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<?xml version="1.0"?> |
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<document xmlns="http://maven.apache.org/XDOC/2.0" |
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" |
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xsi:schemaLocation="http://maven.apache.org/XDOC/2.0 https://maven.apache.org/xsd/xdoc-2.0.xsd"> |
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<properties> |
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<title>Commons Compress TAR package</title> |
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<author email="dev@commons.apache.org">Apache Commons Team</author> |
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</properties> |
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<body> |
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<section name="The TAR package"> |
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<p>In addition to the information stored |
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in <code>ArchiveEntry</code> a <code>TarArchiveEntry</code> |
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stores various attributes including information about the |
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original owner and permissions.</p> |
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<p>There are several different dialects of the TAR format, maybe |
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even different TAR formats. The tar package contains special |
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cases in order to read many of the existing dialects and will by |
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default try to create archives in the original format (often |
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called "ustar"). This original format didn't support file names |
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longer than 100 characters or bigger than 8 GiB and the tar |
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package will by default fail if you try to write an entry that |
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goes beyond those limits. "ustar" is the common denominator of |
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all the existing tar dialects and is understood by most of the |
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existing tools.</p> |
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<p>The tar package does not support the full POSIX tar standard |
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nor more modern GNU extension of said standard.</p> |
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<subsection name="Long File Names"> |
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<p>The <code>longFileMode</code> option of |
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<code>TarArchiveOutputStream</code> controls how files with |
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names longer than 100 characters are handled. The possible |
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choices are:</p> |
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<ul> |
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<li><code>LONGFILE_ERROR</code>: throw an exception if such a |
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file is added. This is the default.</li> |
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<li><code>LONGFILE_TRUNCATE</code>: truncate such names.</li> |
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<li><code>LONGFILE_GNU</code>: use a GNU tar variant now |
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referred to as "oldgnu" of storing such names. If you choose |
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the GNU tar option, the archive can not be extracted using |
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many other tar implementations like the ones of OpenBSD, |
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Solaris or MacOS X.</li> |
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<li><code>LONGFILE_POSIX</code>: use a PAX <a |
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href="http://pubs.opengroup.org/onlinepubs/009695399/utilities/pax.html#tag_04_100_13_03">extended |
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header</a> as defined by POSIX 1003.1. Most modern tar |
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implementations are able to extract such archives. <em>since |
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Commons Compress 1.4</em></li> |
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</ul> |
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<p><code>TarArchiveInputStream</code> will recognize the GNU |
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tar as well as the POSIX extensions (starting with Commons |
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Compress 1.2) for long file names and reads the longer names |
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transparently.</p> |
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</subsection> |
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<subsection name="Big Numeric Values"> |
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<p>The <code>bigNumberMode</code> option of |
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<code>TarArchiveOutputStream</code> controls how files larger |
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than 8GiB or with other big numeric values that can't be |
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encoded in traditional header fields are handled. The |
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possible choices are:</p> |
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<ul> |
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<li><code>BIGNUMBER_ERROR</code>: throw an exception if such an |
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entry is added. This is the default.</li> |
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<li><code>BIGNUMBER_STAR</code>: use a variant first |
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introduced by Jörg Schilling's <a |
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href="http://developer.berlios.de/projects/star">star</a> |
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and later adopted by GNU and BSD tar. This method is not |
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supported by all implementations.</li> |
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<li><code>BIGNUMBER_POSIX</code>: use a PAX <a |
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href="http://pubs.opengroup.org/onlinepubs/009695399/utilities/pax.html#tag_04_100_13_03">extended |
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header</a> as defined by POSIX 1003.1. Most modern tar |
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implementations are able to extract such archives.</li> |
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</ul> |
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<p>Starting with Commons Compress 1.4 |
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<code>TarArchiveInputStream</code> will recognize the star as |
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well as the POSIX extensions for big numeric values and reads them |
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transparently.</p> |
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</subsection> |
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<subsection name="File Name Encoding"> |
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<p>The original ustar format only supports 7-Bit ASCII file |
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names, later implementations use the platform's default |
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encoding to encode file names. The POSIX standard recommends |
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using PAX extension headers for non-ASCII file names |
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instead.</p> |
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<p>Commons Compress 1.1 to 1.3 assumed file names would be |
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encoded using ISO-8859-1. Starting with Commons Compress 1.4 |
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you can specify the encoding to expect (to use when writing) |
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as a parameter to <code>TarArchiveInputStream</code> |
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(<code>TarArchiveOutputStream</code>), it now defaults to the |
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platform's default encoding.</p> |
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<p>Since Commons Compress 1.4 another optional parameter - |
