maven-exec-plugin JVM arguments aren't getting passed to the VM

<plugin>
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>exec-maven-plugin</artifactId>
    <!-- WARNING: This configuration must be run with "mvn exec:exec" not "mvn exec:java". -->
    <!-- It is impossible to write a configuration that is compatible with both exec:java and exec:exec -->
    <configuration>
        <executable>java</executable>
        <arguments>
        <argument>-Xms256m</argument>
        <!-- Most examples run (potentially slower) with max heap of 128 MB (so -Xmx128m), but 1 example's dataset requires 1.5 GB -->
        <argument>-Xmx1536m</argument>
        <argument>-server</argument>
        <argument>-classpath</argument>
        <classpath />
        <argument>org.optaplanner.examples.app.OptaPlannerExamplesApp</argument>
        </arguments>
    </configuration>
</plugin>

package org.optaplanner.examples.app;

import java.lang.management.ManagementFactory;
import java.lang.management.RuntimeMXBean;
import java.util.Arrays;
import java.util.List;

public class OptaPlannerExamplesAppTest {

    public static void main(String[] args) {
        System.out.println("Command-line arguments: " + Arrays.toString(args));
        RuntimeMXBean runtimeMxBean = ManagementFactory.getRuntimeMXBean();
        List<String> arguments = runtimeMxBean.getInputArguments();
        System.out.println("JVM arguments: " + arguments);
        System.exit(0);
    }
}