Heroku is installing the wrong JDK for my App and failing to deploy it

Question

I'm trying to deploy my spring boot app (JDK 15.0.1) on heroku through github, but I get the error message

Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.1:compile (default-compile) on project conference-demo: Fatal error compiling: invalid target release: 15 -> [Help 1]

I already added a file called system.properties to my application resources folder, with the following content

java.runtime.version =15

And here's the pom

<?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.myPackage</groupId>
    <artifactId>conference-demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>myApp</name>
    <description>Something</description>
    <properties>
        <java.version>15</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.23</version>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

But heroku still seems to be trying to deploy with Java 1.8. The first message the build log shows is

-----> Building on the Heroku-20 stack

-----> Java app detected

-----> Installing JDK 1.8... done

Is there something I'm missing?

Answer 1

In your pom.xml add your own context as below. Paste this code in effective POM before the root element ends, after declaring dependencies, and adjust the version as per your requirement.

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.5.1</version>
    <configuration>
        <source>1.8</source>
        <target>1.8</target>
    </configuration>
</plugin>