Hibernate 与 struts2 集成 - 会话为空答案

【问题标题】：Hibernate integration with struts2 - session is nullHibernate 与 struts2 集成 - 会话为空
【发布时间】：2017-02-17 18:13:57
【问题描述】：

我正在按照这个 Struts2 教程将它提出的 Struts2 项目与休眠集成： https://www.tutorialspoint.com/struts_2/struts_hibernate.htm 我一步一步按照说明进行操作，唯一更改的是库，因为我使用的是更新后的库。

当我尝试执行我的操作时，我的 DAO 类中出现 NullPointerException。出于某种原因，我的 Session 对象始终为空。

这是我的 bean 类：

package com.fabioSarmiento.hibernate;

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;
   }
}

这是我的 DAO：

package com.fabioSarmiento.hibernate;

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", "deprecation" })
    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) {
        session.save(student);
    }
}

我的休眠配置文件：

<?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>
   <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
   <property name="hibernate.connection.url">jdbc:mysql://localhost/mydb</property>
   <property name="hibernate.connection.username">root</property>
   <property name="hibernate.connection.password">root</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.fabioSarmiento.hibernate.Student" />
</session-factory>
</hibernate-configuration>

还有我的 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>
    <constant name="struts.devMode" value="true" />
     <package name="myhibernate" extends="hibernate-default">
      <action name="addStudent" method="execute"
         class="com.fabioSarmiento.struts2.AddStudentAction">
         <result name="success" type="redirect">
               listStudents
         </result>
      </action>
      <action name="listStudents" method="listStudents"
         class="com.fabioSarmiento.struts2.AddStudentAction">
         <result name="success">/index.jsp</result>
      </action>

</package>
</struts>

我的 web.xml：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>Struts 2</display-name>
   <welcome-file-list>
      <welcome-file>index.jsp</welcome-file>
   </welcome-file-list>
   <filter>
      <filter-name>struts2</filter-name>
    <filter-class>
     org.apache.struts2.dispatcher.filter.StrutsPrepareAndExecuteFilter
  </filter-class>
   </filter>

   <filter-mapping>
      <filter-name>struts2</filter-name>
      <url-pattern>/*</url-pattern>
   </filter-mapping>
</web-app>

这是我得到的堆栈跟踪：

java.lang.NullPointerException
    at com.fabioSarmiento.hibernate.StudentDAO.getStudents(StudentDAO.java:21)
    at com.fabioSarmiento.struts2.AddStudentAction.listStudents(AddStudentAction.java:29)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at ognl.OgnlRuntime.invokeMethod(OgnlRuntime.java:873)
    at ognl.OgnlRuntime.callAppropriateMethod(OgnlRuntime.java:1539)
    at ognl.ObjectMethodAccessor.callMethod(ObjectMethodAccessor.java:68)
    at com.opensymphony.xwork2.ognl.accessor.XWorkMethodAccessor.callMethodWithDebugInfo(XWorkMethodAccessor.java:96)
    at com.opensymphony.xwork2.ognl.accessor.XWorkMethodAccessor.callMethod(XWorkMethodAccessor.java:88)
    at ognl.OgnlRuntime.callMethod(OgnlRuntime.java:1615)
    at ognl.ASTMethod.getValueBody(ASTMethod.java:91)
    at ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:212)
    at ognl.SimpleNode.getValue(SimpleNode.java:258)
    at ognl.Ognl.getValue(Ognl.java:467)
    at ognl.Ognl.getValue(Ognl.java:431)
    at com.opensymphony.xwork2.ognl.OgnlUtil$3.execute(OgnlUtil.java:351)
    at com.opensymphony.xwork2.ognl.OgnlUtil.compileAndExecuteMethod(OgnlUtil.java:403)
    at com.opensymphony.xwork2.ognl.OgnlUtil.callMethod(OgnlUtil.java:349)
    at com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:436)
    at com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:291)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:252)
    at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:177)
    at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.doIntercept(ConversionErrorInterceptor.java:139)
    at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:133)
    at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:192)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:88)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:248)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:99)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:139)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at org.apache.struts2.interceptor.ProfilingActivationInterceptor.intercept(ProfilingActivationInterceptor.java:105)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:253)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:155)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at org.apache.struts2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:120)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:174)
    at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:171)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:195)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:193)
    at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247)
    at org.apache.struts2.factory.StrutsActionProxy.execute(StrutsActionProxy.java:54)
    at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:564)
    at org.apache.struts2.dispatcher.ExecuteOperations.executeAction(ExecuteOperations.java:81)
    at org.apache.struts2.dispatcher.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:143)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:108)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:620)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:349)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:784)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:802)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1410)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:745)

这是一个关于如何进行集成的好教程吗？我一直在研究，我发现的实现有很多差异。但是这个教程对学习struts部分非常有用，我没有任何问题，现在我无法完成这节课。

我该如何解决这个问题？

感谢您的帮助和耐心，我是 hibernate 新手，我正在尝试自学。

【问题讨论】：

com.fabioSarmiento.hibernate.StudentDAO.getStudents(StudentDAO.java:21) 中有什么
嗨 RamPrakash，感谢您对以下行发表评论：students = session.createQuery("from student").list();
是的，它是空的。但我不确定为什么。而且我不知道如何测试我的应用程序是否真的在尝试连接到我的数据库。我在不使用休眠的情况下尝试了相同的连接，效果很好
你是否使用 Spring 将会话工厂的依赖注入到你的 DAO 中？
@Will 不，我没有使用任何 Spring 库，目标是仅使用 hibernate 和 struts。如果我使用 Spring 会更好吗？

标签： java hibernate struts2

【解决方案1】：

使用 createQuery 时，我认为 Model 对象类区分大小写。

尝试将第 21 行更改为大写 Student：

students = session.createQuery("from Student").list();

【讨论】：

嗨 shokulei，它没有改变任何东西：/
代码更改后是否重启了容器（Tomcat、JBoss等）？还是您配置了热部署？
我已经配置了热部署，但我还是重新启动了它......同样的错误。由于某种原因，我的会话对象为空
您也可以发布您的 web.xml 吗？
我刚刚做了:)你会在上面找到它

【解决方案2】：

好的，现在可以了！我使用了@shokulei 提供的库（他的回答中的链接）。之后，我不得不更改我的 web.xml 以使用 org.apache.struts2.dispatcher.FilterDispatcher 作为过滤器类。最后，我必须禁用 MySQL 中的一个参数才能使 SSL 连接成为可选。谢谢大家的帮助！！

【讨论】：