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52 lines
3.3 KiB
Markdown
52 lines
3.3 KiB
Markdown
---
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title: PSI Files
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---
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A PSI (Program Structure Interface) file is the root of a structure representing the contents of a file as a hierarchy of elements in a particular programming language.
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The [`PsiFile`](upsource:///platform/core-api/src/com/intellij/psi/PsiFile.java) class is the common base class for all PSI files, while files in a specific language are usually represented by its subclasses. For example, the [`PsiJavaFile`](upsource:///java/java-psi-api/src/com/intellij/psi/PsiJavaFile.java) class represents a Java file, and the [`XmlFile`](upsource:///xml/xml-psi-api/src/com/intellij/psi/xml/XmlFile.java) class represents an XML file.
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Unlike `VirtualFile` and `Document`, which have application scope (even if multiple projects are open, each file is represented by the same `VirtualFile` instance), PSI has project scope (the same file is represented by multiple `PsiFile` instances if the file belongs to multiple projects open at the same time).
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## How do I get a PSI file?
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* From an action: `e.getData(LangDataKeys.PSI_FILE)`.
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* From a VirtualFile: `PsiManager.getInstance(project).findFile()`
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* From a Document: `PsiDocumentManager.getInstance(project).getPsiFile()`
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* From an element inside the file: `psiElement.getContainingFile()`
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* To find files with a specific name anywhere in the project, use `FilenameIndex.getFilesByName(project, name, scope)`
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## What can I do with a PSI file?
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Most interesting modification operations are performed on the level of individual PSI elements, not files as a whole.
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To iterate over the elements in a file, use `psiFile.accept(new PsiRecursiveElementWalkingVisitor()...);`
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## Where does a PSI file come from?
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As PSI is language-dependent, PSI files are created through the [`Language`](upsource:///platform/core-api/src/com/intellij/lang/Language.java) object, by using the `LanguageParserDefinitions.INSTANCE.forLanguage(language).createFile(fileViewProvider)` method.
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Like documents, PSI files are created on demand when the PSI is accessed for a particular file.
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## How long do PSI files persist?
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Like documents, PSI files are weakly referenced from the corresponding `VirtualFile` instances and can be garbage-collected if not referenced by anyone.
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## How do I create a PSI file?
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The [`PsiFileFactory`](upsource:///platform/core-api/src/com/intellij/psi/PsiFileFactory.java) `createFileFromText()` method creates an in-memory PSI file with the specified contents.
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To save the PSI file to disk, use the [`PsiDirectory`](upsource:///platform/core-api/src/com/intellij/psi/PsiDirectory.java) `add()` method.
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## How do I get notified when PSI files change?
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`PsiManager.getInstance(project).addPsiTreeChangeListener()` allows you to receive notifications about all changes to the PSI tree of a project.
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## How do I extend PSI?
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PSI can be extended to support additional languages through custom language plugins. For more details on developing custom language plugins, see the [Custom Language Support](/reference_guide/custom_language_support.md) reference guide.
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## What are the rules for working with PSI?
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Any changes done to the content of PSI files are reflected in documents, so all [rules for working with documents](documents.md#what-are-the-rules-of-working-with-documents) (read/write actions, commands, read-only status handling) are in effect.
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