The source code of Sweet Home 3D, the online version and the viewer are in the same project available here.

To build the jar files of the online version, select the file build.xml and choose Run > External Tools > External Tools Configuration… menu item in Eclipse. In the dialog that opens, select the target named applet in the Targets tab, and select a JDK (not a JRE) in the JRE tab, then click on Run.
As explained in README.TXT file, note that you'll need first to create or retrieve a keys.keytool file to sign the applet jars (this keystore file will have to be named keys.p12 in the coming version).