空指针异常如何处理

package com.imooc.test;import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.Transaction;import com.imooc.entity.Grade;
import com.imooc.entity.Student;
import com.imooc.util.HibernateSessionFactory;public class Test {
public static void main(String[] args) {
Session session = null;
Transaction ts = null;
try {
session = HibernateSessionFactory.getSession();   //16行
ts = session.beginTransaction();                                //17行
Grade g = new Grade("Java一班");
Student stu1 = new Student("李四","男");
Student stu2 = new Student("张三","女");
stu1.setGrade(g);
stu2.setGrade(g);
session.save(stu1);
session.save(stu2);
ts.commit();
} catch (HibernateException e) {
ts.rollback();
e.getStackTrace();
}
HibernateSessionFactory.closeSession();
}
}空指针异常：一月 08, 2017 4:49:03 下午 org.hibernate.cfg.Configuration configure
INFO: HHH000043: Configuring from resource: /hibernate.cfg.xml
一月 08, 2017 4:49:03 下午 org.hibernate.cfg.Configuration getConfigurationInputStream
INFO: HHH000040: Configuration resource: /hibernate.cfg.xml
一月 08, 2017 4:49:03 下午 org.hibernate.cfg.Configuration addResource
INFO: HHH000221: Reading mappings from resource: com/imooc/entity/Student.hbm.xml
%%%% Error Creating SessionFactory %%%%
org.hibernate.InvalidMappingException: Unable to read XML
at org.hibernate.internal.util.xml.MappingReader.readMappingDocument(MappingReader.java:109)
at org.hibernate.cfg.Configuration.add(Configuration.java:478)
at org.hibernate.cfg.Configuration.add(Configuration.java:474)
at org.hibernate.cfg.Configuration.add(Configuration.java:647)
at org.hibernate.cfg.Configuration.addResource(Configuration.java:730)
at org.hibernate.cfg.Configuration.parseMappingElement(Configuration.java:2109)
at org.hibernate.cfg.Configuration.parseSessionFactory(Configuration.java:2081)
at org.hibernate.cfg.Configuration.doConfigure(Configuration.java:2061)
at org.hibernate.cfg.Configuration.doConfigure(Configuration.java:2014)
at org.hibernate.cfg.Configuration.configure(Configuration.java:1929)
at org.hibernate.cfg.Configuration.configure(Configuration.java:1908)
at com.imooc.util.HibernateSessionFactory.rebuildSessionFactory(HibernateSessionFactory.java:71)
at com.imooc.util.HibernateSessionFactory.getSession(HibernateSessionFactory.java:55)
at com.imooc.test.Test.main(Test.java:16)
Caused by: org.dom4j.DocumentException: http://hibernate.org/dtd/hibernate-mapping-3.0.dtd%20 Nested exception: http://hibernate.org/dtd/hibernate-mapping-3.0.dtd%20
at org.dom4j.io.SAXReader.read(SAXReader.java:484)
at org.hibernate.internal.util.xml.MappingReader.readMappingDocument(MappingReader.java:78)
... 13 more