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将业务层改写为单例模式出现空指针异常

来源：5-2 项目作业

Wonwayshon

2021-02-05 15:47:00

我用debug追踪到似乎是CategoryController中属性位置

CategoryBizImpl.getInstance()

部分的问题，麻烦老师帮忙看看如何解决。

05-Feb-2021 15:35:40.662 涓ラ噸 [http-nio-8080-exec-4] org.apache.catalina.core.StandardWrapperValve.invoke 鍦ㄨ矾寰勪负/imoocBook鐨勪笂涓嬫枃涓紝Servlet[globalController]鐨凷ervlet.service锛堬級寮曞彂浜嗗叿鏈夋牴鏈師鍥犵殑寮傚父Servlet鎵ц鎶涘嚭涓�涓紓甯�
    java.lang.NullPointerException
        at com.imooc.ibook.global.DaoFactory.getDao(DaoFactory.java:32)
        at com.imooc.ibook.biz.impl.CategoryBizImpl.<clinit>(CategoryBizImpl.java:11)
        at com.imooc.ibook.controller.CategoryController.<init>(CategoryController.java:14)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
        at java.lang.Class.newInstance(Class.java:442)
        at com.imooc.ibook.global.GlobalController.service(GlobalController.java:36)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
        at com.imooc.ibook.global.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:25)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:543)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
        at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:690)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
        at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:616)
        at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
        at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:818)
        at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1626)
        at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
        at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
        at java.lang.Thread.run(Thread.java:748)

package com.imooc.ibook.controller;

import com.imooc.ibook.biz.CategoryBiz;
import com.imooc.ibook.biz.impl.CategoryBizImpl;
import com.imooc.ibook.entity.Category;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.List;

public class CategoryController {
    private CategoryBiz categoryBiz=CategoryBizImpl.getInstance();

    //list.do
    public void list(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        List<Category> categoryList=categoryBiz.getAllCategoried();
        request.setAttribute("categoryList",categoryList);
        request.getRequestDispatcher("/WEB-INF/pages/admin/category_list.jsp").forward(request,response);
    }

    //toAdd.do
    public void toAdd(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.getRequestDispatcher("/WEB-INF/pages/admin/category_add.jsp").forward(request,response);
    }

    //add.do
    public void add(HttpServletRequest request, HttpServletResponse response) throws IOException {
        String name=request.getParameter("name");
        Category category=new Category();
        category.setName(name);
        categoryBiz.addCategory(category);
        response.sendRedirect(request.getContextPath()+"/admin/Category/list.do");
    }

    //delete.do
    public void delete(HttpServletRequest request, HttpServletResponse response) throws IOException {
        String id=request.getParameter("id");
        categoryBiz.deleteCategory(Integer.parseInt(id));
        response.sendRedirect(request.getContextPath()+"/admin/Category/list.do");
    }
}
package com.imooc.ibook.biz.impl;

import com.imooc.ibook.biz.CategoryBiz;
import com.imooc.ibook.dao.CategoryDao;
import com.imooc.ibook.entity.Category;
import com.imooc.ibook.global.DaoFactory;

import java.util.List;

public class CategoryBizImpl implements CategoryBiz {
    private static CategoryDao categoryDao=DaoFactory.getInstance().getDao(CategoryDao.class);
    private static CategoryBizImpl categoryBizImpl;

    private CategoryBizImpl(){};

    public static CategoryBizImpl getInstance(){
        if(categoryBizImpl==null){
            categoryBizImpl=new CategoryBizImpl();
        }
        return categoryBizImpl;
    }

    @Override
    public List<Category> getAllCategoried() {
        return categoryDao.selectAllCategories();
    }

    @Override
    public void addCategory(Category category) {
        categoryDao.insertCategory(category);
    }

    @Override
    public void deleteCategory(long id) {
        categoryDao.deleteCategory(id);
    }
}
package com.imooc.ibook.dao;

import com.imooc.ibook.entity.Category;
import org.apache.ibatis.annotations.*;

import java.util.List;

public interface CategoryDao {

    @Select("SELECT * FROM category")
    @Results({
            @Result(property = "id",column = "id"),
            @Result(property = "name",column = "name"),
            @Result(property = "create_time",column = "create_time"),
            @Result(property = "update_time",column = "update_time")
    })
    List<Category> selectAllCategories();

    @Insert("INSERT category VALUES(NULL,#{name},NOW(),NOW())")
    void insertCategory(Category category);

    @Delete("DELETE * FROM category WHERE id=#{id}")
    void deleteCategory(long id);
}
package com.imooc.ibook.biz;

import com.imooc.ibook.entity.Category;

import java.util.List;

public interface CategoryBiz {
    //获取全部分类
    List<Category> getAllCategoried();

    //添加分类
    void addCategory(Category category);

    //根据id删除分类
    void deleteCategory(long id);
}
package com.imooc.ibook.global;

import javax.servlet.GenericServlet;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class GlobalController extends GenericServlet {
    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
        HttpServletRequest request= (HttpServletRequest) servletRequest;
        HttpServletResponse response= (HttpServletResponse) servletResponse;
        String servletPath=request.getServletPath();
        String methodName=null;
        String controllerPath="com.imooc.ibook.controller.";
        if(servletPath.indexOf("/admin")!=-1){
            servletPath=servletPath.substring(7);
        }else{
            servletPath=servletPath.substring(1);
        }
        int slashIndex=servletPath.indexOf("/");
        if(slashIndex!=-1){
            controllerPath=controllerPath+servletPath.substring(0,slashIndex)+"Controller";
            methodName=servletPath.substring(slashIndex+1,servletPath.indexOf(".do"));
        }else{
            controllerPath=controllerPath+"DefaultController";
            methodName=servletPath.substring(0,servletPath.indexOf(".do"));
        }
        try {
            Class controllerClass=Class.forName(controllerPath);
            Object controllerObject=controllerClass.newInstance();
            Method controllerMethod=controllerClass.getMethod(methodName,HttpServletRequest.class,HttpServletResponse.class);
            controllerMethod.invoke(controllerObject,request,response);
        } catch (ClassNotFoundException | NoSuchMethodException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        }
    }
}
package com.imooc.ibook.entity;

public class Category {
    private Long id;
    private String name;
    private String createTime;
    private String updateTime;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCreateTime() {
        return createTime;
    }

    public void setCreateTime(String createTime) {
        this.createTime = createTime;
    }

    public String getUpdateTime() {
        return updateTime;
    }

    public void setUpdateTime(String updateTime) {
        this.updateTime = updateTime;
    }
}

​

写回答

2回答

好帮手慕阿满

2021-02-05

已采纳

同学你好，如下需要去掉try中的SqlSessionFactory，三个sessionFactory是同一个对象，try中的sessionFactory是初始化sessionFactory，如果try中重新定义了sessionFactory，那么getDao()方法中的sessionFactory没有初始化，也就是null，使用其调用方法时就会出现空指针异常。

http://img.mukewang.com/climg/601d03de09ef036406350536.jpg

祝学习愉快~

0

好帮手慕阿满

2021-02-05

同学你好，报错提示最终出现DaoFactory类的getDao()方法中，如：

http://img.mukewang.com/climg/601d01fe095ae2c607370106.jpg

建议同学将DaoFactory类的代码也贴一下。

祝学习愉快~

0
honwayshon
hpre class="brush:java;toolbar:false">​package com.imooc.ibook.global;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;


public class DaoFactory {
private static SqlSessionFactory sqlSessionFactory;
private static DaoFactory daoFactory;

private DaoFactory(){
SqlSessionFactoryBuilder sqlSessionFactoryBuilder=new SqlSessionFactoryBuilder();
try {
SqlSessionFactory sqlSessionFactory=sqlSessionFactoryBuilder.build(Resources.getResourceAsReader("/mybatis-config.xml"));
} catch (IOException e) {
e.printStackTrace();
}
}

public static DaoFactory getInstance(){
if(daoFactory==null){
daoFactory=new DaoFactory();
}
return daoFactory;
}

public <T> T getDao(Class<T> daoClass){
SqlSession sqlSession=sqlSessionFactory.openSession(true);
return sqlSession.getMapper(daoClass);
}
}
h021-02-05
共1条回复

0 学习 · 8016 问题

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