java.lang.NullPointerException错哪了？

Question

public class EditAction extends ActionSupport { /**
 * 
 */
private static final long serialVersionUID = 1L;
private String subName;
private Byte subType;
private Byte subDifficult;
private String subIntroduction;
private String subUsecomputer;
private String subNo;
private Byte subDirection;
private String subTaskname;
private String subFrom;
private String subReform;
private String subOther;
private  Students students;


@Override

public String execute() throws Exception{
try{


SubjectInfoDaoImpl subDao=new SubjectInfoDaoImpl();
Teachers tea  ;
HttpServletRequest request = ServletActionContext .getRequest() ;
String subid = request .getParameter("subId") ;
            Subjectinfo info = new Subjectinfo();

            info=subDao.findById(subid);
 // int Id = Integer.parseInt (request.getParameter ("subId")); 

      //  info.setSubId(Id);

HttpSession session = request .getSession() ;
tea = (Teachers) session .getAttribute("teacher") ;

info .setSubName(this .getSubName());
info.setSubType(this .getSubType());  
info.setSubDifficult(this.getSubDifficult());//
info .setSubIntroduction(this .getSubIntroduction()) ;//
info.setSubFrom(this.getSubFrom());//
info.setSubTaskname(this.getSubTaskname());//
info .setSubNo(this .getSubNo());//
info .setSubDirection(this .getSubDirection()) ;//
info .setSubReform(this .getSubReform()) ;//
info .setSubOther(this .getSubOther()) ;//
info .setSubUsecomputer(this .getSubUsecomputer()) ;
info .setTeachers(tea);

subDao.updateSubject(info);


return SUCCESS;
}catch(Exception e){
e.printStackTrace();
return ERROR;
}
} public String getSubName() {
return subName;
} public void setSubName(String subName) {
this.subName = subName;
} public Byte getSubType() {
return subType;
} public void setSubType(Byte subType) {
this.subType = subType;
} public Byte getSubDifficult() {
return subDifficult;
} public void setSubDifficult(Byte subDifficult) {
this.subDifficult = subDifficult;
} public String getSubIntroduction() {
return subIntroduction;
} public void setSubIntroduction(String subIntroduction) {
this.subIntroduction = subIntroduction;
} public String getSubUsecomputer() {
return subUsecomputer;
} public void setSubUsecomputer(String subUsecomputer) {
this.subUsecomputer = subUsecomputer;
} public String getSubNo() {
return subNo;
} public void setSubNo(String subNo) {
this.subNo = subNo;
} public Byte getSubDirection() {
return subDirection;
} public void setSubDirection(Byte subDirection) {
this.subDirection = subDirection;
} public String getSubTaskname() {
return subTaskname;
} public void setSubTaskname(String subTaskname) {
this.subTaskname = subTaskname;
} public String getSubFrom() {
return subFrom;
} public void setSubFrom(String subFrom) {
this.subFrom = subFrom;
} public String getSubReform() {
return subReform;
} public void setSubReform(String subReform) {
this.subReform = subReform;
} public String getSubOther() {
return subOther;
} public void setSubOther(String subOther) {
this.subOther = subOther;
} public Students getStudents() {
return students;
} public void setStudents(Students students) {
this.students = students;
}
}

Answer 1

具体点，将所有的EXCEPTION信息全贴出来

Answer 2

String subid = request .getParameter("subId") ;
info=subDao.findById(subid);
可能subid=null，所以上面这句就会抛出java.lang.NullPointerException

