Three uses for Kotlin When

Three uses for Kotlin When

Replace the if when

By Patrick Luck

Introduction

The kotlin when extension function is part of the Kotlin standard library and it is used to replace both the switch statement as well as the if-else statements. It is critical that you understand when to use the kotlin when function because when used properly, when can greatly increase the readability of your code. There are three primary different uses where using kotlin when is practical. The first case is comparing a value and then executing the proper branch of code when a true condition has been found. The second involves returning a value based on a try condition. Finally, when is also useful for exception handling.

Like switch and if-else, kotlin when allows you to specify a default case that will execute when none of the specified conditions have been found. A default case is optional as long as when is not being used to return a value. In other words, should you use the kotlin when function to return a value, you will be forced to include an else block in the function or the compiler will flag an error. However, as long as you are not returning a value by using kotlin when, you will not be required to have a default case. Let’s take a look at a few examples of using when in a real program.

Executing a block of code based on a condition

fun throwException(name : String)

fun throwException(name: String){
    when (name) {
        "RuntimeException" -> throw RuntimeException("RuntimeException")
        "IllegalArgumentException" -> throw IllegalArgumentException("IllegalArgumentException")
        "IndexOutOfBoundsException" -> throw IndexOutOfBoundsException("IndexOutOfBoundsException")
        else -> println("Not an exception")
    }

}

Explanation

Here is a function that throws an exception based on the name String parameter. In this case, we are using the kotlin when function to replace a switch or an if-else-else-if block. As you can see, the kotlin when function makes the code highly readable. We start by passing the name variable to when which allows the function to compare the value in name to the values listed on the left side of the ->. Our first value is “RuntimeException” so when name == “RuntimeException” the code to the right of the -> will execute and a RuntimeException is thrown.

The same logic holds true for the two other cases as well. When name == “IllegalArgumentException” the code to the right of -> next to “IllegalArgumentException” is executed and an IllegalArgumentException is thrown by the JVM. The same is also the case for “IndexOutOfBoundsException”.

Finally, we also have an else in this when function. The else acts like a default in a Java switch statement or as an else in an if block. In our case, when name isn’t “RuntimeException”, “IllegalArgumentException”, or “IndexOutOfBoundsException”, then the code to the right of the -> next to the else block executed and we print “Not an exception” to the console.

Exception Handling

fun handleException(name : String)

fun handleException(name : String){
    try {
        throwException(name)
    } catch (e : Exception){
        when (e) {
            is IllegalArgumentException -> println("Handling an IllegalArgumentException")
            is IndexOutOfBoundsException -> println("Handling an IndexOutOfBoundsException")
            is RuntimeException -> println("Handling a Runtime Exception")
        }
    }
}

Explanation

This is an example of when we are using the kotlin when function for exception handling. Developers who are familiar with Java will likely remember using multiple catch blocks for each kind of exception that they wanted to handle. Every kind of unique exception type had to have its own catch block until Java 7 when multi-catch handlers were introduced for when you wanted to use the same code to handle different exception types. However, using a unique catch block for every kind of or groups of exceptions was cumbersome and lead to a lot of boiler plate in your code.

Kotlin addressed this issue by allowing the when function to be combined with the is operator. Keep in mind that “is” is used to compare the type of an object with a class to see if object is of a specific type and return true or false accordingly. That means we have a boolean operation here which allows it to be used with when. For example, when e is IllegalArgumentException, we execute the code to the right of the -> and print “Handling an IllegalArgumentException”. Not only does this improve the readability of the code by allowing for a plain english construct, but we also do away with all of the catch blocks that we would have needed in Java.

Returning a value

fun returnFromWhen(name : String): Class?

fun returnFromWhen(name : String): Class? {
    return when(name){
        "RuntimeException" -> RuntimeException::class.java
        "IllegalArgumentException" -> IllegalArgumentException::class.java
        "IndexOutOfBoundsException" -> IndexOutOfBoundsException::class.java
        else -> {
            println("Returning a null value")
            null
        }
    }
}

Explanation

Our final case is for using when to return a value. This is a powerful construct because it allows us to avoid declaring intermediate variables in a function just for the purpose of returning a value. It also allows us to avoid multiple exit points in a function which many developers consider to be a bad practice since it can be prone to bugs.

Since when is a function, it can be combined with the return keyword to return a value. Should you decide to use this feature, you will need to keep in mind that the returning value needs to be the last statement in a block of code following the -> in each case of the when. The type of return value also had to be declared in the calling function as well, which is what we have in the function declaration.

