Compile and execute a single file to generate code before compiling all the code together - maven

Question

A maven project with some generated files:
my.package.R // generated from resources
my.package.ClassThatGeneratesRFromResources
my.package.AClassThatUsesR
my.package.AnotherClassThatUsesR
my.package.YetAnotherClassThatUsesR
When building, I would like to compile ClassThatGeneratesRFromResources, execute it (thus generating the R.java class) and then compile everything else together.
I can make this work with modules, a reactor, and isolating ClassThatGeneratesRFromResources in its own little module. However, I wonder why my ugly, clunky solution is not working. Here is my current POM:
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<executions>
<execution>
<id>
generate-r-file-creator
</id>
<configuration>
<phase>generate-sources</phase>
<goal>compile</goal>
<includes>
<include>my/package/ClassThatGeneratesRFromResources.java</include>
</includes>
</configuration>
</execution>
</executions>
</plugin>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>exec-maven-plugin</artifactId>
<version>1.1.1</version>
<executions>
<execution>
<id>generate-r-files</id>
<phase>process-sources</phase>
<goals>
<goal>java</goal>
</goals>
<configuration>
<mainClass>my.package.ClassThatGeneratesRFromResources</mainClass>
<arguments>
<argument>${basedir}</argument>
</arguments>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>
The result is that the initial compiler run fails, and then the code-generation run fails too (because ClassThatGeneratesRFromResources has not been compiled). Why is my compiler plugin not being triggered? How can I fix this (other than going modular)?