The set of IntelliJ IDEA features which are supported for custom languages includes:
- Syntax highlighting and error highlighting;
- Go to Declaration (including Quick Definition Lookup);
- Code completion;
- Find Usages;
- Rename and Safe Delete refactorings;
- Code formatter;
- Code inspections and intentions;
- Structure View (including the File Structure Popup);
- Surround With;
- Go to Class, Go to Symbol;
- Brace matching;
- Code folding;
- Comment Code;
- To Do view;
- and much more.
In addition, IDEA provides a powerful framework on which additional intelligence features, like refactorings and code analysis, can be implemented.
If you have any questions or comments related to the Language API or any other aspects of IntelliJ IDEA plugin development, feel free to ask them in the jetbrains.intellij.openapi newsgroup on the news.jetbrains.com news server, or in the corresponding Web forum. The newsgroup is monitored by JetBrains developers who will be able to help you with the development.
The information in this document has been updated to cover the API changes and new features of IntelliJ IDEA 8.0.
Registering a File Type
The first step in developing a custom language plugin is registering a file type the language will be associated with. IDEA determines the type of a file by looking at its file name. Thus, a custom language can only be associated with specific file names or extensions - it is not currently possible to create a language which will be applied to files with specific content, like, for example, a specific XML root namespace.
A custom language file type is a class derived from LanguageFileType, which passes a Language implementation class to its base class constructor. To register a file type, the plugin developer provides an implementation of the FileTypeFactory interface, which is registered via the com.intellij.fileTypeFactory extension point.
To verify that the file type is indeed registered correctly, you can implement the LanguageFileType.getIcon() method and verify that the correct icon is displayed for files which have the extension associated with your file type.
Implementing a Lexer
The lexer (lexical analyzer) defines how the contents of a file is broken into tokens. The lexer serves as a foundation for nearly all of the features of custom language plugins, from basic syntax highlighting to advanced code analysis features. The API for the lexer is defined by the Lexer interface.
IDEA invokes the lexer in three main contexts, and the plugin can provide different lexer implementations for these contexts:
- 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.
The lexer used for syntax highlighting can be invoked incrementally to process only the changed part of a file, whereas lexers used in other contexts are always called to process an entire file, or a complete language construction embedded in a file in a different language. An important requirement for a syntax highlighting lexer, required for incremental lexing, is that its state must be represented by a single integer number (returned from Lexer.getState()). That state will be passed to the Lexer.start() method, along with the start offset of the fragment to process, when lexing is resumed from the middle of a file. Lexers used in other contexts can always return 0 from the getState() method if their state has a more complex internal representation.
The easiest way to create a lexer for a custom language plugin is to use JFlex. IDEA contains adapter classes (FlexLexer and FlexAdapter) that adapt JFlex lexers to the IDEA lexer API. The Plugin Development package includes a patched version of JFlex 1.4.1 (tools/jflex) and lexer skeleton file (tools/jflex/idea-flex.skeleton) which can be used for creating lexers compatible with FlexAdapter. The patched version of JFlex provides a new command line option --charat which changes the JFlex generated code so that it works with the IDEA skeleton (which passes the source data for lexing as a CharSequence and not as an array of characters).
Note that lexers, and in particular JFlex-based lexers, need to be created in such a way that they always match the entire contents of the file, without any gaps between tokens, and generate special tokens for characters which are not valid at their location. Lexers must never abort prematurely because of an invalid character.
Types of tokens for lexers used in IDEA are defined by instances of IElementType. A number of token types common for all languages are defined in the TokenType interface; custom language plugins should reuse these token types wherever applicable. For all other token types, the plugin needs to create new IElementType instances and associate with the language in which the token type is used. The same IElementType instance should be returned every time a particular token type is encountered by the lexer.
An important feature which can be implemented at lexer level is mixing languages within a file (for example, embedding fragments of Java code in some template language). If a language supports embedding its fragments in another language, it needs to define the chameleon token types for different types of fragments which can be embedded, and these token types need to implement the IChameleonElementType interface. The lexer of the enclosing language needs to return the entire fragment of the embedded language as a single chameleon token, of the type defined by the embedded language. To parse the contents of the chameleon token, IDEA will call the parser of the embedded language through a call to IChameleonElementType.parseContents().
Implementing a Parser and PSI
Parsing files in IDEA is a two-step process. First, an abstract syntax tree (AST) is built, defining the structure of the program. AST nodes are created internally by IDEA and are represented by instances of the ASTNode class. Each AST node has an associated element type (IElementType instance), and the element types are defined by the language plugin. The top-level node of the AST tree for a file needs to have a special element type, implementing the IFileElementType interface.