The kotlin when function works the same as it does in the other two cases. We pass a variable to it and then compare it to the separate cases. The only difference is that the final statement in the code of the -> needs to be a return value of some sort. In our case, we are returning Class objects that extend from RuntimeException. Our function has also been declared as nullable so that we can return null. After each case in the when block, we return a Class object, except for the else case which returns null.

You will notice also that the else part has { } that wraps multiple statements. This can always be done with the kotlin when function and will allow you to execute a block of code when it is needed.

Conclusion

As you may have noticed, the kotlin when function is a great tool to use when you need to increase the readability of your code by allowing you to avoid if else statements. It is also more powerful than the Java switch statement, as you are free to use any boolean condition in the when statement. The most common patterns for using kotlin when is to execute a block of code, exception handling, and returning a value.

Many developers execute a block of code using kotlin when just as if they are using a Java switch statement. In this case, we are checking a value against different conditions and then acting accordingly. Using kotlin when in this fashion is more flexible than using a switch statement, because you are not limited to just numeric or String values. Kotlin when allows you to do any legal comparisons which makes it more powerful than a Java switch statement.

The kotlin when function is also used to avoid a messy list of catch blocks when you are handling exceptions. Instead, you can use the “is” operator to check the type of your exception object and respond as needed to the exception. This allows for more compact exception handling than what you can normally achieve in Java.

Finally, since kotlin when is a function, you can use it to return a value. This allows you to avoid having multiple exit points in your function and you can avoid needing to declare an unnecessary variable. Once again, this makes your code more concise and readable that what you can normally achieve in other programming languages.

Github

You can find the entire code for this post at https://github.com/archer920/Koltin-When-Exception

Sources

  1. https://kotlinlang.org/docs/reference/control-flow.html
  2. http://www.baeldung.com/kotlin-when
  3. https://www.programiz.com/kotlin-programming/when-expression
  4. https://antonioleiva.com/when-expression-kotlin/
  5. https://www.tutorialkart.com/kotlin/when-expression-in-kotlin/
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Consuming REST with Spring and Kotlin

Spring 5 came with official support for Kotlin, a JVM language developed by Jetbrains which focuses on code clarity and conciseness. Many web applications today are a mockup of content from other websites, which are usually exposed with a web service. Consuming a web service is really easy when you use Spring’s RestTemplate class. This tutorial is an adaptation of the one found here, which has been modified to use the Kotlin language.

Project Structure

You will want to setup your project with a folder structure that is similar to the one shown in the screenshot below.

consuming_rest

build.gradle

Next you will want to use a dependency management system, either gradle or maven, which will see to the details of downloading your dependencies. We use gradle in this tutorial.

buildscript {
    ext.kotlin_version = '1.2.30'

    repositories {
        mavenCentral()
    }
    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath("org.springframework.boot:spring-boot-gradle-plugin:2.0.1.RELEASE")
    }
}

group 'com.stonesoupprogramming'
version '1.0-SNAPSHOT'

apply plugin: 'kotlin'
apply plugin: 'org.springframework.boot'
apply plugin: 'io.spring.dependency-management'

bootJar {
    baseName = 'gs-consuming-rest'
    version =  '0.1.0'
}

repositories {
    mavenCentral()
}

dependencies {
    compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
    compile group: 'org.jetbrains.kotlin', name: 'kotlin-reflect', version: '$kotlin_version'
    compile("org.springframework.boot:spring-boot-starter")
    compile("org.springframework:spring-web")
    compile("com.fasterxml.jackson.core:jackson-databind")
}

compileKotlin {
    kotlinOptions.jvmTarget = "1.8"
}
compileTestKotlin {
    kotlinOptions.jvmTarget = "1.8"
}

ConsumingRest.kt

Now we are going to write a simple Kotlin application which will make an HTTP GET request to a server and deserialize the JSON into Kotlin object. We will use two data classes and then write a main function. Here is the code.

package com.stonesoupprogramming

import com.fasterxml.jackson.annotation.JsonIgnoreProperties
import org.springframework.web.client.RestTemplate

/**
 * The following data classes are instantiated by Jackson and converted from JSON to
 * objects. Normally, the class needs to have all of the properties in the JSON, but
 * we can change this by using the ignoreUnknown = true argument
 */
@JsonIgnoreProperties(ignoreUnknown = true)
data class Value(var id: Long = 0, var quote: String = "")

@JsonIgnoreProperties(ignoreUnknown = true)
data class Quote(var type : String = "", var value : Value? = null)

fun main (args : Array){
    //Create a new RestTemplate and use getForObject to make a GET request
    //to the server and return an instance of Quote representing the response
    val quote = RestTemplate().getForObject("http://gturnquist-quoters.cfapps.io/api/random", Quote::class.java)

    //Print the response to the console
    println(quote)
}

Our application begin by declaring two data classes which are Value and Quote. We annotate these classes with @JsonIgnoreProperties which allows the JSON deserializer to skip over JSON properties that are not present in our data classes. Otherwise, we would get a runtime exception. The JSON deserializer uses reflection (java based) to instantiate objects from the data classes so we need some form of a default constructor. We can do this a number of different ways in Kotlin, but I chose to use default values the properties in the data classes.