Answer 3

public class Subjectinfo  implements java.io.Serializable { // Fields private Integer subId;
private Teachers teachers;
private Students students;
private String subName;
private Byte subType;
private Byte subDifficult;
private String subIntroduction;
private String subUsecomputer;
private String subNo;
private Byte subDirection;
private String subTaskname;
private String subFrom;
private String subReform;
private String subOther;
private Set stutemps = new HashSet(0);
// Constructors public Set getStutemps() {
return stutemps;
} public void setStutemps(Set stutemps) {
this.stutemps = stutemps;
...}
<hibernate-mapping>
    <class name="hibernate.model.Subjectinfo" table="subjectinfo" catalog="subjectinfo">
        <id name="subId" type="java.lang.Integer">
            <column name="sub_id" />
            <generator class="native" />
        </id>
        <many-to-one name="teachers" class="hibernate.model.Teachers" fetch="select">
            <column name="tea_id" not-null="true" />
        </many-to-one>
        <many-to-one name="students" class="hibernate.model.Students" fetch="join">
            <column name="stu_id" />
        </many-to-one>
        <property name="subName" type="java.lang.String">
            <column name="sub_name" length="100" not-null="true" />
        </property>
        <property name="subType" type="java.lang.Byte">
            <column name="sub_type"  not-null="true" />
        </property>
        <property name="subDifficult" type="java.lang.Byte">
            <column name="sub_difficult" not-null="true" />
        </property>
        <property name="subIntroduction" type="java.lang.String">
            <column name="sub_introduction" length="1000" not-null="true" />
        </property>
        <property name="subUsecomputer" type="java.lang.String">
            <column name="sub_usecomputer" length="10">
                <comment>涓???烘?舵?</comment>
            </column>
        </property>
        <property name="subNo" type="java.lang.String">
            <column name="sub_no" length="10" />
        </property>
        <property name="subDirection" type="java.lang.Byte">
            <column name="sub_direction"  />
        </property>
        <property name="subTaskname" type="java.lang.String">
            <column name="sub_taskname" length="20" />
        </property>
        <property name="subFrom" type="java.lang.String">
            <column name="sub_from" length="20" />
        </property>
        <property name="subReform" type="java.lang.String">
            <column name="sub_reform" length="20" />
        </property>
        <property name="subOther" type="java.lang.String">
            <column name="sub_other" length="100" />
        </property>
        <set name="stutemps" inverse="true">
         <key>
         <column name="sub_id"></column>
         </key>
         <one-to-many class="hibernate.model.Stutemp"/>
        </set>    </class>
</hibernate-mapping>
[color=#FF0000]Action
public class EditAction extends ActionSupport {[/color] /** 
* 
*/ 
private static final long serialVersionUID = 1L; 
private String subName; 
private Byte subType; 
private Byte subDifficult; 
private String subIntroduction; 
private String subUsecomputer; 
private String subNo; 
private Byte subDirection; 
private String subTaskname; 
private String subFrom; 
private String subReform; 
private String subOther; 
private  Students students; @Override public String execute() throws Exception{ 
try{ 
SubjectInfoDaoImpl subDao=new SubjectInfoDaoImpl(); 
Teachers tea  ; 
HttpServletRequest request = ServletActionContext .getRequest() ; 
String subid = request .getParameter("subId") ; 
            Subjectinfo info = new Subjectinfo();             info=subDao.findById(subid); 
HttpSession session = request .getSession() ; 
tea = (Teachers) session .getAttribute("teacher") ; 

info .setSubName(this .getSubName()); 
info.setSubType(this .getSubType());  
info.setSubDifficult(this.getSubDifficult());// 
info .setSubIntroduction(this .getSubIntroduction()) ;// 
info.setSubFrom(this.getSubFrom());// 
info.setSubTaskname(this.getSubTaskname());// 
info .setSubNo(this .getSubNo());// 
info .setSubDirection(this .getSubDirection()) ;// 
info .setSubReform(this .getSubReform()) ;// 
info .setSubOther(this .getSubOther()) ;// 
info .setSubUsecomputer(this .getSubUsecomputer()) ; 
info .setTeachers(tea); subDao.updateSubject(info); 
return SUCCESS; 
}catch(Exception e){ 
e.printStackTrace(); 
return ERROR; 
} 
get() set().....
} 
public void updateSubject(Object obj)
{
Session session=HibernateUtil.createSession();
Transaction tx=session.beginTransaction();
session.update(obj);
tx.commit();
HibernateUtil.closeSession();
}

