Loading a spring config at server startup

Question

I have a jsp login page for identifying users. To configure JdbcTemplate , I set up its properties with spring under "WEB-INF/applicationContext.xml" , so for loading spring context , i added some tags in web.xml . But , autowiring jdbcTemplate bean in UserDao class is causing a NullPointerException !

applicationContext.xml

 <context:annotation-config />
 <context:component-scan base-package="com.firstapp.dao"/>

 <bean id="datasource" 
      class="org.springframework.jdbc.datasource.DriverManagerDataSource">  
     <property name="driverClassName" 
     value="oracle.jdbc.driver.OracleDriver"  />  
     <property name="url" value="jdbc:oracle:thin:@localhost:1521:xe" />  
     <property name="username" value="system" />  
     <property name="password" value="oracle" />  
 </bean>  

<bean id="jdbcTemplate" 
      class="org.springframework.jdbc.core.JdbcTemplate">  
      <property name="dataSource" ref="datasource"></property>  
 </bean>  

class DAO

 public class UserDAO 
  {
   @Autowired
   private JdbcTemplate jdbcTemplate;
   ....}

class Service

  public class UserService  
   {
      private UserDAO  userDAO  = new UserDAO ();       
    ....}

web.xml

<context-param>
  <param-name>contextConfigLocation</param-name>
  <param-value>/WEB-INF/applicationContext.xml</param-value>
</context-param>

<listener>
  <listener-class>
     org.springframework.web.context.ContextLoaderListener
  </listener-class>
</listener>

Answer 1

I bet your UserDAO class is not defined as a Spring Bean. In that case you have two options: define your bean in XML or annotating it.

Option 1: XML

applicationContext.xml

<bean id="userDao" class="[package of UserDAO class].UserDAO">
    <property name="jdbcTemplate" ref="jdbcTemplate"/>
</bean>

In that case you won't need @Autowired on the field.

Option 2: annotation-based

applicationContext.xml

<context:annotation-config/>
<context:component-scan base-package="[package of UserDAO class]"/>

UserDAO.java

@Component
public class UserDAO {
    @Autowired
    private JdbcTemplate jdbcTemplate;
}

Please also note that it is not recommended to have fields autowired directly. You can find more on this topic here.

DummyClass.java

@Component // must be a bean as well
public class DummyClass {
    private final UserDAO userDao;

    @Autowired
    public DummyClass(UserDAO userDao) {
        this.userDao = userDao;
    }
}

Option 3: manually extract Bean from the application context

I want to emphasize that this method is not recommended and you should avoid it and use dependency injection correctly. If you need to use that method - the architecture of your application is probably wrong.

SpringBeanUtil.java

@Component
public class SpringBeanUtil implements ApplicationContextAware {
    private static ApplicationContext context;

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        context = applicationContext;
    }

    public static <T> T getBean(Class<T> beanClass) {
        return context.getBean(beanClass);
    }
}

UserDAO.java

public class UserDAO {
    private JdbcTemplate jdbcTemplate = SpringBeanUtil.getBean(JdbcTemplate.class);
}