使用DOM方式操作XML文件，即是和DOM樹打交道的過程：在構建XML文件時，首先構建一棵DOM樹，然后將該樹狀結構寫成XML文件；在解析XML文件時，首先將源XML文件解析成一棵DOM樹，然后遍歷這棵DOM樹、或從DOM樹中查找需要的信息。
關于DOM樹中節點類型、不同節點具有的接口、特性、限制等信息可以參考《DOM樹節點解析》，本文只關注如何構建XML文件與解析XML文件。在構建和解析XML文件中，都以w3school中的books.xml文件的內容為例：
<?xml version="1.0" encoding="UTF-8"?>
<bookstore>
<book category="children">
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
<book category="cooking">
<title lang="en">Everyday Italian</title>
<author>Giada De Laurentiis</author>
<year>2005</year>
<price>30.00</price>
</book>
<bookcategory="web"cover="paperback" >
<title lang="en">Learning XML</title>
<author>Erik T. Ray</author>
<year>2003</year>
<price>39.95</price>
</book>
<book category="web">
<title lang="en">XQuery Kick Start</title>
<author>James McGovern</author>
<author>Per Bothner</author>
<author>Kurt Cagle</author>
<author>James Linn</author>
<author>Vaidyanathan Nagarajan</author>
<year>2003</year>
<price>49.99</price>
</book>
</bookstore>
我們都知道Java是一門面向對象的語言，因而我們需要盡量以面向對象的思想我編寫代碼，面向對象編程其中一個比較重要的特點就是基于對象編程，因而我們在編寫這個測試代碼時，也盡量的基于對象操作，而不是像過程式的語言，有一點信息做一點操作。
在這里，對XML文件中定義的book元素，我們使用Book對象與其對應：
public class Book {
private String category;
private String cover;
private TitleInfo title;
private List<String> authors;
private int year;
private double price;
...
public static class TitleInfo {
private String title;
private String lang;
...
}
}
根據XML文件定義構建Book實例：
public class W3CBooksBuilder {
public static List<Book> buildBooks() {
List<Book> books = new ArrayList<Book>();
books.add(buildHarrayBook());
books.add(builcEverydayItalian());
books.add(buildLearningXML());
books.add(buildXQueryKickStart());
return books;
}
public static Book buildHarrayBook() {
Book book = new Book();
book.setCategory("children");
book.setTitle(new TitleInfo("Harry Potter", "en"));
book.setAuthors(Arrays.asList("J K. Rowling"));
book.setYear(2005);
book.setPrice(29.99);
return book;
}
public static Book builcEverydayItalian() {
...
}
public static Book buildLearningXML() {
...
}
public static Book buildXQueryKickStart() {
...
}
}
DOM解析XML文件
DOM使用DocumentBuilder類來解析XML文件，它提供parse方法，將XML文件解析成一棵DOM樹，并返回Document實例：
public Document parse(InputStream is);
public Document parse(InputStream is, String systemId);
public Document parse(String uri);
public Document parse(File f);
public abstract Document parse(InputSource is);
DocumentBuilder類還提供了判斷當前解析器是否存在命名空間解析、驗證等配置，以及提供了設置EntityResolver、ErrorHandler的接口。這里使用EntityResolver和ErrorHandler只是重用SAX的API，并不表示DOM解析的內部實現一定要基于SAX，然而貌似JDK自帶的DOM解析內部使用的引擎就是SAX。T_T
public abstract boolean isNamespaceAware();
public abstract boolean isValidating();
public abstract void setEntityResolver(EntityResolver er);
public abstract void setErrorHandler(ErrorHandler eh);
DocumentBuilder提供了 構建Document實例的工廠方法，在以編程方式構建DOM樹時，首先需要構建Document實例，繼而使用Document實例構建其余節點類型，而構建Document實例需要通過DocumentBuilder類來實現：
public abstract Document newDocument();
最后，DocumentBuilder還提供了一些額外的方法，比如重置DocumentBuilder實例的狀態，以重用該DocumentBuilder；獲取DOMImplementation實例；獲取Schema實例；判斷XInclude處理模式。
public void reset();
public abstract DOMImplementation getDOMImplementation();
public Schema getSchema();
public boolean isXIncludeAware();
