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HowTo create an ImageJ plugin jar file: Pitfalls and Tips

Brief documentation on how to create an ImageJ jar file plugin.

I recently created my first ImageJ plugin, and wanted to make it a jar file. Seemingly easy, I did not find any documentation on the exact process, which cost me nearly 2 hours and a trip to the ImageJ source code! In an effort to make life easier for you, gentle reader, I've documented some of the pitfalls.

  1. Plugins can go in the default package or a class with a package declaration.
  2. Be sure to include an '_' in the filename of the jar file! This is apparently undocumented but very important.
  3. Create a 'plugins.config' file as part of your jar. This tells ImageJ where to locate your plugin in it's menus.
  4. The whitespace separating each element in a line must be a space (\s) character and not a tab (\t).

The plugins.config has a format like this:

  # This is a comment
  # Each entry is menuname, "Command text", classname

  # This puts a command named "Foo" in the Plugins menu associated
  # with the "FancyPluginClass" class in the default package.
  Plugins, "Foo", FancyPluginClass
  #This puts a command named "Foo" in the Plugins>Moo menu associated with the "FooClass" class.
  Plugins>Moo, "Foo", FooClass
  #This puts a command named "Left" in the Plugins>Feet menu associated with the class
  Plugins>Feet, "Left",
  #This puts a separator between menu items
  Plugins>Feet, "-"
  #This puts a command named "Right" in the Plugins>Feet menu associated with the class
  Plugins>Feet, "Right",

plugins.config file pitfall

For each line, there should be no space after the class name. For example:

File>New, "Blob1", my.test.Test1
File>New, "Blob2", my.test.Test2 

These two lines might look same, but the first one works and the second not. This is because there is a space in the second line after my.test.Test2. It compiles Ok, but in runtime there will be an error message:

Plugin of class not found: “my.test.Test2”
(java.lang.ClassNotFoundException: my.test.Test2)

Which will then be a bug that is very difficult to debug. Check the trailing space first!


  • JAR Plugin demo documentation There is information on how to package plugins in JAR files in the JAR Plugin demo at:
  • Jar File naming The jar file itself must have an underscore in it, otherwise it won't be registered!
  • Plugin Architecture
    • detailed explanation on the architecture of plugin in Fiji site.

I hope this howto saves you all some time!

howto/plugins/howto_create_an_imagej_plugin_jar_file.txt · Last modified: 2019/04/12 13:13 by

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