笔记：XML-解析文档-DOM

要处理XML文档，就要先解析（parse）他，解析器时这样一个程序，读入一个文件，确认整个文件具有正确的格式，然后将其分解成各种元素，使得程序员能够访问这些元素，Java库提供了两种XML解析器：

• 像文档对象模型（Document Object Model，DOM）解析器这样的树型解析器，他们将读入的XML文档转换成树结构
• 像XML简单API（Simple API for XML，SAX）解析器这样的流机制解析器，他们读入XML文档时生成相应的事件

DOM解析器对于实现我们的大多数目的来说都更容易一些，如果需要处理很长的文档，用他来生成树结构会消耗大量的内存，或者只需要对于某些元素感兴趣，而不关心他们的上下文，那么可以考虑使用SAX解析器。DOM解析器的接口已经被W3C标准化了，org.w3c.dom包中包含了这些接口类型的定义，比如：Document、Element等，不同的实现者都编写了实现这些接口的DOM解析器，Java
XML
处理API（Java
API
for
XML
Processing，JAXP）库使得实际上可以以插件形式使用这些解析器中的任意一个。要读入一个XML文档，首先需要一个DocumentBuilder对象，可以从
DocumentBuilderFactory
中得到这个对象，代码如下：

DocumentBuilderFactory factory=DocumentBuilderFactory.newInstance();

DocumentBuilder builder=factory.newDocumentBuilder();

现在，可以从文件中读入某个文档：

 File f = …;

Document doc = builder.parse(f);

或者，可以用一个URL：

URL u = …;

Document doc = builder.parse(u);

甚至可以使用一个任意的输入流：

InputStream in = …;

Document doc = builder.parse(in);

Document 对象时XML文档的树型结构在内存的表现，他由实现了 Node 接口及其各子接口的类对象构成，子接口层次结构如下：

DOM
解析XML文档的代码示例如下：

• XML文档内容：

  <?xml version="1.0" encoding="utf-8" ?>

  <root>

          <title>

                  <font enabled="false">

                          <!-- 字体名称 -->

                          <name>Helvetica</name>

                          <size>36</size>

                  </font>

                  <data>

                          <![CDATA[xml document root node <root.../>]]>

                  </data>

          </title>

  </root>

• 解析代码：

  public static
  void
  main(String[] args) {

                  DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();

                  try {

                          DocumentBuilder builder = factory.newDocumentBuilder();

                          Path xmlPath = Paths.get("E:\\IDEA Workspace\\exampleiostream\\src\\main\\java\\org\\drsoft\\examples\\xml", "appParse.xml");

                          InputStream xmlStream = Files.newInputStream(xmlPath, StandardOpenOption.READ);

                          Document xmlDocument = builder.parse(xmlStream);

                          StringBuilder sb = new
  StringBuilder();

                          sb.append("<?xml version=\"" + xmlDocument.getXmlVersion() + "\"
  encodin=\""

  + xmlDocument.getXmlEncoding() + "\"
  ?>");

                          Element root = xmlDocument.getDocumentElement();

                          sb.append("<" + root.getTagName() + ">");

                          NodeList nodeList = root.getChildNodes();

                          for (int i = 0; i < nodeList.getLength(); i++) {

                                  if (nodeList.item(i) instanceof Element) {

                                          sb.append(parseChildNodes((Element) nodeList.item(i)));

                                  }

                          }

                          sb.append("</" + root.getTagName() + ">");

                          System.out.println(sb.toString());

                  } catch (ParserConfigurationException e) {

                          e.printStackTrace();

                  } catch (IOException ex) {

                          ex.printStackTrace();

                  } catch (SAXException e) {

                          e.printStackTrace();

                  }

          }

   
   

          private static String parseChildNodes(Element element) {

                  StringBuilder sb = new
  StringBuilder();

                  sb.append("<" + element.getTagName());

                  NamedNodeMap map = element.getAttributes();

                  for (int j = 0; j < map.getLength(); j++) {

                          sb.append(" " + map.item(j).getNodeName() + "=\"" + map.item(j).getNodeValue() + "\"");

                  }

                  sb.append(" >");

                  if (element.hasChildNodes()) {

                          NodeList list = element.getChildNodes();

                          for (int i = 0; i < list.getLength(); i++) {

                                  if (list.item(i) instanceof Element) {

                                          Element curElement = (Element) list.item(i);

                                          sb.append(parseChildNodes(curElement));

                                          continue;

                                  }

                                  if (list.item(i) instanceof CDATASection) {

                                          CDATASection cdataSection = (CDATASection) list.item(i);

                                          sb.append("<![CDATA[").append(cdataSection.getData()).append("]]>");

                                          continue;

                                  }

                                  if (list.item(i) instanceof Comment) {

                                          Comment comment = (Comment)list.item(i);

                                          sb.append("<!-- "+comment.getData()+" -->");

                                          continue;

                                  }

                                  if (list.item(i) instanceof Text) {

                                          Text curText = (Text) list.item(i);

                                          sb.append(curText.getData().trim());

                                          continue;

                                  }

                          }

                  }

                  sb.append("</" + element.getTagName() + ">");

                  return sb.toString();

          }