我有一个简单的Java Web应用程序,该应用程序从数据库接收一些信息并在Web浏览器中显示该信息。 Hibernate用于与servlet和jsp文件中的数据库进行交互。一切都按我的意愿进行,但我有些不懂。

数据库很简单-2个表:问答。表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:

Question.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;
    }
}


Answer.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>


question.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>


answer.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>


因此,在问题中有答案的集合。因此,答案会变得很懒惰。我想在我的jsp文件中使用Hibernate对象,所以我使用this technique:该过滤器用于创建会话,并且该会话在servlet和coresponding jsp文件中都使用。

这是我的过滤器的代码:

HibernateFilter.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() {
    }
}


在我的应用程序中,有两个servlet和两个对应的jsp文件-第一个显示ID = 1的问题,第二个显示所有问题。这是此servlet,与之相关的jsp文件和tomcat配置的代码:

GetOneQuestion.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>


GetAllQuestion.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>




1)为什么即使我已经在过滤器中调用“ session.beginTransaction()”,但为什么我仍需要在servlet中调用“ session.beginTransaction()”方法?如果我从另一个调用一个服务程序,那么我还必须在第二个servlet中调用此方法吗?还是在每次与数据库交互之前必须调用此方法?

2)我没有在过滤器中调用“ sessionFactory.getCurrentSession()。openSession()”或“ sessionFactory.getCurrentSession()。getCurrentSession()”,而是在servlet中仍然调用了“ sessionFactory.getCurrentSession()。getCurrentSession()”。获取似乎是在过滤器中创建的会话。怎么会这样

3)如果我取消注释“ session.getTransaction()。commit();”行在jsp文件的GetOneQuestion类中,我收到LazyInitializationException:即使无法在此jsp文件中进行延迟加载,“也无法初始化代理-没有会话”,因为我在那里没有使用任何Answer对象。是什么原因导致此异常?即使没有延迟加载,会话也必须打开才能与Hibernate对象进行任何交互?

最佳答案

您需要在应用程序启动时配置一次会话工厂(!),而不是在每个servlet中进行配置。
事务提交后,您需要在过滤器中关闭会话。
您不需要在servlet中创建事务(或者您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程序类的静态字段中,并在servlet中从中获取当前会话。

关于你的问题


因为你做错了。您在Servlet中创建一个新的会话工厂。
您获得一个新的会话(由于不正确的会话工厂创建)。在正常情况下,getCurrentSession()返回由ThreadLocal绑定到当前线程的会话(并非总是如此,这取决于配置)。
您有LazyInitializationExceptionquestion.getAnswers()

10-08 00:08
查看更多