Question

I have a project where I have an interface, an Abstract class implementing the same interface and then a set of concrete classes which implement this interface and extend the Abstract Class.

public interface Invoice
{
   void process();
}

@component
public abstract class AbstractInvoice(){

    @Resource
    protected Writer writer;

    protected validateInvoice(){
        //some implementation
    }
}

@Component
public Class TypeAInvoice() extends AbstractInvoice implements Invoice{

    @Override
    public void process(){
        //... some code
        writer.write();
    }
}

public Interface Writer(){
    public void write();
}

@Component
public class CDWriter implements Writer{
    @Override 
    public void write() { /* implementation.....*/}
}

Spring file has a component scan for the package.

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

I am using a factory to get an instance of TypeAInvoice invoice
Now calling invoice.process() gets a NPE when getting to write.write()

I am not sure what am I missing here. I tried to see the component scan and scope and could not find anything conceptually wrong.

Answer 1

I am using a factory to get an instance of TypeAInvoice invoice

Depending on what your Factory does, this may be the problem. If the Factory creates a new TypeAInvoice, Spring wiring doesn’t apply. You have to query the Spring context for the Bean. One way (though not very pretty) is to use ContextLoader:

return ContextLoader.getCurrentWebApplicationContext().getBean(TypeAInvoice.class)

I’d say static Factories and Spring don’t go to well together. Spring stands for the Inversion of Control pattern, while Factories stand for the Service Locator pattern. I’d suggest that you get rid of your factories and autowire your Spring Beans.