[//]: # (title: PSI Files)

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A PSI (Program Structure Interface) file is the root of a structure representing a file's contents as a hierarchy of elements in a particular programming language.

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.

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.

## How do I get a PSI file?

| Context                          | API                                                                                                                                                                      |
|----------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| [Action](basic_action_system.md) | [`AnActionEvent.getData(CommonDataKeys.PSI_FILE)`](upsource:///platform/editor-ui-api/src/com/intellij/openapi/actionSystem/AnActionEvent.java)                          |
| [Document](documents.md)         | [`PsiDocumentManager.getInstance(project).getPsiFile()`](upsource:///platform/core-api/src/com/intellij/psi/PsiDocumentManager.java)                                     |
| [PSI Element](psi_elements.md)   | [`PsiElement.getContainingFile()`](upsource:///platform/core-api/src/com/intellij/psi/PsiElement.java) (may return `null` if the PSI element is not contained in a file) |
| [Virtual File](virtual_file.md)  | [`PsiManager.getInstance(project).findFile()`](upsource:///platform/core-api/src/com/intellij/psi/PsiManager.java)                                                       |
| File Name                        | [`FilenameIndex.getFilesByName(project, name, scope)`](upsource:///platform/indexing-api/src/com/intellij/psi/search/FilenameIndex.java)                                 |

## What can I do with a PSI file?

Most interesting modification operations are performed on the level of individual PSI elements, not files as a whole.

To iterate over the elements in a file, use

```java
psiFile.accept(new PsiRecursiveElementWalkingVisitor() {
  // visitor implementation ...
});
```

See also [Navigating the PSI](navigating_psi.md).

## Where does a PSI file come from?

As PSI is language-dependent, PSI files are created using the [`Language`](upsource:///platform/core-api/src/com/intellij/lang/Language.java) instance:

```java
LanguageParserDefinitions.INSTANCE
    .forLanguage(MyLanguage.INSTANCE)
    .createFile(fileViewProvider);
```

Like documents, PSI files are created on-demand when the PSI is accessed for a particular file.

## How long do PSI files persist?

Like documents, PSI files are weakly referenced from the corresponding `VirtualFile` instances and can be garbage-collected if not referenced by anyone.

## How do I create a PSI file?

The [`PsiFileFactory`](upsource:///platform/core-api/src/com/intellij/psi/PsiFileFactory.java) `createFileFromText()` method creates an in-memory PSI file with the specified contents.

To save the PSI file to disk, use the [`PsiDirectory`](upsource:///platform/core-api/src/com/intellij/psi/PsiDirectory.java) `add()` method.

## How do I get notified when PSI files change?

`PsiManager.getInstance(project).addPsiTreeChangeListener()` allows you to receive notifications about all changes to the PSI tree of a project.
Alternatively, register [`PsiTreeChangeListener`](upsource:///platform/core-api/src/com/intellij/psi/PsiTreeChangeListener.java) in `com.intellij.psi.treeChangeListener` extension point.

> Please see [`PsiTreeChangeEvent`](upsource:///platform/core-api/src/com/intellij/psi/PsiTreeChangeEvent.java) Javadoc for common problems when dealing with PSI events.
>
{type="note"}

## How do I extend PSI?

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](custom_language_support.md) reference guide.

## What are the rules for working with PSI?

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.