Google is committed to advancing racial equity for Black communities. See how.

AtomicFile

public class AtomicFile
extends Object

java.lang.Object
   ↳ androidx.core.util.AtomicFile


Static library support version of the framework's AtomicFile, a helper class for performing atomic operations on a file by writing to a new file and renaming it into the place of the original file after the write has successfully completed.

Atomic file guarantees file integrity by ensuring that a file has been completely written and sync'd to disk before renaming it to the original file. Previously this is done by renaming the original file to a backup file beforehand, but this approach couldn't handle the case where the file is created for the first time. This class will also handle the backup file created by the old implementation properly.

Atomic file does not confer any file locking semantics. Do not use this class when the file may be accessed or modified concurrently by multiple threads or processes. The caller is responsible for ensuring appropriate mutual exclusion invariants whenever it accesses the file.

Summary

Public constructors

AtomicFile(File baseName)

Create a new AtomicFile for a file located at the given File path.

Public methods

void delete()

Delete the atomic file.

void failWrite(FileOutputStream str)

Call when you have failed for some reason at writing to the stream returned by startWrite().

void finishWrite(FileOutputStream str)

Call when you have successfully finished writing to the stream returned by startWrite().

File getBaseFile()

Return the path to the base file.

FileInputStream openRead()

Open the atomic file for reading.

byte[] readFully()

A convenience for openRead() that also reads all of the file contents into a byte array which is returned.

FileOutputStream startWrite()

Start a new write operation on the file.

Inherited methods

Public constructors

AtomicFile

public AtomicFile (File baseName)

Create a new AtomicFile for a file located at the given File path. The new file created when writing will be the same file path with ".new" appended.

Parameters
baseName File

Public methods

delete

public void delete ()

Delete the atomic file. This deletes both the base and new files.

failWrite

public void failWrite (FileOutputStream str)

Call when you have failed for some reason at writing to the stream returned by startWrite(). This will close the current write stream, and delete the new file.

Parameters
str FileOutputStream

finishWrite

public void finishWrite (FileOutputStream str)

Call when you have successfully finished writing to the stream returned by startWrite(). This will close, sync, and commit the new data. The next attempt to read the atomic file will return the new file stream.

Parameters
str FileOutputStream

getBaseFile

public File getBaseFile ()

Return the path to the base file. You should not generally use this, as the data at that path may not be valid.

Returns
File

openRead

public FileInputStream openRead ()

Open the atomic file for reading. You should call close() on the FileInputStream when you are done reading from it.

You must do your own threading protection for access to AtomicFile.

Returns
FileInputStream

Throws
FileNotFoundException

readFully

public byte[] readFully ()

A convenience for openRead() that also reads all of the file contents into a byte array which is returned.

Returns
byte[]

Throws
IOException

startWrite

public FileOutputStream startWrite ()

Start a new write operation on the file. This returns a FileOutputStream to which you can write the new file data. The existing file is replaced with the new data. You must not directly close the given FileOutputStream; instead call either finishWrite(FileOutputStream) or failWrite(FileOutputStream).

Note that if another thread is currently performing a write, this will simply replace whatever that thread is writing with the new file being written by this thread, and when the other thread finishes the write the new write operation will no longer be safe (or will be lost). You must do your own threading protection for access to AtomicFile.

Returns
FileOutputStream

Throws
IOException