We’re using Scala with Spring Boot, so creating Singleton objects is as simple as using the Scala ‘object’ keyword instead of the ‘class’ keyword, and filling it with whatever functions are needed. So far, it seems that Singleton objects can do the same thing as #Service classes, and also has the extra benefit of avoiding circular dependency errors during dependency injection. Is there any reason to use #Service classes over Singletons?
Annotating classes with #Service means that a Spring-managed bean will be created for such classes. By default, they are also singletons. However, being Spring-managed beans make it possible to use Spring-related features, such as:
Dependency injection (being this bean injected in other Spring-managed beans and also inject other Spring-managed beans into this one);
Spring AOP (allowing, for example, the usage of #Transactional);
Application context changes awareness (you could make your #Service implement ApplicationListener so that you can do something based on Application related events);
etc...
Related
Reading some source code and I stumbled upon this spring bean
#Service
public class ValidationService {...}
Which is instantiated manually using the new() operator in many places
ex:
var validationService=new ValidationService();
Now I am thinking to refactor all of these instantiations and replace it with proper spring injection.
It made me think about what would happen if we instantiated a bean using the new() operator. Will it create a new instance of the bean or will it just retrieve the spring managed instance ?
When the object is instantiated manually, it acts like a normal Java object.
Spring is not involved, so all the "spring features" (Inversion of control, dependency injection, etc..) not available.
How does constructor method dependency injection work if I'm not even initializing the class anywhere or passing in arguments. Does Spring take care of this somewhere for me? Would really like to know more about this, if anyone wouldn't mind sharing/teaching me. How does this work on the heap? Would there be a reference to the class being constructed and the dependency so I would have two instances of different objects or just one? I found this answer How does constructor dependency injection work?, but it doesn't really answer my question in great detail.
Example:
#Controller
#RequestMapping
public class SampleController{
private final SomeRepository someRepository;
pulbic SampleController(SomeRepository someRepository) throws Exception{
// will someRepository ever be null?
if(someRepository == null)
throw new Exception("I told you it wouldn't work!");
this.someRepository = someRepository;
}
Answering your question: yes. Spring manages all the Beans on your behalf. You can declare a Bean in multiple ways:
XML (the old way);
Using #Bean (but in this case you would actually instantiate the Bean);
Using #Component, #Service, #Repository or #Controller (the later 3 are basically specializations of the first).
In your case, you have annotated SampleController with #Controller, so now Spring knows that it will need to provide this Bean for you by managing it in the Spring IoC container. However, so that Spring can create the Bean it will also understand that it needs another Bean to be available, in your case SomeRepository. If such a Bean already exists in Spring IoC Container then Spring provides it to your SampleController constructor. For this to work, SomeRepository must also be a Spring-managed Bean, and thus defined by one of the previous three ways.
It is worth mentioning that if for example you would have multiple SomeRepository Beans in your application (imagine an interface having multiple implementations, being each one a different Bean), you would need to "tell" Spring which one you want to be used in SampleController, otherwise you would get an error. There are multiple ways to do this:
Using #Qualifier annotations;
Using #Primary on one of the implementations to mark it as the "default" in case you don't explicitly "say" which one you want.
You can check examples in https://www.baeldung.com/spring-qualifier-annotation.
This is a very high-level description, but you can find some good articles online about this or even check the reference documentation.
I'm just starting to pick up Spring, coming from a purely Java EE background, so stuff like IOC and dependency injection is all kinda new to me. As per the Spring docs, I understand that for any class to call on instances of its dependencies, I can choose to autowire those dependencies as a form of dependency injection. And that this is actually aside from the usual instance declaration we always do in java like below: Animal animal1 = new Animal()
In my one of my little test Spring boot project services, I noticed that I end up doing both dependency injection and normal class instantiation. I use JPA Repository to craft my repo layer, and autowire the repo classes so I can use them like eg.
#Autowired
customerAccountRepo
This is all fine. I also have a DAO customerMembershipValidity whose attributes are other declared POJOs and has some public helper functions within to set the DAO's attributes. In order to use this DAO, however, I find myself creating multiple instances of the DAO the traditional way, instantiating
CustomerMembershipValidity customerMembershipValidity1 = new CustomerMembershipValidity() multiple times throughout the service to call on public helper methods like customerMembershipValidity.setNewExpiry(). I didnt think there would be a need to autowire this since I'm dealing with a DAO or POJO, not another service... or should I?