DocumentBuilder是一個抽象類，要獲取DocumentBuilder實例，需要使用DocumentBuilderFactory。DocumentBuilderFactory提供了多種查找DocumentBuilder實現類的方法；DocumentBuilderFactory本身也是抽象類，它提供了兩個靜態方法來創建DocumentBuilderFactory實例：
public static DocumentBuilderFactory newInstance();
public static DocumentBuilderFactory newInstance(String factoryClassName, ClassLoader classLoader);
不帶參數的newInstance()方法使用以下步驟查找DocumentBuilderFactory的實現類：
1. 查看系統屬性中是否存在javax.xml.parsers.DocumentBuilderFactory為key的定義，如果存在，則使用該key定義的值作為DocumentBuilderFactory的實現類。
2. 查找${java.home}/lib/jaxp.properties屬性文件中是否存在javax.xml.parsers.DocumentBuilderFactory為key的定義，若存在，則使用該屬性文件中以該key定義的值作為DocumentBuilderFactory的實現類。
3. 查找當前ClassPath（包括jar包中）下是否存在META-INF/services//javax.xml.parsers.DocumentBuilderFactory文件的定義（ServiceProvider），若存在，則讀取該文件中的第一行的值作為DocumentBuilderFactory的實現類。
4. 若以上都沒有找到，則使用默認的DocumentBuilderFactory的實現類：
com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl
在找到相應的DocumentBuilderFactory實現類后，實例化該實現類，并返回DocumentBuilderFatory實例。這里的查找機制和XMLReaderFactory查找XMLReader實現類以及commons-logging查找LogFactory的機制很像。
對帶參數的newInstance()方法，直接使用參數中提供的DocumentBuilderFactory實現類以及ClassLoader來創建DocumentBuilderFactory實例。
最后，在系統屬性中將jaxp.debug設置為true可以打開調試信息。
在創建DocumentBuilderFactory實例后，如其名所示，它可以用于獲取DocumentBuilder實例，另外，DocumentBuilderFactory還提供了配置解析器的方法：
public abstract DocumentBuilder newDocumentBuilder();
public void setNamespaceAware(boolean awareness);
public boolean isNamespaceAware();
public void setValidating(boolean validating);
public boolean isValidating();
public void setIgnoringElementContentWhitespace(boolean whitespace);
public boolean isIgnoringElementContentWhitespace();
public void setExpandEntityReferences(boolean expandEntityRef);
public boolean isExpandEntityReferences();
public void setIgnoringComments(boolean ignoreComments);
public boolean isIgnoringComments();
public void setCoalescing(boolean coalescing);
public boolean isCoalescing();
public void setXIncludeAware(final boolean state);
public boolean isXIncludeAware();
public abstract void setAttribute(String name, Object value);
public abstract Object getAttribute(String name);
public abstract void setFeature(String name, boolean value);
public abstract boolean getFeature(String name);
public Schema getSchema();
public void setSchema(Schema schema);
在創建出DocumentBuilderFactory，使用該factory創建DocumentBuilder實例后，就可以使用該DocumentBuilder解析XML文件成一個Document實例，而通過該Document實例就可以遍歷、查找DOM樹，從而獲得想要的信息。在下面的例子中，遍歷DOM樹，創建多個Book實例：
public class W3CBooksDOMReader {
private static DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
private String booksXmlFile;
public W3CBooksDOMReader(String booksXmlFile) {
this.booksXmlFile = booksXmlFile;
}
public List<Book> parse() {
Document doc = parseXmlFile();
Element root = doc.getDocumentElement();
NodeList nodes = root.getElementsByTagName("book");
List<Book> books = new ArrayList<Book>();
for(int i = 0; i < nodes.getLength(); i++) {
books.add(parseBookElement((Element)nodes.item(i)));
}
return books;
}
private Document parseXmlFile() {
File xmlFile = new File(booksXmlFile);
if(!xmlFile.exists()) {
throw new RuntimeException("Cannot find xml file: " + booksXmlFile);
}
try {
DocumentBuilder builder = factory.newDocumentBuilder();
