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。一切都按我的意愿行事，但我不了解一些事情。
数据库很简单 - 2个表：问题和答案。表之间的关系是一对多的：一个问题可以有很多答案。
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中
<?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>
因此在问题中有一系列答案。因此，会有一些懒惰的Answers。我想在我的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，此会话用于servlet和相应的jsp文件。
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() {
}
}
我的应用程序中有两个servlet和两个相应的jsp文件 - 第一个显示ID = 1的问题，第二个显示所有问题。这是这个servlet的代码，相应的jsp文件和tomcat的配置：
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>
的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）为什么我需要调用＆＃34; session.beginTransaction（）＆＃34; servlet中的方法，即使我已经调用＆＃34; session.beginTransaction（）＆＃34;在过滤器？如果我从另一个servler调用一个servler，我也必须在第二个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）我没有调用＆＃34; sessionFactory.getCurrentSession（）。openSession（）＆＃34;或＆＃34; sessionFactory.getCurrentSession（）。getCurrentSession（）＆＃34;在过滤器中，我调用＆＃34; sessionFactory.getCurrentSession（）。getCurrentSession（）＆＃34;在servlet中仍然获得似乎在过滤器中创建的会话。怎么会这样？
3）如果我取消注释line＆＃34; session.getTransaction（）。commit（）;＆＃34;在来自jsp文件的GetOneQuestion类中，我收到了LazyInitializationException：＆＃34;无法初始化代理 - 没有Session＆＃34;即使这个jsp文件中没有延迟加载，因为我不在那里使用任何Answer对象。是什么导致这种例外？即使没有延迟加载，Session也必须打开与Hibernate对象的任何交互？
答案 0 :(得分：1)
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，并从servlet中获取当前会话。
关于您的问题
getCurrentSession()返回绑定到当前线程的会话ThreadLocal（并非总是如此，它取决于配置）。LazyInitializationException question.getAnswers()。