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openSession(),获取当前会话(),开始事务(),延迟加载

servlets jsp hibernate java

nik · 技术社区 · 9 年前

我有一个简单的java web应用程序,它从数据库接收一些信息,并在web浏览器中显示这些信息。Hibernate用于在servlet和jsp文件中与数据库交互。所有的工作都是我想要的,但有些事情我不懂。

数据库很简单-2个表:问题和答案。表格之间的关系是一对多的:一个问题可以有多个答案。

这是java类的代码问答:

问题.java

package app;

import java.util.Set;

public class Question {
    Long id = null;
    String text = "";
    Set<Answer> answers = null;

    public Question() {        
    }    

    public void setId(Long id) {
        this.id = id;
    }
    public void setText(String text) {
        this.text = text;
    }
    public void setAnswers(Set<Answer> answers) {
        this.answers = answers;
    }
    public Long getId() {
        return id;
    }
    public String getText() {
        return text;
    }
    public Set<Answer> getAnswers() {
        return answers;
    }
}

答案.java

package app;

public class Answer {
    Long id = null;    
    String text = "";
    Question question = null;

    public Answer() {
    }
    public void setId(Long id) {
        this.id = id;        
    }
    public void setText(String text) {
        this.text = text;
    }
    public void setQuestion(Question question) {
        this.question = question;
    }    
    public Long getId() {
        return id;
    }
    public String getText() {
        return text;
    }
    public Question getQuestion() {
        return question;
    } 
}

这是Hibernate的配置:

hibernate.cfg.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
  <session-factory>
    <property name="hibernate.connection.driver_class">com.microsoft.sqlserver.jdbc.SQLServerDriver</property>
    <property name="hibernate.connection.url">jdbc:sqlserver://localhost;databaseName=Test</property>
    <property name="hibernate.connection.username">sa</property>
    <property name="hibernate.connection.password">password</property>

    <property name="hibernate.current_session_context_class">thread</property>

    <mapping resource="question.hbm.xml"/>
    <mapping resource="answer.hbm.xml"/>
  </session-factory>
</hibernate-configuration>

问题.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="app">
    <class name="app.Question" table="Question">
        <id column="id" name="id" type="java.lang.Long">
            <generator class="native"/>
        </id>
        <property column="text" name="text" not-null="true" type="java.lang.String"/>
        <set name="answers">
            <key column="question_id"/>
            <one-to-many class="app.Answer"/>
        </set>
    </class>
</hibernate-mapping>

答案.hbm.xml

    <?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="app">
    <class name="app.Answer" table="Answer">
        <id column="id" name="id" type="java.lang.Long">
            <generator class="native"/>
        </id>
        <property column="text" name="text" not-null="true" type="java.lang.String"/>
        <many-to-one class="app.Question" column="question_id" name="question" not-null="true"/>
    </class>
</hibernate-mapping>

所以在问题中有一组答案。正因为如此,才会有一大堆懒惰的答案。我想在jsp文件中使用Hibernate对象,所以我使用 this technique :过滤器用于创建会话,该会话用于servlet和相应的jsp文件中。

这是我的过滤器的代码:

休眠过滤器.java

package app;

import java.io.IOException;
import javax.servlet.*;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class HibernateFilter implements Filter {  

    private SessionFactory sessionFactory;  

    public void doFilter(ServletRequest request,  
                         ServletResponse response,  
                         FilterChain chain)  
            throws IOException, ServletException {    
        try {
            sessionFactory.getCurrentSession().beginTransaction();  
            chain.doFilter(request, response);  
            sessionFactory.getCurrentSession().getTransaction().commit();  
        } catch (Exception ex) { 
            sessionFactory.getCurrentSession().getTransaction().rollback();
            ex.printStackTrace();   
        }  
    }  

    public void init(FilterConfig filterConfig) throws ServletException { 
        sessionFactory = new Configuration().configure().buildSessionFactory();  
    }  

    public void destroy() {
    }    
}

我的应用程序中有两个servlet和两个对应的jsp文件——第一个显示ID为1的问题,第二个显示所有问题。这是这个servlet的代码,对应的jsp文件和tomcat的配置:

