If you have written a web application, the next logical step is to make it available offline. The long-term solution is clear: You give your web application an offline mode, which will hopefully be complemented by explicit application management in web browsers. Short- to mid-term, though, that is often not feasible, because the server provides crucial functionality to the client.
Thus, I was looking for a different solution. One option I had seen was Hudson’s self-executable WAR file: When you execute java -jar hudson.war, an embedded web server starts and you can immediately try out Hudson. This approach had two limitations that I didn’t like. First, I wanted my solution to be acceptable for end users, so I wanted a graphical user interface. Second, the embedded web server extracted the WAR file to a temporary directory and did so again for each startup, wasting time. In contrast, my solution does the following things:
- The application is a JAR file, the WAR is embedded inside. I opted against a single binary for JAR and WAR, because packaging the JAR as a nice desktop application adds platform-specific data, anyway, and makes it often impossible to deploy the result as a WAR.
- When the JAR starts up, a Swing user interface is shown. If this hasn’t been done before, the WAR file is extracted to a directory next to the JAR file. The idea is that there is a dedicated folder for the application in which the JAR resides. The location of the JAR is determined via this trick. The WAR file (=ZIP format) in the classpath is extracted to the file system using the standard Java API.
- Next, a web server is started and pointed to the web application directory in the file system (none of the web servers I’ve seen is able to serve a WAR file directly, let alone one that is embedded inside a JAR file). I used Winstone, because it is so small. There are even smaller ones, but those won’t allow you to use servlets. Obviously, any embeddable Java servlet container will do.
- A button allows the user to open the starting page of the web application in the default web browser. java.awt.Desktop (Java 6) allows you to do this. Currently, localhost and a fixed port is used. In the future, the port should be configurable and one could maybe automatically switch to a different port if the default one is occupied.
- Finally, I also produced a Mac OS X application. Such an application is a folder with the file name extension “.app” and the JAR file inside. This has the nice side effect of hiding the extracted WAR directory.