Inheritance (Late Binding) via Dependency Injection in Java

Question

I am using Spring DI to wire my components and I came across this issue.

I have a BaseService class which has multiple implementations. And the layer above it, has a builder which calls the service to get data to populate POJOs. Service implementation I need to call (ServiceA,ServiceB) changes according to the type of POJO I need to build.

In such case, how can I autowire the service, as it requires late binding the service. How can I tackle this kind of scenario? (Example in Spring DI would really help)

I read similar questions but could not find the answer. And I read that SOA patterns such as Service Host provide different solutions to exact use case.

Please help. Thanks

Answer 1

How about using a FactoryBean:

public class BuilderFactory implements FactoryBean<Builder> {

  @Autowired
  private ApplicationContext appContext;

  ...

  @Override
  public Builder getObject() {
      Builder builder = new Builder();      
      switch(something()) { 
         case "foo":
             builder.service = new ServiceA(); 
             break;
         case "bar":
             builder.service= new ServiceB();
             break;
         ...
         default:
             //handle cases where it's unclear which type to create

         }
     return builder;
  }

}

where Builder instances have a public/package-private field BaseService service that gets called in their getData(), buildPojos() and wherever other methods.

(you could also use static factory methods to instantiate Builder if you want this field to be private)

Answer 2

You can use ServiceLocatorFactoryBean. In your case you would do something like this:

public interface BaseServiceLocator {

   BaseService lookup(String qualifier); //use whatever qualifier type makes sense here
}

<bean id="serviceLocatorFactoryBean"
    class="org.springframework.beans.factory.config.ServiceLocatorFactoryBean">
    <property name="serviceLocatorInterface"
              value="your.package.BaseServiceLocator" />
</bean>

Then your builder would look something like this:

public class Builder {

  @Autowired
  private BaseServiceLocator baseServiceLocator;

  @Override
  public YourReturnType businessMethod() {
      SomeData data = getData();
      BaseService baseService = baseServiceLocator(data.getType()); //here I am assuming that getType() is a String

      //whatever
  }