思路：通过解析xml将各个类存入容器内，然后通过map存放各个标签的内容，通过
url-pattern的内容找到servlet-name,通过servlet-name的内容，找到对应的类servlet-class,然后通过servlet-class内的内容（包名.类名)反射得到对应的类，然后输出

web.xml

<?xml version="1.0" encoding="UTF-8"?>  
 <web-app>
 <servlet>
    <servlet-name>login</servlet-name>
    <servlet-class>hello.RegisterServlet</servlet-class>
 </servlet>
     <servlet>
    <servlet-name>reg</servlet-name>
    <servlet-class>hello.LoginServlet</servlet-class>
 </servlet>   
 <servlet-mapping>
    <servlet-name>login</servlet-name>
    <url-pattern>/login</url-pattern> 
    <url-pattern>/g</url-pattern> 
 </servlet-mapping>  
 <servlet-mapping>
    <servlet-name>reg</servlet-name>
    <url-pattern>/reg</url-pattern> 
 </servlet-mapping>
 </web-app>

xml:

public class xmll {

public static void main(String[] args) throws Exception{
    //SAX解析
    //1、获取解析工厂
    SAXParserFactory factory=SAXParserFactory.newInstance();
    //2、从解析工厂获取解析器
    SAXParser parse =factory.newSAXParser();
    //3、编写处理器
    //4、加载文档 Document 注册处理器
    WebHandler handler=new WebHandler();
    //5、解析
    parse.parse(Thread.currentThread().getContextClassLoader()
    .getResourceAsStream("hello\\web.xml")
    ,handler);

    //获取数据
    WebContext context=new WebContext(handler.getEntitys(),handler.getMappings());
    //假设你输入了 /login
    String className=context.getClz("/login");
    //反射
    Class clz=Class.forName(className);  //className返回的是servlet-class的内容，即包名.类名
    Servlet servlet=(Servlet)clz.getConstructor().newInstance();
    servlet.service();
}

}
class WebHandler extends DefaultHandler{
    private List<Entity> entitys  = new ArrayList<Entity>();
    private List<Mapping> mappings = new ArrayList<Mapping>();
    private Entity entity ;
    private Mapping mapping ;
    private String tag; //存储操作标签
    private boolean isMapping = false;

@Override
public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {
    if(null!=qName) {
        tag = qName; //存储标签名
        if(tag.equals("servlet")) {
            entity = new Entity();
            isMapping = false;
        }else if(tag.equals("servlet-mapping")) {
            mapping = new Mapping();
            isMapping = true;
        }
    }
}

@Override
public void characters(char[] ch, int start, int length) throws SAXException {
    String contents = new String(ch,start,length).trim();
    if(null!=tag) { //处理了空
        if(isMapping) { //操作servlet-mapping
            if(tag.equals("servlet-name")) {
                mapping.setName(contents);
            }else if(tag.equals("url-pattern")) {
                mapping.addPattern(contents); //添加的是url对应的内容，不是标签,并且内容不能相同
                //因为一个类有多个url的内容，所以加入集合，要不然要两个变量来保存，且这两个url都是对应的一个name一个class
            }
        }else { //操作servlet
            if(tag.equals("servlet-name")) {
                entity.setName(contents);
            }else if(tag.equals("servlet-class")) {
                entity.setClz(contents);
            }
        }           
    }
}

@Override
public void endElement(String uri, String localName, String qName) throws SAXException {
    if(null!=qName) { 
        if(qName.equals("servlet")) {
            entitys.add(entity);
        }else if(qName.equals("servlet-mapping")) {
            mappings.add(mapping);
        }
    }
    tag = null; //tag丢弃了
}

public List<Entity> getEntitys() {
    return entitys;
}

public List<Mapping> getMappings() {
    return mappings;
}   

}

webContext:

public class WebContext {
    private List<Entity> entitys=null;
    private List<Mapping> mappings=null;
    //key:servlet-name   value:servlet-class
    private Map<String,String> entityMap=new HashMap<String,String>();
    //key:url-pattern value :servlet-name
    private Map<String,String> mappingMap=new HashMap<String,String>();

public WebContext(List<Entity> entitys,List<Mapping> mappings)
{
    this.entitys=entitys;
    this.mappings=mappings;

    for(Entity entity:entitys)
    {
        entityMap.put(entity.getName(), entity.getClz());
    }
    for(Mapping mapping:mappings)
    {
        for(String pattern:mapping.getPatterns())
        {
            mappingMap.put(pattern, mapping.getName());
        }
    }
}
public String getClz(String pattern)
{
    String name=mappingMap.get(pattern);

    return entityMap.get(name);
}

}

Servlet:

public interface Servlet {

void service();
}

LoginServlet:

public class LoginServlet implements Servlet {

public void service()
{
    System.out.println("LoginServlet");
}
}

RegisterServlet:

public class RegisterServlet implements Servlet {
    public void service()
    {
        System.out.println("RegisterServlet");
    }

}