Next we have a main function. It starts by creating a new RestTemplate object and calling its getForObject method. The getForObject requires a web address and a class of the object to return. Then we call println on the returned Quote object to print the output to the console.

RestTemplate has methods for just about every HTTP verb so while this example only uses GET, you can do POST, PUT, DELETE and other common web requests.

Output

The output will different every time you run the application, but here is what I got when I ran it.

14:47:14.091 [main] WARN org.springframework.http.converter.json.Jackson2ObjectMapperBuilder - For Jackson Kotlin classes support please add "com.fasterxml.jackson.module:jackson-module-kotlin" to the classpath
14:47:14.156 [main] DEBUG org.springframework.web.client.RestTemplate - Created GET request for "http://gturnquist-quoters.cfapps.io/api/random"
14:47:14.247 [main] DEBUG org.springframework.web.client.RestTemplate - Setting request Accept header to [application/json, application/*+json]
14:47:14.321 [main] DEBUG org.springframework.web.client.RestTemplate - GET request for "http://gturnquist-quoters.cfapps.io/api/random" resulted in 200 (OK)
14:47:14.322 [main] DEBUG org.springframework.web.client.RestTemplate - Reading [class com.stonesoupprogramming.Quote] as "application/json;charset=UTF-8" using [org.springframework.http.converter.json.MappingJackson2HttpMessageConverter@cc43f62]
Quote(type=success, value=Value(id=1, quote=Working with Spring Boot is like pair-programming with the Spring developers.))

Conclusion

Here is the source document for this tutorial.

https://spring.io/guides/gs/consuming-rest/

You can get the code at my github at this address: https://github.com/archer920/consuming-rest

Kotlin Scheduling Tasks with Spring Boot

Kotlin is fully interoperable with Spring Boot which makes Spring and Kotlin a perfect companion to one another. Spring brings a high level platform that can be used for making just about any enterprise grade application, while Kotlin offers language features that make your code concise and readable. Both Kotlin and Spring do a great job of reducing boilerplate in your code so that you can write an application quickly and get to the point.

This tutorial is based on Scheduling Tasks found on the Spring website is an adapation of the tutorial for Kotlin. We will be using Kotlin, Spring Boot, and Gradle. You can find the code here.

Project Structure

You should setup your project to use this folder structure.

scheduling_tasks

build.gradle

Here is the full code for your gradle.build file. Notice that will bring in both Kotlin and Spring libraries so that we can build the project.

buildscript {
    ext.kotlin_version = '1.2.30'

    repositories {
        mavenCentral()
    }
    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath "org.springframework.boot:spring-boot-gradle-plugin:2.0.1.RELEASE"
    }
}

group 'com.stonesoupprogramming'
version '1.0-SNAPSHOT'

apply plugin: 'kotlin'
apply plugin: 'java'
apply plugin: 'eclipse'
apply plugin: 'idea'
apply plugin: 'org.springframework.boot'
apply plugin: 'io.spring.dependency-management'

repositories {
    mavenCentral()
}

bootJar {
    baseName = 'gs-scheduling-tasks'
    version =  '0.1.0'
}

sourceCompatibility = 1.8
targetCompatibility = 1.8

dependencies {
    compile "org.springframework.boot:spring-boot-starter"
    compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
    compile group: 'org.jetbrains.kotlin', name: 'kotlin-reflect', version: '1.2.30'
    testCompile "junit:junit"
}

compileKotlin {
    kotlinOptions.jvmTarget = "1.8"
}
compileTestKotlin {
    kotlinOptions.jvmTarget = "1.8"
}

SchedulingTasks.kt

Here is the Kotlin code followed by an explanation.

package com.stonesoupprogramming.schedulingtasks

import org.slf4j.LoggerFactory
import org.springframework.boot.SpringApplication
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.scheduling.annotation.EnableScheduling
import org.springframework.scheduling.annotation.Scheduled
import org.springframework.stereotype.Component
import java.time.LocalDateTime
import java.time.format.DateTimeFormatter

