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  • Developing Custom Language Plugins for IntelliJ IDEA

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  • Syntax highlighting: The lexer is returned from the implementation of the SyntaxHighlighterFactory interface which is registed in the com.intellij.lang.syntaxHighlighterFactory extension point. the lexer is expected to be returned from Language.getSyntaxHighlighter().getHighlightingLexer();
  • Building the syntax tree of a file: the lexer is expected to be returned from ParserDefinition.createLexer(), and the ParserDefinition interface is registered in the com.intellij.lang.parserDefinition extension point.
  • Building the index of the words contained in the file: If the lexer-based words scanner implementation is used, the lexer is passed to the DefaultWordsScanner constructor.


A very helpful tool for debugging the PSI implementation is the PsiViewer plugin. It can show you the structure of the PSI built by your plugin, the properties of every PSI element and highlight the text range of every PSI element.

Syntax Highlighting and Error Highlighting

The class used in IDEA to specify how a particular range of text should be highlighted is called TextAttributesKey. An instance of this class is created for every distinct type of item which should be highlighted (keyword, number, string and so on). The TextAttributesKey defines the default attributes which are applied to items of the corresponding type (for example, keywords are bold, numbers are blue, strings are bold and green). The mapping of the TextAttributesKey to specific attributes used in an editor is defined by the EditorColorsScheme class, and can be configured by the user if the plugin provides an appropriate configuration interface. Highlighting from multiple TextAttributeKey items can be overlaid - for example, one key may define an item's boldness and another its color.

The syntax and error highlighting is performed on multiple levels. The first level of syntax highlighting is based on the lexer output, and is provided through the SyntaxHighlighter interface. The syntax highligher returns the TextAttributeKey instances for each token type which needs special highlighting. For highlighting lexer errors, the standard TextAttributeKey for bad characters (HighligherColors.BAD_CHARACTER) can be used.

The second level of error highlighting happens during parsing. If a particular sequence of tokens is invalid according to the grammar of the language, the PsiBuilder.error() method can be used to highlight the invalid tokens and display an error message showing why they are not valid.

The third level of highlighting is performed through the Annotator interface. If the plugin returns an instance of Annotator from Language.getAnnotator(), the annotator is called during the background highlighting pass to process the elements in the PSI tree of the custom language. The annotator can analyze not only the syntax, but also the semantics of the text in the language, and thus can provide much more complex syntax and error highlighting logic. The annotator can also provide quick fixes to problems it detects

to be continued