Dependency injection

Question 1

What is dependency injection?

Answer 1

Dependency Injection is a design pattern which Spring implemented through it’s IoC container to create and implements object dependency using configuration files. The objects can be injected through constructor or setter injection.

Question 2

What are the main advantages of dependency injection?

Answer 2

Main advantage:
Have a loosely coupled system:
More testable code
It promotes re-usability 
It promotes more readable code ( thanks to config files )

Question 3

What is component scanning used for ?

Answer 3

Spring xml setting or annotation that define which are the packages where should look over classes annotated with certain stereotype

Question 4

Which are the default stereotype used in component scanning ?

Answer 4

@component ( first introduced in spring )
@service
@repository
@controller

Any annotation which extend these is also used

Question 5

What are meta-annotations?

Answer 5

Are used to annotate methods and properties

Question 6

How to use constructor or setter injection in xml configuration ?

Answer 6

The properties of a xml element can be injected or through

constructor injection
Or through the setter injection .

Question 7

How to create an application context for xml configuration ?

Answer 7

ClassPathXmlApplicationContext
FileSystemXmlApplicationContext
XmlWebApplicationContext

Question 8

How to create an application context for java annotated configuration

Answer 8

AnnotationApplicationContext

Question 9

What is @autowired used for?

Answer 9

It inject the beans depending on where is appearing. It can appear on a class field or a class method or on the constructor

Question 10

What is an interface?

Answer 10

Is a contract for defining the main methods and constants for its implementers, developers can code different kind of implementations

Question 11

Which are the main advantages of an interface?

Answer 11

It makes code more flexible

It supports selectable multiple inheritance

Users can call methods without knowing how the client implements the main logic

Question 12

What is meant by “container”?

Answer 12

Spring IoC container is the core of the framework for managing , wiring and configuring beans

Question 13

How do you create a container ?

Answer 13

ApplicationContext is used to create a container.

ApplicationContext extends BeanFactory

Question 14

Which packages provide the main core components for a Spring container?

Answer 14

org. springframework.beans

org. springframework.context

Question 15

Who is managing the lifecycle of beans?

Answer 15

The container is the lifecycle manager and is taking care of initialize , use and destroy beans

Question 16

Which are the steps of initialize a bean?

Answer 16

• beans are loaded
• post process beans definition ( load into beanfactory )
• each bean is created
• setters are called
• bean post processor ( before init )
• init method is called
• bean post processor ( after init )
• bean is ready for use

Question 17

What is the purpose of BeanFactoryPostProcessor?

Answer 17

This class can be used to modify the configuration metadata before instantiate a bean

Question 18

Why is not ok to use BeanFactory getBean method when implementing BeanFactoryPostProcessor?

Answer 18

Because is violating the normal bean lifecycle

Question 19

BeanPostProcessor before init when is called?

Answer 19

Every registered BeanPostProcessor is called in order before @PostConstruct ( init method ) method if one is returning null the process stops there and nothing else is called

Question 20

BeanPostProcessor after init when is called?

Answer 20

Every registered implementation is called in order starting after the call of @PostConstruct method ( init method ) and if one return null the process stops there

Question 21

Which are the possible scopes for a bean?

Answer 21

• singleton
• prototype
• session
• request
• global session
• application

Question 22

Which is the default scope for a bean?

Answer 22

If nothing is specified the default scope is singleton

Question 23

Define the singleton scope

Answer 23

A single instance of the object will be created inside framework

Question 24

Define the prototype scope

Answer 24

A new instance of the object will be created each time the bean is referenced

Question 25

Define the session scope

Answer 25

It is used in web environment and a new instance will be created per user web HTTP session

Question 26

Define the request scope

Answer 26

It is used in web environment and a new instance will be created per each request

Question 27

Define the global session scope

Answer 27

Same as session and used in web applications, typically used in portlet context

Question 28

Define the application scope

Answer 28

It is used in web applications and Scopes a single bean definition in a ServletContext

Question 29

Which are the short versions of constructor injection in xml configuration ?

Answer 29

For constructor injection we have

• c:[name_property]-ref for bean reference
• c:[name_property] for scalar values
• c:_[pos] for scalar value (ex: c:_0)
• c:_[pos]-ref for bean references

Question 30

How to import the short version of constructor injection?

Answer 30

In xml beans element an xmlns:c attribute should be created with value http://www.springframework.org/schema/c

Question 31

Which are the short versions of setter injection in xml configuration ?

Answer 31

For setter injection we have

• p:[name_property]-ref for bean references
• p:[name_property] for scalar values

Question 32

How to import the short version of setter injection?

Answer 32

In xml beans element an xmlns:p attribute should be created with value http://www.springframework.org/schema/p

Question 33

What is an initialization method ?

Answer 33

Is a method which is called after all the properties are injected and the bean initialized

Question 34

How you can declare an initialization method for a bean in Spring?

Answer 34

Through the xml configuration attribute init-method in the bean element

Through the @PostConstruct annotation in the bean class if annotations are enabled or AnnotationApplicationContext is initialized

Through the implementation of the InitializingBean interface

Question 35

What is a destroy method?

Answer 35

A destroy method is used to release the resources before the class is destroyed

Question 36

How a destroy method is declared in Spring?

Answer 36

• by using the @PreDestroy annotation on method if annotations are enabled
• by using the destroy-method attribute on the bean element
• by implementing the DisposableBean interface

Question 37

When the destroy method is called ?

Answer 37

After the context.close method is called and beans and resources are released

Question 38

What is happening if I inject 1977/10/58 in a date property ?

Answer 38

Spring will automatically convert the date and will add the days to the next month giving 27 nov 1977 like a date

Question 39

Are beans lazy or eagerly instantiated by default ?

Answer 39

All the beans are eagerly instantiated by default. This consume more time at the startup but ensure that once all the beans are loaded no errors will appear later.

Question 40

How you can set a bean to load in lazy mode?

Answer 40

In order to load a bean in a lazy init way the attribute on the bean lazy-init should be true. If this attribute is set to default than the default-lazy-init attribute will be checked.

Question 41

How does the @Qualifier annotation complement the use of @Autowired?

Answer 41

is helping @Autowired by configuring the id of the bean that should be auto wired if more than one of the same type exist in the configuration file

Question 42

How can a list or set be injected ?

Answer 42

In order to inject a list or a set the or element should be used. Inside the list or set than ref or bean or null element should be used

Question 43

How do you inject a map in a setter property ?

Answer 43

In order to inject the map the right element is for each element of the map exist the element or if the element is a reference

Question 44

How to use Spring until for map and lists?

Answer 44

First of all xmlns:util should be created and spring-until.xsd imported after the follow code can be written