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<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"><head><title>JAR Files</title><link rel="stylesheet" href="core.css" type="text/css"/><meta name="generator" content="DocBook XSL Stylesheets V1.74.0"/></head><body><div class="sect1" title="JAR Files"><div class="titlepage"><div><div><h1 class="title"><a id="learnjava3-CHP-3-SECT-5"/>JAR Files</h1></div></div></div><p><span class="emphasis"><em>Java Archive (JAR) files</em></span> are Java’s suitcases. They are the standard and portable way to pack up all the parts of your Java application into a compact bundle for distribution or installation. You can put whatever you want into a JAR file: Java class files, serialized objects, data files, images, audio, etc. A JAR file can also carry one or more digital signatures that attest to its integrity and authenticity. A signature can be attached to the file as a whole or to individual items in the file.</p><p>The Java runtime system can load class files directly from an archive in your <code class="literal">CLASSPATH</code>, as described earlier. Nonclass files (data, images, etc.) contained in your JAR file can also be retrieved from the classpath by your application using the <a id="I_indexterm3_id645658" class="indexterm"/><code class="literal">getResource()</code> method (described in <a class="xref" href="ch12.html" title="Chapter 12. Input/Output Facilities">Chapter 12</a>). Using this facility, your code doesn’t have to know whether any resource is in a plain file or a member of a JAR archive. Whether a given class or data file is an item in a JAR file or an individual file on the classpath, you can always refer to it in a standard way and let Java’s class loader resolve the location.</p><div class="sect2" title="File Compression"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-3-SECT-5.1"/>File Compression</h2></div></div></div><p><a id="I_indexterm3_id645686" class="indexterm"/> <a id="I_indexterm3_id645694" class="indexterm"/>Items stored in JAR files are compressed with the standard <a id="I_indexterm3_id645705" class="indexterm"/>ZIP file compression. Compression makes downloading classes over a network much faster. A quick survey of the standard Java distribution shows that a typical class file shrinks by about 40 percent when it is compressed. Text files such as HTML or ASCII containing English words often compress to one-tenth their original size or less. (On the other hand, image files don’t normally get smaller when compressed as most common image formats are themselves a compression format.)</p><p>Java also has an archive format called <span class="emphasis"><em>Pack200</em></span>, which is optimized specifically for Java class bytecode and can achieve over four times greater compression of Java classes than ZIP alone. We’ll talk about Pack200 later in this chapter.</p></div><div class="sect2" title="The jar Utility"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-3-SECT-5.2"/>The jar Utility</h2></div></div></div><p><a id="idx10116" class="indexterm"/> <a id="idx10117" class="indexterm"/> <a id="idx10127" class="indexterm"/>The <span class="emphasis"><em>jar</em></span> utility provided with the JDK is a simple tool for creating and reading JAR files. Its user interface isn’t particularly friendly. It mimics the Unix <a id="I_indexterm3_id645782" class="indexterm"/><span class="emphasis"><em>tar</em></span> (tape archive) command. If you’re familiar with <span class="emphasis"><em>tar</em></span>, you’ll recognize the following incantations:</p><div class="variablelist"><dl><dt><span class="term"><code class="literal">jar -cvf jarFile path [ path ] [ ... ]</code></span></dt><dd><p>Create <em class="replaceable"><code>jarFile</code></em> containing <em class="replaceable"><code>path</code></em>(s).</p></dd><dt><span class="term"><code class="literal">jar -tvf jarFile [ path ] [ ... ]</code></span></dt><dd><p>List the contents of <em class="replaceable"><code>jarFile</code></em>, optionally showing just <em class="replaceable"><code>path</code></em>(s).</p></dd><dt><span class="term"><code class="literal">jar -xvf jarFile [ path ] [ ... ]</code></span></dt><dd><p>Extract the contents of <em class="replaceable"><code>jarFile</code></em>, optionally extracting just <em class="replaceable"><code>path</code></em>(s).</p></dd></dl></div><p>In these commands, the flag letters <em class="replaceable"><code>c</code></em>, <em class="replaceable"><code>t</code></em>, and <em class="replaceable"><code>x</code></em> tell <span class="emphasis"><em>jar</em></span> whether it is creating an archive, listing an archive’s contents, or extracting files from an archive. The <em class="replaceable"><code>f</code></em> means that the next argument is the name of the JAR file on which to operate. The optional <em class="replaceable"><code>v</code></em> flag tells <span class="emphasis"><em>jar</em></span> to be verbose when displaying information about files. In verbose mode, you get information about file sizes, modification times, and compression ratios.