Dependency Injection - Spring

#java #spring

Basics

Spring has lot of modules
start.spring.io lists many spring modules which you can configure
spring boot is useful to build RESTfull web services. It makes developing spring applications easy by auto configuring default components. It is opinionated framework
fat jar -> also includes tomcat. better solution if you want to deploy on cloud platforms

Dependency Injection

Assume mail sender example, where we can have mailSender of type SMTP, POPM or MockMail.
We can have an interface MailSender which defines the common functionality and we implement the interface in SMTP, POPM or Mock class.
*Problem: object creation will be like: *
- private MailSender = new SMTPMailSender()
- private MailSender = new POPMMailSender()
- private MailSender = new MockMailSender()
- we need to hardcode the type while creating object
- can we automate this?
- Add @Component annotation to any class which implements MailSender
- Add @Autowired annotation when creating object
- how does it work:
  - When application starts all the beans gets setup
    - Beans: annotations at top level, whose object will be created and stored in Application Context
    - Beans examples: @Component, @RestController
  - When application is running and when we create objects variables for which we have annotations like @Autowired. Objects which were already created from application context are assigned to current variables.
  - i.e those objects from application context are injected into these variables and hence dependency injection
@Autowired also works on methods(setters, constructors, getters, any methods..)
Multiple beans problem: when we create two classes from same interface and add @component annotation to both classes, and create object using interface using @autowired annotation, spring will be confused to choose between beans
- one solution is to use camel-case variable names while creating objects which resemble the name of the class you want (ex: smtpMailSender)
- you can also add name param to @component -> @component("mockMail"), and create object using the name mockMail
- use extra @primary annotation, the bean which has this will get preference
- you can also use @qualifier annotation to class, and also use @qualifier in setter or constructor, ...etc
Prefer constructor injection
@resource and @inject are similar to @autowired, but @autowired is very powerful, so use @autowired
Specializations of @component annotations
- @Controller
- @Service
- @Repository
- @Configuration
- @SpringBootApplication(scanBasePackageClasses = {default.class, abc.class}) - scans through the package and puts any dependency injection components into application context
- @RestController
- ..... many others
what if you import a dependency and you have to create object from there, you cannot add @component annotation dependency package
- Create a new config class, which has methods to return objects from the external dependency
- the new config class must have @Configuration annotation
- and methods must have @Bean configuration
- names of the methods can be used for object variable names
use @Value annotation to access variables from .properties or .yaml files
an external config file takes more preference over internal config(application.properties)
**Profiles: **A spring application can be configure to behave differently in different environments using profiles
- ex environments: dev, test, prod
- provide in application.properties:
  - spring.profiles.active: book, dev
- you can also use particular beans in particular environments!
  - just use @profile("envName"), can accepts bool, !envName
- @Conditional annotation, include or exclude beans based on arbitrary conditions
  - @ConditionalOnClass, OnProperty, OnMissingBean ..etc
  - configure bean based on presence of application property => @ConditionalOnProperty
In @Configuration classes, when we try to create same bean again from some method, spring still returns the cached bean only.
- Caching does not happen when we use @Component annotation
Always use @Configuration Classes