/**
 * Mark this class an injectable component so that the Spring environment will create
 * an instance of this class when it starts up.
 */
@Component
class ScheduleTasks {

    private val logger = LoggerFactory.getLogger(ScheduleTasks::class.java)

    /**
     * This @Schedule annotation run every 5 seconds in this case. It can also
     * take a cron like syntax.
     * See https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/scheduling/support/CronSequenceGenerator.html
     */
    @Scheduled(fixedRate = 5000)
    fun reportTime(){
        logger.info("The time is now ${DateTimeFormatter.ISO_LOCAL_TIME.format(LocalDateTime.now())}")
    }
}

@SpringBootApplication
//Required to tell Spring to run tasks marked with @Scheduled
@EnableScheduling
open class Application

fun main(args : Array){
    SpringApplication.run(Application::class.java)
}

When run, you will get this output on your console every five seconds.

2018-04-06 18:51:21.868  INFO 20294 --- [pool-1-thread-1] c.s.schedulingtasks.ScheduleTasks        : The time is now 18:51:21.865
2018-04-06 18:51:26.858  INFO 20294 --- [pool-1-thread-1] c.s.schedulingtasks.ScheduleTasks        : The time is now 18:51:26.858

Explanation

So how does the code work? The ScheduleTasks class is annotaded with @Component, which the Spring environment scans for on start up and instantiates the class. At this point, an instance of ScheduleTasks lives in the ApplicationContent. You will notice that the ScheduleTasks::reportTime function is annotated with @Scheduled which defaults to a fix rate or can use a CRON like syntax.

You can’t annotate a method and expect it to run without turning on scheduling. That is why the Application class is annotated with @EnableScheduling. This will tell Spring to scan all container managed classes and look for the @Scheduled annotation. The Spring environment will do the job of making sure that the methods run at the proper time.

Code

You can get the code for this tutorial at my GitHub: https://github.com/archer920/scheduling-tasks

Sources

https://spring.io/guides/gs/scheduling-tasks/
https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/scheduling/support/CronSequenceGenerator.html

Build a RESTful Web Service with Kotlin

Introduction

Spring and Kotlin combine together to create a powerhouse when it comes to rapid application development. The Spring project is a powerful framework that allows you to develop an application quickly with as little boilerplate and configuration code as possible. Kotlin is a language that is developed Jetbrains that focuses on code readability and conciseness. This guide will show you how to build a RESTful web service using Spring Boot and Kotlin.

Getting Started

We will use a Maven project to mranage the resources that this application will need. Your project will need the following folder skeleton before you can continue.

Maven

After you have created your project skeleton you can continue.

pom.xml

The pom.xml file is used by Maven to manage all of your project dependencies. You can copy and paste this code into your pom.xml file, which will pull in all of the Spring Boot and Kotlin dependencies.



    4.0.0

    stonesoupprogramming
    BuildingRESTfulWebService
    1.0-SNAPSHOT

    
        1.2.31
    

    
        org.springframework.boot
        spring-boot-starter-parent
        2.0.0.RELEASE
    

    
        
            org.jetbrains.kotlin
            kotlin-stdlib-jdk8
            ${kotlin.version}
        
        
            org.jetbrains.kotlin
            kotlin-reflect
            ${kotlin.version}
        
        
            org.reflections
            reflections
            0.9.10
        
        
            org.jetbrains.kotlin
            kotlin-test
            ${kotlin.version}
            test
        

        
            org.springframework.boot
            spring-boot-starter-web
        
        
            org.springframework.boot
            spring-boot-starter-test
            test
        
        
            com.jayway.jsonpath
            json-path
            test
        
    

    
        src/main/kotlin
        
            
                org.springframework.boot
                spring-boot-maven-plugin
            
            
                org.jetbrains.kotlin
                kotlin-maven-plugin
                ${kotlin.version}
                
                    
                        compile
                        compile
                        
                            compile
                        
                    
                    
                        test-compile
                        test-compile
                        
                            test-compile
                        
                    
                
                
                    1.8
                
            
        
    

    
        
            spring-releases
            https://repo.spring.io/libs-release
        
    
    
        
            spring-releases
            https://repo.spring.io/libs-release
        
    

Application.kt

Kotlin is a language that is meant to be concise, which plays to our advantage. We will hold all of our classes inside of the Application.kt file.

package com.stonesoupprogramming.spring.boot

import org.springframework.boot.SpringApplication
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RequestParam
import org.springframework.web.bind.annotation.RestController
import java.util.concurrent.atomic.AtomicLong