</p><p>Subsequent items on the command line (i.e., anything aside from the letters telling <span class="emphasis"><em>jar</em></span> what to do and the file on which <span class="emphasis"><em>jar</em></span> should operate) are taken as names of archive items. If you’re creating an archive, the files and directories you list are placed in it. If you’re extracting, only the filenames you list are extracted from the archive. (If you don’t list any files, <span class="emphasis"><em>jar</em></span> extracts everything in the archive.)</p><p>For example, let’s say we have just completed our new game, <span class="emphasis"><em>spaceblaster</em></span>. All the files associated with the game are in three directories. The Java classes themselves are in the <span class="emphasis"><em>spaceblaster/game</em></span> directory, <span class="emphasis"><em>spaceblaster/images</em></span> contains the game’s images, and <span class="emphasis"><em>spaceblaster/docs</em></span> contains associated game data. We can pack all this in an archive with this command:</p><a id="I_3_tt90"/><pre class="programlisting"> <code class="o">%</code> <strong class="userinput"><code><code class="n">jar</code> <code class="o">-</code><code class="n">cvf</code> <code class="n">spaceblaster</code><code class="o">.</code><code class="na">jar</code> <code class="n">spaceblaster</code></code></strong></pre><p>Because we requested verbose output, <em class="filename">jar</em> tells us what it is doing:</p><a id="I_3_tt91"/><pre class="programlisting"> <code class="nl">adding:</code><code class="n">spaceblaster</code><code class="o">/</code> <code class="o">(</code><code class="n">in</code><code class="o">=</code><code class="mi">0</code><code class="o">)</code> <code class="o">(</code><code class="n">out</code><code class="o">=</code><code class="mi">0</code><code class="o">)</code> <code class="o">(</code><code class="n">stored</code> <code class="mi">0</code><code class="o">%)</code> <code class="nl">adding:</code><code class="n">spaceblaster</code><code class="o">/</code><code class="n">game</code><code class="o">/</code> <code class="o">(</code><code class="n">in</code><code class="o">=</code><code class="mi">0</code><code class="o">)</code> <code class="o">(</code><code class="n">out</code><code class="o">=</code><code class="mi">0</code><code class="o">)</code> <code class="o">(</code><code class="n">stored</code> <code class="mi">0</code><code class="o">%)</code> <code class="nl">adding:</code><code class="n">spaceblaster</code><code class="o">/</code><code class="n">game</code><code class="o">/</code><code class="n">Game</code><code class="o">.</code><code class="na">class</code> <code class="o">(</code><code class="n">in</code><code class="o">=</code><code class="mi">8035</code><code class="o">)</code> <code class="o">(</code><code class="n">out</code><code class="o">=</code><code class="mi">3936</code><code class="o">)</code> <code class="o">(</code><code class="n">deflated</code> <code class="mi">51</code><code class="o">%)</code> <code class="nl">adding:</code><code class="n">spaceblaster</code><code class="o">/</code><code class="n">game</code><code class="o">/</code><code class="n">Planetoid</code><code class="o">.</code><code class="na">class</code> <code class="o">(</code><code class="n">in</code><code class="o">=</code><code class="mi">6254</code><code class="o">)</code> <code class="o">(</code><code class="n">out</code><code class="o">=</code><code class="mi">3288</code><code class="o">)</code> <code class="o">(</code><code class="n">deflated</code> <code class="mi">47</code><code class="o">%)</code> <code class="nl">adding:</code><code class="n">spaceblaster</code><code class="o">/</code><code class="n">game</code><code class="o">/</code><code class="n">SpaceShip</code><code class="o">.</code><code class="na">class</code> <code class="o">(</code><code class="n">in</code><code class="o">=</code><code class="mi">2295</code><code class="o">)</code> <code class="o">(</code><code class="n">out</code><code class="o">=</code><code class="mi">1280</code><code class="o">)</code> <code class="o">(</code><code class="n">deflated</code> <code class="mi">44</code><code class="o">%)</code> <code class="nl">adding:</code><code class="n">spaceblaster</code><code class="o">/</code><code class="n">images</code><code class="o">/</code> <code class="o">(</code><code class="n">in</code><code class="o">=</code><code class="mi">0</code><code class="o">)</code> <code class="o">(</code><code class="n">out</code><code class="o">=</code><code class="mi">0</code><code class="o">)</code> <code class="o">(</code><code class="n">stored</code> <code class="mi">0</code><code class="o">%)</code> <code class="nl">adding:</code><code class="n">spaceblaster</code><code class="o">/</code><code class="n">images</code><code class="o">/</code><code class="n">spaceship</code><code class="o">.