Hello everyone, Kotlin JPA Entity constructor issu...

# server

[Joaquim Costa]

Hello everyone,
Kotlin JPA Entity constructor issue in Java.

I working on a spring/java server application. I started refactoring some entities to use Kotlin, but I am having some challenges invoking default constructor from Java class.
This is a maven project using kotlin-maven-plugin with jpa, and spring.
For some reason i am able to invoke the no arg constructor in java class as new Person(), and when this happens the default values of that object are all set to null, instead of preserving the value that i defined in the Person class.

i expect the new Person() to yield a syntax error similar to when it is invoked in Kotlin class, or at least if the default constructor is used/allowed to be invoked from java it should use the default values in the fields. What am i missing?

>The no-arg compiler plugin generates an additional zero-argument constructor for classes with a specific annotation.

The generated constructor is synthetic so it can’t be directly called from Java or Kotlin, but it can be called using reflection.

https://kotlinlang.org/docs/reference/compiler-plugins.html#no-arg-compiler-plugin

example Person.kt

@Entity class Person(
  @Id var id:Long?=0,
  var type:String="A"
){
  var description:String="none"
}

[nfrankel]

could you share your build configuration?
especially the part where you configure the no-arg-constructor plugin?

[Joaquim Costa]

<?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> <https://maven.apache.org/xsd/maven-4.0.0.xsd>">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.4.2</version>
    <relativePath/> <!-- lookup parent from repository -->
  </parent>
  <groupId>com.example</groupId>
  <artifactId>demo</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <name>demo</name>
  <description>Demo project for Spring Boot</description>
  <properties>
    <java.version>11</java.version>
    <kotlin.version>1.4.21</kotlin.version>
  </properties>
  <dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-data-jpa</artifactId>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
      <groupId>com.h2database</groupId>
      <artifactId>h2</artifactId>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-test</artifactId>
      <scope>test</scope>
    </dependency>
    <dependency>
      <groupId>org.jetbrains.kotlin</groupId>
      <artifactId>kotlin-stdlib-jdk8</artifactId>
      <version>${kotlin.version}</version>
    </dependency>
    <dependency>
      <groupId>org.jetbrains.kotlin</groupId>
      <artifactId>kotlin-test</artifactId>
      <version>${kotlin.version}</version>
      <scope>test</scope>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.module</groupId>
      <artifactId>jackson-module-kotlin</artifactId>
    </dependency>
    <dependency>
      <groupId>org.jetbrains.kotlin</groupId>
      <artifactId>kotlin-reflect</artifactId>
    </dependency>
  </dependencies>

  <repositories>
    <repository>
      <releases>
        <enabled>true</enabled>
      </releases>
      <snapshots>
        <enabled>false</enabled>
      </snapshots>
      <id>bintray.kotlin.eap</id>
      <name>Bintray Kotlin EAP Repository</name>
      <url><https://dl.bintray.com/kotlin/kotlin-eap></url>
    </repository>
  </repositories>

  <pluginRepositories>
    <pluginRepository>
      <releases>
        <enabled>true</enabled>
      </releases>
      <snapshots>
        <enabled>false</enabled>
      </snapshots>
      <id>bintray.kotlin.eap</id>
      <name>Bintray Kotlin EAP Repository</name>
      <url><https://dl.bintray.com/kotlin/kotlin-eap></url>
    </pluginRepository>
  </pluginRepositories>


  <build>
    <plugins>
      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
      </plugin>
      <plugin>
        <groupId>org.jetbrains.kotlin</groupId>
        <artifactId>kotlin-maven-plugin</artifactId>
        <version>${kotlin.version}</version>
        <dependencies>
          <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-allopen</artifactId>
            <version>${kotlin.version}</version>
          </dependency>
          <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-noarg</artifactId>
            <version>${kotlin.version}</version>
          </dependency>
        </dependencies>
        <executions>
          <execution>
            <id>compile</id>
            <phase>compile</phase>
            <goals>
              <goal>compile</goal>
            </goals>
            <configuration>
              <sourceDirs>
                <sourceDir>${project.basedir}/src/main/kotlin</sourceDir>
                <sourceDir>${project.basedir}/src/main/java</sourceDir>
              </sourceDirs>
            </configuration>
          </execution>
          <execution>
            <id>test-compile</id>
            <phase>test-compile</phase>
            <goals>
              <goal>test-compile</goal>
            </goals>
            <configuration>
              <sourceDirs>
                <sourceDir>${project.basedir}/src/test/kotlin</sourceDir>
                <sourceDir>${project.basedir}/src/test/java</sourceDir>
              </sourceDirs>
            </configuration>
          </execution>
        </executions>
        <configuration>
          <jvmTarget>1.8</jvmTarget>
          <compilerPlugins>
            <plugin>jpa</plugin>
            <plugin>spring</plugin>
<!--            <plugin>no-arg</plugin>-->
          </compilerPlugins>
<!--          <pluginOptions>-->
<!--            <option>jpa:invokeInitializers=true</option>-->
<!--            <option>no-arg:invokeInitializers=true</option>-->
<!--          </pluginOptions>-->
        </configuration>
      </plugin>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.5.1</version>
        <executions>
          <!-- Replacing default-compile as it is treated specially by maven -->
          <execution>
            <id>default-compile</id>
            <phase>none</phase>
          </execution>
          <!-- Replacing default-testCompile as it is treated specially by maven -->
          <execution>
            <id>default-testCompile</id>
            <phase>none</phase>
          </execution>
          <execution>
            <id>java-compile</id>
            <phase>compile</phase>
            <goals>
              <goal>compile</goal>
            </goals>
          </execution>
          <execution>
            <id>java-test-compile</id>
            <phase>test-compile</phase>
            <goals>
              <goal>testCompile</goal>
            </goals>
            <configuration>
              <skip>${maven.test.skip}</skip>
            </configuration>
          </execution>
          <execution>
            <id>compile</id>
            <phase>compile</phase>
            <goals>
              <goal>compile</goal>
            </goals>
          </execution>
          <execution>
            <id>testCompile</id>
            <phase>test-compile</phase>
            <goals>
              <goal>testCompile</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
</project>

[Joaquim Costa]

this sample project shows the issue: https://github.com/joaquimscosta/javakoltinjpa-demo
see MyJavaService.java