Why deployment fails for spring boot open jdk 15 in heroku

Question

Heroku supports open JDK 15 https://devcenter.heroku.com/changelog-items/1887

I have a system.properties that contain java.runtime.version=15 in my resource. Here my 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 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>2.4.3</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.ma</groupId>
    <artifactId>currencyconverter</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>currencyconverter</name>
    <description>Demo project for Spring Boot</description>
    
    <properties>
        <java.version>15</java.version>
    </properties>
    
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <!-- For Json Parser -->
        <dependency>
            <groupId>com.google.code.gson</groupId>
            <artifactId>gson</artifactId>
            <version>2.8.6</version>
        </dependency>

        <!-- For swagger-annotations -->
        <dependency>
            <groupId>io.swagger</groupId>
            <artifactId>swagger-annotations</artifactId>
            <version>1.5.17</version>
        </dependency>

        <!-- springfox-swagger2 -->
        <dependency>
            <groupId>io.springfox</groupId>
            <artifactId>springfox-swagger2</artifactId>
            <version>3.0.0</version>
        </dependency>

        <!-- springfox-core -->
        <dependency>
            <groupId>io.springfox</groupId>
            <artifactId>springfox-core</artifactId>
            <version>3.0.0</version>
        </dependency>

        <!-- springfox-swagger-ui -->
        <dependency>
            <groupId>io.springfox</groupId>
            <artifactId>springfox-swagger-ui</artifactId>
            <version>3.0.0</version>
        </dependency>

        <!-- Swagger dependency -->
        <dependency>
            <groupId>io.springfox</groupId>
            <artifactId>springfox-boot-starter</artifactId>
            <version>3.0.0</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>

When the deployment starts in Heroku first few line of logs:-

-----> Building on the Heroku-20 stack
-----> Java app detected
-----> Installing JDK 1.8... done
-----> Executing Maven
$ ./mvnw -DskipTests clean dependency:list install
[INFO] Scanning for projects...
[INFO] Downloading from central:
https://repo.maven.apache.org/maven2/org/springframework/boot/spring-boot-starter-parent/2.4.3/spring-boot-starter-parent-2.4.3.pom

Why is it installing JDK 1.8?

Last few line of the log:-

  [ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.8.1:compile (default-compile) on project currencyconverter: Fatal error compiling: invalid target release: 15 -> [Help 1]
   [ERROR] 
   [ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
   [ERROR] Re-run Maven using the -X switch to enable full debug logging.
   [ERROR] 
   [ERROR] For more information about the errors and possible solutions, please read the following articles:
   [ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
   ERROR: Failed to build app with Maven
   We're sorry this build is failing! If you can't find the issue in application code,
   please submit a ticket so we can help: https://help.heroku.com/
   Push rejected, failed to compile Java app.
   Push failed

How do I deploy the JDK 15 app in Heroku?

Answer 1

I have a system.properties that contain java.runtime.version=15 in my resource.

You placed it in /src/main/resources.

You are supposed to place it in the root folder / of your git repo.

---

In my case the content of my system.properties is java.runtime.version=10.0.2 and is placed in the root directory of my git repo.

In the build.log immediately after Java app detected it should install the specified Java version: