SpringMVC+Hibernate:手动向表插入数据时获得NullPointerException

Question

当我试图用Hibernate手动插入MySql表时，我会收到这个错误。我试图插入一些默认数据到我的应用程序，但当手动插入时，我得到空指针异常。我是java编程的新手，所以不能理解如何修复这个错误。如果有人能帮我做这件事，我真的很感激…

User_Role类：

package com.sanjay31321.sys.model;


import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity @Table(name="user_role")
public class User_Role {

    @Id @Column @GeneratedValue(strategy=GenerationType.AUTO)
    private int id;

    @Column(name="role_name")
    private String name;

    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

}

import org.springframework.beans.factory.annotation.Autowired;

import com.sanjay31321.sys.model.User_Role;
import com.sanjay31321.sys.service.UserRoleService;

public class InsertUserRole {
    private static final Logger logger = LoggerFactory.getLogger(InsertUserRole.class);

    @Autowired
    private UserRoleService userRoleService;

    public void insert() {
        User_Role role = new User_Role();

        role.setId(1);
        role.setName("ROLE_ADMIN");
        logger.info("id : "+ role.getId() + " | Role : " + role.getName());
        userRoleService.addUserRole(role);
    }
}

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.NullPointerException
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:965)
    org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:855)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:641)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:829)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
root cause
java.lang.NullPointerException
com.sanjay31321.sys.preset.data.InsertUserRole.Insert（InsertUserRole.java:22)
com.sanjay31321.sys.preset.defaultdatainstall.install（defaultdatainstall.java:23)
com.sanjay31321.sys.controller.DefaultDataController.PostInstall（DefaultDataController.java:31)
Sun.Reflect.NativeMethodAccessorImpl.Invoke0（本机方法）
Sun.Reflect.NativeMethodAccessorImpl.Invoke（NativeMethodAccessorImpl.java:57)
Sun.Reflect.DelegatingMethodAccessorImpl.Invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.Reflect.Method.Invoke(method.java:601)
org.springframework.web.method.support.InvocableHandlerMethod.Invoke(InvocableHandlerMethod.java:219)
org.springframework.web.method.support.InvocableHandlerMethod.InvokeForRequest（InvocableHandlerMethod.java:132)
org.springframework.web.servlet.mvc.method.annotation.ServletInvoCableHandlerMethod.InvokeAndHandle（ServletInvoCableHandlerMethod.java:104)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.InvokeHandleMethod（RequestMappingHandlerAdapter.java:745)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.HandleInternal（RequestMappingHandlerAdapter.java:686)
org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.Handle（AbstractHandlerMethodAdapter.java:80)
org.springframework.web.servlet.dispatcherServlet.doDispatch（dispatcherServlet.java:925)
org.springframework.web.servlet.DispatcherServlet.DoService（DispatcherServlet.java:856)
org.springframework.web.servlet.frameworkservlet.processrequest（frameworkservlet.java:953)
org.springframework.web.servlet.frameworkservlet.doPost（frameworkservlet.java:855)
服务(httpservlet.java:641)
org.springframework.web.servlet.frameworkservlet.service（frameworkservlet.java:829)
服务(httpservlet.java:722)
DefaultDataController类
package com.sanjay31321.sys.controller;

import java.util.Locale;

import javax.validation.Valid;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Controller;
import org.springframework.validation.BindingResult;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

import com.sanjay31321.sys.preset.DefaultDataInstall;
import com.sanjay31321.sys.preset.model.Install;

@Controller
public class DefaultDataController {
    private static final Logger logger = LoggerFactory.getLogger(DefaultDataController.class);

    @RequestMapping(value = "/install", method = RequestMethod.GET)
    public String getinstall(Locale locale, Install install) {
        logger.info("Welcome to Install Default Settings page ! GET METHOD : The client locale is {}.", locale);
        return "install";
    }

    @RequestMapping(value="/install", method=RequestMethod.POST)
    public String postinstall(@Valid  Install install, BindingResult result, Locale locale){
        logger.info("Welcome to Install Default Settings page ! POST METHOD : The client locale is {}.", locale);
        DefaultDataInstall settings = new DefaultDataInstall();
        settings.install();
        return "install";
    }
}
DefaultDataInstall类
我已经附上了你要求的课程。
                

Answer 1

我在Github上看到你的代码了。

显示该错误是因为InsertUserRole中的UserRoleService为空。因此不能对其调用任何方法。

为什么？

您可以使用DefaultDataInstall中的New关键字创建InsertUserRole对象。@autowired只有在defaultdatainstall也被Spring实例化时才起作用。Spring只有在创建对象时才能在对象中注入某些东西。

如何修复它。

实例化preset包中的所有对象，即将它们标记为@component。

另一种方法是在控制器中注入ApplicationContext，并将其传递给正在创建的对象，然后从AppContext获取服务。

@Controller
public class DefaultDataController {
    private static final Logger logger = LoggerFactory.getLogger(DefaultDataController.class);

    @Autowired private ApplicationContext applicationContext;   

    @RequestMapping(value="/install", method=RequestMethod.POST)
    public String postinstall(@Valid  Install install, BindingResult result, Locale locale){
        logger.info("Welcome to Install Default Settings page ! POST METHOD : The client locale is {}.", locale);       
        DefaultDataInstall settings = new DefaultDataInstall(applicationContext);//create a constructor which acceps ApplicationContext object
        settings.install();
        return "install";
    }

    //other code
}

public class DefaultDataInstall {

    private static final Logger logger = LoggerFactory.getLogger(DefaultDataInstall.class);
    private ApplicationContext applicationContext;

    public DefaultDataInstall(ApplicationContext applicationContext){
        this.applicationContext = applicationContext;
    }

    public void install() {

        InsertUserRole role = new InsertUserRole();
        role.setUserRoleService(applicationContext.getBean("userRoleServiceImpl"));
        role.insert();
        logger.info("User Role data is installed");

        //other stuff
    }
}

现在您可以使用传递的服务来调用它们上的方法

public class InsertUserRole {
    private static final Logger logger = LoggerFactory.getLogger(InsertUserRole.class);

    private UserRoleService userRoleService;

    public void setUserRoleService(UserRoleService){
        this.userRoleService = userRoleService;
    }

    //other code
}