return builder.parse(xmlFile);
} catch(Exception ex) {
throw new RuntimeException("Failed to create DocumentBuilder instance", ex);
}
}
private Book parseBookElement(Element bookElement) {
String category = bookElement.getAttribute("category");
String cover = bookElement.getAttribute("cover");
NodeList nodes = bookElement.getElementsByTagName("title");
String lang = ((Element)nodes.item(0)).getAttribute("lang");
// First way to get content of an Element
String title = ((Text)((Element)nodes.item(0)).getFirstChild()).getData().trim();
List<String> authors = new ArrayList<String>();
nodes = bookElement.getElementsByTagName("author");
for(int i = 0; i < nodes.getLength(); i++) {
// Second way to get content of an Element
String author = nodes.item(0).getTextContent().trim();
authors.add(author);
}
nodes = bookElement.getElementsByTagName("year");
int year = Integer.parseInt(nodes.item(0).getTextContent().trim());
nodes = bookElement.getElementsByTagName("price");
double price = Double.parseDouble(nodes.item(0).getTextContent().trim());
Book book = new Book();
book.setCategory(category);
book.setCover(cover);
book.setTitle(new TitleInfo(title, lang));
book.setAuthors(authors);
book.setYear(year);
book.setPrice(price);
return book;
}
public String getBooksXmlFile() {
return booksXmlFile;
}
public static void main(String[] args) {
W3CBooksDOMReader reader = new W3CBooksDOMReader("resources/xmlfiles/w3c_books.xml");
List<Book> books = reader.parse();
System.out.println("result:");
for(Book book : books) {
System.out.println(book);
}
}
}
DOM構建XML文件
將對象實例序列化成XML文件，首先需要構建DOM樹，即要構建Document實例，然后將該Document實例寫入的XML文件中。如上節所述，可以使用DocumentBuilder類來創建Document實例，然后根據對象實例（Book實例）和需要的XML格式構建節點和節點的排布即可，這里不再詳述。
要將對象序列化成XML文件還要處理的另一個問題是如何將Document實例寫入到指定的XML文件中，在Java中提供了Transformer接口來做這件事情。這屬于XLST（EXtensible Stylesheet Language）的范疇，不過這里不打算對其做詳細介紹，主要關注如何將Document實例輸出成XML文件。
Transformer提供了transform方法將Document實例寫入指定的流中：
public abstract void transform(Source xmlSource, Result outputTarget);
其中Source接口定義了輸入源，它可以是DOMSource，也可以是SAXSource，或者是自定義的其他Source子類，這里主要介紹DOMSource。Source接口定義了systemId屬性，它表示XML源的位置，XML源不是從URL中獲取的源來說，它為null。具體定義如下：
public interface Source {
public void setSystemId(String systemId);
public String getSystemId();
}
DOMSource是對Source的一個具體實現，它接收Node、systemId信息：
public class DOMSource implements Source {
private Node node;
private String systemID;
public DOMSource() { }
public DOMSource(Node n) {
setNode(n);
}
public DOMSource(Node node, String systemID) {
setNode(node);
setSystemId(systemID);
}
...
}
Result是對輸出目的的抽象，即將輸入源轉換成目的源。同Source接口，Result接口也定義了systemId屬性，表示目的文件位置，如果目的源不是URL，則改值為null：
public interface Result {
public void setSystemId(String systemId);
public String getSystemId();
}
JDK中提供了多種Result的實現，如DOMResult、StreamResult等。這里只介紹StreamResult，表示其輸出目的是流，我們可以提供Writer、OutputStream等實例來接收這些輸出：
public class StreamResult implements Result {
public StreamResult() {
}
public StreamResult(OutputStream outputStream) {
setOutputStream(outputStream);
}
public StreamResult(Writer writer) {
setWriter(writer);
}
public StreamResult(String systemId) {
this.systemId = systemId;
}
public StreamResult(File f) {
setSystemId(f.toURI().toASCIIString());
}
...
