Creating a simple application

The application that we are going to create is a book catalog management system. It will keep a record of books that were published, who were the authors, the reviewers, publishing houses, and so forth. We will name our project BookPub, and apply the following steps:

Clicking on the Generate Project button will download the bookpub.zip archive, which we will extract in our working directory. In the newly created bookpub directory, we will see a build.gradle file that defines our build. It already comes preconfigured with the right version of a Spring Boot plugin and libraries and even includes the extra starters, which we have chosen.

The following is the code of the build.gradle file:

dependencies {
  compile("org.springframework.boot:spring-boot-starter-data-jpa")
  compile("org.springframework.boot:spring-boot-starter-jdbc")
  runtime("com.h2database:h2")
  testCompile("org.springframework.boot:spring-boot-starter-test") 
}

We have selected the following starters:

As you can see, the runtime("com.h2database:h2") dependency is a runtime one. This is because we don't really need, and probably don't even want, to know the exact kind of a database to which we will connect at the compile time. Spring Boot will autoconfigure the needed settings and create appropriate beans once it detects the presence of the org.h2.Driver class in the classpath when the application is launched. We will look into the inner workings of how and where this happens later in this chapter.

The data-jpa and jdbc are Spring Boot starter artifacts. If we look inside these dependency jars once they are downloaded locally by Gradle, or using Maven Central online file repository, we will find that they don't contain any actual classes, only the various metadata. The two containing files that are of particular interest to us are pom.xml and spring.provides. Let's first look at the spring.provides file in the spring-boot-starter-jdbc.jar artifact, with the following content:

provides: spring-jdbc,spring-tx,tomcat-jdbc

This tells us that by having this starter as our dependency, we will transitively get the spring-jdbc, spring-tx, and tomcat-jdbc dependency libraries in our build. The pom.xml file contains the proper dependency declarations that will be used by Gradle or Maven to resolve the needed dependencies during the build time. This also applies to our second starter: spring-boot-starter-data-jpa. This starter will transitively provide us with the spring-orm, hibernate-entity-manager, and spring-data-jpa libraries.

At this point, we have enough libraries/classes in our application classpath so as to give Spring Boot an idea of what kind of application we are trying to run and what are the kind of facilities and frameworks that need to be configured automatically by Spring Boot in order to stitch things together.


Earlier, we mentioned that the presence of the org.h2.Driver class in the classpath will trigger Spring Boot to automatically configure the H2 database connection for our application. To see exactly how this will happen, let's start by looking at our newly created application template, specifically at BookPubApplication.java located in the src/main/java/org/test/bookpub directory in the root of the project, as follows:

package org.test.bookpub;

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

@SpringBootApplication
public class BookPubApplication {

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

This is effectively our entire and fully runnable application. There's not a whole lot of code here and definitely no mention about configuration or databases anywhere. The key to making magic is the @SpringBootApplication meta-annotation. In order to understand what actually happens, we can take a look inside the code definition for this annotation, where we will find the real annotations that will direct Spring Boot to set things up automatically:

@Configuration
@EnableAutoConfiguration
@ComponentScan
public @interface SpringBootApplication {…}

Let's go through the following list of annotations:

The SpringApplication.run(BookPubApplication.class, args); line in the main method basically creates a Spring application context that reads the annotations in BookPubApplication.class and instantiates a context, which is similar to how it would have been done if instead of using Spring Boot we would have stuck with the regular Spring Framework.