Best practices for parsing XML - android

My application shall parse XML received via HTTP. As far as I understand there are three major ways of parsing XML:
SAX
DOM
XmlPullParser
It is said that SAX is the fastest of these while DOM is not optimal for larger XML documents. But what is a large XML document in terms of parsing? What would be a recommended parser for the following?
XML document size between 1-5 kB
Easy traversing through the document, i.e. I need to know not only the current element but also the parent elements.

As far as I understand there are three major ways of parsing XML:
- SAX
- DOM
- XmlPullParser
Wrong! Neither of those is the best way. What you really want is annotation based parsing using the Simple XML Framework. To see why follow this logic:
Java works with objects.
XML can be represented using Java objects. (see JAXB)
Annotations could be used to map that XML to your Java objects and vice versa.
The Simple XML Framework uses Annotations to allow you to map your Java and XML together.
Simple XML is capable of running on Android (unlike JAXB).
You should use Simple XML for all of your XML needs on Android.
And to help you do exactly that I will point you to my own blog post that explains exactly how to use the Simple library on Android.
Unless you have a 100MB XML file then Simple will be more than fast enough for you. It is for me, I use it on all of my Android XML projects.
N.B. I should point out that if you require the user to download XML files that are more than 1MB on Android then you may want to rethink your strategy. You might be doing it wrong.

I'm afraid this is a case of, it depends ...
As a rule of thumb, using Java to build a DOM tree from an XML document will consume between 4 and 10 times that document's native size (assuming Western text and UTF-8 encoding), depending on the underlying implementation. So if speed and memory-use are not critical it will not be a problem for the small documents you mention.
DOM is generally regarded as quite an unpleasant way to work with XML. For background you might want to look at Elliotte Rusty Harold's presentation: What's Wrong with XML APIs (and how to fix them).
However, using SAX can be even more tedious as the document is processed one item at a time. SAX however is fast and consumes very little memory. If you can find a pull parser you like then by all means try that.
Another approach (not super-efficient, but clean and maintainable) is to build an in-memory tree of your XML (using DOM, say) and then use XPath expressions to select the information you are interested in.

Related

In android is doing XPath evaluate or Pull parser faster?

There is a remote xml file and I need to get set of xml nodes from it and show in an activity. One way is to download it completely and then do an xpath evaluate and get node set as follows
// using node I can drill further down to collect my relevant info
NodeList names = (NodeList) xPath.evaluate("/library/artists/name", new InputSource(new FileReader(getDowloadedXMLFileLocation())), XPathConstants.NODESET);
The other way is to use a pull parser which would be a bit more code I guess to do a simple job to extract a nodeset . I am wondering which method performs better in memory and speed ?
Customized code versus a generic solution, equates to high performance versus ease of use.
XPath (and XLST, and JDOM etc) will construct a DOM like model, and iterate it. This will take more RAM and CPU. XPath is easy, but can be really slow for large files. Also it requires you to have the complete file at hand. Use this for small files.
PullParser on other hand is like reading a file only at places you are interested, skipping the rest. Also, only one node is loaded in RAM at a time. There are no limits on how large the input file is, or is it fully available or being streamed over a medium. Use this for large or streamed files.
An excellent answer I found at Best practices for parsing XML
You can also use XmlPullParser to parse xml. But I suggest if possible, use json instead of XML, which decrease your memory and Google provide its wrapper mechanism in GSON which is best at all.
Links.
1) http://vtd-xml.sourceforge.net/

What should I be using for an XML parser in my Android App?

I want to create an App that uses a potentially large xml file. It will also modify and ideally be able to traverse in reverse.
I know there is SAX, DOM, and the XML pull parser. The pull parser is out, unless I spend memory on creating my own tree of objects which does not seem feasible.
That leaves SAX and DOM unless there is another parser out there that can do what I want. Highly improbable, I know.
Yes, I saw this answer: https://stackoverflow.com/questions/7498616/which-xml-parser-should-i-use-for-android
Thoughts on having tree like usability without having to use DOM?
There are a lot of options when it comes to parsing XML. But it depends on your own requirements that which parser you can use when. For that you need to know the basic differences between the parser. Here is some basic information i have provided.
SAX parser is one where your code is notified as the parser walks through the XML tree,
and you are responsible for keeping track of state and constructing any objects you might want to keep track of the data as the parser marches through.
DOM parser reads the entire document and builds up an in-memory representation that you can query for different elements. Often, you can even construct XPath queries to pull out particular pieces.
And as you said you are having large file and also if you want faster performance i suggest that you should use StAX parser. Here is link for that.
Hope this will help you...
Also refer this link.
DOM is better for most of the cases where it will load all the XML at a time. But If the XML size is very big then we should go for SAX parser where it will read for the tag from the start of the XML every time.
If the XML is really big then it is better to filter from the server end by sending the requirements in the request or else we can go for pagination which is suggestible.

Most efficient way to modify xml

I'm writing an app that needs to modify xml documents. More precisely it need to add attributes and add/delete nodes. These documents are relatively small, 30-50K at most. From what I'm reading the best way to read XML is use the SAX parser, but does that apply to modifying XML as well? The DOM parser seems to be the easiest to manipulate XML with, but, obviously, uses more battery and memory.
Just looking for the most efficient way (uses the least battery/memory) to manipulate XML on an Android device.
I suggest XOM - It is very memory efficient and support dual streaming/tree-based API.
The most efficient way to modify XML is to use VTD-XML(http://vtd-xml.sf.net)...it is the only way to do so called incremental update..

Parsers used in Android

Which all parsers are used in Android for XML parsing? Right now I know only SAX, XMLPullParser and DOM parsers. It will be really great if someone can tell the efficiency comparison for parsers used.
Thanks,
Stone
this guy has talked exactly for what you have asked.
http://www.ibm.com/developerworks/opensource/library/x-android/
though about efficiency you can find it while he is closing
SAX:
1. Parses node by node
2. Doesnt store the XML in memory
3. We cant insert or delete a node
4. Top to bottom traversing
DOM
1. Stores the entire XML document into memory before processing
2. Occupies more memory
3. We can insert or delete nodes
4. Traverse in any direction.
check this:
SAX parser vs XMLPull parser
http://www.developer.com/ws/article.php/3824221/Android-XML-Parser-Performance.htm
http://www.differencebetween.net/technology/difference-between-sax-and-dom/
Another XML Parser that I've found very useful is the Simple Java XML Parser (SJXP) available at http://www.thebuzzmedia.com/software/simple-java-xml-parser-sjxp/. It uses the XPP3 pull parser, and aims for efficiency while still being very simple to use.
The source code is also available, with great inline commenting if you want to see how it works.

is there some built in resources for letting the user update an xml file in Android?

I want to keep some information in a xml file, and I want to let the user update that file. Later I will parse and use that information in my app.
Before rolling my own code to create the UI to let the user do this, I was wondering if Android already has something along the lines I could use?
Android doesn't provide XML generation code itself, but there are plenty of Java resources for it, such as JAXP.
Take a look at using the DOM parser library, that Android has as standard. There are a number of tutorials for this online. For general parsing you might tend to use the SAX parser library due to the fact that it has lower memory requirements, but the DOM parser library appears to contain all of the standard methods that you would use to modify the DOM structure once it's in memory. Just as an example, the Node class has an appendChild() method.
Once you've modified the DOM in memory, hopefully there is some way of persisting the modified Document object for later use (e.g. persist to file), though I have no first-hand experience of doing that.

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