cool and it is doing lot of things automatically for me. But still I am not fully understanding how it is all really working behind the scenes. Right?

I can understand. Watching a magic show is fun normally, but not in Software Development. Don’t worry, we will be looking at each of those things and explain in detail how things are happening behind the scenes in future articles. But I don’t want to overwhelm you by dumping everything onto you right now in this article.

Summary

In this article we had a quick overview of various Spring configuration styles and understand the complexity of configuring Spring applications. Also, we had a quick look at SpringBoot by creating a simple web application.

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