2016-01-21 by Vladimir Schneider

The Best IntelliJ Plugin Examples

The best source of examples of IntelliJ plugins is the intellij-community source code. It is also the biggest code base of plugins because most of the product functionality is implemented as extensions to the base architecture.

Figuring out how to find your way around is a challenge. I constantly find myself distracted by interesting code that I know I will need in the future. It is very much like looking things up in an encyclopedia (that's Wikipedia on paper for those born after the internet revolution) and spending an hour reading other interesting stuff, never getting to the item that you intended to look up.

As an example, I will take you through the steps I used to figure out how and where to add "Collapse by default" options that I could see in the GUI of IDEA but had no clue how to go about implementing these for my plugin.


I am assuming you have the IntelliJ IDEA installed and have gotten to the point where you can search the source, if not yet debug your own copy, of intellij-community.

Goal: Implement collapse by default configuration

I wanted to implement user configurable "Collapse by default" settings for markdown elements in my plugin. These are available for Java and other custom languages in settings/preferences under Editor > General > Code Folding

Now, I could see that plugins extend this list but where to start? Choose the most unlikely text to be repeated all over the source code and search intellij-community for it. I chose I18n strings as a promising candidate. Search in path for it at the root of the project:

Hit find and keep your fingers crossed. You can also use the preview tab but I prefer to see the results in the find tool window that way I can explore various options without having to search again. Pay dirt! We have the bundle that is used for the localization of strings:

Now we repeat the process but this time searching for the key checkbox.collapse.i18n.messages

Again, pay dirt JavaCodeFoldingOptionsProvider sounds like the class that handles customization of folding settings. In general it will be a provider for settings or a handler for actions.

Now what the hell are those screaming snake case ids in strings? IntelliJ uses Java reflection API a lot so I suspect that these are properties of a settings provider. Well, search in path to the rescue:

I was on a lucky streak: JavaCodeFoldingSettingsBase looks perfect for what we are looking for.

Now, I already know that both the JavaCodeFoldingSettingsBase and JavaCodeFoldingOptionsProvider need to have extension entries in the plugin.xml but which ones? Easiest part of the search. Just put your cursor on the identifier and use the Navigate to Declaration. For JavaCodeFoldingSettingsBase you are first taken to abstract JavaCodeFoldingSettings class and from there to the IdeaPlugin.xml file's entry for the service provider.

And the same procedure to find the entry for JavaCodeFoldingSettingsBase:

Now it is only a matter of implementing your own classes and registering them in your plugin.xml file.

90% of the time the hard part is finding what 3 lines of code to implement and how to get the IDE to know that those lines exist. Writing the code itself is easy. Sometimes it takes the implementation of multiple providers to have a working feature, as in the case of IntelliLang language injections, but that I will leave for another blog.


Constructive comments and suggestions are welcome. I can also be reached me at vladimir@vladsch.com