问题
Well,I have been trying to add an orm layer for past 3 weeks and i havent been able to do so.I was trying to learn from the tutorial.I have done whatever the tutorial says .Here is the code
Student.java
package com.codinghazard.actions;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;
@Entity
@Table(name="student")
public class Student {
@Id
@GeneratedValue
private int id;
@Column(name="last_name")
private String lastName;
@Column(name="first_name")
private String firstName;
private int marks;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public int getMarks() {
return marks;
}
public void setMarks(int marks) {
this.marks = marks;
}
}
StudentDAO.java
package com.codinghazard.actions;
import java.util.ArrayList;
import java.util.List;
import org.hibernate.Session;
import org.hibernate.Transaction;
import com.googlecode.s2hibernate.struts2.plugin.
annotations.SessionTarget;
import com.googlecode.s2hibernate.struts2.plugin.
annotations.TransactionTarget;
public class StudentDAO {
@SessionTarget
Session session;
@TransactionTarget
Transaction transaction;
@SuppressWarnings("unchecked")
public List<Student> getStudents()
{
List<Student> students = new ArrayList<Student>();
try
{
students = session.createQuery("from Student").list();
}
catch(Exception e)
{
e.printStackTrace();
}
return students;
}
public void addStudent(Student student)
{
try
{
session.save(student);
}
catch(Exception e)
{
System.out.println("Exception is "+e);
}
}
}
AddStudentAction.java
package com.codinghazard.actions;
import java.util.ArrayList;
import java.util.List;
import com.opensymphony.xwork2.ActionSupport;
import com.opensymphony.xwork2.ModelDriven;
import com.codinghazard.actions.Student;
import com.codinghazard.actions.StudentDAO;
@SuppressWarnings("serial")
public class AddStudentAction extends ActionSupport
implements ModelDriven<Student>{
Student student = new Student();
List<Student> students = new ArrayList<Student>();
StudentDAO dao = new StudentDAO();
public Student getModel() {
return student;
}
public String execute()
{
dao.addStudent(student);
return "success";
}
public String listStudents()
{
students = dao.getStudents();
return "success";
}
public Student getStudent() {
return student;
}
public void setStudent(Student student) {
this.student = student;
}
public List<Student> getStudents() {
return students;
}
public void setStudents(List<Student> students) {
this.students = students;
}
}
hibernate.cfg.xml
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory name="java:hibernate/SessionFactory">
<property name="hibernate.connection.driver_class">c
om.mysql.jdbc.Driver
</property>
<property name="hibernate.connection.url">
jdbc:mysql://localhost/driverregistration
</property>
<property name="hibernate.connection.username">root</property>
<property name="hibernate.connection.password">admin</property>
<property name="hibernate.connection.pool_size">10</property>
<property name="show_sql">true</property>
<property name="dialect">
org.hibernate.dialect.MySQLDialect
</property>
<property name="hibernate.hbm2ddl.auto">update</property>
<mapping class="com.codinghazard.actions.Student" />
</session-factory>
</hibernate-configuration>
struts.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE struts PUBLIC
"-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
"http://struts.apache.org/dtds/struts-2.0.dtd">
<struts>
<!-- http://localhost:8080/test/user/login -->
<constant name="struts.devMode" value="true" />
<package name="user" namespace="/user" extends="struts-default,hibernate-default">
<action name="addStudent" method="execute"
class="com.codinghazard.actions.AddStudentAction">
<result name="success" type="redirect">
listStudents
</result>
</action>
<action name="listStudents" method="listStudents"
class="com.codinghazard.actions.AddStudentAction">
<result name="success">pages/student.jsp</result>
</action>
<action name="userinput">
<result>pages/input.jsp</result>
</action>
<action name="userinputaction" class="com.codinghazard.actions.CustomerAction"
method="execute">
<result name="success" type="chain" >fetchDriver</result>
</action>
<action name="fetchDriver" class="com.codinghazard.actions.customerEmailAction"
method="getDriver">
<result name="success" type="chain" >customeremail</result>
</action>
<action name="customeremail"
class="com.codinghazard.actions.customerEmailAction"
method="call">
<result>pages/emailsuccess.jsp</result>
<result name="error">pages/error.jsp</result>
</action>
<action name="login">
<result>pages/login.jsp</result>
</action>
<action name="loginaction" class="com.codinghazard.actions.LoginAction" >
<result name="success">pages/userExists.jsp</result>
<result name="error">pages/notExists.jsp</result>
