The name of the cache directory. Can be null.
If this value is null, no cache file will be saved. If this value is set, the JpipRasterImageViewer object will save the cache file when Close is called. The cache file holds all of the meta and image data. This cache file will be used the next time the image is opened so that less data will need to be sent by the server.
For an example that demonstrates this value being set to save cache data, refer to JpipRasterImageViewer.