Can an annotated Spring MVC Controller also be annotated with #Component/#Service type of annotations and be used both as a controller and as a bean?
Edit: placing more emphasis on the software design aspect, and updating the API link to SpringV3
As mentioned in other answers, this is not an ideal approach to Spring MVC, but nevertheless the controller will already be available for autowiring in your ApplicationContext.
It's already a Bean in your ApplicationContext, so you can auto-wire it by type. There's no need to add an #Component annotation.
From the Spring API Docs: "This annotation serves as a specialization of #Component, allowing for implementation classes to be autodetected through classpath scanning."
http://static.springsource.org/spring/docs/3.0.x/api/org/springframework/stereotype/Controller.html
The same holds true for #Service.
Although I've done it myself, I would not normally recommend this design approach.
If possible, refactor the required functionality into a separate bean, which can then be (auto-)wired into both the #Controller, and any other bean, as required.
If, as you have commented, you are 'cornered' into this decision (as I was, by previous design choices), then so be it.
HTH
It can but it shouldn't. A web controller should be an entry point, nothing else.
Any reusable logic it performs should be in a dedicated service layer, not in the controller itself
No, sounds like it's doing too much. One or the other, not both. I don't know if it's possible (I doubt it), but I'm sure it's not advisable.
I think that you should hear more about patterns like Front Controller, MVC, DAO and Multitier architecture and so on.
Related
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
Is there any way (existing framework) to configure DI in Java without having to add annotations to classes where they "don't belong"?
In my research, it looks like to accomplish constructor injection we need to add some annotation to the constructor, like this:
#Inject // or #Autowired for Spring Boot
public MyInjectedClass(IThing1 thing1, IThing2 thing2) { ... }
Spring Boot also suggests adding #Service to implementations in order to register them with DI for injection.
Adding these annotations brings DI into the injected class (MyInjectedClass and any controllers, service implementations, etc.). Those classes should have no concept of DI, they should just be given the things they ask for in the constructor and not care about how they got there.
I know you can use the #Configuration attribute in Spring Boot, but this still feels not very configurable or powerful. Is there any way to accomplish this using Java without annotations? E.g. like how SimpleInjector, AutoFac, etc. work in C#? (Configured via code, not XML or config files, and not via annotations.)
If not, I'm curious why such a different approach has been taken between Java and C# in regard to DI.
Update I've offended someone in the comments but that was not my intent at all. I am asking a legitimate question. Let me highlight my concern/question in another way.
Suppose I have the following packages/classes. I have marked each case where each type of DI will appear. By "appear" I mean these marked classes/packages have some relation to the DI framework being used.
[A]: annotation-based DI (3rd party package dependency)
[C]: configuration-based DI
data_access [A]
repositories
PersonRepository [A]
logic [A]
people
CreatePersonCommandHandler [A]
UpdatePersonCommandHandler [A]
services [A] [C]
controllers
PersonController [A] [C]
I hope this helps to illustrate my true question. The footprint of annotation-based DI extends into the far reaches of the stack, whereas configuration-based DI is only used in the outermost (entry) application, where the composition root lives.
This question already has answers here:
What's the difference between #Component, #Repository & #Service annotations in Spring?
(31 answers)
Closed 4 years ago.
When to use service or component in spring?
For example, is a module responsible for sending email or common business logic a "service" or a "component"?
and what's the difference?
Is a service able to call the other services?
Is there any transaction problem? or a service should call the components only?
Someone told me that a service should never call the other services and should only call the components instead, which means Controller->Service->Component->DAO, but I found many people share the concept of Controller->Service->DAO with no component.
Is there any system design criteria about this topic in Spring?
In order to "configure" Spring so it can provide you with the instances of the classes you need, you are supposed to tell Spring what objects are involved and how they are built. To do this you can use an xml configuration file or through annotations
In case you take the annotation approach (IMHO a much better and simpler one) you can use #Component to annotate the class. This is like telling Spring: "Hey! I want you to know that you may need an instance of this class. Maybe because I request it, maybe because something I requested needs it". So annotating a class with #Component just let Spring know that it exists
There are other annotations that do the same:
#Controller (and #RestController)
#Service
#Repository
They all inform Spring that the class is involved in the DI context. But they also have semantic meaning:
#Controller = #Component belonging to Presentation Layer
#Service = #Component belonging to Service/Use Case Layer
#Repository = #Component belonging to Persistence Layer
You can find more info in this question
Should a service be able to call the other services?
I don't see any problem with that. If any of your services requires to do some actions that are already performed by other you surely want to avoid code duplication. As long as you respect the architecture layers dependency (never going up) you'll be fine.
About this you can check this article about Clean Architecture
#Component is generic for other stereotypes.
So you can replace
#Repository, #Service, #Controller with #Component and nothing will change. But for better readability you should use #Repository, #Service, #Controller
How would you extract something prior 2.5 version from .xml config? It bothers me because if #Autowired is removed from my arsenal I would not really know what to do.
Say I want to use some DAO implementation.
In service class I usually write:
#Autowired
someDaoInterface generalDao;
Then I typically call
generalDao.someInterfaceMethod(someParam param);
How would I extract implementation from config in Spring 2.0 to use this method?
Is it as dumb as just: new ApplicationContext(pathToXml) and then use .getBean or there is other way?
Why do I ask for taking bean out from configuration file?
Because in Spring MVC how can you perform your logic without getting beans out from the application context.
If you have #Controller handler then you need to make calls to the service classes' methods? So they should be somehow retrieved from the context and the only way so far is using #Autowired? Then I would also want to populate Service classes as I stated in previous example with DAO classes and they also need to be retrieved from the application context, so I would be able to write logic for service classes themself. How would people do it in the past?
I see the #Autowired as the only mean of taking something out, not because it is convenient to wire automatically - I am perfectly ok with XML.
You still have option to wire it explicitely via property or constructor parameter. (Anyway, autowired is not going to work if there is ambiguity in your container )
Of course, you can use application context and getBean() in your java code, but it violates DI pattern and makes all the spring stuff useless. Purpose of DI is to decouple your business loginc from implementation details - it's not business logic it's how and where it dependencies come from. Dependencies are just there.
By using ApplicationContext.getBean() you are breaking this pattern, and introduce dependency to:
spring itself
your configuration names
After you done this, you can as well drop use of DI and spring because you just voided all the advandages DI is providing to you. (BTW, #Autowired also introduces dependency to spring, and violates DI pattern, it also implies that there is only one instance available)
Also, answer is: in ideal case there shall be no reference to spring in your code at all.
No imports, no annotations - just interfaces of collaborating entities.
I am new to Spring AOP, I want some help to
i have MultiActionController ,this is annotaion based.
i defined pointcut for method and advisor in spring configuration file.
i am calling that method from my controller.
Now my question is: How to define pointcut annotation in controller?
Please give me some hints.
Thanks in Advance.
The whole point of AOP is that you don't define the pointcuts in the target classes, but in external aspect classes (or in case of XML-based Spring AOP, in XML).
So if you have both your controller and the aspects in the same ApplicationContext (and the pointcuts match the controller methods), the advices should be applied automatically.
However, there is a caveat:
Spring AOP will only work the standard (JDK-proxy based) way if you use interfaces to back your controller.
If you do use interfaces consider this note (source):
Note
When using controller interfaces (e.g. for AOP proxying), make sure to consistently put all your mapping annotations - such as #RequestMapping and #SessionAttributes - on the controller interface rather than on the implementation class.
And if you don't, you must set proxy-target-class="true" as described in the section Proxying mechanisms.