What is a bean, IoC and DI?

Question

I recently started learning spring, started reading and met with bean, ioc and di, I don’t quite understand how they work, I have ready-made spring programs I wrote myself, but how ioc worked there I don’t understand what the difference is normal class and what is it?) can you please explain with simple examples? I will be glad to all the answers, thank you very much

Answer 1

Here goes a quick explanation, as you have already made an application. This is in the context of a Spring app, because those 3 concepts apply differently depending on the framework/context you're in.

IOC is Inversion Of Control. It means that the application won't manage it's lifecycle/flow of control itself. The framework (Spring) will. So you just tell the framework how you want (some) elements of your app to work together.

DI is dependency injection. It's a specific kind of IOC where the framework will manage the dependencies that an object uses (you can call dependency: a service).

Bean is an object managed by the Framework.

Here is part of a applicationContext.xml:

 <beans>
  <bean id="foo" class="x.y.Foo">
      <constructor-arg ref="bar"/>
      <constructor-arg ref="baz"/>
  </bean>

  <bean id="bar" class="x.y.Bar"/>
  <bean id="baz" class="x.y.Baz"/>

</beans>

It's going to use this file to instanciate the classes Foo, Bar and Baz (IoC), and inject both instance of Bar and Baz into Foo (DI). Those instance are thus Beans, managed by Spring (IoC).

If you ever need a bean (a service), you'll have to ask the Framework (using something like context.getBean(foo)): you're not supposed to do a new Foo() anywhere. Spring keeps an internal map of all these instances.

Again, it's in the context of a Spring app. These definitions will slightly differ if you're on a JEE application, for instance.