private String systemId;
private OutputStream outputStream;
private Writer writer;
}
除了transform方法，Transformer類還提供了其他的方法用于配置Transformer在轉換時用到的信息（只提供接口定義，不詳述）：
public void reset();
public abstract void setParameter(String name, Object value);
public abstract Object getParameter(String name);
public abstract void clearParameters();
public abstract void setURIResolver(URIResolver resolver);
public abstract URIResolver getURIResolver();
public abstract void setOutputProperties(Properties oformat);
public abstract Properties getOutputProperties();
public abstract void setOutputProperty(String name, String value);
public abstract String getOutputProperty(String name);
public abstract void setErrorListener(ErrorListener listener);
public abstract ErrorListener getErrorListener();
類似DocumentBuilder，Transformer通過TransformerFactory創建，而TransformerFactory的創建如同DocumentBuilderFactory的創建以及查找機制，所不同的是TransformerFactory的屬性名為：javax.xml.transform.TransformerFactory，其默認實現類為：com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl，而且它也提供了兩個獲取TransformerFactory實例的方法，這里不再詳述：
public static TransformerFactory newInstance();
public static TransformerFactory newInstance(String factoryClassName, ClassLoader classLoader);
TransformerFactory提供了創建Transformer和Templates的方法，同時也提供了在創建這兩個實例時可以設置的一些配置方法：
public abstract Transformer newTransformer(Source source);
public abstract Transformer newTransformer();
public abstract Templates newTemplates(Source source);
public abstract Source getAssociatedStylesheet(Source source, String media,
String title, String charset);
public abstract void setURIResolver(URIResolver resolver);
public abstract URIResolver getURIResolver();
public abstract void setFeature(String name, boolean value);
public abstract boolean getFeature(String name);
public abstract void setAttribute(String name, Object value);
public abstract Object getAttribute(String name);
public abstract void setErrorListener(ErrorListener listener);
public abstract ErrorListener getErrorListener();
最后，提供一個完整的例子，使用本文開始時創建的List<Book>實例序列化成XML文件：
public class W3CBooksDOMWriter {
private static DocumentBuilder docBuilder;
private static Transformer transformer;
static {
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
try {
docBuilder = factory.newDocumentBuilder();
} catch(Exception ex) {
throw new RuntimeException("Create DocumentBuilder instance failed.", ex);
}
TransformerFactory transFactory = TransformerFactory.newInstance();
try {
transformer = transFactory.newTransformer();
} catch(Exception ex) {
throw new RuntimeException("Create Transformer instance failed.", ex);
}
transformer.setOutputProperty(OutputKeys.ENCODING, "UTF-8");
transformer.setOutputProperty(OutputKeys.INDENT, "yes");
}
private List<Book> books;
public W3CBooksDOMWriter(List<Book> books) {
this.books = books;
}
public void toXml(Writer writer) throws Exception {
Document doc = buildDOMTree();
writeToXmlFile(writer, doc);
}
public Document buildDOMTree() {
Document doc = docBuilder.newDocument();
Element root = doc.createElement("bookstore");
doc.appendChild(root);
for(Book book : books) {
Element bookElement = buildBookElement(doc, book);
root.appendChild(bookElement);
}
return doc;
}
public Element buildBookElement(Document doc, Book book) {
Element bookElement = doc.createElement("book");
bookElement.setAttribute("category", book.getCategory());
bookElement.setAttribute("cover", book.getCover());
TitleInfo title = book.getTitle();
Element titleElement = doc.createElement("title");
titleElement.setAttribute("lang", title.getLang());
titleElement.setTextContent(title.getTitle());
bookElement.appendChild(titleElement);
for(String author : book.getAuthors()) {
Element authorElement = doc.createElement("author");
authorElement.setTextContent(author);
bookElement.appendChild(authorElement);
}
Element yearElement = doc.createElement("year");
yearElement.setTextContent(String.valueOf(book.getYear()));
bookElement.appendChild(yearElement);
Element priceElement = doc.createElement("price");
priceElement.setTextContent(String.valueOf(book.getPrice()));
bookElement.appendChild(priceElement);
return bookElement;
}
public void writeToXmlFile(Writer writer, Document doc) throws Exception {
DOMSource source = new DOMSource(doc);
StreamResult result = new StreamResult(writer);
transformer.transform(source, result);
}
public static void main(String[] args) throws Exception {
StringWriter writer = new StringWriter();
List<Book> books = W3CBooksBuilder.buildBooks();
W3CBooksDOMWriter domWriter = new W3CBooksDOMWriter(books);
domWriter.toXml(writer);
System.out.println(writer.toString());
}
}