Application class

We define our  SpringApplication class to launch our Spring Boot application from the Kotlin code. It reads the application configuration from application.yml in our case, but supports being read from various sources.

We use @SpringBootApplication to launch our application as a Spring Boot application and @EnableTransactionManagement to enable database transaction support for databases:

    /**
      * To launch Spring Boot application and hold the application   
       level properties
      */
    @SpringBootApplication
    @EnableTransactionManagement
    class Application {

      /**
        * To deserialize-serialize the PostGIS data structures
        */
      @Bean
      fun objectMapper(): ObjectMapper =
       Jackson2ObjectMapperBuilder()
       .modulesToInstall(PostGISModule())
                      
       .serializationInclusion(JsonInclude.Include.NON_NULL)
       .build()

      /**
        * Configuring transaction support
        */
      @Bean
      fun transactionManager(@Qualifier("dataSource") dataSource:   
       DataSource) = SpringTransactionManager(dataSource)

      /**
        * Initialize our web app each time it runs
        */
      @Bean
      fun init(mr: MessageRepository) = CommandLineRunner {
        mr.createTable()
        mr.deleteAll()
      }
    }

    /**
      * Launch the Spring Boot application
      */
    fun main(args: Array<String>) {
      SpringApplication.run(Application::class.java, *args)
    }

To configure transaction support with the database we simply need to specify the @EnableTransactionManagement annotation and pass PlatformTransactionManager with the transactionManager bean using the Application class:

    @Bean
    fun transactionManager(@Qualifier("dataSource") dataSource:  
     DataSource) = SpringTransactionManager(dataSource)

Moreover, we need to configure our object mapper (Jackson) with an additional module for mapping PostGIS data structures: 

    @Bean
    fun objectMapper(): ObjectMapper =
     Jackson2ObjectMapperBuilder()
     .modulesToInstall(PostGISModule())
     .serializationInclusion(JsonInclude.Include.NON_NULL)
     .build()

You can specify your main function for JVM outside the  Application class, such functions that are not part of any class are called Top-level functions in Kotlin and they come from the functional side of Kotlin.