Using XML in a Java Context

These three example applications cover the core features of the main XOM package and are representative of its straightforward approach to XML processing.

There also are smaller nu.xom.canonical, nu.xom.converters, nu.xom.xinclude, and nu.xom.xslt packages to support XInclude, XSLT, canonical XML serialization, and conversions between the XOM model for XML and the one used by DOM and SAX.

Listing 20.11 contains an application that works with XML from a dynamic source: RSS feeds of recently updated Web content from the producer of the feed. The RssFilter application searches the feed for specified text in headlines, producing a new XML document that contains only the matching items and shorter indentation. It also modifies the feed's title and adds an RSS 0.91 document type declaration if one is needed.

Listing 20.11 The Full Text of RssFilter.java

 1: import nu.xom.*;
2: 
3: public class RssFilter {
4:   public static void main(String[] arguments) {
5: 
6:     if (arguments.length < 2) {
7:       System.out.println("Usage: java RssFilter

rssFile searchTerm");
8:       System.exit(-1);
9:     }
10: 
11:     // Save the RSS location and search term
12:     String rssFile = arguments[0];
13:     String searchTerm = arguments[1];
14:     
15:     try {
16:       // Fill a tree with an RSS file's XML

data
17:       // The file can be local or something on

the
18:       // Web accessible via a URL.
19:       Builder bob = new Builder();
20:       Document doc = bob.build(rssFile);
21: 
22:       // Get the file's root element (<rss>)
23:       Element rss = doc.getRootElement();
24: 
25:       // Get the element's version attribute
26:       Attribute rssVersion =

rss.getAttribute("version");
27:       String version = rssVersion.getValue();
28:       
29:       // Add the DTD for RSS 0.91 feeds, if

needed
30:       if ( (version.equals("0.91")) &

(doc.getDocType() == null) ) {
31:         DocType rssDtd = new DocType("rss",
32:           "http://my.netscape.com/publish/

formats/rss-0.91.dtd");
33:         doc.insertChild(rssDtd, 0);
34:       }
35: 
36:       // Get the first (and only) <channel>

element
37:       Element channel =

rss.getFirstChildElement("channel");
38: 
39:       // Get its <title> element
40:       Element title =

channel.getFirstChildElement("title");
41:       Text titleText = (Text)title.getChild(0);
42: 
43:       // Change the title to reflect the search

term
44:       titleText.setValue(titleText.getValue() +

": Search for " +
45:         searchTerm + " articles");
46: 
47:       // Get all of the <item> elements and

loop through them
48:       Elements items =

channel.getChildElements("item");
49:       for (int i = 0; i < items.size(); i++) {
50:         // Get an <item> element
51:         Element item = items.get(i);
52:         
53:         // Look for a <title> element inside it
54:         Element itemTitle =

item.getFirstChildElement("title");
55:         
56:         // If found, look for its contents
57:         if (itemTitle != null) {
58:           Text itemTitleText = (Text)

itemTitle.getChild(0);
59:           
60:           // If the search text is not found in

the item,
61:           // delete it from the tree
62:           if (itemTitleText.toString().indexOf

(searchTerm) == -1)
63:             channel.removeChild(item);
64:         }
65:       }
66: 
67:       // Display the results with a serializer
68:       Serializer output = new Serializer

(System.out);
69:       output.setIndent(2);
70:       output.write(doc);
71:     } catch (Exception exc) {
72:       System.out.println("Error: " +

exc.getMessage());
73:       exc.printStackTrace();
74:     }
75:   }
76: }

One feed that can be used to test the application is the one from the author's weblog, Workbench. The following command searches it for items that mention the word "Java":

java RssFilter http://www.cadenhead.org/workbench/ 

rss.xml Java

Comments in the application's source code describe its functionality.

XOM's design is strongly informed by one overriding principle: enforced simplicity.

On the Web site for the class library, Harold states that XOM "should help inexperienced developers do the right thing and keep them from doing the wrong thing. The learning curve needs to be really shallow, and that includes not relying on best practices that are known in the community but are not obvious at first glance."

The new class library is useful for Java programmers whose Java programs require a steady diet of XML.