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为什么在SpringDAO方法的JSP页面中出现nullpointerexception？

spring dependency-injection

为什么在SpringDAO方法的JSP页面中出现nullpointerexception？,spring,dependency-injection,repository,Spring,Dependency Injection,Repository,在这里，每当我试图使用Hibernate访问或调用SpringMVC项目中JSP页面中的DAO方法（在@Repository类中编写）时，它都会显示NullPointerException 当我在控制器类中访问这个方法时，它工作得很好，它只是在JSP页面中抛出一个异常 在这里，我得到了期待 严重：Servlet[jsp]的Servlet.service（）引发异常 java.lang.NullPointerException 位于com.lms.service.BookDetailsServic

在这里，每当我试图使用Hibernate访问或调用SpringMVC项目中JSP页面中的DAO方法（在@Repository类中编写）时，它都会显示NullPointerException

当我在控制器类中访问这个方法时，它工作得很好，它只是在JSP页面中抛出一个异常

在这里，我得到了期待

严重：Servlet[jsp]的Servlet.service（）引发异常 java.lang.NullPointerException 位于com.lms.service.BookDetailsServiceImpl.GetBookStatusForLibraryById（BookDetailsServiceImpl.java:62）

这是我的密码

在pom.xml中

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
  
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.libmansystem</groupId>
  <artifactId>LibraryManagementSystems</artifactId>
  <packaging>war</packaging>
  <version>1.0</version>
  <name>LibraryManagementSystems Maven Webapp</name>
  <url>http://maven.apache.org</url>
  
  <properties>
     
    <springframework.version>5.0.2.RELEASE</springframework.version>
    <springsecurity.version>5.0.0.RELEASE</springsecurity.version>
    
    <hibernate.version>5.4.14.Final</hibernate.version>
    <mysql.connector.version>8.0.20</mysql.connector.version>
    <c3po.version>0.9.5.5</c3po.version>
  
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
    
  </properties>
  
  
  
  
  <dependencies>
  
   <!-- Spring MVC support -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webmvc</artifactId>
            <version>${springframework.version}</version>
        </dependency>
    
    <!-- Spring Transactions -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-tx</artifactId>
            <version>${springframework.version}</version>
        </dependency>
        
        <!-- Spring ORM -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-orm</artifactId>
            <version>${springframework.version}</version>
        </dependency>

        <!-- Hibernate Core -->
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>${hibernate.version}</version>
        </dependency>
        
        <!-- Add MySQL and C3P0 support -->

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>${mysql.connector.version}</version>
        </dependency>
        
        <dependency>
            <groupId>com.mchange</groupId>
            <artifactId>c3p0</artifactId>
            <version>${c3po.version}</version>
        </dependency>
    
  
      <!-- Servlet, JSP and JSTL support -->
        <dependency>
            <groupId>javax.servlet.jsp.jstl</groupId>
            <artifactId>jstl-api</artifactId>
            <version>1.2</version>
            <exclusions>
                <exclusion>
                    <groupId>javax.servlet</groupId>
                    <artifactId>servlet-api</artifactId>
                </exclusion>
                <exclusion>
                    <groupId>javax.servlet.jsp</groupId>
                    <artifactId>jsp-api</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
    
        <dependency>
            <groupId>org.glassfish.web</groupId>
            <artifactId>jstl-impl</artifactId>
            <version>1.2</version>
            <exclusions>
                <exclusion>
                    <groupId>javax.servlet</groupId>
                    <artifactId>servlet-api</artifactId>
                </exclusion>
                <exclusion>
                    <groupId>javax.servlet.jsp</groupId>
                    <artifactId>jsp-api</artifactId>
                </exclusion>
                <exclusion>
                    <groupId>javax.servlet.jsp.jstl</groupId>
                    <artifactId>jstl-api</artifactId>
                </exclusion>
            </exclusions>
        </dependency> 
    
  
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
  
  
  </dependencies>


  
  <build>
    <finalName>LibraryManagementSystems</finalName>
    
    <pluginManagement>
            <plugins>
               <plugin>
                 <!-- Add maven co-ordinates for : maven-war-plugin -->
                 
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-war-plugin</artifactId>
                 <version>3.2.3</version>
               
               </plugin>
            </plugins>
      </pluginManagement>
      
  </build>

</project>
Dao接口和类

public interface BookDetailsDAO {

    public BookStatusForLibrarian getBookStatusForLibrarianById(int statusId);
     
}




@Repository
public class BookDetailsDAOImpl implements BookDetailsDAO{

    
    
    private SessionFactory sessionFactory;
    