For now, the code seems functional when I do my unit-testing, but I would like to know if this would harm the overall longevity and sensibility of the code, or if it's forseeable to end up breaking in E2E when I run the Spring Boot application.
I have a few recommendations:
DAO should be a singleton Spring Bean that you inject into service bean(s) that need it. Make sure there is no mutable shared state in your DAO - just database operations.
The DAOs I write might use mapper functions to map ResultSet to objects/collections, but I don't see the need for helpers.
Spring preference is to use constructor injection. You should not use setter or autowired attribute to inject dependencies.
If you use new, that means the object is not under Spring's control. It's appropriate to call new for objects in method scope, but not a Spring Bean like a DAO.
Unit testing and production are two different things. I prefer to leave Spring out of unit testing. I call new for JUnit tests, but not production code. Once I've tested the DAO, I can use mocks for services that depend on it.
You're smart to take up Spring and leave Java EE behind. It's a twenty year old dead standard.
From a software design perspective, when should we use #Component instead of a traditional Java class (that needs to be explicitly instantiated by 'new')? For example, if we need to create a class that is one of the following patterns:
Adapter
Bridge
Façade
Strategy
Translator
Should the class have the #Component annotation (or any Spring derivative annotation such as #Repository/#Controller/#Service)?
Spring applies the Inversion of Control principle, which drills down to that the framework handles stuff for you, so you don't have to worry about it.
By using #Component on the class you let Spring create a bean for you.
This way Spring can, for example, inject this bean on runtime when you need it. (For example by Autowiring your constructor).
It is up to you to decide if you want to make use of this functionality for your class. A facade for example could very well be a Spring component, this way you could possibly inject an API implementation that is exposed via a facade on runtime, without the need to think about the dependency injection implementation.
I would not recommend using this annotation on a DTO or model class for example. These classes mostly consist of data and don't fit the need to be managed by Spring.
Other interesting related questions that can help you decide when to create a component:
What's the difference between #Component, #Repository & #Service annotations in Spring?
Spring: #Component versus #Bean
I'm studying spring beans and came across #Lookup, it says:
If we happen to decide to have a prototype Spring bean, then we are
almost immediately faced with the problem of how will our singleton
Spring beans access these prototype Spring beans?
hmm, I don't get it, because when I studied scope=prototype it says:
4.4.2 The prototype scope
The non-singleton, prototype scope of bean deployment results in the creation of a new bean instance every time a
request for that specific bean is made
so it seems i misinterpreted the words:
a request for that specific bean is made
actually programming in spring framework every line of the code is inside of some bean (i.e. #controller, #Service, etc), isn't it?
And almost all of them are singletons, isn't it?
So if I need prototype I just make scope=prototype and almost everytime it's injected to another bean (i.e. #controller, #Service, etc) isn't it?
So please give a real world scenarios, 1) when one should use #Lookup and 2) when it's not needed
Ok for the 1) the scenario:
#Component
#Scope("prototype")
public class SchoolNotification {
// ... prototype-scoped state
}
#Component
public class StudentServices {
// ... member variables, etc.
#Lookup
public SchoolNotification getNotification() {
return null;
}
// ... getters and setters
}
Please, show me scenario for the 2) case, and explain please the difference
Thank u
The implicit Bean scope in Spring is Singleton.
That means for a JVM instance, only a single instance of a Bean exists in memory (theoretically).
When you #Autowire a Prototype-scoped Bean inside a Singleton-scoped Bean, that Prototype one becomes a sort-of-singleton. Just think about it; a Singleton gets created, its injectable fields get Autowired, and that is it, the instance lives forever along with all its fields (keep in mind those Prototype-scoped fields are "pure" instances, they're not proxied).
#Lookup
is a proxy-driven annotation. What that means is Spring will extend your class using JDK proxies or CGLIB proxies, and it will override/implement the #Lookup-annotated method, providing its own version which uses a BeanFactory#getBean each time it is invoked.
The documentation is clear on this point
An annotation that indicates 'lookup' methods, to be overridden by the
container to redirect them back to the BeanFactory for a getBean call.
Thus, that means a fresh Bean instance is returned every time.
Just for your knowledge, another approach for working with Prototype-scoped Beans inside "other"-scoped Beans is using ProxyFactoryBean. The only difference is that the proxy is created at configuration-time, and then made available for direct #Autowireing, thus not requiring the definition of a #Lookup method, which sometimes is not wanted (usually by folks that are obsessed with clean code, like me).