Spring Injection - Interfaces and Classes

Question

Hi,

I want to be able to call Interfaces in my class, but have spring instantiate them with the right implementation class behind the scenes.

I.e. Normally you can do:

IClass clz = new Class();

I want to have the line IClass clz; preferable in the middle of a method (or as one of the attributes if it can't be done), where clz is instantiated to the class I want by Spring.

The reason I'd like to do it this way is because I want to be able to be able to change which implementation I want to use simply by editing the context files.

Even better, would be knowing how to do the above with class contructors that expect parameters. i.e. new Class(ar1, arg2);

I hope this makes sense. Any help would be much appreciated.

Answer 1

You can make your class implement BeanFactoryAware and then Spring will inject the bean factory in your class. If you then want to get an instance of a class implementing your interface you say something like:

beanFactory.getBean(IClass.class);

If there are multiple beans that implement the same interface you will have to resolve by name. To create a new object each time you ask this, set the bean scope of the bean you're asking for to "prototype".

Answer 2

You can include code such as:

ApplicationContext context = new ClassPathXmlApplicationContext("spring-config.xml");
IClass clz = (IClass) context.getBean("beanName");

Not saying this is better per se than Gerco's answer btw, just it's an option, depending what you want to do.

You can also implement the ApplicationContextAware interface: I've found that using ApplicationContext gives me beans with filled-in properties, e.g. if you have an app.properties file which contains key/value property pairs which you expect to be resolved within the Spring config, beans retrieved via BeanFactory calls may not resolve those.

See this previous SO topic for more info.