java - NullPointerException...数据已插入但无法检索数据 [Mysql DB]

Question

SEVERE: Servlet.service() for servlet dispatcher threw exception java.lang.NullPointerException at com.rajs.apps.controller.UserController.users(UserController.java:16) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:483) at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:175) at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:446) at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:434) at ........

applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:jpa="http://www.springframework.org/schema/data/jpa"
    xmlns:context="http://www.springframework.org/schema/context" xmlns:tx="http://www.springframework.org/schema/tx"
    xmlns:jdbc="http://www.springframework.org/schema/jdbc" xmlns:mvc="http://www.springframework.org/schema/mvc"
    xsi:schemaLocation="http://www.springframework.org/schema/jdbc http://www.springframework.org/schema/jdbc/spring-jdbc-4.1.xsd
        http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-4.1.xsd
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.1.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.1.xsd
        http://www.springframework.org/schema/data/jpa http://www.springframework.org/schema/data/jpa/spring-jpa-1.3.xsd
        http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.1.xsd">


    <context:component-scan base-package="com.rajs.apps">
        <context:exclude-filter type="annotation"
            expression="org.springframework.stereotype.Controller" />
    </context:component-scan>

    <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource"
        destroy-method="close">
        <property name="driverClassName" value="com.mysql.jdbc.Driver" />
        <property name="url" value="jdbc:mysql://localhost:3306/rba" />
        <property name="username" value="root" />
        <property name="password" value="root" />
    </bean>

    <bean
        class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"
        id="entityManagerFactory">
        <property name="packagesToScan" value="com.rajs.apps.entity" />
        <property name="dataSource" ref="dataSource" />
        <property name="jpaProperties">
            <props>
                <prop key="hibernate.hbm2ddl.auto">create</prop>
                <prop key="hibernate.show_sql">true</prop>
                <prop key="hibernate.dialect">org.hibernate.dialect.MySQLDialect</prop>
            </props>
        </property>

        <property name="persistenceProvider">
            <bean class="org.hibernate.jpa.HibernatePersistenceProvider" />
        </property>

    </bean>

    <tx:annotation-driven transaction-manager="transactionManager" />

     <bean class="org.springframework.orm.jpa.JpaTransactionManager"
        id="transactionManager">
        <property name="dataSource" ref="dataSource" />
     </bean>

     <jpa:repositories base-package="com.rajs.apps.repository" />
    </beans>

UserController.java

package com.rajs.apps.controller;

import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;

import com.rajs.apps.service.UserService;

@Controller
public class UserController {

    private UserService userService;

    @RequestMapping("/users")
    public String users(Model model){
        model.addAttribute("users", userService.findAll());
        return "users";

    }


}

UserService.java

package com.rajs.apps.service;

import java.util.List;

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

import com.rajs.apps.entity.User;
import com.rajs.apps.repository.UserRepository;


@Service
public class UserService {

    @Autowired
    private UserRepository userRepository;

    public List<User> findAll(){

        return userRepository.findAll();

    }

}

UserRepository.java

package com.rajs.apps.repository;
import org.springframework.data.jpa.repository.JpaRepository;
import com.rajs.apps.entity.User;
public interface UserRepository extends JpaRepository<User, Integer> {
}

Answer 1

添加

@Autowire

在 Controller 中

私有(private) UserService userService;

基本上你的 userService 是 null 导致 nullpointer