</code><code class="na">gif</code> <code class="o">(</code><code class="n">in</code><code class="o">=</code><code class="mi">6174</code><code class="o">)</code> <code class="o">(</code><code class="n">out</code><code class="o">=</code><code class="mi">5936</code><code class="o">)</code> <code class="o">(</code><code class="n">deflated</code> <code class="mi">3</code><code class="o">%)</code> <code class="nl">adding:</code><code class="n">spaceblaster</code><code class="o">/</code><code class="n">images</code><code class="o">/</code><code class="n">planetoid</code><code class="o">.</code><code class="na">gif</code> <code class="o">(</code><code class="n">in</code><code class="o">=</code><code class="mi">23444</code><code class="o">)</code> <code class="o">(</code><code class="n">out</code><code class="o">=</code><code class="mi">23454</code><code class="o">)</code> <code class="o">(</code><code class="n">deflated</code> <code class="mi">0</code><code class="o">%)</code> <code class="nl">adding:</code><code class="n">spaceblaster</code><code class="o">/</code><code class="n">docs</code><code class="o">/</code> <code class="o">(</code><code class="n">in</code><code class="o">=</code><code class="mi">0</code><code class="o">)</code> <code class="o">(</code><code class="n">out</code><code class="o">=</code><code class="mi">0</code><code class="o">)</code> <code class="o">(</code><code class="n">stored</code> <code class="mi">0</code><code class="o">%)</code> <code class="nl">adding:</code><code class="n">spaceblaster</code><code class="o">/</code><code class="n">docs</code><code class="o">/</code><code class="n">help1</code><code class="o">.</code><code class="na">html</code> <code class="o">(</code><code class="n">in</code><code class="o">=</code><code class="mi">3592</code><code class="o">)</code> <code class="o">(</code><code class="n">out</code><code class="o">=</code><code class="mi">1545</code><code class="o">)</code> <code class="o">(</code><code class="n">deflated</code> <code class="mi">56</code><code class="o">%)</code> <code class="nl">adding:</code><code class="n">spaceblaster</code><code class="o">/</code><code class="n">docs</code><code class="o">/</code><code class="n">help2</code><code class="o">.</code><code class="na">html</code> <code class="o">(</code><code class="n">in</code><code class="o">=</code><code class="mi">3148</code><code class="o">)</code> <code class="o">(</code><code class="n">out</code><code class="o">=</code><code class="mi">1535</code><code class="o">)</code> <code class="o">(</code><code class="n">deflated</code> <code class="mi">51</code><code class="o">%)</code></pre><p><span class="emphasis"><em>jar</em></span> creates the file <span class="emphasis"><em>spaceblaster.jar</em></span> and adds the directory <span class="emphasis"><em>spaceblaster</em></span>, adding the directories and files within <span class="emphasis"><em>spaceblaster</em></span> to the archive. In verbose mode, <span class="emphasis"><em>jar</em></span> reports the savings gained by compressing the files in the archive.</p><p>We can unpack the archive with this command:</p><a id="I_3_tt92"/><pre class="programlisting"> <code class="o">%</code> <strong class="userinput"><code><code class="n">jar</code> <code class="o">-</code><code class="n">xvf</code> <code class="n">spaceblaster</code><code class="o">.</code><code class="na">jar</code></code></strong></pre><p>Likewise, we can extract an individual file or directory with:</p><a id="I_3_tt93"/><pre class="programlisting"> <code class="o">%</code> <strong class="userinput"><code><code class="n">jar</code> <code class="o">-</code><code class="n">xvf</code> <code class="n">spaceblaster</code><code class="o">.</code><code class="na">jar</code></code></strong><strong class="userinput"><code><em class="replaceable"><code> filename</code></em></code></strong></pre><p>But, of course, you normally don’t have to unpack a JAR file to use its contents; Java tools know how to extract files from archives automatically. We can list the contents of our JAR with the command:</p><a id="I_3_tt94"/><pre class="programlisting"> <code class="o">%</code> <strong class="userinput"><code><code class="n">jar</code> <code class="o">-</code><code class="n">tvf</code> <code class="n">spaceblaster</code><code class="o">.