Heroku not allowing creating/reading from process

I was working on a project involving reading from another process when I noticed that it kept throwing java.util.NoSuchElementException: No line found, so I made a Maven test application to check that it was Heroku's fault instead of my program's.

src/main/java/com/test/App.java:

package com.test;
import java.util.Arrays;
import java.util.Scanner;
import java.io.IOException;
public class App{
public static void main(String[] args) throws IOException{
System.out.println(new Scanner(new ProcessBuilder(Arrays.asList("java", "A")).start().getInputStream()).nextLine());
}
}

A.java:

public class A{
public static void main(String[] args){
System.out.println("Test");
}
}

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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.test</groupId>
  <artifactId>test</artifactId>
  <version>1.0</version>

  <name>test</name>
  <!-- FIXME change it to the project's website -->
  <url>http://www.example.com</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.7</maven.compiler.source>
    <maven.compiler.target>1.7</maven.compiler.target>
  </properties>

  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.11</version>
      <scope>test</scope>
    </dependency>
  </dependencies>

  <build>
    <pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->
      <plugins>
        <!-- clean lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#clean_Lifecycle -->
        <plugin>
          <artifactId>maven-clean-plugin</artifactId>
          <version>3.1.0</version>
        </plugin>
        <!-- default lifecycle, jar packaging: see https://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_jar_packaging -->
        <plugin>
          <artifactId>maven-resources-plugin</artifactId>
          <version>3.0.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-compiler-plugin</artifactId>
          <version>3.8.0</version>
        </plugin>
        <plugin>
          <artifactId>maven-surefire-plugin</artifactId>
          <version>2.22.1</version>
        </plugin>
        <plugin>
          <artifactId>maven-jar-plugin</artifactId>
          <version>3.0.2</version>
          <configuration>
            <archive>
              <manifest>
                <mainClass>com.test.App</mainClass>
              </manifest>
            </archive>
          </configuration>
        </plugin>
        <plugin>
          <artifactId>maven-install-plugin</artifactId>
          <version>2.5.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-deploy-plugin</artifactId>
          <version>2.8.2</version>
        </plugin>
        <!-- site lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#site_Lifecycle -->
        <plugin>
          <artifactId>maven-site-plugin</artifactId>
          <version>3.7.1</version>
        </plugin>
        <plugin>
          <artifactId>maven-project-info-reports-plugin</artifactId>
          <version>3.0.0</version>
        </plugin>
      </plugins>
    </pluginManagement>
  </build>
</project>

After javac A.java, mvn install, and java -jar target/test-1.0.jar, it prints "Test" on my computer, but throws a NoSuchElementException in Heroku.

Full Stacktrace:

Exception in thread "main" java.util.NoSuchElementException: No line found
at java.util.Scanner.nextLine(Scanner.java:1540)
at com.test.App.main(App.java:7)

Does anyone know what's causing this and how to fix it?

Edit: Attatched Github repo to hopefully make it clearer

Edit Edit: Oddly, if I put A.java in src/main/java and use the command from @Malax's answer, it works, but I still don't get why Heroku isn't seeing the java file outside of src. Maybe it's trimming them out to save space?

The issue occurs because you're calling nextLine on Scanner without checking if there even is a line. You need to check with hasNext to ensure nextLine will not fail.

The underlying reason why you get no lines is that you're not specifying a classpath when invoking java. java will not be able to find your compiled A class without it and fail with something like

Error: Could not find or load main class A
Caused by: java.lang.ClassNotFoundException: A

on STDERR which you don't capture with getInputStream. You will be able to read the error message when you use getErrorStream instead (but then you won't get the STDOUT messages).

This has nothing to do with Heroku and I can only speculate why this is working for you locally. To provide java a classpath, you can use -cp:

ProcessBuilder processBuilder = new ProcessBuilder(Arrays.asList("java", "-cp", "target/classes", "A"));
Scanner scanner = new Scanner(processBuilder.start().getInputStream());
while (scanner.hasNext()) {
System.out.println(scanner.nextLine());
}