Rome v0.3 Tutorial, Using Rome to aggregate many syndication feeds into a single one

Software requirements: J2SE 1.4+, JDOM B10 and Rome 0.3.

Rome represents syndication feeds (RSS and Atom) as instances of the com.rometools.rome.synd.SyndFeedI interface. The SyndFeedI interfaces and its properties follow the Java Bean patterns. The default implementations provided with Rome are all lightweight classes.

Rome includes parsers to process syndication feeds into SyndFeedI instances. The SyndFeedInput class handles the parsers using the correct one based on the syndication feed being processed. The developer does not need to worry about selecting the right parser for a syndication feed, the SyndFeedInput will take care of it by peeking at the syndication feed structure. All it takes to read a syndication feed using Rome are the following 2 lines of code:

SyndFeedInput input = new SyndFeedInput();
SyndFeedI feed = input.build(new InputStreamReader(feedUrl.openStream()));

Obtaining the char based input stream has been simplified in this tutorial, the sample code in the distribution is a little more complex to handle charset encodings properly.

The first line creates a SyndFeedInput instance that will work with any syndication feed type (RSS and Atom versions). The second line instructs the SyndFeedInput to read the syndication feed from the InputStream of a URL pointing to the feed. The SyndFeedInput.build() method returns a SyndFeedI instance that can be easily processed.

Rome also includes generators to create syndication feeds out of SyndFeedI instances. The SyndFeedOutput class does this generation. The SyndFeedOutput will generate a syndication feed of the feed type indicated by the SyndFeedI object being output. The following two lines of code show how to create a syndication feed output from a SyndFeedI instance:

SyndFeedOutput output = new SyndFeedOutput();
output.output(feed,new PrintWriter(System.out));

The first line creates a SyndFeedOutput instance that will produce syndication feeds. It can output feeds of any type (rss_0.9, rss_0.91, rss_0.92, rss_0.93, rss_0.94, rss_1.0, rss_2.0 & atom_0.3), the SyndFeedI feedType property indicates the type. The second line writes the SyndFeedI as a syndication feed into the application's output.

SyndFeedI can also be created and populated within the code. For example:

SyndFeedI aggrFeed = new SyndFeed();
aggrFeed.setFeedType("rss_1.0");
aggrFeed.setTitle("Aggregated Feed");
aggrFeed.setDescription("Anonymous Aggregated Feed");
aggrFeed.setAuthor("anonymous");
aggrFeed.setLink("http://www.anonymous.com");

The snipped of code above creates a SyndFeedI instance using the default implementation provided by Rome, sets the feed type to RSS 1.0, sets the title, description, author and link properties of the feed.

SyndFeedI properties can be modified, assigned to other SyndFeedI instances, removed, etc. It's important to remember that the getters/setters semantics defined for all SyndFeedI properties (and properties of its properties) is a copy by reference, not by value. In other words if you alter the property of a SyndFeedI property you are altering the SyndFeedI. Or, another example, if you assign the list of entries of one SyndFeedI instance to another SyndFeedI isntance and then you modify the list or any of the entries in the list, you are modifying the entries of both SyndFeedI instances.

The following lines of code show how to copy the entries of a SyndFeedI instance being read (inFeed) into a SyndFeedI instance being created within the application (feed):

package com.rometools.rome.samples;

import java.net.URL;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.List;
import java.util.ArrayList;

import com.rometools.rome.feed.synd.SyndFeedI;
import com.rometools.rome.feed.synd.SyndFeed;
import com.rometools.rome.io.SyndFeedOutput;
import com.rometools.rome.io.SyndFeedInput;

/**
 * It aggregates a list of RSS/Atom feeds (they can be of different types)
 * into a single feed of the specified type.
 * <p>
 * @author Alejandro Abdelnur
 *
 */
public class FeedAggregator {

    public static void main(String[] args) {
        boolean ok = false;
        if (args.length>=2) {
            try {
                String outputType = args[0];

                SyndFeedI feed = new SyndFeed();
                feed.setFeedType(outputType);

                feed.setTitle("Aggregated Feed");
                feed.setDescription("Anonymous Aggregated Feed");
                feed.setAuthor("anonymous");
                feed.setLink("http://www.anonymous.com");

                List entries = new ArrayList();
                feed.setEntries(entries);

                for (int i=1;i<args.length;i++) {
                    URL inputUrl = new URL(args[i]);

                    SyndFeedInput input = new SyndFeedInput();
                    SyndFeedI inFeed = input.build(new InputStreamReader(inputUrl.openStream()));

                    entries.addAll(inFeed.getEntries());

                }

                SyndFeedOutput output = new SyndFeedOutput();
                output.output(feed,new PrintWriter(System.out));

                ok = true;
            }
            catch (Exception ex) {
                System.out.println("ERROR: "+ex.getMessage());
            }
        }

        if (!ok) {
            System.out.println();
            System.out.println("FeedAggregator aggregates different feeds into a single one.");
            System.out.println("The first parameter must be the feed type for the aggregated feed.");
            System.out.println(" [valid values are: rss_0.9, rss_0.91, rss_0.92, rss_0.93, ]");
            System.out.println(" [                  rss_0.94, rss_1.0, rss_2.0 & atom_0.3  ]");
            System.out.println("The second to last parameters are the URLs of feeds to aggregate.");
            System.out.println();
        }
    }

}