Question

我试图学习冬眠和春天。我想做的第一件事是使用Hibernate从数据库中获取一些数据。

我要做的是从数据库获取所有数据，但我得到空指针异常。

这是我的代码;

City.java（在com.hopyar.dao包下）

@Entity
@Table(name = "Cities")
public class City implements Serializable{

    private static final long serialVersionUID = 2637311781100429929L;

    @Id
    @GeneratedValue
    @Column(name = "c_id")
    int id;

    @Column(name = "c_name")
    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;
    }
}

CityService.java（在com.hopyar.service包下）

@Service
public class CityService {

    @PersistenceContext
    private EntityManager em;

    @Transactional
    public List<City> getAllCities(){
        List<City> result = em.createQuery("Select c From Cities c", City.class)
                .getResultList(); // This is where I get the exeption.
        System.out.println();
        return result;
    }
}

spring-context.xml（在webapp / WEB-INF下）

<?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:mvc="http://www.springframework.org/schema/mvc" xmlns:context="http://www.springframework.org/schema/context"
  xmlns:tx="http://www.springframework.org/schema/tx"
  xsi:schemaLocation="http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.2.xsd
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-3.2.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.2.xsd">

  <!-- Enable annotation-based Spring MVC controllers (eg: @Controller annotation) -->
  <mvc:annotation-driven/>

  <!-- Classpath scanning of @Component, @Service, etc annotated class -->
  <context:component-scan base-package="com.hopyar" />

  <!-- Resolve view name into jsp file located on /WEB-INF -->
  <bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
    <property name="prefix" value="/WEB-INF/" />
    <property name="suffix" value=".jsp" />
  </bean>

  <!-- MySQL Datasource with Commons DBCP connection pooling -->
  <bean class="org.apache.commons.dbcp.BasicDataSource" id="dataSource">
    <property name="driverClassName" value="com.mysql.jdbc.Driver"/>
    <property name="url" value="jdbc:mysql://localhost:3306/myproject"/>
    <property name="username" value="username"/>
    <property name="password" value="password"/>
  </bean>

  <!-- EntityManagerFactory -->
  <bean class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean" id="entityManagerFactory">
    <property name="persistenceUnitName" value="persistenceUnit"/>
    <property name="dataSource" ref="dataSource"/>
  </bean>

  <!-- Transaction Manager -->
  <bean class="org.springframework.orm.jpa.JpaTransactionManager" id="transactionManager">
    <property name="entityManagerFactory" ref="entityManagerFactory"/>
  </bean>

  <!-- Enable @Transactional annotation -->
  <tx:annotation-driven/>

</beans>

Main.java（在com.hopyar.test包下）

public class Main {

    public static void main(String[] args) {
        CityService service = new CityService();

        List<City> cities = service.getAllCities();

        System.out.println(cities.size());

    }

}

Answer 1

您正在将service实例化为new CityService()，这是错误的，因为您绕过了Spring。这意味着您的注释不会被截获，em为空。您需要从春季背景中获取service。

CityService service = applicationContext.getBean("cityService");

Answer 2

您可以尝试在CityService上使用自动装配的注释，Spring将实例化它。

无法使用Hibernate从数据库中获取所有数据

2 个答案: