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<title>What's with all the cache/nocache stuff and weird filenames?</title>
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<h2 id="What's_with_all_the_cache/nocache_stuff_and_weird_filenames">What's
with all the cache/nocache
stuff and weird filenames?</h2>
<p>During its bootstrap process, a Google Web Toolkit application goes
through a series of sometimes cryptically-named files. These files,
generated by the GWT Compiler, usually
seem strange to developers new to GWT. To effectively deploy a GWT
application, however, it is necessary to understand these files so that
they can be placed appropriately on the web server.</p>
<p>The important files produced by the GWT Compiler are:</p>
<ul>
<li>
<i><Module Name></i><tt>.nocache.js</tt>
</li>
<li>
<i><alphanumeric></i><tt>.gwt.rpc</tt>
</li></ul>
<p>Each of the items listed above is described below. However, first
it's important to understand the concept of deferred binding since that
notion is at the heart of the bootstrap process injected into the <tt><Module
Name>.nocache.js</tt> file, so you
might want to read a bit about <a
href="http://code.google.com/webtoolkit/doc/latest/DevGuideCodingBasics.html#DevGuideDeferredBinding">deferred
binding</a> before continuing.</p>
<p>Before explaining what each file does, it's also useful to summarize
the overall bootstrap procedure for a GWT application:</p>
<ol>
<li>The browser loads and processes the host HTML page.</li>
<li>When the browser encounters the page's <tt><script
src="<Module Name>.nocache.js"></tt> tag, it immediately
downloads and executes the JavaScript code in the
file.</li>
<li>The <tt>.nocache.js</tt> file contains JavaScript code that resolves
the Deferred Binding configurations (such as browser detection, for
instance) and then uses a lookup table
generated by the GWT Compiler to locate one of the <tt>.cache.html</tt>
files to use.</li>
<li>The JavaScript code in <tt>.nocache.js</tt> then creates a hidden <tt><iframe></tt>,
inserts it to the host page's DOM, and loads the <tt>.cache.html</tt>
file into that
iframe.</li>
<li>The <tt>.cache.html</tt> file contains the actual program logic of
the GWT application.</li>
</ol>
<p>That's the process in a nutshell. For an example the bootstrap
process for a complete GWT application, check out the <a
href="http://code.google.com/webtoolkit/doc/latest/DevGuideOrganizingProjects.html#DevGuideBootstrap">Developer
Guide example</a>. The sections below describe each of the GWT
application files in detail.</p>
<h5 id="The_.cache.html_Files">The .cache.html Files</h5>
<p>The "cache" files contain your application's logic. If you were to
look inside a <tt>.cache.html</tt> file, you would see that it is
JavaScript code wrapped in a thin HTML wrapper. You might wonder why the
GWT Compiler doesn't
simply emit it as a JavaScript .js file. The reason for this is that
certain browsers do not correctly handle compression of pure-JavaScript
files in some circumstances. This
would effectively mean that users unfortunate enough to be using such a
browser would download the .js file uncompressed. Since the GWT mantra
is no-compromise, high-performance
AJAX code, the GWT Compiler wraps the JavaScript in an HTML file to
wiggle around this browser quirk.</p>
<p>They are named according to the MD5 sum of their contents. This
guarantees deterministic behavior by the GWT Compiler: if you recompile
your application without changing code,
the contents of the output will not change, and so the MD5 sums will
remain the same. Conversely, if you do change your source code, the
output JavaScript code will likewise
change, and so the MD5 sums and thus the filenames will change.</p>
<p>Because of this uniqueness guarantee, it is safe (and indeed
preferable) for browsers to cache these files, which is reflected in
their <tt>.cache.html</tt> file extension.</p>
<h5 id="The_.nocache.js_File">The .nocache.js File</h5>
<p>The "nocache" file is where <a
href="http://code.google.com/webtoolkit/doc/latest/DevGuideCodingBasics.html#DevGuideDeferredBinding">Deferred
Binding</a> occurs. Before the application can run, any
dynamically-bound code must be resolved. This might include
browser-specific versions of classes, the specific set of string
constants appropriate to the user's selected language, and so on. In
Java, this would be handled by simply loading an appropriate
service-provider class that implements a particular interface. To
maximize performance and minimize download size, however, GWT does this
selection up-front in the "nocache" file.</p>
<p>The reason the file is named ".nocache.html" is to indicate that the
file should never be cached. That is, it must be downloaded and executed
again each time the browser starts
the GWT application. The reason it must be re-downloaded each time is
that the GWT Compiler regenerates it each time, but under the same file
name. If the browsers were allowed to
cache the file, they might not download the new version of the file,
when the GWT application was recompiled and redeployed on the server. To
help prevent caching, the code in
<tt>gwt.js</tt> actually appends an HTTP GET parameter on the end of
file name containing a unique timestamp. The browser interprets this as a
dynamic HTTP request, and thus
should not load the file from cache.</p>
<h4 id="Summary">Summary</h4>
<p>That is the story behind the somewhat strange GWT file names. The <tt><module>.nocache.js</tt>
performs the deferred binding
resolution and selects a cache file based on the execution context
</p><p>
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