Informa不僅提供了對不同版本的RSS Feed source的讀入和解析，同樣也提供了將channel object導出為不同協議版本的XML文件的功能。這個功能是通過exporters包下的各個導出類來完成的。目前僅支持對RSS協議的導出，不支持Atom協議、OPML協議的導出。
所有的導出類都實現了ChannelExporterIF接口
/** *//**
* A channel exporter is used to write channel objects in an
* implementation dependent way to an implicit destination.</p>
*
* @author Niko Schmuck (niko@nava.de)
*/
public interface ChannelExporterIF 
{
/** *//**
* Writes the given channel to an implicit implementation dependent
* destination.
*
* @param channel - The channel to be exported/written;
* an object implementing ChannelIF.
* @throws IOException - Thrown if writing the channel fails.
*/
void write(ChannelIF channel) throws IOException;
}
★RSS_0_91_Exporter
在RSS_0_91_Exporter中提供了幾種構造方法
public RSS_0_91_Exporter(String filename) throws IOException {
this(new File(filename), "utf-8");
}
public RSS_0_91_Exporter(File file) throws IOException {
this(file, "utf-8");
}
public RSS_0_91_Exporter(File file, String encoding) throws IOException {
this.writer = new OutputStreamWriter(new FileOutputStream(file),
encoding);
this.encoding = encoding;
}
public RSS_0_91_Exporter(Writer writer, String encoding) {
this.writer = writer;
this.encoding = encoding;
}
真正的生成XML文件的過程其實很簡單，就是按照RSS 0.9.1規范的格式將channel object的屬性映射到XML文件的element去。
// create items for channel
Collection<ItemIF> items = channel.getItems();
Iterator<ItemIF> it = items.iterator();
while (it.hasNext()) {
ItemIF item = (ItemIF) it.next();
Element itemElem = new Element("item");
itemElem.addContent(new Element("title").
setText(item.getTitle()));
if (item.getLink() != null) {
itemElem.addContent(new Element("link").
setText(item.getLink().toString()));
}
if (item.getDescription() != null) {
itemElem.addContent(new Element("description").
setText(item.getDescription()));
}
if (item.getDate() != null) {
itemElem.addContent(new Element("pubDate").
setText(ParserUtils.formatDate(item.getDate())));
}
channelElem.addContent(itemElem);
}
這是write(Channel)方法的其中一個片段，是用于導出channel中的item節點的。
★RSS_2_0_Exporter
和RSS 0.9.1的導出類似，2.0版本的導出同樣需要實現ChannelExporterIF接口，同樣提供了和上面一樣參數類型的構造方法。不同的是在2.0的版本，由于新增了幾個定義，所以增加了對應的構建方法。例如下面的getCategoryElements方法
// ------------------------------------------------------------
// Build a hierarchical category String from CategoryIF
// ------------------------------------------------------------
private Element getCategoryElements(Element elem, CategoryIF category,
StringBuffer catString) {
StringBuffer l_catString;
if (catString == null || catString.length() < 1)
l_catString = new StringBuffer(category.getTitle());
else
l_catString = catString.append("/").append(category.getTitle());
Collection<CategoryIF> categories = category.getChildren();
if (categories.size() == 0) {
elem.addContent(new Element("category").setText(l_catString
.toString()));
} else {
Iterator<CategoryIF> catIt = categories.iterator();
while (catIt.hasNext()) {
CategoryIF childCat = (CategoryIF) catIt.next();
elem = getCategoryElements(elem, childCat, l_catString);
}
}
return elem;
}
這個方法采用遞歸的方式，獲取一個channel所屬于的category，因為在Category對象中，關系是以級聯的方式存在的，要把這種對象的級聯關系展開為平行的字符串，只能通過遞歸的方式訪問到最底層的category對象，得到其title后，再用分隔符"/"拼接后返回給上一層。
在2.0的write方法中，新增了對Namespace的支持
Namespace dcNs = Namespace.getNamespace("dc", NS_DC);