</code><code class="na">jar</code></code></strong></pre><p>Here’s the output; it lists all the files, their sizes, and their creation times:</p><a id="I_3_tt95"/><pre class="programlisting"> <code class="mi">0</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">12</code><code class="o">:</code><code class="mi">18</code><code class="o">:</code><code class="mi">54</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">META</code><code class="o">-</code><code class="n">INF</code><code class="o">/</code> <code class="mi">1074</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">12</code><code class="o">:</code><code class="mi">18</code><code class="o">:</code><code class="mi">54</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">META</code><code class="o">-</code><code class="n">INF</code><code class="o">/</code><code class="n">MANIFEST</code><code class="o">.</code><code class="na">MF</code> <code class="mi">0</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">12</code><code class="o">:</code><code class="mi">09</code><code class="o">:</code><code class="mi">24</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">spaceblaster</code><code class="o">/</code> <code class="mi">0</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">11</code><code class="o">:</code><code class="mi">59</code><code class="o">:</code><code class="mi">32</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">spaceblaster</code><code class="o">/</code><code class="n">game</code><code class="o">/</code> <code class="mi">8035</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">12</code><code class="o">:</code><code class="mi">14</code><code class="o">:</code><code class="mi">08</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">spaceblaster</code><code class="o">/</code><code class="n">game</code><code class="o">/</code><code class="n">Game</code><code class="o">.</code><code class="na">class</code> <code class="mi">6254</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">12</code><code class="o">:</code><code class="mi">15</code><code class="o">:</code><code class="mi">18</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">spaceblaster</code><code class="o">/</code><code class="n">game</code><code class="o">/</code><code class="n">Planetoid</code><code class="o">.</code><code class="na">class</code> <code class="mi">2295</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">12</code><code class="o">:</code><code class="mi">15</code><code class="o">:</code><code class="mi">26</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">spaceblaster</code><code class="o">/</code><code class="n">game</code><code class="o">/</code><code class="n">SpaceShip</code><code class="o">.</code><code class="na">class</code> <code class="mi">0</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">12</code><code class="o">:</code><code class="mi">17</code><code class="o">:</code><code class="mi">00</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">spaceblaster</code><code class="o">/</code><code class="n">images</code><code class="o">/</code> <code class="mi">6174</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">12</code><code class="o">:</code><code class="mi">16</code><code class="o">:</code><code class="mi">54</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">spaceblaster</code><code class="o">/</code><code class="n">images</code><code class="o">/</code><code class="n">spaceship</code><code class="o">.</code><code class="na">gif</code> <code class="mi">23444</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">12</code><code class="o">:</code><code class="mi">16</code><code class="o">:</code><code class="mi">58</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">spaceblaster</code><code class="o">/</code><code class="n">images</code><code class="o">/</code><code class="n">planetoid</code><code class="o">.</code><code class="na">gif</code> <code class="mi">0</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">12</code><code class="o">:</code><code class="mi">10</code><code class="o">:</code><code class="mi">02</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">spaceblaster</code><code class="o">/</code><code class="n">docs</code><code class="o">/</code> <code class="mi">3592</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">12</code><code class="o">:</code><code class="mi">10</code><code class="o">:</code><code class="mi">16</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">spaceblaster</code><code class="o">/</code><code class="n">docs</code><code class="o">/</code><code class="n">help1</code><code class="o">.</code><code class="na">html</code> <code class="mi">3148</code> <code class="n">Thu</code> <code class="n">May</code> <code class="mi">15</code> <code class="mi">12</code><code class="o">:</code><code class="mi">10</code><code class="o">:</code><code class="mi">02</code> <code class="n">PDT</code> <code class="mi">2003</code> <code class="n">spaceblaster</code><code class="o">/</code><code class="n">docs</code><code class="o">/</code><code class="n">help2</code><code class="o">.