In spring when to use new keyword and autowired keyword - spring

Question

i am new to spring. i am confused when to use autowired keyword and new keyword.while using the model class in spring we use new keyword rather than Autowired keyword

Answer 1

The new keyword allocated new memory space in the heap for the object. Then the constructor will initialize the state of the object (properties of the object).
The #Autowired asks Spring framework to inject the requests bean by type.
Now, you have built an application which has a controller, service and dao layer. These layers basically doesn't hold state of the application/business data. They wrap functionalities. Business logic/Logic to talk to db , etc. So from Service when you want to call a save(entity) in dao layer you will do -> new DAO().save(entity)...... This is a wastage of memory as the you creating new Object everytime just to use the functionality. An efficient way to do so is -> Create singleton Service instance -> one instance for entire application and then inject them in other places to use the functionality.
Spring handles that for you.. #Service/#REstController/#Component basically tells Spring to create an instance of that class for you - default scope is Singleton. And wherever you mention -> #Autowired, it injects that bean (by type) to the requested class for you to use the functionality.
This is basically inversion of control. Way I keep this is mind is handling the responsibility/control of creating instances and injecting them in places you need is inversed from you to the framework. And also you removing the strong coupling by injecting by type (interface generally) and spring finds the impl of the interface and injects it whereevr you mentioned #Autowired. In case there are multiple impls you need to name each impl (or default name is the class name with firstLetter small case - please confirm this once cause I dont remember correctly) and use #Qualified(name) to mention to spring which impl you want to inject.
Now your model/entity -> These are basically Java beans -> having properties and accesor methods to access those properties. These will hold state of your application. So every time a new User registers you need a new User object to pass around the layers to perform some logic and then save to the DB. So you will create a new Object with new keyword.

Answer 2

The typical use of #Autowire is to automatically fill a property, when initializing a bean, with a singleton dependency. If it is really a dependency that should be initialized once and reused.

Answer 3

to answer your question how to check scope of bean, there is ConfigurableApplicationContext which sub class of ApplicationContext, which again is the subclass of BeanFactory. There is a method on configurableApplicationContext::getBeanFactory which returns you the BeanFactory. You can invoke various methods on it to check the scope of various beans.
Here is the code snippet I wrote:
#Component
public class ScopeTestService {
#Autowired
private ConfigurableApplicationContext applicationContext;
public void testScope() {
String scope = applicationContext.getBeanFactory().getBeanDefinition("scopeTestService").getScope();
System.out.println(scope);
}
}
You can person various permutation combinations to check. You can change using #Scope("scope type") annotation along with #Component.
The result for the above code:
2020-01-29 02:24:56.125 INFO 57500 --- [ main] org.hibernate.dialect.Dialect : HHH000400: Using dialect: org.hibernate.dialect.MySQL8Dialect
2020-01-29 02:24:57.604 INFO 57500 --- [ main] j.LocalContainerEntityManagerFactoryBean : Initialized JPA EntityManagerFactory for persistence unit 'default'
2020-01-29 02:24:58.066 INFO 57500 --- [ main] c.example.demo.JpaTutorialsApplication : Started JpaTutorialsApplication in 7.023 seconds (JVM running for 7.837)
singleton
2020-01-29 02:24:58.077 INFO 57500 --- [ Thread-2] j.LocalContainerEntityManagerFactoryBean : Closing JPA EntityManagerFactory for persistence unit 'default'
2020-01-29 02:24:58.082 INFO 57500 --- [ Thread-2] com.zaxxer.hikari.HikariDataSource : HikariPool-1 - Shutdown initiated...
2020-01-29 02:24:58.098 INFO 57500 --- [ Thread-2] com.zaxxer.hikari.HikariDataSource : HikariPool-1 - Shutdown completed.