JAXB API is designed to make it easy to read the whole XML document into a single tree of JAXB objects. This is the typical use case, but in some situations this is not desirable. Perhaps:
This section discusses several advanced techniques to deal with these situations.
4.4.1. Processing a document by chunk
When a document is large, it's usually because there's repetitive parts in it. Perhaps it's a purchase order with a large list of line items, or perhaps it's an XML log file with large number of log entries.
This kind of XML is suitable for chunk-processing; the main idea is to use the StAX API, run a loop, and unmarshal individual chunks separately. Your program acts on a single chunk, and then throws it away. In this way, you'll be only keeping at most one chunk in memory, which allows you to process large documents.
See the streaming-unmarshalling example and the partial-unmarshalling example in the JAXB RI distribution for more about how to do this. The streaming-unmarshalling example has an advantage that it can handle chunks at arbitrary nest level, yet it requires you to deal with the push model --- JAXB unmarshaller will "push" new chunk to you and you'll need to process them right there.
In contrast, the partial-unmarshalling example works in a pull model (which usually makes the processing easier), but this approach has some limitations in databinding portions other than the repeated part.
4.4.2. Processing a live stream of XML
The techniques discussed above can be used to handle this case as well, since they let you unmarshal chunks one by one. See the xml-channel example in the JAXB RI distribution for more about how to do this.
4.4.3. Creating virtual infosets
For further advanced cases, one could always run a streaming infoset conversion outside JAXB API and basically curve just the portion of the infoset you want to data-bind, and feed it as a complete infoset into JAXB API. JAXB API accepts XML infoset in many different forms (DOM, SAX, StAX), so there's a fair amount of flexibility in choosing the right trade off between the development effort in doing this and the runtime performance.
For more about this, refer to the respective XML infoset API.