Answer 4

页面<body>题目修改页面： <form name="form21" method="post" action="EditeSubject.action">
<table width="950" border="1">  <tr bgcolor="#E3E3E3">
    <td width="198" bgcolor="#E6E6E6"><div align="right">题目:</div></td>
    <td colspan="5" bgcolor="#E6E6E6"><div align="left">
   
        <label>
          <input type="text" name="subName" id="subName" value='<s:property  value="%{info.subName}"/>'>
        </label>
     
    </div>
      <div align="left"></div></td>
  </tr>
  <tr bgcolor="#E3E3E3">
    <td bgcolor="#E6E6E6"><div align="right">题目类型：</div></td>
    <td width="151" bgcolor="#E6E6E6"><div align="left">
      <label>
     
        <s:if test='info.subType==1'>
          <input type="radio" name="subType"  value="1" id="subType" checked>
        </s:if>
        <s:else>
          <input type="radio" name="subType" value="1" id="subType" >
        </s:else>
        工程设计 </label>
    </div></td>
    <td width="123" bgcolor="#E6E6E6"><div align="left">
      <s:if test='info.subType==2'>
        <input type="radio" name="subType" value="2" id="subType" checked>
      </s:if>
      <s:else>
        <input type="radio" name="subType" value="2" id="subType" >
      </s:else>
      工程技术</div></td>
    <td width="124" bgcolor="#E6E6E6"><div align="left">
      <s:if test="info.subType==3">
        <input type="radio" name="subType" value="3" id="subType" checked>
      </s:if>
      <s:else>
        <input type="radio" name="subType"value="3"  id="subType" >
      </s:else>
      实验研究</div></td>
    <td width="122" bgcolor="#E6E6E6"><div align="left">
      <s:if test='info.subType==4'>
        <input type="radio" name="subType" value="4" id="subType" checked>
      </s:if>
      <s:else>
        <input type="radio" name="subType" value="4" id="subType" >
      </s:else>
      软件开发</div></td>
    <td width="192" bgcolor="#E6E6E6"><div align="left">
      <s:if test='info.subType==5'>
        <input type="radio" name="subType" value="5" id="subType" checked>
      </s:if>
      <s:else>
        <input type="radio" name="subType" value="5" id="subType" >
      </s:else>
      理论研究</div></td>
  </tr>
  <tr>
    <td bgcolor="#E6E6E6"><div align="right">题目难度：</div></td>
    <td colspan="2" bgcolor="#E6E6E6"><div align="left">
     
        <s:if test="info.subDifficult==1">
        <input type="radio" name="subDifficult" value="1"  id="subDifficult" checked>
        </s:if>
        <s:else>
          <input type="radio" name="subDifficult" value="1" id="subDifficult" >
          </s:else>
        A 
    
        <s:if test="info.subDifficult==2">
        <input type="radio" name="subDifficult" value="2" id="subDifficult" checked>
       </s:if>
        <s:else>
          <input type="radio" name="subDifficult" value="2" id="subDifficult" >
        </s:else>
    B  </div></td>
    <td bgcolor="#E6E6E6"><div align="right">上机时数：</div></td>
    <td colspan="2" bgcolor="#E6E6E6"><div align="left">
      <label>
          <input type="text" name="subUsecomputer" value='<s:property  value="%{info.subUsecomputer}"/>'id="subUsecomputer">
        </label>
     
    </div></td>
    
  </tr>
  <tr bgcolor="#E3E3E3">
    <td rowspan="5" bgcolor="#E6E6E6">题目来源于科研课题（该课题的情况）</td>
    <td bgcolor="#E6E6E6"><div align="right">课题编号：</div></td>
    <td bgcolor="#E6E6E6"><div align="left">
      
