Factory design pattern and Spring

Question

I am wondering what is the current best practice as to the use of factory pattern within the context of Spring framework in using dependency injection. My wonder arises about whether the factory pattern is still relevant nowadays in light of the use of Spring dependency injection. I did some searching and see some past discussion (Dependency Injection vs Factory Pattern) but seem there is different view.

I see in some real life project in using a Map to hold all the beans and rely on autowiring to create those beans. When the bean is needed, it get it via the map using the key.

public abstract class Service {
  //some methods
}

@Component
public class serviceA extends Service {
  //implementation
}

@Component
public class serviceB extends Service {
  //implementation
}

Map<String, Service> services; 

But I see there is some difference among the two approaches.

Using the above method, all beans are created on application start up and the creation of object is handled by the framework. It also implies there is only one bean for each type.

While for factory pattern, the factory class creates the object on request. And it can create a new object for each request.

I think a deeper question may be, when Spring framework is used in a project, should it be strived to not create any object inside a class, which means the factory pattern ( or any creational design patterns?) should not be used, as Spring is supposed to be the central handler of the objects dependency ?

question from:https://stackoverflow.com/questions/65879813/factory-design-pattern-and-spring

Answer 1

The answer to this question can be really deep and broad, I'll try to provide some points that hopefully will help.

First off, spring stores its beans (singletons) in the ApplicationContext. Essentially this is the map you're talking about. In a nutshell, it allows getting the bean by name, type, etc.

ApplicationContext, while being a really important concept, is not the whole Spring, in fact Spring framework allows much more flexibility:

You say, using a map implies that all the beans will be created at the beginning of the application and there is one instance of the bean.

Spring has a concept of Lazy beans, basically supporting a concept of beans being actually created only when they're required for the first time, so Spring supports the "delayed" beans initialization

Spring also allows more than one instance of a bean per type. So this map is more "advanced". For example you can create more than one implementation of the interface and use declare both as beans. As long as you provide enough information about what bean should be injected to the class that might use them (for example with a help of qualifiers suppored in spring), you're good to go. In addition, there are features in spring IoC container that allow injecting all registered implementations of an interface into a list:

interface Foo {}
@Component
class FooImpl1 implements Foo {}
@Component
class FooImpl2 implements Foo {}

class Client {
   @Autowired
   List<Foo> allFoos;
}

Now you say:

While for factory pattern, the factory class creates the object on request. And it can create a new object for each request.

Actually Spring can create objects per request. Not all beans have to be singletons, in general spring has a concept of scopes for this purposes.

For example, scope prototype means that Spring will create a bean upon each usage. In particular one interesting usage that spring supports in variety of ways is Injecting prototype bean into singleton. Some solutions use exactly like a factory (read about annotation @Lookup others rely on auto-generated proxy in runtime (like javax.inject.Provider). Prototype scope beans are not held in the application context, so here again spring goes beyond a simple map abstraction.

Last feature that you haven't mentioned is that sometimes even for singletons the initialization can be a little bit more complicated then calling a constructor with Parameters. Spring can address that by using Java Configurations:

@Configuration
public class MyConfig {
   public SomeComplicatedObject foo(@Value("...") config, Bar bar) {

       SomeComplicatedObject obj = new SomeComplicatedObject() // lets pretend this object is from some thirdparty, it only has no-op constructor, and you can't place spring annotations on it (basically you can't change it):
        obj.setConfig(config);
        obj.setBar(bar);
        return obj;
   }
}

The method foo here initializes the object SomeComplicatedObject and returns it. This can be used instead of factories to integrate "legacy" code (well, java configurations go way beyond this, but its out of scope for this question).

So bottom line, you Spring as an IoC container can provide many different ways to deal with object creation, in particular it can do everything that factory design pattern offers.

Now, I would like to also refer to your last sentense:

I think a deeper question may be, when Spring framework is used in a project, should it be strived to not create any object inside a class, which means the factory pattern ( or any creational design patterns?) should not be used, as Spring is supposed to be the central handler of the objects dependency ?

Indeed you don't have to use Factory Pattern when using Spring, since (as I hopefully have convinced you) provides everything that factory can do and more.

Also I agree that spring is supposed to be the central handler of the objects dependency (unless there are also parts of the application which are written in a different manner so you have to support both :) )

I don't think we should avoid using "new" altogether, not everything should/can be a bean, but I do see (from my subjective experience, so this is arguable) that you use it much less leaving the creation of most of the objects to Spring.

Should we avoid a usage of any creation design pattern? I don't think so, sometimes you can opt for implementing "builder" design pattern for example, its also a creational pattern but spring doesn't provide a similar abstraction.