Syntax and Error Highlighting

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.

tip

Looking up existing TextAttributeKey

To inspect applied TextAttributesKey (s) in the editor for the element at the caret, use Jump to Colors and Fonts action.

The underlying TextAttributeKey's external name for items in Settings | Editor | Color Scheme can be inspected using UI Inspector.

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 Settings | Editor | Color Scheme by providing an implementation of ColorSettingPage registered in com.intellij.colorSettingsPage extension point. To look up the external name for a setting in the IDE, use UI Inspector.

The File | Export | Files or Selection to HTML feature uses the same syntax highlighting mechanism as the editor. Thus, it will work automatically for custom languages that provide a syntax highlighter.

Examples:

• ColorSettingsPage for Properties language plugin

• Custom Language Support Tutorial: Color Settings Page

note

See note about Language Defaults and support for additional color schemes in Color Scheme Management.

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:

• SyntaxHighlighter implementation for Properties language plugin

• Custom Language Support Tutorial: Syntax Highlighter

tip

Creating highlighted code sample

Use HtmlSyntaxInfoUtil to create Lexer-based highlighted code samples, e.g., for usage in documentation.

The second level of error highlighting happens during parsing. If according to the grammar of the language a particular sequence of tokens is invalid, 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.

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. If highlighting data requires invoking external tools, use External Annotator instead.

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+)

note

See also Code Inspections which offer more fine-grained control and some additional features.

holder.newAnnotation(HighlightSeverity.WARNING, "Invalid code") // or HighlightSeverity.ERROR
    .withFix(new MyFix(psiElement))
    .create();

holder.newSilentAnnotation(HighlightSeverity.INFORMATION)
    .range(rangeToHighlight)
    .textAttributes(MyHighlighter.EXTRA_HIGHLIGHT_ATTRIBUTE)
    .create();

If the custom language employs external tools for validating files in the language (for example, using 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).

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().

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

annotate(PsiFile) {
  <<highlight all relevant identifiers>>
}

this approach should be used:

annotate(PsiIdentifier) {
  <<highlight this identifier if it's relevant>>
}

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