3.2XML中的XPath和XML、對象之間的轉換

（XPath、XMl和對象之間的轉換）

1、XPath的介紹

XPath 是一門在 XML 文檔中查找信息的語言。類似於數據庫的SQL。

XPath 是通過元素和屬性進行查找

XPath簡化了Dom4j查找節點的過程
使用XPath必須導入jaxen-1.1-beta-6.jar
否則出現NoClassDefFoundError: org/jaxen/JaxenException

2、Xpath語法：

3、XPath進行信息查詢的代碼：

package com.test;

import java.io.File;
import java.util.List;

import org.dom4j.Document;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;

public class TestXPath {
	public static void main(String[] args) {

		File xmlFile = new File(
				"D:\\WorkSpace6.5\\java1603\\Eclipse\\Xml_Project\\src\\student.xml");

		SAXReader saxReader = new SAXReader();
		String xpath = "";
		List<Element> stuList = null;
		Element element = null;
		try {
			Document document = saxReader.read(xmlFile);
			Element root = document.getRootElement();
			/**
			 * 1：通過XPath來查找節點
			 */
			// 1：查找XML所有的student
			xpath = "/root/student";
			stuList = document.selectNodes(xpath);
			System.out.println(stuList.size());

			/**
			 * 2：使用相對路徑，從root來查詢
			 */
			stuList = root.selectNodes("student");
			System.out.println(stuList.size());
			/**
			 * 3：獲取所有name爲student元素對象，不考慮位置
			 */
			stuList = document.selectNodes("//student");
			System.out.println(stuList.size());

			// 2：查詢sudent元素中id = 4的元素節點
			element = (Element) document.selectSingleNode("//student[@id='4']");
			System.out.println(element.attributeValue("id"));

			// 3：查詢性別爲男的元素節點
			stuList = document.selectNodes("/root/student[stu_sex='男']");
			System.out.println(stuList.size());
			// 4：查詢名稱中包含"a"的元素節點,類似like語句。
			stuList = document
					.selectNodes("/root/student[contains(stu_name,'a')]");
			System.out.println(stuList.size());

			// 4：查詢名稱中包含"a"的元素節點,或者年齡大於20歲
			stuList = document
					.selectNodes("/root/student[contains(stu_name,'a') or stu_age>=20]");
			System.out.println(stuList.size());
			// 4：查詢名稱中包含"a"的元素節點,並且年齡大於20歲
			stuList = document
					.selectNodes("/root/student[contains(stu_name,'a')][stu_age>=20]");
			System.out.println(stuList.size());

		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

4、XMl與對象之間的轉換

   XStream對象相當Java對象和XML之間的轉換器，轉換過程是雙向的。創建XSteam對象的方式很簡單，只需要new XStream()即可。

              Java到xml，用toXML()方法。  
              Xml到Java，用fromXML()方法。 

 在沒有任何設置默認情況下，java到xml的映射，是java成員名對應xml的元素名，java類的全名對應xml根元素的名字。而實際中，往往是xml和java類都有了，要完成相互轉換，必須進行別名映射。
別名配置包含三種情況： 
 1、類別名，用alias(String name, Class type)。 
 2、類成員別名，用aliasField(String alias, Class definedIn, String fieldName) 
 3、類成員作爲屬性別名，用 aliasAttribute(Class definedIn, String attributeName, String alias)

package com.test;

import java.io.FileOutputStream;

import com.bean.StudentBean;
import com.thoughtworks.xstream.XStream;
import com.thoughtworks.xstream.io.xml.DomDriver;
import com.thoughtworks.xstream.io.xml.XmlFriendlyNameCoder;
import com.thoughtworks.xstream.io.xml.XppDriver;

/**
 * XML與Java對象之間的轉換
 * 
 * @author ctd
 *
 */
public class XmlToJava {
	public static void main(String[] args) {
		XmlToJava.objectToXML();

		// XmlToJava.xmlToObject();
	}

	private static void objectToXML() {
		StudentBean student = new StudentBean();
		student.setStu_id(1);
		student.setStu_name("admin");
		student.setStu_sex("男");
		student.setStu_content("備註內容");
		student.setStu_age(20);

		/**
		 * 將對象轉爲XML字符串/XML文件
		 * 
		 * 1：通過字符串的拼接
		 * 
		 * 2：通過X-Stream對象來進行轉換，得包含xstream-1.4.5.jar
		 */
		/**
		 * 解決下劃線的問題
		 */
		XStream stream = new XStream(new XppDriver(new XmlFriendlyNameCoder(
				"_-", "_")));
		/**
		 * 更改根節點的名稱
		 */
		stream.alias("root", StudentBean.class);
		/**
		 * 將id做爲屬性
		 */
		stream.aliasAttribute(StudentBean.class, "stu_id", "id");
		System.out.println(stream.toXML(student));

		/**
		 * 生成一個文件
		 */
		try {
			FileOutputStream fileOutputStream = new FileOutputStream(
					"E:\\test.xml");
			fileOutputStream.write("<?xml version=\"1.0\" encoding=\"UTF-8\"?>"
					.getBytes());
			fileOutputStream.write("\n".getBytes());
			stream.toXML(student, fileOutputStream);
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	private static void xmlToObject() {
		XStream stream = new XStream(new DomDriver());
		stream.alias("root", StudentBean.class);
		stream.aliasAttribute(StudentBean.class, "stu_id", "id");

		StringBuffer xml = new StringBuffer();
		xml.append("<root id='2'>");
		xml.append("  <stu_name>admin</stu_name>");
		xml.append("  <stu_sex>男</stu_sex>");
		xml.append("  <stu_age>20</stu_age>");
		xml.append("  <stu_content>備註內容</stu_content>");
		xml.append("</root>");
		StudentBean studentBean = (StudentBean) stream.fromXML(xml.toString());

		System.out.println(studentBean.getStu_id() + "\t"
				+ studentBean.getStu_name());
	}
}

studentBean.java

package com.bean;

public class StudentBean {
	private int stu_id;
	private String stu_name;
	private String stu_sex;
	private int stu_age;
	private String stu_content;

	public int getStu_id() {
		return stu_id;
	}

	public void setStu_id(int stu_id) {
		this.stu_id = stu_id;
	}

	public String getStu_name() {
		return stu_name;
	}

	public void setStu_name(String stu_name) {
		this.stu_name = stu_name;
	}

	public String getStu_sex() {
		return stu_sex;
	}

	public void setStu_sex(String stu_sex) {
		this.stu_sex = stu_sex;
	}

	public int getStu_age() {
		return stu_age;
	}

	public void setStu_age(int stu_age) {
		this.stu_age = stu_age;
	}

	public String getStu_content() {
		return stu_content;
	}

	public void setStu_content(String stu_content) {
		this.stu_content = stu_content;
	}

}