/**
 * This class gets converted into JSON and serves as our data model.
 * We can use Kotlin's data class feature to make it only one line of code.
 */
data class Greeting (val id : Long, val content : String)

/**
 * The @RestController annotation tells the Spring Environment to
 * use this class to handle REST requests. That means that it will handle
 * HTTP requests but does not use a view technology to write the response.
 * Instead, an instance of Greeting is simply converted into JSON and written
 * to the HTTP response body.
 */
@RestController
class GreetingController {

    private val counter : AtomicLong = AtomicLong()

    /**
     * The @RequestMapping signals that this method will handle
     * HTTP requests to /greeting. We can narrow it down to GET, POST, PUT, etc
     * when we want different methods to handle different requests
     * at this endpoint.
     *
     * The name parameter is annotated with @RequestParam which has
     * two arguments. The name argument maps the request parameter name to
     * the name argument in this method. The defaultValue will populate
     * name with the value "World" if the request does not have a name argument.
     */
    @RequestMapping("/greeting")
    fun greeting(@RequestParam(value="name", defaultValue="World") name : String) : Greeting {
        return Greeting(counter.incrementAndGet(), "Hello $name")
    }
}

/**
 * The @SpringBootApplication is a meta annotation that makes
 * this application executable.
 */
@SpringBootApplication
open class Application

/**
 * Now we just need an entry point to the program.
 */
fun main(args : Array){
    SpringApplication.run(Application::class.java, *args)
}

Let’s break the code down into each piece.

Greeting

Greeting is a data class that has two fields, id and content. Kotlin introduced data classes to cut down on boilerplate code when using POJOs (Plain old java object). It will have all of the getters, equals, hashcode, and toString() as well as a constructor. This class will get converted into JSON and written to the response body later on in the application.

GreetingController

Spring works on a Model, View, Controller architecture so it uses Controller classes to map web requests to backend code. In this case, we are using @RestController to specify that we are not using a view technology to generate HTML and are instead going to write JSON to the HTML response body.

This class only has one method, greeting, which is annotated with @RequestMapping. You will use @RequestMapping to map HTTP requests to a method in the class. In our case, we are mapping all requests (GET, PUT, POST, DELETE) to /greeting to our greeting method. The greeting method has one argument, name, which is also annotated with @RequestParam.

The @RequestParam has two arguments, value which specifies the name of the argument in the request and the default value if the argument is not present in the request. In our case, we also called the request parameter name and we have it default to World. Inside of the method, we return a new instance of Greeting and then return it. The Spring environment will see to the details of converting it to JSON and writing it to the response.

Application

We also have an empty Application class that is marked with the @SpringBootApplication annotation. This is a meta-annotation that pulls in all of the annotations that are needed to make this program executable. We using it in the main function to start the program.

Finishing

After you start the application, you can point your browser to

http://localhost:8080/greeting and then http://localhost:8080/greeting?name=User to see the JSON output of this application.

Sources

https://spring.io/guides/gs/rest-service/

https://kotlinlang.org/docs/reference/

Source

The source code for this project is available on my github here: https://github.com/archer920/BuildingRESTfulWebService

Kotlin JDBC – RowSet Interface

The RowSet interface is a sub-interface of ResultSet and is used provide finer grade control over JDBC result sets. The RowSet interface has its own sub-interfaces that provide different features depending on the type of interface.

RowSet Sub-Interfavces

Interface Brief Description
JdbcRowSet A RowSet that is capable of being used as a JavaBeans component. The JdbcRowSet maintains a connection to the underlying database and makes the ResultSet scrollable and updateable.
CachedRowSet Caches rows in memory, allowing for the application to work on the ResultSet without maintaing an active connection to the database.
WebRowSet : CachedRowSet An extension of CachedRowSet, the WebRowSet provides XML capabilities
JoinRowSet : WebRowSet Extends WebRowSet to provide SQL JOIN capabilities.
FilterRowSet : WebRowSet Extends WebRowSet to provide filtering capabilities

Example Program

Below is an example program that demonstrates how to create an instance of RowSet.