Namespace syNs = Namespace.getNamespace("sy", NS_SY);
Namespace adminNs = Namespace.getNamespace("admin", NS_ADMIN);
其余的生成過程和0.9.1的基本相同，除了在category方面
if (channel.getCategories() != null) {
Collection<CategoryIF> categories = channel.getCategories();
Iterator<CategoryIF> catIt = categories.iterator();
while (catIt.hasNext()) {
CategoryIF cat = (CategoryIF) catIt.next();
channelElem = getCategoryElements(channelElem, cat, null);
}
}
★RSS_1_0_Exporter
RSS_1_0_Exporter導出和0.9.1和2.0協議的過程有些不同，區別在于item方面和部分屬性，這里重點介紹一下item方面的導出過程
// ===========================================
Element itemsElem = new Element("items", defNs);
Element seqElem = new Element("Seq", rdfNs);
Collection<ItemIF> items = channel.getItems();
Iterator<ItemIF> it = items.iterator();
while (it.hasNext()) {
ItemIF item = (ItemIF) it.next();
Element itemElem = new Element("li", rdfNs);
if (item.getLink() != null) {
itemElem.setAttribute("resource", item.getLink().toString());
}
seqElem.addContent(itemElem);
}
itemsElem.addContent(seqElem);
channelElem.addContent(itemsElem);
rootElem.addContent(channelElem);
// item-by-item en detail
items = channel.getItems();
it = items.iterator();
while (it.hasNext()) {
ItemIF item = (ItemIF) it.next();
Element itemElem = new Element("item", defNs);
if (item.getLink() != null) {
itemElem.setAttribute("about",
item.getLink().toString(), rdfNs);
}
itemElem.addContent(new Element("title", defNs).
setText(item.getTitle()));
if (item.getLink() != null) {
itemElem.addContent(new Element("link", defNs).
setText(item.getLink().toString()));
}
if (item.getDescription() != null) {
itemElem.addContent(new Element("description", dcNs).
setText(item.getDescription()));
}
if (item.getDate() != null) {
itemElem.addContent(new Element("date", dcNs).
setText(ParserUtils.formatDate(item.getDate())));
}
rootElem.addContent(itemElem);
}
和0.9.1和2.0協議不同，1.0的RSS對于item的描述有2次，第一次是summary，第二次是detail。
<items>
<rdf:Seq>
<rdf:li resource="http://xml.com/pub/2000/08/09/xslt/xslt.html" />
<rdf:li resource="http://xml.com/pub/2000/08/09/rdfdb/index.html" />
</rdf:Seq>
</items>
<item rdf:about="http://xml.com/pub/2000/08/09/xslt/xslt.html">
<title>Processing Inclusions with XSLT</title>
<link>http://xml.com/pub/2000/08/09/xslt/xslt.html</link>
<description>
Processing document inclusions with general XML tools can be
problematic. This article proposes a way of preserving inclusion
information through SAX-based processing.
</description>
</item>
<item rdf:about="http://xml.com/pub/2000/08/09/rdfdb/index.html">
<title>Putting RDF to Work</title>
<link>http://xml.com/pub/2000/08/09/rdfdb/index.html</link>
<description>
Tool and API support for the Resource Description Framework
is slowly coming of age. Edd Dumbill takes a look at RDFDB,
one of the most exciting new RDF toolkits.
</description>
</item>
可以看到上面的<items>是大概地告知了RSS閱讀器這個channel下面有多少個item，順序如何。下面每一個<item>則是對上面提到的item的詳細描述。根據規范，<item>節點的rdf:about屬性值必須和rdf:li的resource屬性值一樣。
上面的代碼就是先找出item的列表，構建<items>節點及其子節點<rdf:seq>，后面是構建<item>節點。所以有2次的節點迭代操作。
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生活就像打牌，不是要抓一手好牌，而是要盡力打好一手爛牌。
posted on 2010-01-04 10:15
Paul Lin 閱讀(415)
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