A null pointer exception is often prompted when the action is a dependency injection

Sometimes, when we inject a dependency into the DAO layer, service layer, and action, we often prompt for null pointer exceptions, that is, injection failures. ..

This is every bit abnormal, normal use ~ ~ What is going on ... The reasons are as follows:

In the case of dependency injection, the injection of the DAO layer class is not the same as the action. A little bit different.

Our example explains the subtle differences between the two injections:

Example of a DAO layer injection:

<bean id= "Loginregistserviceimpl" class= "Com.serviceImpl.LoginRegistServiceImpl" >  
   <constructor-arg Name= "Udao" ><ref bean= "Userdaoimpl"/></constructor-arg>  
   </bean>  
     
   <bean id= " ServiceManager "class=" Com.serviceImpl.ServiceManager ">  
   <property name=" LRS "><ref bean=" Loginregistserviceimpl "/></property>  
   </bean>  

Take a look at the injection of action:

    <bean id= "loginregistaction" class= "com.action.LoginRegistAction" >  
       <property name= "ServiceManager" ><ref bean= "ServiceManager"/></property>  
    </bean>  

The focus is on: [HTML] view plain copy <property name= "ServiceManager" ><ref bean= "ServiceManager"/></ Property>

In action injection, you must require that the name of the variable defined by the action be consistent with the ID name of the injected bean, and no injection fails ~ ~ ~