XML读写之DOM

　　本文先使用DOM方式写一个简单地小例子，亲测可以运行，然后在末尾分析其优缺点。

1.准备

• 新建一个Java Project
• 新建一个Java类名称为DOMTest.java
• 新建一个xml文件，名称为student.xml，内容如下：

<?xml version="1.0" encoding="UTF-8"?>
<student_info>
    <student id="1">
        <name>张三</name>
        <gender>男</gender>
        <age>16</age>
    </student>
    <student id="2">
        <name>李四</name>
        <gender>女</gender>
        <age>15</age>
    </student>
</student_info>

• 由于是Maven工程，所有xml都存放在src/main/resources下面

2.编码

• Bean文件：StudentBean.java

public class StudentBean {
    private String id;
    private String name;
    private String gender;
    private Integer age;

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "StudentBean [id=" + id + ",name=" + name + ",gender=" + gender + ",age=" + age + "]";
    }

}

• XML读写类：DOMTest.java

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.util.Iterator;
import java.util.Vector;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.transform.OutputKeys;
import javax.xml.transform.Result;
import javax.xml.transform.Source;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerConfigurationException;
import javax.xml.transform.TransformerException;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.TransformerFactoryConfigurationError;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;

import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.w3c.dom.Text;
import org.xml.sax.SAXException;

import bean.StudentBean;

public class DOMTest {
    // 创建一个Vector来保存每个student对象
    public static Vector<StudentBean> studentsVector = new Vector<StudentBean>();

    public static void main(String[] args) {
        // 创建一个Vector来存放StudentBean对象
        Vector<StudentBean> v = readXMLFile("src/main/resources/student.xml");
        Iterator<StudentBean> it = v.iterator();
        while (it.hasNext()) {
            System.out.println(it.next());
        }
        writeXMLFile("src/main/resources/college.xml");
        updateXMLFile("src/main/resources/student.xml");
    }

    // 从XML文件中读数据
    public static Vector<StudentBean> readXMLFile(String fileName) {
        // 为解析XML做准备，首先创建DocumentBuilderFactory实例
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = null;
        Document doc = null;
        try {
            // 指定DocumentBuilder
            db = dbf.newDocumentBuilder();
            doc = db.parse(fileName);
        } catch (ParserConfigurationException e) {
            // 输出异常信息然后退出
            e.printStackTrace();
            System.exit(1);
        } catch (SAXException e) {
            e.printStackTrace();
            System.exit(1);
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
        // 下面开始解析XML文件过程
        // 先取根元素
        Element root = doc.getDocumentElement();
        // 获取student个数
        NodeList students = root.getElementsByTagName("student");
        System.out.println("XML文件中有" + students.getLength() + "个对象！");
        for (int i = 0; i < students.getLength(); i++) {
            Element st = (Element) students.item(i);
            // 创建一的StudentBean对象用来保存XML中学生的属性
            StudentBean sb = new StudentBean();
            // 获取student节点的属性值赋给Bean对象
            // 注意，属性值是在节点括号里面的
            sb.setId(st.getAttribute("id"));
            // st是一个student节点，在这个节点内获取所有标记名为name的元素，保存为NodeList
            NodeList names = st.getElementsByTagName("name");
            // 单一student节点下只有一个name节点，所以只需要取names第一个元素即可
            Element name = (Element) names.item(0);
            Node t = name.getFirstChild();
            sb.setName(t.getNodeValue());
            NodeList genders = st.getElementsByTagName("gender");
            Element gender = (Element) genders.item(0);
            Node n = gender.getFirstChild();
            sb.setGender(n.getNodeValue());
            NodeList ages = st.getElementsByTagName("age");
            Element age = (Element) ages.item(0);
            Node g = age.getFirstChild();
            sb.setAge(Integer.parseInt(g.getNodeValue()));
            studentsVector.add(sb);
        }
        return studentsVector;
    }

