How do I extract a tar file in Java?

Note: This functionality was later published through a separate project, Apache Commons Compress, as described in another answer. This answer is out of date.

I haven't used a tar API directly, but tar and bzip2 are implemented in Ant; you could borrow their implementation, or possibly use Ant to do what you need.

Gzip is part of Java SE (and I'm guessing the Ant implementation follows the same model).

GZIPInputStream is just an InputStream decorator. You can wrap, for example, a FileInputStream in a GZIPInputStream and use it in the same way you'd use any InputStream:

InputStream is = new GZIPInputStream(new FileInputStream(file));

(Note that the GZIPInputStream has its own, internal buffer, so wrapping the FileInputStream in a BufferedInputStream would probably decrease performance.)

What about using this API for tar files, this other one included inside Ant for BZIP2 and the standard one for GZIP?

Here's a version based on this earlier answer by Dan Borza that uses Apache Commons Compress and Java NIO (i.e. Path instead of File). It also does the uncompression and untarring in one stream so there's no intermediate file creation.

public static void unTarGz( Path pathInput, Path pathOutput ) throws IOException {
    TarArchiveInputStream tararchiveinputstream =
        new TarArchiveInputStream(
            new GzipCompressorInputStream(
                new BufferedInputStream( Files.newInputStream( pathInput ) ) ) );

    ArchiveEntry archiveentry = null;
    while( (archiveentry = tararchiveinputstream.getNextEntry()) != null ) {
        Path pathEntryOutput = pathOutput.resolve( archiveentry.getName() );
        if( archiveentry.isDirectory() ) {
            if( !Files.exists( pathEntryOutput ) )
                Files.createDirectory( pathEntryOutput );
        }
        else
            Files.copy( tararchiveinputstream, pathEntryOutput );
    }

    tararchiveinputstream.close();
}