A story about a Spring Boot project written in Java that supports Kotlin


The engineers of the NOREL team are modernizing the technology used, and there was a talk that "I want to write a system written in Java / Spring Boot in Kotlin anyway", so I will proceed with correspondence so that it can be developed in Kotlin. I am.

I often see articles such as starting with Kotlin (Gradle) from the beginning, but I could not find many articles that correspond to Kotlin (Maven) while it was written in Java, so I tried to deal with it.

Existing environment

Basically, Spring Boot implements only REST API.

Policy to support Kotlin

--Since development in Java is progressing in parallel, converting all code from Java to Kotlin will be postponed this time. --Controller / Service can be written in Kotlin --Leave the data access layer such as JPA as Java ――I want to make everything Kotlin in the end

Also, this work was done on IntelliJ.

Migration work

pom.xml settings

--For the pom.xml settings, I referred to the following Kotlin official website.

--Definition of Kotlin version


+    <kotlin.version>1.2.10</kotlin.version>
+    <kotlin.compiler.incremental>true</kotlin.compiler.incremental>

--Added Kotlin dependency library


・ ・ ・
+    <dependency>
+      <groupId>org.jetbrains.kotlin</groupId>
+      <artifactId>kotlin-stdlib</artifactId>
+      <version>${kotlin.version}</version>
+    </dependency>
+    <dependency>
+      <groupId>org.jetbrains.kotlin</groupId>
+      <artifactId>kotlin-reflect</artifactId>
+      <version>${kotlin.version}</version>
+    </dependency>
+    <dependency>
+      <groupId>org.jetbrains.kotlin</groupId>
+      <artifactId>kotlin-test</artifactId>
+      <version>${kotlin.version}</version>
+      <scope>test</scope>
+    </dependency>

--Build settings

Originally I didn't use maven-compiler-plugin, but each package is ready for development in both Java and Kotlin.

I'm using maven-compiler-plugin to separate by.

In addition, spring is defined as compilerPlugins so that various annotations used in Spring can be used. As mentioned in the documentation, defining a spring plugin seems to eliminate the need to define all-open.


-    <sourceDirectory>src/main/java</sourceDirectory>
-    <testSourceDirectory>src/test/java</testSourceDirectory>
+      <plugin>
+        <artifactId>kotlin-maven-plugin</artifactId>
+        <groupId>org.jetbrains.kotlin</groupId>
+        <version>${kotlin.version}</version>
+        <configuration>
+          <compilerPlugins>
+            <plugin>spring</plugin>
+          </compilerPlugins>
+        </configuration>
+        <executions>
+          <execution>
+            <id>compile</id>
+            <goals> <goal>compile</goal> </goals>
+            <configuration>
+              <sourceDirs>
+                <sourceDir>${project.basedir}/src/main/kotlin</sourceDir>
+                <sourceDir>${project.basedir}/src/main/java</sourceDir>
+              </sourceDirs>
+            </configuration>
+          </execution>
+          <execution>
+            <id>test-compile</id>
+            <goals> <goal>test-compile</goal> </goals>
+            <configuration>
+              <sourceDirs>
+                <sourceDir>${project.basedir}/src/test/kotlin</sourceDir>
+                <sourceDir>${project.basedir}/src/test/java</sourceDir>
+              </sourceDirs>
+            </configuration>
+          </execution>
+        </executions>
+        <dependencies>
+          <dependency>
+            <groupId>org.jetbrains.kotlin</groupId>
+            <artifactId>kotlin-maven-allopen</artifactId>
+            <version>${kotlin.version}</version>
+          </dependency>
+        </dependencies>
+      </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-compiler-plugin</artifactId>
+        <version>3.5.1</version>
+        <executions>
+          <!-- Replacing default-compile as it is treated specially by maven -->
+          <execution>
+            <id>default-compile</id>
+            <phase>none</phase>
+          </execution>
+          <!-- Replacing default-testCompile as it is treated specially by maven -->
+          <execution>
+            <id>default-testCompile</id>
+            <phase>none</phase>
+          </execution>
+          <execution>
+            <id>java-compile</id>
+            <phase>compile</phase>
+            <goals> <goal>compile</goal> </goals>
+          </execution>
+          <execution>
+            <id>java-test-compile</id>
+            <phase>test-compile</phase>
+            <goals> <goal>testCompile</goal> </goals>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>

Now you are ready to write Kotlin on IntelliJ.

Support for STS (Eclipse)

When I asked the members who usually use STS to check the correspondence so far, it did not work well and I was a little addicted to it, so I will also describe how to deal with STS.

--Install the Kotlin Plugin for Eclipse from the Eclipse marketplace.

--Since src / main / kotlin is not recognized as a build path, check the kotlin folder with Add File from Source of Projects> Properties> Java Build Path. Same for src / test / kotlin


--Right-click on the project and run Configure Kotlin> Add Kotlin Nature

In the figure below, you can not select it because it has already been added Kotlin Nature, but you can select it before execution.


Now it compiles on STS and you can develop on Kotlin.

Write Kotlin

Convert from Java code to Kotlin code

Since the conversion function of IntelliJ is excellent, I basically converted it with IntelliJ. It's scary to put them all together at once, so I converted one code or several codes at a time.

Select the code you want to convert and convert it to Kotlin code with Code> Convert Java File to Kotlin File.


Also, since the package is still under src / main / java, move it under the same package under src / main / kotlin. This is also nicely refactored with Refactor> Move ... in IntelliJ.

DI (Autowired etc.)

For example, the code below

public class HogeService {
    private HogeRepository hogeRepository;

In Kotlin, you can also declare it with lateinit, but this time I defined it with constructor injection.

class HogeService(
    private val hogeRepository: HogeRepository
) {

Around Lombok

When using Lombok, I sometimes got a compile error when converting to Kotlin.

Basically, the class that converts to Kotlin and the class that is referenced from Kotlin are solved by Delombok.

Use the IntelliJ lombok plugin. It will remove the Lombok conversion from Refactor> Delombok.


--Slf4j annotation

You can omit the logger declaration and write code that outputs logs suddenly. If you use Kotlin, the following code cannot resolve the log variable and a compile error will occur.

public class HogeService {
    public void hoge() {

In kotlin, it can be used by initializing the logger with companion object.

class HogeService {
    companion object {
        private val log = LoggerFactory.getLogger(HogeService::class.java)

Some people have tried the DI method with annotations, so I would like to consider making it easier. http://saiya-moebius.hatenablog.com/entry/2017/11/08/033932

--Data annotation

I read the following Lombokized Java code from Kotlin, and when I try to get the hoge, it says that it is private and not accessible. In this case, as described above, it was converted to Delombok. (If everything becomes Kotlin, you don't have to use Lombok in the first place ...)

@EqualsAndHashCode(callSuper = false)
@ToString(callSuper = true)
public class Hoge extends AbstractEntity {
	@Column(name = "hoge")
	private String hoge;

About the future

At this point, some code is now Kotlinized and works. However, I've had problems several times while I was actually writing the code, so it's possible that I'll continue to have problems. I think that I should add more and more how to deal with it.

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