Dependency injection

Interview Answer

1. Two types of IOC container:

   • BeanFactory : lazy loaded, XML
   • ApplicationContext : eager loaded, XML & annotation

2. IOC container life cycle:

   1. When Spring application starts, IOC container is instantiated(BeanFactory or Application Context)
   2. IOC container read the bean class from config files such as XML or annotations like @ComponentScan
   3. Create bean instance based on the Bean definition
   4. When a bean is required, inject the required dependencies.
   5. Post-Post-initialization Pre-destruction and bean call back will be executed, then is the bean destruction when spring application shut down, then the container will shut down.

3. Bean scopes:

   • singleton (default)
   • prototype
   • request
   • session
   • global-session

4. Three types of dependency injection: (Optional)

What is dependency injection?

Dependency injection is an implementation of IoC principle (invert the control of the object creation to the spring framework) to achieve loose coupling. It is the main advantage of Spring framework. It makes the code more extendable.

Two types of IOC containers

1. BeanFactory

   BeanFactory is lazy loaded, beans are loaded on-demand. BeanFactory only supports XML based configuration.

2. ApplicationContext

   ApplicationContext extends the BeanFactory interface, It is eager loaded, that is, beans are loaded on start-up. ApplicationContext supports both XML and annotation based configuration. In spring boot we generally use Application Context as the IOC container

IOC container life cycle

1. When Spring application starts, IOC container is instantiated(BeanFactory or Application Context)

2. IOC container read the bean class from config files such as XML or annotations like @ComponentScan

3. Create bean instance based on the Bean definition

4. When a bean is required, inject the required dependencies.

5. Post-Post-initialization Pre-destruction and bean call back will be executed, then is the bean destruction when spring application shut down, then the container will shut down.

Bean scopes

Five types of bean scopes

• singleton (default)

• prototype

• request

• session

• global-session

Singleton

Singleton means only one instance of the bean will be created for the entire application. There at most one instance exists in the container. This is the default scope for the spring beans. We can use @Scope to specify the scope of the bean.

Singleton is not thread-safe, we can create a thread-safe singleton.

Question: Given a singleton bean A and a prototype bean B, how do we inject A into B? What will be the behaviors of the beans? (not sure)

Each B will keep the same instance of the A, if A is modified, it might lead to some conflict with other B instances. If we need a different instance of singleton, we can create an instance manually.

Question: Given a singleton bean A and a prototype bean B, how do we inject B into A? What will be the behaviors of the beans? (not sure)

There will be only one instance of B, because A is singleton, it will only create one instance of B.

Three types of dependency injection

Use @Autowired to inject dependency, Setter injection is recommended if circular dependency exists. constructor injection is recommended if we want to inject immutable class.

1. constructor injection
2. Setter injection
   make setter injection mandatory: @Required, recently recommend constructor
3. Field injection

Circular dependency & Field injection

Constructor injection sometimes causes circular dependency, the field injection cannot be injected to the immutable class also only support in Spring framework, and it is tight coupling.

Dependency ambiguity

If there are multiple beans that implement the same interface, the container will not know which one to inject, We can use @Primary to determine which bean has higher priority, or use @Qualifier to specify an exact bean