Question

I need to write an XML parsing class that I can reuse throughout my Android application and from what I’ve read a SAXParser is the best for a mobile application. I am using this guide:

http://www.jondev.net/articles/Android_XML_SAX_Parser_Example

And the type of document I wish to parse is a feed from the Blogger GData API – example would be:

<?xml version='1.0' encoding='UTF-8'?>
<?xml-stylesheet href="http://www.blogger.com/styles/atom.css" type="text/css"?>
<feed xmlns='http://www.w3.org/2005/Atom' xmlns:openSearch='http://a9.com/-/spec/opensearch/1.1/' xmlns:gd='http://schemas.google.com/g/2005' gd:etag='W/&quot;CUIGRnc4fyp7ImA9Wx9SEEg.&quot;'>
    <id>tag:blogger.com,1999:user-464300745974.blogs</id>
    <updated>2010-11-29T17:58:47.937Z</updated>
    <title>Tim's Blogs</title>
    <link rel='http://schemas.google.com/g/2005#feed' type='application/atom+xml' href='http://www.blogger.com/feeds/blogid/blogs'/>
    <link rel='self' type='application/atom+xml' href='http://www.blogger.com/feeds/blogid/blogs'/>
    <link rel='alternate' type='text/html' href='http://www.blogger.com/profile/blogid'/>
    <author>
        <name>Tim</name>
        <uri>http://www.blogger.com/profile/blogid</uri>
        <email>[e-mail]</email>
    </author>
    <generator version='7.00' uri='http://www.blogger.com'>Blogger</generator>
    <openSearch:totalResults>2</openSearch:totalResults>
    <openSearch:startIndex>1</openSearch:startIndex>
    <openSearch:itemsPerPage>25</openSearch:itemsPerPage>
    <entry gd:etag='W/&quot;DUIBQHg-cCp7ImA9Wx9TF0s.&quot;'>
        <id>tag:blogger.com,1999:user-464300745974.blog-blogid</id>
        <published>2010-06-22T10:59:38.603-07:00</published>
        <updated>2010-11-26T02:32:31.658-08:00</updated>
        <title>Application Testing Blog</title>
        <summary type='html'>This blog is for testing the Android application.</summary>
        <link rel='self' type='application/atom+xml' href='http://www.blogger.com/feeds/blogid/blogs/blogid'/>
        <link rel='alternate' type='text/html' href='http://devrum.blogspot.com/'/>
        <link rel='http://schemas.google.com/g/2005#feed' type='application/atom+xml' href='http://devrum.blogspot.com/feeds/posts/default'/>
        <link rel='http://schemas.google.com/g/2005#post' type='application/atom+xml' href='http://www.blogger.com/feeds/blogid/posts/default'/>
        <link rel='http://schemas.google.com/blogger/2008#template' type='application/atom+xml' href='http://www.blogger.com/feeds/blogid/template'/>
        <link rel='http://schemas.google.com/blogger/2008#settings' type='application/atom+xml' href='http://www.blogger.com/feeds/blogid/settings'/>
        <author>
            <name>Tim</name>
            <uri>http://www.blogger.com/profile/blogid</uri>
            <email>[e-mail]</email>
        </author>
    </entry>
    <entry gd:etag='W/&quot;C08HRXo4eSp7ImA9Wx9TE0o.&quot;'>
        <id>tag:blogger.com,1999:user-464300745974.blog-515600026106499737</id>
        <published>2010-06-22T10:59:00.328-07:00</published>
        <updated>2010-11-21T12:37:14.431-08:00</updated>
        <title>Development Blog</title>
        <summary type='html'>etc</summary>
        <link rel='self' type='application/atom+xml' href='http://www.blogger.com/feeds/blogid/blogs/515600026106499737'/>
        <link rel='alternate' type='text/html' href='http://rumdev.blogspot.com/'/>
        <link rel='http://schemas.google.com/g/2005#feed' type='application/atom+xml' href='http://rumdev.blogspot.com/feeds/posts/default'/>
        <link rel='http://schemas.google.com/g/2005#post' type='application/atom+xml' href='http://www.blogger.com/feeds/515600026106499737/posts/default'/>
        <link rel='http://schemas.google.com/blogger/2008#template' type='application/atom+xml' href='http://www.blogger.com/feeds/515600026106499737/template'/>
        <link rel='http://schemas.google.com/blogger/2008#settings' type='application/atom+xml' href='http://www.blogger.com/feeds/515600026106499737/settings'/>
        <author>
            <name>Tim</name>
            <uri>http://www.blogger.com/etc</uri>
            <email>[e-mail]</email>
        </author>
    </entry>
</feed>

I need to parse the blog IDs and post IDs out of feeds like the above. From any example I find on SAX, they are not generic at all. I’d like to write a reusable one, do you have any examples how I can modify the SAXParser accordingly?

Answer 1

SAX parsers are event driven parsers. You write a handler for every TYPE of XML node (start element, end element, attribute, text, etc..) and then parser iterates XML document and SAX events are sent to you (= methodes in your handler get called).

In your particular case you are looking for two node sequences:

1. <feed><id>
2. <feed><entry><id>

So you have to store some state info inside your handler to know where you are. Here is the code (didn’t try it myself, you’ll have to debug it):

public class DataHandler extends DefaultHandler {

    private Feed feed;
    private Entry currentEntry;
    private boolean isId = false;

    public Feed getFeed() {
        return feed;
    }

    @Override
    public void startElement(String namespaceURI, String localName, String qName, Attributes atts) throws SAXException {
        if (localName.equals("feed")) {
            feed = new Feed();
        } else if (localName.equals("entry")) {
            currentEntry = new Entry();
        } else if (localName.equals("id")) {
            isId = true;
        }
    }

    @Override
    public void endElement(String namespaceURI, String localName, String qName) throws SAXException {
        if (localName.equals("feed")) {
            // </feed> - do nothing, it's the end
        } else if (localName.equals("entry")) {
            // </entry> - save current entry and reset variable
            feed.entries.add(currentEntry);
            currentEntry = null;
        } else if (localName.equals("id")) {
             isId = false;
        }
    }

    @Override
    public void characters(char ch[], int start, int length) {
        if(!isId) return;

        String chars = new String(ch, start, length);
        chars = chars.trim();

        if (currentEntry != null) {
            currentEntry.id = chars;
        } else {
            feed.id = chars;
        }
    }

    private class Feed {
        public String id;
        public List<Entry> entries = new ArrayList<Entry>();
    }

    private class Entry {
        public String id;
    }
}