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>OCJP-DB</groupId>
    <artifactId>ocjpdb</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <kotlin.version>1.2.10</kotlin.version>
        <main.class>stonesoupprogramming.MainKt</main.class>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.apache.derby</groupId>
            <artifactId>derby</artifactId>
            <version>10.14.1.0</version>
        </dependency>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-stdlib-jre8</artifactId>
            <version>${kotlin.version}</version>
        </dependency>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-test</artifactId>
            <version>${kotlin.version}</version>
            <scope>test</scope>
        </dependency>

    </dependencies>

    <build>
        <sourceDirectory>src/main/kotlin</sourceDirectory>
        <plugins>
            <plugin>
                <groupId>org.jetbrains.kotlin</groupId>
                <artifactId>kotlin-maven-plugin</artifactId>
                <version>${kotlin.version}</version>
                <executions>
                    <execution>
                        <id>compile</id>
                        <phase>compile</phase>
                        <goals>
                            <goal>compile</goal>
                        </goals>
                    </execution>
                    <execution>
                        <id>test-compile</id>
                        <phase>test-compile</phase>
                        <goals>
                            <goal>test-compile</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <jvmTarget>1.8</jvmTarget>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>2.6</version>
                <configuration>
                    <archive>
                        <manifest>
                            <addClasspath>true</addClasspath>
                            <mainClass>${main.class}</mainClass>
                        </manifest>
                    </archive>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.2.1</version>
                <executions>
                    <execution>
                        <phase>test</phase>
                        <goals>
                            <goal>java</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <mainClass>${main.class}</mainClass>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

EmployeesRowset.kt

package stonesoupprogramming

import javax.sql.rowset.RowSetProvider

fun main (args : Array<String>){
    val rowSet  = RowSetProvider.newFactory().createWebRowSet()

    with(rowSet){
        //Connection information
        url = "jdbc:derby:stonesoup;create=true"
        username = "admin"
        password = "pw"

        //Command to execute against the database
        command = "SELECT * FROM BURGERS.EMPLOYEES"

        //Execute the command
        execute()

        //Output XML to standard out
        writeXml(System.out)
    }
}

Explanation

This program queries a table in the database and prints the XML to the standard out. We get an instance of WebRowSet by calling createWebRowSet() on line 6. Note that if we wanted a different kind of RowSet, we would just use the corresponding method on RowSetProvider. For example, if we wanted a JdbcRowSet, we would use createJdbcRowSet() instead of createWebRowSet().

The result is a RowSet object. Once we have a RowSet object, we start by populating its properties to establish a connection to the database. In this case, we pass a JDBC connection string, a username, and a password. Next, we set the command property with a SQL string.

The SQL is executed when call the execute() method. A connection is established to the database and then the RowSet object is populated with the results. Since we are using a WebRowSet, we can write the results to XML. The example program passes System.out as the output stream and the results appear on the console.

Kotlin JDBC – Savepoints

The JDBC connection object has the ability to create SavePoint objects that are used to rollback a transaction to a specific point in time. One possible use case is providing users the ability to have “Undo” options while working in a database client program. Of course, we can also use SavePoints in Exception handlers or other areas of the program as needed.

SavePoints are used when the connection’s autoCommit property is set to false. We create a SavePoint like so

val bob = connection.setSavePoint("Bob") //Name is optional

Later on, we can pass the SavePoint to the rollback() method on the connection object.

connection.rollback(bob)

Once the connection is rollback to a SavePoint, any work performed on the conncetion after the SavePoint is lost.

Below is an example program that demonstrates using SavePoints.

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>OCJP-DB</groupId>
    <artifactId>ocjpdb</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <kotlin.version>1.2.10</kotlin.version>
        <main.class>stonesoupprogramming.MainKt</main.class>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.apache.derby</groupId>
            <artifactId>derby</artifactId>
            <version>10.14.1.0</version>
        </dependency>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-stdlib-jre8</artifactId>
            <version>${kotlin.version}</version>
        </dependency>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-test</artifactId>
            <version>${kotlin.version}</version>
            <scope>test</scope>
        </dependency>

    </dependencies>

    <build>
        <sourceDirectory>src/main/kotlin</sourceDirectory>
        <plugins>
            <plugin>
                <groupId>org.jetbrains.kotlin</groupId>
                <artifactId>kotlin-maven-plugin</artifactId>
                <version>${kotlin.version}</version>
                <executions>
                    <execution>
                        <id>compile</id>
                        <phase>compile</phase>
                        <goals>
                            <goal>compile</goal>
                        </goals>
                    </execution>
                    <execution>
                        <id>test-compile</id>
                        <phase>test-compile</phase>
                        <goals>
                            <goal>test-compile</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <jvmTarget>1.8</jvmTarget>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>2.6</version>
                <configuration>
                    <archive>
                        <manifest>
                            <addClasspath>true</addClasspath>
                            <mainClass>${main.class}</mainClass>
                        </manifest>
                    </archive>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.2.1</version>
                <executions>
                    <execution>
                        <phase>test</phase>
                        <goals>
                            <goal>java</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <mainClass>${main.class}</mainClass>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