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<code>addPaxHeadersForNonAsciiNames</code> - of |
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<code>TarArchiveOutputStream</code> controls whether PAX |
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extension headers will be written for non-ASCII file names. |
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By default they will not be written to preserve space. |
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<code>TarArchiveInputStream</code> will read them |
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transparently if present.</p> |
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</subsection> |
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<subsection name="Sparse files"> |
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<p>Prior to Commons Compress 1.20 <code>TarArchiveInputStream</code> would recognize sparse |
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file entries stored using the "oldgnu" format |
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(<code>--sparse-version=0.0</code> in GNU tar) but not |
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able to extract them correctly. Starting with Commons Compress 1.20 |
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all GNU and POSIX variants of sparse files are recognized and |
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can be read.</p> |
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</subsection> |
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<subsection name="Consuming Archives Completely"> |
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<p>The end of a tar archive is signaled by two consecutive |
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records of all zeros. Unfortunately not all tar |
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implementations adhere to this and some only write one record |
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to end the archive. Commons Compress will always write two |
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records but stop reading an archive as soon as finds one |
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record of all zeros.</p> |
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<p>Prior to version 1.5 this could leave the second EOF record |
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inside the stream when <code>getNextEntry</code> or |
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<code>getNextTarEntry</code> returned <code>null</code> |
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Starting with version 1.5 <code>TarArchiveInputStream</code> |
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will try to read a second record as well if present, |
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effectively consuming the archive completely.</p> |
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</subsection> |
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<subsection name="PAX Extended Header"> |
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<p>The tar package has supported reading PAX extended headers |
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since 1.3 for local headers and 1.11 for global headers. The |
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following entries of PAX headers are applied when reading:</p> |
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<dl> |
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<dt>path</dt> |
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<dd>set the entry's name</dd> |
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<dt>linkpath</dt> |
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<dd>set the entry's link name</dd> |
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<dt>gid</dt> |
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<dd>set the entry's group id</dd> |
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<dt>gname</dt> |
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<dd>set the entry's group name</dd> |
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<dt>uid</dt> |
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<dd>set the entry's user id</dd> |
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<dt>uname</dt> |
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<dd>set the entry's user name</dd> |
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<dt>size</dt> |
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<dd>set the entry's size</dd> |
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<dt>mtime</dt> |
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<dd>set the entry's modification time</dd> |
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<dt>SCHILY.devminor</dt> |
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<dd>set the entry's minor device number</dd> |
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<dt>SCHILY.devmajor</dt> |
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<dd>set the entry's major device number</dd> |
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</dl> |
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<p>in addition some fields used by GNU tar and star used to |
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signal sparse entries are supported and are used for the |
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<code>is*GNUSparse</code> and <code>isStarSparse</code> |
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methods.</p> |
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<p>Some PAX extra headers may be set when writing archives, |
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for example for non-ASCII names or big numeric values. This |
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depends on various setting of the output stream - see the |
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previous sections.</p> |
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<p>Since 1.15 you can directly access all PAX extension |
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headers that have been found when reading an entry or specify |
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extra headers to be written to a (local) PAX extended header |
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entry.</p> |
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<p>Some hints if you try to set extended headers:</p> |
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<ul> |
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<li>pax header keywords should be ascii. star/gnutar |
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(SCHILY.xattr.* ) do not check for this. libarchive/bsdtar |
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(LIBARCHIVE.xattr.*) uses URL-Encoding.</li> |
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<li>pax header values should be encoded as UTF-8 characters |
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(including trailing <code>\0</code>). star/gnutar |
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(SCHILY.xattr.*) do not check for this. libarchive/bsdtar |
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(LIBARCHIVE.xattr.*) encode values using Base64.</li> |
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<li>libarchive/bsdtar will read SCHILY.xattr headers, but |
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will not generate them.</li> |
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<li>gnutar will complain about LIBARCHIVE.xattr (and any |
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other unknown) headers and will neither encode nor decode |
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them.</li> |
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</ul> |
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</subsection> |
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<subsection name="Random Access"> |
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<p>Starting with Commons Compress 1.21 the tar package |
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contains a <code>TarFile</code> class that provides random |
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access to archives. Except for the ability to access entries |
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out of order <code>TarFile</code> is not superior to |
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<code>TarArchiveInputStream</code>.</p> |
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</subsection> |
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</section> |
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</body> |
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</document> |
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