获取一个问题.java

package app;

import java.io.IOException;
import javax.servlet.*;
import javax.servlet.http.*;
import org.hibernate.Session;
import org.hibernate.cfg.Configuration;

public class GetOneQuestion extends HttpServlet { 

    @Override
    public void doGet(HttpServletRequest request, 
            HttpServletResponse response) 
            throws ServletException, IOException {  
        try {
            Session session = new Configuration().configure()
                    .buildSessionFactory().getCurrentSession();

            session.beginTransaction();
            Question question = (Question)session.load(Question.class, 1L);            
            //session.getTransaction().commit();

            request.setAttribute("oneQuestion", question); 
        } catch (Exception ex) {            
            ex.printStackTrace();
            request.setAttribute("oneQuestion", null);
        }             
        RequestDispatcher view = request.getRequestDispatcher("/oneQuestion.jsp");      
        view.forward(request, response);
    }                                                                               
}

oneQuestion.jsp

<%@page import="app.Answer"%>
<%@page import="app.Question"%>
<html>
    <body>        
        <%
            Question question = (Question)request.getAttribute("oneQuestion");            
            out.print("<br>" + question.getText() + "<br><br>");             
        %>           
    </body>
</html>

获取所有问题.java

package app;

import java.io.IOException;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.List;
import org.hibernate.*;
import org.hibernate.cfg.Configuration;

public class GetAllQuestion extends HttpServlet { 

    @Override
    public void doGet(HttpServletRequest request, 
            HttpServletResponse response) 
            throws ServletException, IOException { 
        try {                 
            Session session = new Configuration().configure()
                    .buildSessionFactory().getCurrentSession();

            session.beginTransaction();        
            Query query = session.createQuery("from Question"); 
            List all = query.list();                    

            request.setAttribute("allQuestion", all);                                                        
        } catch (Exception ex) {            
            ex.printStackTrace();
            request.setAttribute("allQuestion", null);
        }             
        RequestDispatcher view = request.getRequestDispatcher("/allQuestion.jsp");      
        view.forward(request, response);
    }                                                                               
}

allQuestion.jsp

<%@page import="app.Answer"%>
<%@page import="app.Question"%>
<%@page import="java.util.List"%>
<html>
    <body>       
        <%
            List all = (List)request.getAttribute("allQuestion");

            for (Object object : all) {
                Question question = (Question)object;
                out.print("<br>Question " + question.getId());   

                for (Answer answer : question.getAnswers()) {
                    out.print("<br>" + answer.getText() + "<br>");  
                }
            }
        %>           
    </body>
</html>

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd">
    <filter>
        <filter-name>HibernateFilter</filter-name>
        <filter-class>app.HibernateFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>HibernateFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <servlet>
        <servlet-name>getAllQuestion</servlet-name>
        <servlet-class>app.GetAllQuestion</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>getAllQuestion</servlet-name>
        <url-pattern>/getAllQuestion</url-pattern>
    </servlet-mapping>
    <servlet>
        <servlet-name>getOneQuestion</servlet-name>
        <servlet-class>app.GetOneQuestion</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>getOneQuestion</servlet-name>
        <url-pattern>/getOneQuestion</url-pattern>
    </servlet-mapping>
    <session-config>
        <session-timeout>
            30
        </session-timeout>
    </session-config>
    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
    </welcome-file-list>
</web-app>

问题

1) 为什么我需要在servlet中调用“session.beginTransaction()”方法,即使我已经在过滤器中调用了“session.beginTraction()”?如果我从另一个servlet调用一个servlet,我还必须在第二个servlet中调用这个方法?或者我必须在每次与数据库交互之前调用此方法?

2) 我没有在过滤器中调用“sessionFactory.getCurrentSession().openSession)”或“session Factory.getCurrentSeession().getCurrenSession”,但我在servlet中调用了“sessiionFactory/getCurrent会话().ggetCurrentsession(()”,并且仍然获得了在过滤器中创建的会话。这怎么可能?

3) 如果我取消注释行“session.getTransaction().commit();”在jsp文件的GetOneQuestion类中,我收到LazyInitializationException:“无法初始化代理-没有会话”,即使这个jsp文件中没有延迟加载,因为我在那里没有使用任何Answer对象。此异常的原因是什么?会话必须打开,以便与Hibernate对象进行任何交互,即使没有延迟加载?

1 回复 | 直到 9 年前

v.ladynev 9 年前

您需要配置一次会话工厂(!)在应用程序启动时,而不是在每个servlet中。
事务提交后,您需要关闭过滤器中的会话。
您不需要在servlet中创建事务(或者不需要在过滤器中创建事务)。

您可以将会话工厂存储在应用程序初始化器类的静态字段中,并在servlet中从中获取当前会话。

关于您的问题

因为你犯了一个错误。您可以在servlet中创建一个新的会话工厂。
您获得了一个新会话(因为会话工厂创建不正确)。在正常情况下 getCurrentSession() 返回绑定到当前线程的会话 ThreadLocal (并非总是如此,这取决于配置)。
你有 LazyInitializationException 具有 question.getAnswers() .