    @Autowired
    public BookDetailsDAOImpl(SessionFactory sessionFactory)
    {
        this.sessionFactory = sessionFactory;
    }
    
    
    public BookDetailsDAOImpl()
    {
    }
    

    @Override
    public BookStatusForLibrarian getBookStatusForLibrarianById(int statusId) {
        
        Session theSession = sessionFactory.getCurrentSession();
        
        
        BookStatusForLibrarian theBookStatusForLibrarian = theSession.get(BookStatusForLibrarian.class, statusId);
        
        
        return theBookStatusForLibrarian;
    }



}

public interface BookDetailsService {
    
    public BookStatusForLibrarian getBookStatusForLibrarianById(int statusId);
    
} 





@Service
public class BookDetailsServiceImpl implements BookDetailsService {

    @Autowired
    BookDetailsDAO theBookDetailsDAO;
    
    

    @Override
    @Transactional
    public BookStatusForLibrarian getBookStatusForLibrarianById(int statusId) {

        System.out.println("inside service ");
        
        return theBookDetailsDAO.getBookStatusForLibrarianById(statusId);
    }



}
我的服务接口和类

public interface BookDetailsDAO {

    public BookStatusForLibrarian getBookStatusForLibrarianById(int statusId);
     
}




@Repository
public class BookDetailsDAOImpl implements BookDetailsDAO{

    
    
    private SessionFactory sessionFactory;
    
    @Autowired
    public BookDetailsDAOImpl(SessionFactory sessionFactory)
    {
        this.sessionFactory = sessionFactory;
    }
    
    
    public BookDetailsDAOImpl()
    {
    }
    

    @Override
    public BookStatusForLibrarian getBookStatusForLibrarianById(int statusId) {
        
        Session theSession = sessionFactory.getCurrentSession();
        
        
        BookStatusForLibrarian theBookStatusForLibrarian = theSession.get(BookStatusForLibrarian.class, statusId);
        
        
        return theBookStatusForLibrarian;
    }



}

public interface BookDetailsService {
    
    public BookStatusForLibrarian getBookStatusForLibrarianById(int statusId);
    
} 





@Service
public class BookDetailsServiceImpl implements BookDetailsService {

    @Autowired
    BookDetailsDAO theBookDetailsDAO;
    
    

    @Override
    @Transactional
    public BookStatusForLibrarian getBookStatusForLibrarianById(int statusId) {

        System.out.println("inside service ");
        
        return theBookDetailsDAO.getBookStatusForLibrarianById(statusId);
    }



}
我的控制器

@Controller
public class HomePageController {

    
    @GetMapping("/homepage")
    public String getHomePage()
    {
        
        return "homepage";
    }

}
在我的JSP页面中，我遇到了异常

/WEB-INF/view/homepage.jsp

<%@page import="com.lms.service.NonServiceMethod"%>
<%@page import="com.lms.dao.BookDetailsDAOImpl"%>
<%@page import="com.lms.service.BookDetailsServiceImpl"%>
<%@page import="com.lms.service.BookDetailsService"%>
<%@page import="com.lms.entity.BookStatusForLibrarian"%>

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>   
<%@ taglib prefix="form" uri="http://www.springframework.org/tags/form"  %> 

<%@ page isELIgnored="false"%>
    
<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>Show All Book</title>

      
</head>
    <body>
    
            
            <%
            
            
            System.out.println("getting exception while calling Spring DAO method (which is written in @Repository class) in JSP page");
            
            BookDetailsServiceImpl theBookDetailsService = new BookDetailsServiceImpl();
            
            BookStatusForLibrarian getBookStatus= theBookDetailsService.getBookStatusForLibrarianById(3);

            
            %>
            
        
         
    </body>

</html> 


展示所有书籍
在这里，我得到了期待

严重：Servlet[jsp]的Servlet.service（）引发异常 java.lang.NullPointerException 位于com.lms.service.BookDetailsServiceImpl.GetBookStatusForLibraryById（BookDetailsServiceImpl.java:62）

请帮助我。

如果您在JSP中创建服务，它不是Springbean。Spring没有进行依赖注入，您的DAO也没有注入到您的服务中（NullPointerException）。将模型（BookStatusForLibrary）传递到视图（JSP）的正确方法是使用控制器

@Controller
public class HomePageController {

    @Autowired
    BookDetailsService bookDetailsService;

    @GetMapping("/homepage")
    public String getHomePage(Model model) {
        BookStatusForLibrarian bookStatus = bookDetailsService.getBookStatusForLibrarianById(3);
        model.addAttribute("bookStatus", bookStatus);
        return "homepage";
    }

}
然后您可以直接在JSP中使用bookStatus，而不必像这样：
${bookStatus.id}

请您添加代码。您好，先生，我刚刚编辑了我的问题并添加了完整的代码。