How to parse large xml file

Question

I need to parse large xml file which contains following tag. The size of the file is upto 10MB-50MB or more.
<departments>
<department>
<a_depart id="124">
<b_depart id="Bss_253">
<bss_depart id="253">
<attributes>
<name_one>abc</name_one>
</attributes>
</bss_depart id="253">
</b_depart id="Bss_253">
</a_depart id="124">
</department>
<department>
<a_depart id="124">
<b_depart id="Bss_254">
<mss_depart id="253">
          <attributes>
          <name_one>abc</name_one>
          <name_two>xyz</name_one>
          </attributes>
     </mss_depart>
     </b_depart>
</a_depart>
</department>
<department>
<a_depart id="124">
<b_depart id="Bss_254">
<mss_depart id="255">
          <attributes>
          <name_one>abc</name_one>
          <name_two>xyz</name_one>
          </attributes>
     </mss_depart>
     </b_depart>
</a_depart>
</department>
<department>
<a_depart id="125">
<b_depart id="Bss_254">
<mss_depart id="253">
          <attributes>
          <name_one>abc</name_one>
          <name_two>xyz</name_one>
          </attributes>
     </mss_depart>
     </b_depart>
</a_depart>
</department>
I want to get the infomation for that xml file. like mss_depart id=233, building xpath dyanmically for every id and loading
that using dom4j. which is very very slow.
Is there any other solution for that to read the data using sax parser only.
I want to execute the xpath or data for the following way.
//a_depart/@id ------> all the ids of a_depart tags if it returns 3 values say 123,124,125
after that i want to execute
//a_depart[@id='123']/b_depart/@id like this ...to retrive the values of all the levels ...
     I am executing following xpath for every unique ids at all levels.
     List l = doc.selectNodes(xPathForID);
     List l1 = doc.selectNodes(xPathForAttributes+attributes.get(j)+"/text()");
But it is very slow and taking lot of time.
Is there any other way to solve this problem. If any please mail me it is urgent.
I am using jdk1.4 and jdk1.5
Is there any support for sax parser to execute xpath in jdk1.5 direclty, with out using dom4j
Thanks in advance....

Answer 1

I doubt you will find a preexisting solution to your problem.
SAX is usually recommended for processing big files (where "big" is undefined"). It works on big files by avoiding the messy problem of storing the data -- that is left as an exercise to you.
DOM (and its variants) works by building a Document object as the head of the tree of objects for the entire contents. With DOM, you can then use XPath, because there is something to search that is already in memory. To use XPath, you seem to have two choices, build a DOM-ish tree, or if you can find an XPath processor (I'm not sure if one exists) that can process the XML file directly, but it will be slow, since you are looking for "all" occurences of an attribute, and this means you have to read the entire file each time.
It might be worth exploring a hybrid approach -- use SAX to get some information, and build your own objects to store the data. Maybe a HashMap as the main index. But, that will keep you from using XPath, since you do not have the data structures it expects.
A third alternative would be to look at JAXB. It builds Java code from a Schema of your data and then when you import the data, it creates the necessary objects and fills in values. But, I don't think XPath woll work there either.
Dave Patterson