How to Implement the Simple Factory Pattern with Spring 3 Annotations

How can I implement the simple factory pattern using Spring 3 annotations while ensuring proper dependency injection for my service classes?

@Component
public class MyServiceFactory {

    @Autowired
    private ApplicationContext applicationContext;

    public MyService getMyService(String service) {
        MyService myService;

        service = service.toLowerCase();

        if (service.equals("one")) {
            myService = applicationContext.getBean(MyServiceOne.class);
        } else if (service.equals("two")) {
            myService = applicationContext.getBean(MyServiceTwo.class);
        } else if (service.equals("three")) {
            myService = applicationContext.getBean(MyServiceThree.class);
        } else {
            myService = applicationContext.getBean(MyServiceDefault.class);
        }

        return myService;
    }
}

The simple factory pattern is a design pattern that provides a way to create objects without specifying the exact class of object that will be created. In Spring 3, this can be implemented using annotations and proper dependency injection. However, when manually creating instances, dependency injection (like @Autowired) does not occur unless you leverage Spring’s ApplicationContext.

@Component
public class MyServiceFactory {

    @Autowired
    private ApplicationContext applicationContext;

    public MyService getMyService(String service) {
        MyService myService;

        service = service.toLowerCase();

        if (service.equals("one")) {
            myService = applicationContext.getBean(MyServiceOne.class);
        } else if (service.equals("two")) {
            myService = applicationContext.getBean(MyServiceTwo.class);
        } else if (service.equals("three")) {
            myService = applicationContext.getBean(MyServiceThree.class);
        } else {
            myService = applicationContext.getBean(MyServiceDefault.class);
        }

        return myService;
    }
}

Causes

• When you instantiate classes directly (using `new`), Spring is not aware of these instances and does not manage them, leading to any injected dependencies being null.
• Dependency injection works only for beans that are managed by the Spring container.

Solutions

• Use the Spring `ApplicationContext` to retrieve beans instead of creating them manually.
• Autowire the `ApplicationContext` in your factory class to get instances of your services, which will ensure that all dependencies are correctly injected.

Mistake: Creating service instances manually using `new` instead of retrieving them from Spring.

Solution: Use `applicationContext.getBean(ClassName.class)` to retrieve instances managed by Spring.

Mistake: Not autowiring the `ApplicationContext` in the factory class.

Solution: Make sure to use `@Autowired` to inject the `ApplicationContext` to access Spring-managed beans.