sax 动态切换 抓取感兴趣的内容（把element当做documnet 处理）

由switch 类触发事件

import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;

abstract class DelegateHandler extends DefaultHandler{
    
    public abstract void startDocument(String uri, String localName, String qName, Attributes attributes) throws SAXException;
    
    public abstract void endDocument(String uri, String localName, String qName) throws SAXException;

}

由api 触发事件

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;

class SwitchHandler extends DefaultHandler {

    private final Map<String, DelegateHandler> handlerMap;

    private List<String> path = new ArrayList<String>();
    private String currentHandlerPath = null;

    public SwitchHandler(Map<String, DelegateHandler> handlerMap) {
        this.handlerMap = handlerMap;
    }

    @Override
    public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {
        // update path (increase)
        setPath(qName, true);

        String path = getPath();
        // startDocument
        if (handlerMap.containsKey(path)) {
            handlerMap.get(path).startDocument(uri, localName, qName, attributes);
            currentHandlerPath = path;
        }
        // startElement
        else if (currentHandlerPath != null && path.startsWith(currentHandlerPath)) {
            handlerMap.get(currentHandlerPath).startElement(uri, localName, qName, attributes);
        }
    }

    @Override
    public void characters(char ch[], int start, int length) throws SAXException {
        if (currentHandlerPath != null) {
            handlerMap.get(currentHandlerPath).characters(ch, start, length);
        }
    }

    @Override
    public void endElement(String uri, String localName, String qName) throws SAXException {

        String path = getPath();

        // endDocument
        if (handlerMap.containsKey(path)) {
            handlerMap.get(path).endDocument(uri, localName, qName);
            currentHandlerPath = null;
        }
        // endElement
        else if (currentHandlerPath != null && path.startsWith(currentHandlerPath)) {
            handlerMap.get(currentHandlerPath).endElement(uri, localName, qName);
        }

        // update path (reduce)
        setPath(qName, false);
    }

    public String getPath() {
        StringBuilder sb = new StringBuilder();
        for (String str : this.path) {
            sb.append("/");
            sb.append(str);
        }
        return sb.toString();
    }

    public void setPath(String qName, boolean add) {
        if (add) {
            this.path.add(qName);
        } else {
            this.path.remove(path.size() - 1);
        }
    }

}

具体的实现

import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.io.StringWriter;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;

import org.xml.sax.Attributes;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
import org.xml.sax.XMLReader;

public class SaxTest {
    
    public static void main(String[] args) throws Exception {
        SaxTest saxTest = new SaxTest();
        InputStream in = SaxTest.class.getResourceAsStream("test.xml");
        InputStreamReader inr = new InputStreamReader(in);
        System.out.println(saxTest.exchange(inr));
    }

    public String exchange(Reader reader) throws Exception {

        // xpath
        Map<String, DelegateHandler> handlerMap = new HashMap<String, DelegateHandler>();
        TeacherHandler teacherHandler = new TeacherHandler();
        StudentHandler studentHandler = new StudentHandler();
        handlerMap.put("/class/teacher", teacherHandler);
        handlerMap.put("/class/student", studentHandler);
        SwitchHandler switchHandler = new SwitchHandler(handlerMap);

        // sax
        SAXParserFactory factory = SAXParserFactory.newInstance();
        SAXParser parser = factory.newSAXParser();
        XMLReader xmlReader = parser.getXMLReader();
        xmlReader.setContentHandler(switchHandler);
        xmlReader.parse(new InputSource(reader));

        // data
        StringWriter writer = new StringWriter();
        for (String teacher : teacherHandler.teacherList) {
            writer.write("teacher:" + teacher + "
");
        }
        for (String student : studentHandler.studentList) {
            writer.write("student:" + student + "
");
        }
        return writer.toString();
    }

    private class TeacherHandler extends DelegateHandler {

        List<String> teacherList = new ArrayList<String>();
        StringBuilder characters = new StringBuilder();
        String id = null;
        String name = null;

        @Override
        public void startDocument(String uri, String localName, String qName, Attributes attributes)
                throws SAXException {

        }

        @Override
        public void startElement(String uri, String localName, String qName, Attributes attributes)
                throws SAXException {
        }

        @Override
        public void characters(char[] ch, int start, int length) throws SAXException {
            characters.append(ch, start, length);
        }

        @Override
        public void endElement(String uri, String localName, String qName) throws SAXException {
            if (qName.equals("id")) {
                id = characters.toString().trim();
            } else if (qName.equals("name")) {
                name = characters.toString().trim();
                teacherList.add(id + " = " + name);
            }
            characters.setLength(0);
        }

        @Override
        public void endDocument(String uri, String localName, String qName) throws SAXException {

        }

    }

    private class StudentHandler extends DelegateHandler {

        List<String> studentList = new ArrayList<String>();
        StringBuilder characters = new StringBuilder();
        String id = null;
        String name = null;

        @Override
        public void startDocument(String uri, String localName, String qName, Attributes attributes)
                throws SAXException {

        }

        @Override
        public void startElement(String uri, String localName, String qName, Attributes attributes)
                throws SAXException {
        }

        @Override
        public void characters(char[] ch, int start, int length) throws SAXException {
            characters.append(ch, start, length);
        }

        @Override
        public void endElement(String uri, String localName, String qName) throws SAXException {
            if (qName.equals("id")) {
                id = characters.toString().trim();
            } else if (qName.equals("name")) {
                name = characters.toString().trim();
                studentList.add(id + " = " + name);
            }
            characters.setLength(0);
        }

        @Override
        public void endDocument(String uri, String localName, String qName) throws SAXException {

        }

    }
}

测试文件：

<class>
    <teacher>
        <id>t01</id>
        <name>SUN</name>
    </teacher>
    <teacher>
        <id>t02</id>
        <name>ZHANG</name>
    </teacher>
    <student>
        <id>s01</id>
        <name>Lucy</name>
    </student>
    <student>
        <id>s02</id>
        <name>Lili</name>
    </student>
</class>

结果：

teacher:t01 = SUN
teacher:t02 = ZHANG
student:s01 = Lucy
student:s02 = Lili

补充：

可通过构造函数传入感兴趣的字段，“a,b,c,d”

补充（如果控制顺序）：

import java.util.LinkedHashMap;
import java.util.Map;

public class Main {

    public static void main(String[] args) {
        Map<String, String> linkedHashMap = new LinkedHashMap<String, String>();
        linkedHashMap.put("a", "1");
        linkedHashMap.put("b", "2");
        // 这里不改变原有顺序
        linkedHashMap.put("a", "3");

        System.out.println(linkedHashMap);

    }

}

{a=3, b=2}