A recent issue encountered with FOP 1.0 involved the FOP processor grinding to a halt when a document contained a batch of external images hosted on a separate domain. It appeared to be fine with just two or three images but froze if greater numbers were included. The issue appeared to be centred around a prefetch utility in the image loader. This was attempting to fetch all the images in the document prior to processing and it appeared to be overloading the Apache Server with the request.
This was further substantiated when removing the images and then processing with just a couple, then reprocessing with the next few inserted. This worked because the initial ones had already been cached. However, such a routine is certainly not an efficient work-around.
The resolution was to disable the FOP image caching using a system property,
-Dorg.apache.xmlgraphics.image.loader.impl.AbstractImageSessionContext.no-source-reuse=true which was added to the
JAVAOPTS environment variable. This is easily done by modifying the FOP batch script that is part of the FOP download, adding the property to the appropriate line:
set JAVAOPTS=-Denv.windir=%WINDIR% -Xmx2048m -Dorg.apache.xmlgraphics.image.loader.impl.AbstractImageSessionContext.no-source-reuse=true
The result of this fix is that the processor only fetches the images when needed as seperate requests rather than as a bulk load in one request. The result worked perfectly. No more hanging, and even documents with 100's of images were being processed within a minute.
More information can be found at http://xmlgraphics.apache.org/commons/image-loader.html