public class SingleInstanceArgument
extends AbstractConsoleArgument

Handles arguments relating to the single instance listener feature of ImageJ 1.x.

This ConsoleArgument plugin is unusual in that it tries to go first, passing all command line arguments to an already-running instance of ImageJ as appropriate. In cases where it does so successfully, it then shuts down the application context.

The plugin scans the entire set of arguments to decide whether and how to attempt to contact another running instance of ImageJ.

The --forbid-single-instance flag, in all cases, forbids ImageJ from attempting to contact another running instance.

Conversely, the --force-single-instance flag encourages ImageJ to forward arguments to another running instance even if it otherwise would not do so.

The --portXXX argument sets the communication channel to use when communicating with the other instance. The default is 7, but a different value can be given to create multiple "single instances" of ImageJ on different channels.

If neither --forbid-single-instance nor --force-single-instance is given, the single instance logic is used if all of the following conditions are met:

• NOT running in headless mode.
• Neither the -batch nor -ijpath options have been specified.
• The "Run single instance listener" option in Edit > Options > Misc menu is enabled (on macOS, this option is always disabled unless toggled programmatically somehow in code).
• Another instance of ImageJ is in fact running on the same channel (port 7 by default).

Finally, a note for the debugger: this plugin logs debug output for all code paths, to make it easier to understand how and why the single instance logic is invoked or skipped. One nice way to enable it in a targeted way is to pass -Dscijava.log.level:net.imagej.legacy=debug on the "right hand side" (i.e. after a -- separator argument).