    // 将数据写入XML文件
    public static void writeXMLFile(String fileName) {
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = null;
        Document doc = null;
        try {
            db = dbf.newDocumentBuilder();
        } catch (ParserConfigurationException e) {
            e.printStackTrace();
        }
        doc = db.newDocument();
        // 创建根元素
        Element root = doc.createElement("student_info");
        // 将根元素添加到文档上
        doc.appendChild(root);
        for (int i = 0; i < studentsVector.size(); i++) {
            // 取出studentBean对象
            StudentBean s = (StudentBean) studentsVector.get(i);
            // 创建student元素
            Element stu = doc.createElement("student");
            // 为该student元素增加属性
            stu.setAttribute("id",s.getId());
            // 将stu添加到root元素上
            root.appendChild(stu);
            // 创建name元素
            Element name = doc.createElement("name");
            stu.appendChild(name);
            Text tname = doc.createTextNode(s.getName());
            name.appendChild(tname);
            // 创建gender元素
            Element gender = doc.createElement("gender");
            stu.appendChild(gender);
            Text tgender = doc.createTextNode(s.getGender());
            gender.appendChild(tgender);
            // 创建age元素
            Element age = doc.createElement("age");
            stu.appendChild(age);
            Text tage = doc.createTextNode(s.getAge().toString());
            age.appendChild(tage);
            // 开始将文档树写入XML文件
            try {
                FileOutputStream fos = new FileOutputStream(fileName);
                OutputStreamWriter out = new OutputStreamWriter(fos);
                callWriteXMLFile(doc,out,"UTF-8");
                out.close();
                fos.close();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        System.out.println("数据已经写入XML文档！");
    }

    // 修改XML文件中的某项内容
    public static void updateXMLFile(String fileName) {
        // 为解析XML做准备，首先创建DocumentBuilderFactory实例
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = null;
        Document doc = null;
        try {
            // 指定DocumentBuilder
            db = dbf.newDocumentBuilder();
            doc = db.parse(fileName);
        } catch (ParserConfigurationException e) {
            // 输出异常信息然后退出
            e.printStackTrace();
            System.exit(1);
        } catch (SAXException e) {
            e.printStackTrace();
            System.exit(1);
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
        // 先取根元素
        Element root = doc.getDocumentElement();
        NodeList students = root.getElementsByTagName("student");
        // 修改第二个学生的信息
        Element st = (Element) students.item(1);
        NodeList genders = st.getElementsByTagName("gender");
        Element gender = (Element) genders.item(0);
        Node n = gender.getFirstChild();
        // 修改该节点的值
        n.setNodeValue("中");
        try {
            FileOutputStream fos = new FileOutputStream(fileName);
            OutputStreamWriter out = new OutputStreamWriter(fos);
            callWriteXMLFile(doc,"UTF-8");
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
        System.out.println("XML文档数据已经修改！");
    }

    // 对XML文件进行写和改操作后，需要保存XML文件才能生效
    public static void callWriteXMLFile(Document doc,OutputStreamWriter out,String encoding) {
        Source source = new DOMSource(doc);
        Result result = new StreamResult(out);
        try {
            Transformer xformer = TransformerFactory.newInstance().newTransformer();
            xformer.setOutputProperty(OutputKeys.ENCODING,encoding);
            xformer.transform(source,result);
        } catch (TransformerConfigurationException e) {
            e.printStackTrace();
        } catch (TransformerFactoryConfigurationError e) {
            e.printStackTrace();
        } catch (TransformerException e) {
            e.printStackTrace();
        }
    }

}

3.运行结果

　　控制台输出为：

　　程序自行新建的XML文件college.xml为：

<?xml version="1.0" encoding="UTF-8"?>
<student_info>
    <student id="1">
        <name>张三</name>
        <gender>男</gender>
        <age>16</age>
    </student>
    <student id="2">
        <name>李四</name>
        <gender>女</gender>
        <age>15</age>
    </student>
</student_info>

　　程序修改后的student.xml为：

<?xml version="1.0" encoding="UTF-8"?>
<student_info>
    <student id="1">
        <name>张三</name>
        <gender>男</gender>
        <age>16</age>
    </student>
    <student id="2">
        <name>李四</name>
        <gender>中</gender>
        <age>15</age>
    </student>
</student_info>

4.结束

　　DOM解析方式是把整个XML文档当成一个对象来处理，在解析的时候会把整个文档读入到内存中。它是基于树的结构，通常需要加载整个文档并且构造DOM树，然后才能工作。 　　优点：由于整棵树都在内存中，所以具有随机访问的优点。 　　缺点：整个文档必须一次性解析完，不太适合较大的文档解析。