      <label>
          <input type="text" name="subNo"  value='<s:property  value="%{info.subNo}"/>'id="subNo">
        </label></div></td>
    <td bgcolor="#E6E6E6"><div align="right">研究方向：</div></td>
    <td bgcolor="#E6E6E6"><div align="left">
        <s:if test="info.subDirection==1">
        <input type="radio" name="subDirection" value="1" id="subDirection" checked>
        </s:if>
        <s:else>
          <input type="radio" name="subDirection" value="1" id="subDirection" >
        </s:else>
       横向 
    </div></td>
    <td bgcolor="#E6E6E6"><div align="left">
      <s:if test="info.subDirection==2">
      <input type="radio" name="subDirection" value="2"  id="subDirection" checked>
      </s:if>
      <s:else>
        <input type="radio" name="subDirection" value="2" id="subDirection" >
      </s:else>
    纵向</div></td>
  </tr>
  <tr>
    <td bgcolor="#E6E6E6"><div align="right">课题名称：</div></td>
    <td colspan="4" bgcolor="#E6E6E6"><div align="left">
      
     <label>
          <input type="text" name="subTaskname"  value='<s:property  value="%{info.subTaskname}"/>' id="subTaskname">
        </label></div></td>
  </tr>
  <tr>
    <td bgcolor="#E6E6E6"><div align="right">课题来源：</div></td>
    <td colspan="4" bgcolor="#E6E6E6"><div align="left">
      
     <label>
          <input type="text" name="subFrom"  value='<s:property  value="%{info.subFrom}"/>'id="subFrom">
        </label></div></td>
  </tr>
  <tr>
    <td bgcolor="#E6E6E6"><div align="right">教学、实验改革：</div></td>
    <td colspan="4" bgcolor="#E6E6E6"><div align="left">
   
     <label>
          <input type="text" name="subReform"  value='<s:property  value="%{info.subReform}"/>' id="subReform">
        </label></div></td>
  </tr>
  <tr>
    <td bgcolor="#E6E6E6"><div align="right">其他:</div></td>
    <td colspan="4" bgcolor="#E6E6E6"><div align="left">
     
     <label>
          <input type="text" name="subOther"  value='<s:property  value="%{info.subOther}"/>' id="subOther">
        </label></div></td>
  </tr>
  <tr bgcolor="#E3E3E3">
    <td colspan="6" bgcolor="#E6E6E6">主要任务与要求简述：</td>
  </tr>
  <tr bgcolor="#F3F3F3">
    <td colspan="6" bgcolor="#E6E6E6">&nbsp;&nbsp;&nbsp;&nbsp;
      
        <label>
          <textarea name="subIntroduction" id="subIntroduction"  cols="140" rows="5" />${info.subIntroduction}</textarea>
        </label>
    
     </td>
     
  </tr>
  <tr bgcolor="#F3F3F3">
    <td colspan="6" bgcolor="#E6E6E6"><div align="center">
      <input name="submit"  type="submit" id="submit" value="修改" /> 
   
     </div></td>
     
  </tr>
  
</table>
</form>
</body>
错误
==========================================页面传过来的值====================================
subName:我[color=#FFFF00]地方
subType: 1
subUsercomputer:421
subDifficult:1
介绍: 3333
来源:2
编号:2
其他:2
方向:1
课程名:2
改革:2
老师莫金旺===========================================页面传过来的值====================================
=====================================================Teacher的值获取==============================
=====================================================Teacher的值获取==============================
java.lang.NullPointerException[/color]
at struts2.action.teacher.subject.EditAction.execute(EditAction.java:82)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)

Answer 5

Struts的要求蛮高滴。。表单提交的名称要和actionForm中的属性一样

Answer 6

大致看看了说下我的看法
注意这段代码
info .setTeachers(tea); 
subDao.updateSubject(info); 
注意这个tea应该是teacher吧，看hbm文件是个类型属性，和teacher是多对一关联的
这个tea是从session里拿来的，会是null吗？
如果是null,在执行subDao.updateSubject(info);这段代码的时候
hibernate会反射调用teacher.getid的方法给info的teacherid字段赋值，如果teacher是null，自然就空指针异常了
建议你检查下teacher就是那个tea的值吧

Answer 7

很好，谢谢大家，问题解决了，是因为在点击修改的时候找不到传过来的老师ID，这个问题得在前面一个页面写。<form ...action?subid='<s:property vlae={subId} />