Spring Boot 2.0 Cookbook（Second Edition）

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Getting ready

In our previous example, we created the basic application that will execute a command-line runner by printing a message in the logs. Let's enhance this application by adding a connection to a database.

Earlier, we already added the necessary jdbc and data-jpa starters as well as an H2 database dependency to our build file. Now we will configure an in-memory instance of the H2 database.

In the case of an embedded database, such as H2, Hyper SQL Database ( HSQLDB), or Derby, no actual configuration is required besides including the dependency on one of these in the build file. When one of these databases is detected in the classpath and a DataSource bean dependency is declared in the code, Spring Boot will automatically create one for you.

To demonstrate the fact that just by including the H2 dependency in the classpath, we will automatically get a default database, let's modify our StartupRunner.java file to look as follows:

public class StartupRunner implements CommandLineRunner { 
    protected final Log logger = LogFactory.getLog(getClass()); 
    @Autowired 
    private DataSource ds; 
    @Override 
    public void run(String... args) throws Exception { 
        logger.info("DataSource: "+ds.toString()); 
    } 
}

Now, if we proceed with the running of our application, we will see the name of the datasource printed in the log, as follows:

2017-12-16 21:46:22.067 com.example.bookpub.StartupRunner   
:DataSource: org.apache.tomcat.jdbc.pool.DataSource@4...  {...driverClassName=org.h2.Driver; ... }

So, under the hood, Spring Boot recognized that we've autowired a DataSource bean dependency and automatically created one initializing the in-memory H2 datastore. This is all well and good, but probably not all too useful beyond an early prototyping phase or for the purpose of testing. Who would want a database that goes away with all the data as soon as your application shuts down and you have to start with a clean slate every time you restart the application?