Employees.kt

package stonesoupprogramming

import java.sql.Connection
import java.sql.DriverManager
import java.sql.ResultSet
import java.util.*

fun main(args: Array<String>) {
    val properties = Properties()

    //Populate the properties file with user name and password
    with(properties) {
        put("user", "admin")
        put("password", "pw")
    }

    //Open a connection to the database
    DriverManager
            .getConnection("jdbc:derby:stonesoup;create=true", properties)
            .use { connection ->
                //Set autoCommit to false to manually manage transactions
                connection.autoCommit = false

                createOrTruncateTable(connection)

                //Create an updatable result set
                val rs = connection
                        .createStatement(ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE)
                        .executeQuery("SELECT * FROM BURGERS.EMPLOYEES")

                //Populate the table with data
                with(rs) {
                    moveToInsertRow()
                    updateInt("ID", 1);
                    updateString("NAME", "Bob")
                    insertRow()

                    val bob = connection.setSavepoint("Bob")

                    moveToInsertRow()
                    updateInt("ID", 2)
                    updateString("NAME", "Linda")
                    insertRow()

                    val linda = connection.setSavepoint("Linda")

                    moveToInsertRow()
                    updateInt("ID", 3)
                    updateString("NAME", "Tina")
                    insertRow()

                    val tina = connection.setSavepoint("Tina")

                    print("Enter Bob, Linda, or Tina => ")
                    val choice = readLine()

                    when (choice) {
                        "Bob" -> connection.rollback(bob)
                        "Linda" -> connection.rollback(linda)
                        "Tina" -> connection.rollback(tina)
                    }
                }
                //Commit the transaction
                connection.commit()


                //Read only queries are still transactions
                val rsq = connection
                        .createStatement()
                        .executeQuery("SELECT * FROM BURGERS.EMPLOYEES")
                with(rsq) {
                    while (next()) {
                        println("${getInt("ID")}\t${getString("NAME")}")
                    }
                }
                //So we need to commit this query also even though it doesn't change anything
                connection.commit()
            }
}

private fun createOrTruncateTable(connection: Connection) {
    val metaData = connection.metaData
    if (!metaData.getTables(null, "BURGERS", "EMPLOYEES", null).next()) {
        connection
                .createStatement()
                .executeUpdate("CREATE TABLE BURGERS.EMPLOYEES (ID INT PRIMARY KEY, NAME VARCHAR(255))")
    } else {
        connection
                .createStatement()
                .executeUpdate("TRUNCATE TABLE BURGERS.EMPLOYEES")
    }
    connection.commit()
}

Explanation

Our program begins by establishing a connection, preparing a table, and creating an updatable ResultSet. We start inserting into the table beginning on line 32. Line 38 is where we create our first SavePoint, after inserting Bob into the table. Once we have the bob SavePoint established, we move on and insert Linda. Linda also gets a SavePoint (line 45), followed by Tina (line 52).

The user is present with a choice on line 55. When they enter Bob, the connection is rolled back to Bob, meaning that neither Linda or Tina are inserted into the database. When the user picks Linda, the connection is rolled back to the linda SavePoint, meaning that Bob and Linda are inserted into the database, but not Tina. If Tina is picked, then all three employees are inserted into the database. Line 64 commits the transaction and the inserts are performed into the database.

Kotlin JDBC – Rollback Transactions

JDBC has the ability rollback transactions. This example shows how to rollback a transaction in the case of an exception. Exception rollbacks are a common pattern because in many cases, committing a transaction after an error can leave the database in an inconsistent state. Let’s take a look at a short example of how to rollback a transaction.

connection.autoCommit = false

createOrTruncateTable(connection)

//Create an updatable result set
val rs = connection
         .createStatement(ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE)
         .executeQuery("SELECT * FROM BURGERS.EMPLOYEES")

try {
    /* Lines 32-46 omitted */

    //Commit the previous transaction (lines 32-46)
    connection.commit()

    //Now let's do an insert but have it fail
    with(rs){
        moveToInsertRow()

        updateInt("ID", 3)
        updateString("NAME", "Tina")

        insertRow()
    }
    throw Exception("Simulated")
} catch (e: Exception){
    println("Caught simulated exception. Rolling back...")

    //We can rollback the current transaction. Tina will never
    //get inserted into the database
    connection.rollback()
}

The above code fragement uses an expanded try-catch block. At the start of the code fragement, turn off autoCommit on the connection object and create an updatable ResultSet that let’s us insert rows into the database table. There is an ommitted portion of code that inserts some rows into the database.

