Live-updating a Java AppEngine project's HTML with Maven

I recently switched all of my Java AppEngine projects to use Maven once the Google updated its Eclipse plugin to support Google Cloud SDK (instead of the AppEngine SDK); see the getting-started with Maven guide.  Google Cloud SDK is much more convenient for me because it gets regular updates via "apt-get" as the "google-cloud-sdk" package, plus it has a nice CLI for working with an app (great for emergency fixes via SSH).

Maven is relatively new to me; I tried it years ago, but it didn't play well with the AppEngine plugin for Eclipse, so I gave up.  Now that it's fully supported, I am very happy with it.  tl;dr is that it's the Java compiler and toolkit that you always thought existed, but didn't.

There is exactly one problem with the new Eclipse plugin for AppEngine/Maven support: the test server doesn't respect any of the plugin settings in my "pom.xml" file.  This means that it won't run on the port that I tell it to run on, etc.

That's not a huge deal because I can just run "mvn appengine:run" to run it perfectly via the command line (and I love my command line).

Aside: the command is actually slightly more involved since AppEngine needs Java 7 (as opposed to Java 9, which is what my desktop runs).
JAVA_HOME=/usr/lib/jvm/java-7-oracle/ mvn appengine:run

This worked perfectly while I was editing my Java source files.  The default "pom.xml" has a line in it that tells eclipse to compile its Java code to the appropriate Maven target directory, which means that the development server will reload itself anytime that, effectively, the Java source code changes.

In case you're wondering, that line is:
<!-- for hot reload of the web application--> <outputDirectory>${project.build.directory}/${project.build.finalName}/WEB-INF/classes</outputDirectory>

However, another time, I had to update the HTML portion of my app, and my changes weren't being applied.  I'd change a file, reload a million times, and nothing would work.  Restarting the development server would work, but that was crazy to me: why would the development server support a hot reload for Java source code changes but not for trivial static files (such as the HTML files)?

I spent hours and hours troubleshooting this very thing, and I finally learned how this whole Maven thing works:

1. Maven compiles the application (including the Java code) to a "target".
2. For development purposes, the target is a directory that can be scanned for changes; for production purposes, the target is a "jar" or "war" file, for example.
3. That "outputDirectory" tag in "pom.xml" tells Eclipse to compile its Java changes to the appropriate target directory, allowing a running development server to hot reload the Java changes.

When the target directory is built, all of the static files are copied to it.  When you edit such a file in Eclipse, you're editing the original file in the "src/main/webapp" folder, not the target folder that the development server is running from.

To solve the problem (that is, to add hot reload for static files), you simply need to mirror whatever change you made in the original folder to the copy in the target folder.

I spent too many hours trying to figure out how to do this in Eclipse and eventually settled on a short bash script that continually monitors the source directory using "inotifywait" for changes and then mirrors those changes in the target directory (as I said, I love my command line).

My solution is now to run "mvn appengine:run" in one terminal and this script in another (it uses "pom.xml" to figure out the target directory):

#!/bin/bash

declare -A commands;
commands[inotifywait]=inotify-tools;
commands[xmlstarlet]=xmlstarlet;

for command in "${!commands[@]}"; do
   if ! which "${command}" &>/dev/null; then
      echo "Could not find command: ${command}";
      echo "Please intall it by running:";
      echo "   sudo apt-get install ${commands[$command]}";
      exit 1;
   fi;
done;

artifactId=$( xmlstarlet sel -N my=http://maven.apache.org/POM/4.0.0 --template --value-of /my:project/my:artifactId pom.xml );
version=$( xmlstarlet sel -N my=http://maven.apache.org/POM/4.0.0 --template --value-of /my:project/my:version pom.xml );

sourceDirectory=src/main/webapp/;
sourceDirectoryLength="${#sourceDirectory}";
targetDirectory=target/"${artifactId}-${version}"/;

echo "Source directory: ${sourceDirectory}";
echo "Target directory: ${targetDirectory}";

inotifywait -m -r src/main/webapp/ -e modify -e moved_to -e moved_from -e create -e delete 2>/dev/null | while read -a line; do
   echo "Line: ${line[@]}";
   fullPath="${line[0]}";
   operation="${line[1]}";
   filename="${line[2]}";
   
   path="${fullPath:$sourceDirectoryLength}";

   echo "Operation: ${operation}";
   echo "   ${path} :: ${filename}";

   case "${operation}" in
      CREATE|MODIFY|MOVED_TO)
         cp -v -a -f "${fullPath}${filename}" "${targetDirectory}${path}${filename}";
         ;;
      DELETE|MOVED_FROM)
         rm -v -r -f "${targetDirectory}${path}${filename}";
         ;;
      *)
         echo "Unhandled operation: ${operation}";
         ;;
   esac;
done;