Class JimfsFileSystem

java.lang.Object
java.nio.file.FileSystem
com.google.common.jimfs.JimfsFileSystem
All Implemented Interfaces:
Closeable, AutoCloseable
final class JimfsFileSystem extends FileSystem
FileSystem implementation for Jimfs. Most behavior for the file system is implemented by its default file system view.

Overview of file system design

JimfsFileSystem instances are created by JimfsFileSystems using a user-provided Configuration. The configuration is used to create the various classes that implement the file system with the correct settings and to create the file system root directories and working directory. The file system is then used to create the Path objects that all file system operations use.

Once created, the primary entry points to the file system are JimfsFileSystemProvider, which handles calls to methods in Files, and JimfsSecureDirectoryStream, which provides methods that are similar to those of the file system provider but which treat relative paths as relative to the stream's directory rather than the file system's working directory.

The implementation of the methods on both of those classes is handled by the FileSystemView class, which acts as a view of the file system with a specific working directory. The file system provider uses the file system's default view, while each secure directory stream uses a view specific to that stream.

File system views make use of the file system's singleton JimfsFileStore which handles file creation, storage and attributes. The file store delegates to several other classes to handle each of these:

Paths

The implementation of Path for the file system is JimfsPath. Paths are created by a PathService with help from the file system's configured PathType.

Paths are made up of Name objects, which also serve as the file names in directories. A name has two forms:

  • The display form is used in Path for toString(). It is also used for determining the equality and sort order of Path objects for most file systems.
  • The canonical form is used for equality of two Name objects. This affects the notion of name equality in the file system itself for file lookup. A file system may be configured to use the canonical form of the name for path equality (a Windows-like file system configuration does this, as the real Windows file system implementation uses case-insensitive equality for its path objects.

The canonical form of a name is created by applying a series of normalizations to the original string. These normalization may be either a Unicode normalization (e.g. NFD) or case folding normalization for case-insensitivity. Normalizations may also be applied to the display form of a name, but this is currently only done for a Mac OS X type configuration.

Files

All files in the file system are an instance of File. A file object contains both the file's attributes and content.

There are three types of files:

JimfsFileChannel, JimfsInputStream and JimfsOutputStream implement the standard channel/stream APIs for regular files.

JimfsSecureDirectoryStream handles reading the entries of a directory. The secure directory stream additionally contains a FileSystemView with its directory as the working directory, allowing for operations relative to the actual directory file rather than just the path to the file. This allows the operations to continue to work as expected even if the directory is moved.

A directory can be watched for changes using the WatchService implementation, PollingWatchService.

Regular files

RegularFile makes use of a singleton HeapDisk. A disk is a resizable factory and cache for fixed size blocks of memory. These blocks are allocated to files as needed and returned to the disk when a file is deleted or truncated. When cached free blocks are available, those blocks are allocated to files first. If more blocks are needed, they are created.

Linking

When a file is mapped to a file name in a directory table, it is linked. Each type of file has different rules governing how it is linked.
  • Directory - A directory has two or more links to it. The first is the link from its parent directory to it. This link is the name of the directory. The second is the self link (".") which links the directory to itself. The directory may also have any number of additional parent links ("..") from child directories back to it.
  • Regular file - A regular file has one link from its parent directory by default. However, regular files are also allowed to have any number of additional user-created hard links, from the same directory with different names and/or from other directories with any names.
  • Symbolic link - A symbolic link can only have one link, from its parent directory.

Thread safety

All file system operations should be safe in a multithreaded environment. The file hierarchy itself is protected by a file system level read-write lock. This ensures safety of all modifications to directory tables as well as atomicity of operations like file moves. Regular files are each protected by a read-write lock which is obtained for each read or write operation. File attributes are protected by synchronization on the file object itself.