Syntax and Error Highlighting
The syntax and error highlighting are performed on multiple levels: Lexer, Parser, and Annotator/External Annotator.
Text Attributes Key
How a particular range of text should be highlighted is defined via TextAttributesKey. An instance of this class is created for every distinct type of item that should be highlighted (keyword, number, string literal, etc.).
The TextAttributesKey defines the default attributes applied to items of the corresponding type (for example, keywords are bold, numbers are blue, string literals are bold and green). Highlighting from multiple TextAttributesKey items can be layered — for example, one key may define an item's boldness and another one its color.
Color Settings
The mapping of the TextAttributesKey to specific attributes used in an editor is defined by the EditorColorsScheme class. It can be configured by the user via by providing an implementation of ColorSettingPage registered in com.intellij.colorSettingsPage extension point. To lookup external name for a setting in the IDE, use UI Inspector.
The feature uses the same syntax highlighting mechanism as the editor. Thus, it will work automatically for custom languages that provide a syntax highlighter.
Examples:
Lexer
The first syntax highlighting level is based on the lexer output and is provided through the SyntaxHighlighter interface. The syntax highlighter returns the TextAttributesKey instances for each token type, which needs special highlighting. For highlighting lexer errors HighlighterColors.BAD_CHARACTER should be used.
Examples:
SyntaxHighlighterimplementation for Properties language plugin
Semantic Highlighting
Semantic highlighting provides an additional coloring layer to improve the visual distinction of several related items (e.g., method parameters, local variables).
Register RainbowVisitor in com.intellij.highlightVisitor extension point. Color Settings must implement RainbowColorSettingsPage in addition.
Parser
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 highlight the invalid tokens and display an error message showing why they are not valid.
See Syntax Errors on how to programmatically suppress these errors in certain contexts.
Annotator
The third level of highlighting is performed through the Annotator interface. A plugin can register one or more annotators in the com.intellij.annotator extension point, and these annotators are called during the background highlighting pass to process the elements in the custom language's PSI tree. Attribute language should be set to the Language ID where this annotator applies to.
Annotators can analyze not only the syntax, but also the semantics using PSI, and thus can provide much more complex syntax and error highlighting logic. The annotator can also provide quick fixes to problems it detects. When the file is changed, the annotator is called incrementally to process only changed elements in the PSI tree.
Annotators not requiring information from indexes can be marked dumb aware to work during indexing (e.g., for additional syntax highlighting). (2023.1+)
Errors/Warning
See Inspections topic in IntelliJ Platform UI Guidelines on how to write message texts for highlighting/quick fixes.
To highlight a region of text as a warning or error:
Call createWarningAnnotation()/createErrorAnnotation() on the AnnotationHolder, and optionally calls registerFix() on the returned Annotation object to add a quick fix for the error or warning.
Syntax
To apply additional syntax highlighting:
Call AnnotationHolder.createInfoAnnotation() with an empty message and then Annotation.setTextAttributes().
Examples:
External Annotator
If the custom language employs external tools for validating files in the language (for example, uses the Xerces library for XML schema validation), it can provide an implementation of the ExternalAnnotator interface and register it in com.intellij.externalAnnotator extension point (language attribute must be specified).
The ExternalAnnotator highlighting has the lowest priority and is invoked only after all other background processing has completed. It uses the same AnnotationHolder interface for converting the output of the external tool into editor highlighting.
To skip running specific ExternalAnnotator for given file, register ExternalAnnotatorsFilter extension in com.intellij.daemon.externalAnnotatorsFilter extension point.
To enable running ExternalAnnotator during indexing in dumb mode, it can be marked dumb aware (2023.3).
Controlling Highlighting
Existing highlighting can be suppressed programmatically in certain contexts, see Controlling Highlighting.
To force re-highlighting all open or specific file(s) (e.g., after changing plugin specific settings), use DaemonCodeAnalyzer.restart().
Order of Running Highlighting
Since 2024.1, inspections and annotators do not run sequentially on each PsiElement anymore. Instead, they're run in parallel on all relevant PSI independently with the following consequences.
Independent Annotators
Annotators are run independent of each other: if an annotator found an error, it no longer stops annotating the PsiElement's parents. Effectively, there is "more" highlighting now.
Highlight Range
Producing highlights must be done as close as possible for the relevant PsiElement. For example, instead of
this approach should be used:
The latter version:
performs faster highlighting — it doesn’t have to wait until all other identifiers are visited
removes outdated highlights faster — right after the identifier was visited and the annotator didn't produce a highlighting anymore