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

Issue

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?

Solution

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>

Answered By - Ashok
Answer Checked By - David Goodson (JavaFixing Volunteer)