</code><code class="na">html</code></pre><div class="sect3" title="JAR manifests"><div class="titlepage"><div><div><h3 class="title"><a id="learnjava3-CHP-3-SECT-5.2.1"/>JAR manifests</h3></div></div></div><p>Note that the <span class="emphasis"><em>jar</em></span> command automatically adds a directory called <a id="I_indexterm3_id646058" class="indexterm"/><span class="emphasis"><em>META-INF</em></span> to our archive. The <span class="emphasis"><em>META-INF</em></span> directory holds files describing the contents of the JAR file. It always contains at least one file: <a id="I_indexterm3_id646071" class="indexterm"/><span class="emphasis"><em>MANIFEST.MF.</em></span> The <span class="emphasis"><em>MANIFEST.MF</em></span> file can contain a “packing list” naming the files in the archive along with a user-definable set of attributes for each entry.</p><p>The manifest is a text file containing a set of lines in the form <span class="emphasis"><em>keyword: value</em></span>. The manifest is, by default, empty and contains only JAR file version information:</p><a id="I_3_tt96"/><pre class="programlisting"><code class="n">Manifest</code><code class="o">-</code><code class="nl">Version:</code> <code class="mf">1.0</code> <code class="n">Created</code><code class="o">-</code><code class="nl">By:</code> <code class="mf">1.7</code><code class="o">.</code><code class="mi">0</code><code class="n">_07</code> <code class="o">(</code><code class="n">Oracle</code> <code class="n">Corporation</code><code class="o">)</code></pre><p>It is also possible to sign JAR files with a digital signature. When you do this, digest (checksum) information is added to the manifest for each archived item (as shown next) and the <span class="emphasis"><em>META-INF</em></span> directory holds digital signature files for items in the archive.</p><a id="I_3_tt97"/><pre class="programlisting"> <code class="nl">Name:</code> <code class="n">com</code><code class="o">/</code><code class="n">oreilly</code><code class="o">/</code><code class="n">Test</code><code class="o">.</code><code class="na">class</code> <code class="n">SHA1</code><code class="o">-</code><code class="nl">Digest:</code> <code class="n">dF2GZt8G11dXY2p4olzzIc5RjP3</code><code class="o">=</code> <code class="o">...</code></pre><p>You can add your own information to the manifest descriptions by specifying your own supplemental, manifest file when you create the archive. This is one possible place to store other simple kinds of attribute information about the files in the archive, perhaps version or authorship information.</p><p>For example, we can create a file with the following <span class="emphasis"><em>keyword: value</em></span> lines:</p><a id="I_3_tt98"/><pre class="programlisting"> <code class="nl">Name:</code> <code class="n">spaceblaster</code><code class="o">/</code><code class="n">images</code><code class="o">/</code><code class="n">planetoid</code><code class="o">.</code><code class="na">gif</code> <code class="nl">RevisionNumber:</code> <code class="mf">42.7</code> <code class="n">Artist</code><code class="o">-</code><code class="nl">Temperament:</code> <code class="n">moody</code></pre><p>To add this information to the manifest in our archive, place it in a file called <span class="emphasis"><em>myManifest.mf</em></span> and give the following <span class="emphasis"><em>jar</em></span> command:</p><a id="I_3_tt99"/><pre class="programlisting"> <code class="o">%</code> <strong class="userinput"><code><code class="n">jar</code> <code class="o">-</code><code class="n">cvmf</code> <code class="n">myManifest</code><code class="o">.</code><code class="na">mf</code> <code class="n">spaceblaster</code><code class="o">.</code><code class="na">jar</code> <code class="n">spaceblaster</code></code></strong></pre><p>We included an additional option, <code class="literal">m</code>, which specifies that <span class="emphasis"><em>jar</em></span> should read additional manifest information from the file given on the command line. How does <span class="emphasis"><em>jar</em></span> know which file is which? Because <code class="literal">m</code> is before <code class="literal">f</code>, it expects to find the manifest information before the name of the JAR file it will create. If you think that’s awkward, you’re right; get the names in the wrong order, and <span class="emphasis"><em>jar</em></span> does the wrong thing.</p><p>An application can get this manifest information from a JAR file using the <a id="I_indexterm3_id646197" class="indexterm"/><code class="literal">java.util.jar.Manifest</code> class.