GWT Compiler Output

The GWT compiler generates a series of, sometimes oddly named, files.
As of GWT version 1.4, you should typically see the following types of files:
• .nocache.js (or -xs.nocache.js for cross-site script inclusion)
• .cache.html
• .gwt.rpc
.no-cache.js File
The “nocache.js” file is where Deferred Binding occurs. Before your GWT application can run, any dynamically-bound code must be resolved. This might include browser-specific versions of classes, Internationalization strings for the user’s selected language, etc. The “nocache.js” file includes a lookup table that maps Deferred Binding permutations to .cache.html filenames, e.g. “Firefox in German” and “Safari in English” would both be entries in the lookup table, pointing to different .cache.html files.
The file is named “.nocache.js” to indicate that it should never be cached, i.e. it must be re-downloaded and executed each time the browser starts your GWT application. The file must be re-downloaded each time is because the GWT Compiler regenerates it each time, but under the same file name. Allowing browsers to cache the file, would result in browsers not always downloading the latest version of the file when your application is recompiled and redeployed.
.cache.html Files
These are the files that actually contain your GWT application’s logic. A .cache.html file contains JavaScript code wrapped in a thin HTML wrapper. The GWT Compiler doesn’t simply output JavaScript (.js) files because certain browsers, under certain circumstances, do not properly handle compression of pure-JavaScript files. Wrapping the JavaScript in an HTML file solves the problem, and allows the GWT’s JavaScript output to be compressed.
Each .cache.html file uses the MD5 sum of its contents as its name. This guarantees deterministic behavior by the GWT Compiler. If you recompile your application without changing code, the output JavaScript code will not change, and so the MD5 sums (and the file names) will not change. If you do change your source code, the output JavaScript code will also change, and so the MD5 sums (and the file names) will change. Since these files will always have unique names, it is safe for browsers to cache them, hence the name “cache.html“.
.gwt.rpc File
As of GWT version 1.4, types that implement the java.io.Serializable interface now also qualify for serialization over RPC, but only if they are included in the .gwt.rpc file generated by the GWT compiler. The .gwt.rpc file serves as a serialization policy and indicates which types implementing java.io.Serializable are allowed to be serialized over the wire.