Spring and Kotlin

Currently working for To The New as Senior Software Engineer.
Over 5 years of experience in Spring and Java.
Passionate about delivering knowledge sharing sessions. Refer to the link below for all my presentations. https://slides.com/pulkitpushkarna

A bit about me

What is Spring?

The core of Spring Framework is based on the principle of Inversion of Control (IOC).
IOC is a technique that externalizes the creation and management of Component dependencies.
Spring acts like a container which provides instances of your application classes with all the dependencies they need.
A spring manages resorce is referred to as bean.

Kotlin

Kotlin is a cross-platform, statically typed, general purpose language with type inference.

Kotlin is designed to interoperate with Java

public class Tea {

    public void prepareDrink(){
        System.out.println("Preparing Tea....");
    }
}




public class Restaurant {

    private HotDrink hotDrink;

    public Restaurant(HotDrink hotdrink){
        this.hotDrink = hotDrink;
    }

    public HotDrink getHotDrink() {
        return hotDrink;
    }

}

@Configuration
class JavaConfig {

    @Bean
    HotDrink hotDrink() {
        return new ExpressTea();
    }

    @Bean
    Restaurant restaurant(HotDrink hotDrink) {
        Restaurant restaurant = new Restaurant();
        restaurant.setHotDrink(hotDrink);
        return restaurant;
    }

}

context.initializer.classes=com.jetbrainsconfdemo.kotlinwithspringboot.BeanInitializer

application.properties

Kotlin Config file

import org.springframework.context.support.beans

fun bean() = beans {}

Getting started with Kotlin Bean Configuration

Initializer class

import org.springframework.context.ApplicationContextInitializer
import org.springframework.context.support.GenericApplicationContext

class BeansInitializer : ApplicationContextInitializer<GenericApplicationContext> {

    override fun initialize(context: GenericApplicationContext) =
            beans().initialize(context)

}

Java Config

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;

@Configuration
class JavaConfig {

    @Bean
    @Primary
    HotDrink tea() {
        return new Tea();
    }

    @Bean("hotDrink")
    HotDrink ExpressTea() {
        return new ExpressTea();
    }

    @Bean(name = "restaurant",initMethod = "init")
    @Scope("Prototype")
    @Primary
    Restaurant restaurant(HotDrink hotDrink) {
        Restaurant restaurant = new Restaurant();
        restaurant.setHotDrink(hotDrink);
        return restaurant;
    }

}

Kotlin Config

import org.springframework.context.support.beans

fun beans() = beans {

    bean<Tea>(isPrimary = true)

    bean<ExpressTea>("hotDrink")

    bean<RestaurantK>(name = "restK",
            initMethodName = "init",
            scope = BeanDefinitionDsl.Scope.PROTOTYPE,
            isPrimary = true)
}

Complex class in Java

import java.util.List;
import java.util.Map;
import java.util.Set;

public class Complex {

    List<Integer> list;
    Set<Integer> set;
    Map<Integer,String> map;

    public Complex(List<Integer> list, Set<Integer> set, Map<INteger,String>){
        this.list = list;
        this.set = set;
        this.map = map;
    }

    public List<Integer> getList() {
        return list;
    }

   
    public Set<Integer> getSet() {
        return set;
    }

   

    public Map<Integer, String> getMap() {
        return map;
    }

}

Complex Class in Kotlin

data class ComplexK(val list: List<Int>, 
                    val set: Set<Int>,
                    val map: Map<Int, String>)

Java Config for Complex class

 @Bean
    Complex complex() {
        Map map = new HashMap();
        map.put(1, "One");
        map.put(2, "Two");
        return new Complex(Arrays.asList(1, 2, 3, 4),
                new HashSet(Arrays.asList(1, 1, 2, 3, 4)),
                map);
    }

Kotlin Config for Complex class

bean{
        ComplexK(
                listOf(1,2,3,4),
                setOf(1,2,2,3,4),
                mapOf(1 to "One", 2 to "Two", 3 to "Three"))
    }

application.properties

spring.datasource.url=jdbc:mysql://localhost/rest_with_spring
spring.datasource.username=root
spring.datasource.password=
spring.datasource.driver-class-name=com.mysql.jdbc.Driver
spring.jpa.hibernate.ddl-auto=create

Exceuting code at application start up

package com.springbootwebkotlin.demo.springbootwebkotlin

import org.springframework.boot.ApplicationRunner
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.builder.SpringApplicationBuilder
import org.springframework.context.support.beans

@SpringBootApplication
class SpringBootWebKotlinApplication

fun main(args: Array<String>) {
	SpringApplicationBuilder()
			.initializers(beans {
				bean {
					ApplicationRunner{
						println("Application Started")
					}
				}

			}).sources(SpringBootWebKotlinApplication::class.java).run(*args)

}

Creating Hibernate Entity

import javax.persistence.Entity
import javax.persistence.Id

@Entity
data class Employee(@Id val id: Int,
               val name: String, val age: Int)

import org.springframework.data.repository.CrudRepository

interface EmployeeRepository : CrudRepository<Employee, Int>

Creating Employee Repository

import org.springframework.boot.ApplicationRunner
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.builder.SpringApplicationBuilder
import org.springframework.context.support.beans

@SpringBootApplication
class SpringBootWebKotlinApplication

fun main(args: Array<String>) {
    SpringApplicationBuilder()
            .initializers(beans {
                bean {
                    ApplicationRunner {
                        println("Bootstrap Data....")
                        val employeeRepository = ref<EmployeeRepository>()
                        if(employeeRepository.count()==0L) {
                            listOf(Employee(1, "name1", 21),
                                    Employee(2, "name2", 22),
                                    Employee(3, "name3", 23))
                                    .forEach { e -> employeeRepository.save(e) }
               println("Total number of employees saved : ${employeeRepository.count()}")
                        }
                    }
                }
            }).sources(SpringBootWebKotlinApplication::class.java).run(*args)
}

import org.springframework.data.repository.CrudRepository

interface EmployeeRepository : CrudRepository<Employee, Int> {
    fun findByName(name: String) : Employee
}

import org.springframework.data.repository.query.Param
import org.springframework.web.bind.annotation.PathVariable
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RestController

@RestController
@RequestMapping("/employee")
class EmployeeController(val employeeRepository: EmployeeRepository) {

    @RequestMapping
    fun getEmployee() = employeeRepository.findAll()

    @RequestMapping("/{id}")
    fun findById(@PathVariable id: Int) = employeeRepository.findById(id);

    @RequestMapping("/findByName")
    fun findByName(@Param("name") name: String)
            = employeeRepository.findByName(name)

}