无法从数据库使用休眠获取所有数据
我正在尝试学习Hibernate和Spring。我想要做的第一件事就是使用Hibernate从数据库中获取一些数据。无法从数据库使用休眠获取所有数据
我想要做的是从数据库中获取所有数据,但我得到空指针异常。
这是我的代码;
City.java(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(下com.hopyar.service封装)
@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;
}
}
弹簧context.xml中(下的web应用/ WEB-INF)
<?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(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());
}
}
您正在实例化service
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处理,并且em
为空。你需要从春天的环境中得到你的service
。
CityService service = applicationContext.getBean("cityService");
或者使用一些Spring测试功能创建JUnitTest。这会为你提供一切。 – 2014-12-04 13:25:37
我想我明白了。感谢帮助。 – xxlali 2014-12-04 13:29:07
@xxlali没问题,很高兴我能帮到你。 – 2014-12-04 13:31:34
您可以尝试在CityService上使用autowired注释,并且Spring将实例化它。
堆栈跟踪粘贴请螺纹 – Pracede 2014-12-04 13:19:38
异常 “主” 显示java.lang.NullPointerException \t在com.hopyar.service.CityService.getAllCities(CityService.java:21)在com.hopyar.test.Main.main \t (Main.java:17) 只是这个,但没有别的 – xxlali 2014-12-04 13:21:21
尝试自动装配而不是PersistenceContext – Pracede 2014-12-04 13:34:14