The following code block shows the generated SpringBootApplication class, FirstWebServiceWithKotlinApplication. We made the class open to enable Spring Boot to override it:

    @SpringBootApplication
    open class FirstWebServiceWithKotlinApplication
    fun main(args: Array<String>) {
      SpringApplication
      .run(
         FirstWebServiceWithKotlinApplication::class.java,
         *args)
    }

The following are a few important things to note:

• Package, import, and annotations are the same as that of a Java class.
• The declaration of the main function in Java was public static void main(String[] args). In the preceding example, we are using the Kotlin function syntax. Kotlin does not have static methods. Any function declared outside of a class can be called without needing a class reference.
• Launching SpringApplication in Java is done using SpringApplication.run(FirstWebServiceWithKotlinApplication.class, args).
• :: is used to obtain a Kotlin class runtime reference. So, FirstWebServiceWithKotlinApplication::class gives us a runtime reference to the Kotlin class. To obtain a Java class reference, we need to use the .java property on the reference. So, in Kotlin, the syntax is FirstWebServiceWithKotlinApplication::class.java.
• In Kotlin, * is called a spread operator. It is used when passing an array to a function accepting variable arguments. So, we will use *args to pass the array to the run method.

The application can be launched up by running FirstWebServiceWithKotlinApplication as a Kotlin application.