The AST nodes have a direct mapping to text ranges in the underlying document (the bottom-most nodes of the AST match individual tokens returned by the lexer, and higher level nodes match multiple-token fragments). Operations performed on nodes of the AST tree (inserting, removing, reordering nodes and so on) are immediately reflected as changes to the text of the underlying document.
Second, a PSI (Program Structure Interface) tree is built on top of the AST, adding semantics and methods for manipulating specific language constructs. Nodes of the PSI tree are represented by classes implementing the PsiElement interface and are created by the language plugin in the ParserDefinition.createElement() method. The top-level node of the PSI tree for a file needs to implement the PsiFile interface, and is created in the ParserDefinition.createFile() method.
The process of building the AST and PSI trees for a file is invoked on demand, when some component of IDEA tries to access the PSI for a file. The PSI is always built when a file is opened in the editor, and it can also be built when a file is affected by a multi-file operation (inspection, refactoring, batch reformat and so on). After a document has been changed, building a new PSI is initiated by committing the document. Methods for committing a specific document or all documents are found in the PsiDocumentManager interface. The documents are committed by all IDEA components which need to access the PSI, and in particular by the thread which performs background highlighting.
The base classes for the PSI implementation (PsiFileBase, the base implementation of PsiFile, and ASTWrapperPsiElement, the base implementation of PsiElement) are provided by IDEA. However, these classes are coupled to the internal implementation of IDEA and are located in idea.jar. Because of this, every custom language plugin needs to include idea.jar in its classpath. (If the plugin is built as a DevKit project, idea.jar must be added to the list of JARs in the classpath of the IDEA SDK, and not added as a separate module or project library. Otherwise, the plugin will not work correctly.)
IDEA currently does not provide a ready way to reuse existing language grammars (for example, from ANTLR) for creating custom language parsers. The parsers need to be coded manually, as a recursive descent implementation.
The language plugin provides the parser implementation as an implementation of the PsiParser interface, returned from ParserDefinition.createParser(). The parser receives an instance of the PsiBuilder class, which is used to get the stream of tokens from the lexer and to hold the intermediate state of the AST being built. The parser must process all tokens returned by the lexer up to the end of stream (until PsiBuilder.getTokenType() returns null), even if the tokens are not valid according to the language syntax.
The parser works by setting pairs of markers (PsiBuilder.Marker instances) within the stream of tokens received from the lexer. Each pair of markers defines the range of lexer tokens for a single node in the AST tree. If a pair of markers is nested in another pair (starts after its start and ends before its end), it becomes the child node of the outer pair.
The element type for the marker pair (and for the AST node created from it) is specified when the end marker is set (by a call to PsiBuilder.Marker.done()). Also, it is possible to drop a start marker before its end marker has been set. The drop() method drops only a single start marker without affecting any markers added after it, and the rollbackTo() method drops the start marker and all markers added after it and reverts the lexer position to the start marker. These methods can be used to implement lookahead when parsing.
The method PsiBuilder.marker.precede() is useful for right-to-left parsing when you don't know how many markers you need at a certain position until you read more input. For example, a binary expression a+b+c needs to be parsed as
( (a+b) + c ). Thus, two start markers are needed at the position of the token 'a', but that is not known until the token 'c' is read. When the parser reaches the '+' token following 'b', it can call precede() to duplicate the start marker at 'a' position, and then put its matching end marker after 'c'.
An important feature of PsiBuilder is its handling of whitespace and comments. The types of tokens which are treated as whitespace or comments are defined by the methods getWhitespaceTokens() and getCommentTokens() in the ParserDefinition class. PsiBuilder automatically omits whitespace and comment tokens from the stream of tokens it passes to PsiParser, and adjusts the token ranges of AST nodes so that leading and trailing whitespace tokens are not included in the node.
The token set returned from
ParserDefinition.getCommentTokens() is also used to search for TO DO items.
In order to better understand the process of building a PSI tree for a simple expression, you can refer to the attached diagram.
In general, there is no single right way to implement a PSI for a custom language, and the plugin author can choose the PSI structure and set of methods which are the most convenient for the code which uses the PSI (error analysis, refactorings and so on). However, there is one base interface which needs to be used by a custom language PSI implementation in order to support features like rename and find usages. Every element which can be renamed or referenced (a class definition, a method definition and so on) needs to implement the PsiNamedElement interface, with methods
A number of functions which can be used for implementing and using the PSI can be found in the com.intellij.psi.util package, and in particular in the PsiUtil and PsiTreeUtil classes.
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.
to be continued