Then we commit the first transaction. So far so good. The rows are entered cleanly into the database. Then our example continues by inserting another record into the table. Rather than committing the transaction, we instead throw an Exception to act as if something went wrong with the insertion.

The catch block found at the end of the code fragement shows what to do when an exception is thrown in the middle of a transaction. In our example, we notify the user that we are rolling back the changes. Then we call rollback() on the connection object. Rollback() resets the transaction and the program can act as if the last transaction never happened.

Complete Example

Below is a complete Kotlin program that shows the demonstration code in its entirety.

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>OCJP-DB</groupId>
    <artifactId>ocjpdb</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <kotlin.version>1.2.10</kotlin.version>
        <main.class>stonesoupprogramming.MainKt</main.class>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.apache.derby</groupId>
            <artifactId>derby</artifactId>
            <version>10.14.1.0</version>
        </dependency>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-stdlib-jre8</artifactId>
            <version>${kotlin.version}</version>
        </dependency>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-test</artifactId>
            <version>${kotlin.version}</version>
            <scope>test</scope>
        </dependency>

    </dependencies>

    <build>
        <sourceDirectory>src/main/kotlin</sourceDirectory>
        <plugins>
            <plugin>
                <groupId>org.jetbrains.kotlin</groupId>
                <artifactId>kotlin-maven-plugin</artifactId>
                <version>${kotlin.version}</version>
                <executions>
                    <execution>
                        <id>compile</id>
                        <phase>compile</phase>
                        <goals>
                            <goal>compile</goal>
                        </goals>
                    </execution>
                    <execution>
                        <id>test-compile</id>
                        <phase>test-compile</phase>
                        <goals>
                            <goal>test-compile</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <jvmTarget>1.8</jvmTarget>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>2.6</version>
                <configuration>
                    <archive>
                        <manifest>
                            <addClasspath>true</addClasspath>
                            <mainClass>${main.class}</mainClass>
                        </manifest>
                    </archive>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.2.1</version>
                <executions>
                    <execution>
                        <phase>test</phase>
                        <goals>
                            <goal>java</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <mainClass>${main.class}</mainClass>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

Employees.kt

package stonesoupprogramming

import java.sql.Connection
import java.sql.DriverManager
import java.sql.ResultSet
import java.util.*

fun main(args: Array<String>) {
    val properties = Properties()

    //Populate the properties file with user name and password
    with(properties) {
        put("user", "admin")
        put("password", "pw")
    }

    //Open a connection to the database
    DriverManager
            .getConnection("jdbc:derby:stonesoup;create=true", properties)
            .use { connection ->
                //Set autoCommit to false to manually manage transactions
                connection.autoCommit = false

                createOrTruncateTable(connection)

                //Create an updatable result set
                val rs = connection
                        .createStatement(ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE)
                        .executeQuery("SELECT * FROM BURGERS.EMPLOYEES")

                try{
                    //Populate the table with data
                    with(rs) {
                        moveToInsertRow()

                        updateInt("ID", 1);
                        updateString("NAME", "Bob")

                        insertRow()
                        moveToInsertRow()

                        updateInt("ID", 2)
                        updateString("NAME", "Linda")

                        insertRow()
                    }
                    //Commit the transaction
                    connection.commit()

                    //Now let's do an insert but have it fail
                    with(rs){
                        moveToInsertRow()

                        updateInt("ID", 3)
                        updateString("NAME", "Tina")

                        insertRow()
                    }
                    throw Exception("Simulated")

                } catch (e: Exception){
                    println("Caught simulated exception. Rolling back...")

                    //We can rollback the current transaction. Tina will never
                    //get inserted into the database
                    connection.rollback()
                }


                //Read only queries are still transactions
                val rsq = connection
                        .createStatement()
                        .executeQuery("SELECT * FROM BURGERS.EMPLOYEES")
                with(rsq) {
                    while (next()) {
                        println("${getInt("ID")}\t${getString("NAME")}")
                    }
                }
                //So we need to commit this query also even though it doesn't change anything
                connection.commit()
            }
}

private fun createOrTruncateTable(connection: Connection) {
    val metaData = connection.metaData
    if (!metaData.getTables(null, "BURGERS", "EMPLOYEES", null).next()) {
        connection
                .createStatement()
                .executeUpdate("CREATE TABLE BURGERS.EMPLOYEES (ID INT PRIMARY KEY, NAME VARCHAR(255))")
    } else {
        connection
                .createStatement()
                .executeUpdate("TRUNCATE TABLE BURGERS.EMPLOYEES")
    }
    connection.commit()
}