</action>
<action name="Calculator">
<result>pages/Calculator.jsp</result>
</action>
<action name="email">
<result>pages/email.jsp</result>
</action>
<action name="uploader">
<result>pages/upload.jsp</result>
</action>
<action name="emailer"
class="com.codinghazard.actions.Emailer"
method="execute">
<result name="success">pages/emailsuccess.jsp</result>
<result name="error">pages/error.jsp</result>
</action>
<action name="upload" class="com.codinghazard.actions.uploadFile">
<interceptor-ref name="exception"/>
<interceptor-ref name="i18n"/>
<interceptor-ref name="fileUpload"/>
<param name="allowedTypes">text/plain</param>
<param name="maximumSize">10240</param>
<interceptor-ref name="defaultStack">
</interceptor-ref>
<result name="success">pages/success.jsp</result>
<result name="error">pages/error.jsp</result>
</action>
<action name="Welcome" class="com.codinghazard.actions.WelcomeAction">
<interceptor-ref name="timer" />
<interceptor-ref name="defaultStack"/>
<result name="SUCCESS">pages/Result.jsp</result>
<result name="ERROR">pages/error.jsp</result>
</action>
</package>
</struts>
Here the exception
java.lang.NullPointerException
at com.codinghazard.actions.StudentDAO.getStudents(StudentDAO.java:28)
at com.codinghazard.actions.AddStudentAction.listStudents(AddStudentAction.java:30)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:453)
at com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:292)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:255)
at org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:256)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:265)
at org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:138)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:211)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:211)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:190)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:90)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:243)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:100)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:141)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:145)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:171)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:176)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:192)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:187)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:54)
at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:510)
at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:315)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)
I am a newbie and trying to learn struts hibernate integration.I dont understand why it is trhowing an exception after what i have followed this tutorial http://javatutions.blogspot.com/2013/05/before-we-starts-with-our-first-hello.html
EDIT:what to do if the session is null?
回答1:
Use the @SessionTarget annotation to inject the Hibernate session into the action bean. For this purpose you need the hibernate interceptor to be added to the action config. More about how to use a session and transaction capabilities of the struts2-hiberanate plugin, see in my answer to Struts + Hibernate: @SessionTarget not working. There's also example here, that is well enough to demonstrate the capabilities of the hibernate plugin. The problem that you have is probably related to the package configuration. You don't need to extend struts-default package because it's already extended by the hibernate-default package, or you just have not a hibernate-plugin on the classpath.
回答2:
Answer to your
what to do if the session is null
Normally if you follow the DAO structure, you need to cater for 2 cases before firing any query.
If session exists
Session hibernateSession = sessionFactory.getCurrentSession();If session is null, we create the session
Session hibernateSession = sessionFactory.openSession();
In case of step 1, you may end up getting a null value if the session is not initialized. And in the case of step 2, you will end up with a new session on every request and you will need to close and handle this session carefully otherwise you may end up with many useless sessions left open in the session pool.
So best practice is to check if getCurrentSession() returns a sessions, otherwise create and return a new session with openSession(). Something like
Sessions sess = null
sess = sessionFactory.getCurrentSession();
if(sess == null){
sess = sessionFactory.openSession();
}
Keep in mind that this may not be the most feasible solution in some cases
来源：https://stackoverflow.com/questions/23238455/nullpointerexception-while-trying-to-add-an-orm-layer-using-hibernate