Use Apache Commons IO for directory monitoring

Context and Problem Statement

In JabRef, there is a need to add a directory monitor that will listen for changes in a specified directory.

Currently, the monitor is used to automatically update the LaTeX Citations when a LaTeX file in the LaTeX directory is created, removed, or modified (#10585). Additionally, this monitor will be used to create a dynamic group that mirrors the file system structure (#10930).

Considered Options

Decision Outcome

Chosen option: “Use org.apache.commons.io.monitor”, because comes out best (see below).

Pros and Cons of the Options

java.nio.file.WatchService

Good, because it is a standard Java API for watching directories.
Good, because it does not need polling, it is event-based for most operating systems.
Bad, because:
1. Does not detect files coming together with a new folder (JDK issue: JDK-8162948).
2. Deleting a subdirectory does not detect deleted files in that directory.
3. Access denied when trying to delete the recursively watched directory on Windows (JDK issue: JDK-6972833).
4. Implemented on macOS by the generic PollingWatchService. (JDK issue: JDK-8293067)

io.methvin.watcher.DirectoryWatcher

Good, because it implemented on top of the java.nio.file.WatchService, which is a standard Java API for watching directories.
Good, because it resolves some of the issues of the java.nio.file.WatchService.
- UsesExtendedWatchEventModifier.FILE_TREE on Windows, which resolves issues (1, 3) of the java.nio.file.WatchService.
- On macOS have native implementation based on the Carbon File System Events API, this resolves issue (4) of the java.nio.file.WatchService.
Bad, because issue (2) of the java.nio.file.WatchService is not resolved.

org.apache.commons.io.monitor

Good, because there are no observed issues.
Good, because can handle huge amount of files without overflowing.
Bad, because it uses a polling mechanism at fixed intervals, which can waste CPU cycles if no change occurs.