</p><p>We’ll see more examples of adding information to the JAR manifest in <a class="xref" href="ch22.html" title="Chapter 22. JavaBeans">Chapter 22</a>. The JavaBeans APIs use manifest information to designate which classes are “beans” using a <a id="I_indexterm3_id646218" class="indexterm"/><code class="literal">Java-Bean</code> attribute. This information is used by IDEs that work with JavaBeans.</p></div><div class="sect3" title="Making a JAR file runnable"><div class="titlepage"><div><div><h3 class="title"><a id="learnjava3-CHP-3-SECT-5.2.2"/>Making a JAR file runnable</h3></div></div></div><p>Aside from attributes, you can put a few special values in the manifest file. One of these, <a id="I_indexterm3_id646238" class="indexterm"/><code class="literal">Main-Class</code>, allows you to specify the class containing the primary <code class="literal">main()</code> method for an application contained in the JAR:</p><a id="I_3_tt100"/><pre class="programlisting"> <code class="n">Main</code><code class="o">-</code><code class="nl">Class:</code> <code class="n">com</code><code class="o">.</code><code class="na">oreilly</code><code class="o">.</code><code class="na">Game</code></pre><p>If you add this to your JAR file manifest (using the <code class="literal">m</code> option described earlier), you can run the application directly from the JAR:</p><a id="I_3_tt101"/><pre class="programlisting"> <code class="o">%</code> <strong class="userinput"><code><code class="n">java</code> <code class="o">-</code><code class="n">jar</code> <code class="n">spaceblaster</code><code class="o">.</code><code class="na">jar</code></code></strong></pre><p>More importantly, under Mac OS X, Windows, and other GUI environments, you can simply double-click on the JAR file to launch the application. The interpreter looks for the <code class="literal">Main-Class</code> value in the manifest, then loads the designated class as the application’s startup class.<a id="I_indexterm3_id646296" class="indexterm"/><a id="I_indexterm3_id646304" class="indexterm"/><a id="I_indexterm3_id646311" class="indexterm"/></p></div></div><div class="sect2" title="The pack200 Utility"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-3-SECT-5.3"/>The pack200 Utility</h2></div></div></div><p><a id="I_indexterm3_id646325" class="indexterm"/> <a id="I_indexterm3_id646333" class="indexterm"/> <a id="I_indexterm3_id646339" class="indexterm"/>Pack200 is an archive format that is optimized for storing compiled Java class files. Pack200 is not a new form of compression, but rather a super-efficient layout for class information that eliminates many types of waste and redundancy across related classes. It is effectively a bulk class-file format that deconstructs many classes and reassembles their parts efficiently into one catalog. This then allows a standard compression format like ZIP to work at maximum efficiency on the archive, achieving four or more times greater compression. The Java runtime does not understand the Pack200 format, so you cannot place archives of this type into the classpath. Instead, it is mainly an intermediate format that is very useful for transferring application JARs over the network for applets or other kinds of web-based applications.</p><p><a id="I_indexterm3_id646359" class="indexterm"/>You can convert a JAR to and from Pack200 format with the <span class="emphasis"><em>pack200</em></span> and <span class="emphasis"><em>unpack200</em></span> commands supplied with the JDK.</p><p>For example, to convert <span class="emphasis"><em>foo.jar</em></span> to <span class="emphasis"><em>foo.pack.gz</em></span>, use the <span class="emphasis"><em>pack200</em></span> command:</p><a id="I_3_tt102"/><pre class="programlisting"> <code class="o">%</code> <strong class="userinput"><code><code class="n">pack200</code> <code class="n">foo</code><code class="o">.</code><code class="na">pack</code><code class="o">.</code><code class="na">gz</code> <code class="n">foo</code><code class="o">.</code><code class="na">jar</code></code></strong></pre><p>To convert <span class="emphasis"><em>foo.pack.gz</em></span> to <span class="emphasis"><em>foo.jar</em></span>:</p><a id="I_3_tt103"/><pre class="programlisting"> <code class="o">%</code> <strong class="userinput"><code><code class="n">unpack200</code> <code class="n">foo</code><code class="o">.</code><code class="na">pack</code><code class="o">.</code><code class="na">gz</code> <code class="n">foo</code><code class="o">.</code><code class="na">jar</code></code></strong></pre><p>Note that the Pack200 process completely tears down and reconstructs your classes at the class level, so the resulting <span class="emphasis"><em>foo.jar</em></span> file will not be byte-for-byte the same as the original.</p></div></div></body></html>