I noticed multiple ways in which developers create an Apache Felix Service. Each of the attached snippets seem to work. Would need some help to understand, which syntax is best for which scenario
Sample 1: Service created without interface
Declaration of Service
D
#Component
#Service(ServiceViaClass.class)
public class ServiceViaClass{
}
Using service via #Reference annotation
private ServiceViaClass serviceViaClass;
Sample 2:Service implementing interface. No value attribute for #Service annotation
- Declaration of Service
#Component
#Service
public class ServiceViaInterfaceImpl implements ServiceViaInterface{
}
Using service via #Reference annotation
private ServiceViaInterface serviceViaInterface;
Sample 3: Service implementing interface with value attribute for #Service annotation
- Declaration of Service
#Component
#Service(ServiceViaInterface.class)
public class ServiceViaInterfaceImpl implements ServiceViaInterface{
}
Using service via #Reference annotation
private ServiceViaInterface serviceViaInterface;
A component implements an interface and publishes itself as a service under that interface, so that clients can find the component using only the interface.
Sample 1 — publishing a service using the concrete type of the component — is nearly always useless. The service can only be found using the concrete type, and if clients can see the concrete type then why not just instantiate it directly rather than get an instance from the service registry??
Sample 2 — publishing a service by implementing an interface and then just adding the #Service annotation — is what you should usually do. When you use #Service and the component directly implements an interface, the build tooling infers that your component wants to be published as a service under that interface.
Sample 3 has the exact same effect at runtime as sample 2, it's just a bit more explicit in the code. Some people prefer this because it's explicit, others (including me) dislike it because it is redundant.
Related
I am trying to implement a Spring Boot REST API but I was asked to use a interface as dependency but no impl, and I don't know how to achieve this. The way I implemented was to have service classes for my entities and there I would just call the repository in my methods. I would like an example of implementation like this.
I watched some youtube tutorials but they all used impl classes
Your controller should have a field of your interface type, with the injecting annotation (in spring it's #Autowired). The DI framework will do the heavy-lifting on startup and inject the correct implementation at runtime
#Controller
public class MyController {
#Autowired
private MyInterface myInterface;
....
}
For this to work, your framework needs to recognize the concrete class. In spring you can achieve this in multiple ways - scanning package paths, xml configuration files and more.
Check the spring documentation to see which way suits you best
i am new to spring
in my spring web project DAO and service they created the interface and implementing it created class. can we liminate interface.
The whole point of using interface is to create abstraction. When you call you DAO from Service, your service is not aware of the actual implementation of DAO/ cases in which you have multiple implementations of your DAO interface, your service is not aware of the actual impl being used.
With spring you will use - Autowire to inject the dependency. You will refer your Dependency using interface.
#Component/#Service
class ServiceImpl {
#Autowired
private DAOInterface dao;
// Rest of code
}
Spring knows - to create the instance of this ServiceImpl it needs to inject a concrete impl of DAOInterface type. In case you have concrete implementation, it does so. In case you have multiple impls - you need to define by bean name which implementation you need using #Qualifier.
Interface is kind of a contract for your impl classes. Other than abstracting the actual impl, it helps you decoupling the layers by not directly having a dependency on the concrete classes. This is a good design pattern
If a class implements ServiceControl, ServiceSuspend, ServiceShutdown, ServiceCustomCommand, ServicePowerEvent and/or ServiceSessionChange, do those actions still have to be wired up through HostConfigurators WhenXXX methods?
If the service class implements the interfaces, such as ServiceControl, then there is no need to configure the WhenStarted and other methods since Topshelf will use the interface methods.
From the docs: http://topshelf.readthedocs.io/en/latest/configuration/config_api.html#simple-service
In spring why the interface is injected rather than the class implementing the interface?
In fact even if you use interface a class is injected. Spring looks for a bean - class which implements the interface.
In some cases like e.g. Spring Data dynamic proxy is created for the interfaces which in fact a class to listen interface's methods calls and do some logic on the calls.
If you set a breakpoint and check what is really injected you will find a class (either your class or some Proxy object)
I am reading the Spring Hibernate CRUD tutorial from this url
http://viralpatel.net/blogs/spring3-mvc-hibernate-maven-tutorial-eclipse-example/
Please can anyone tell me why in ContactController.java, ContactService interface is autowired instead of the class ContactServiceImpl.
Similarly in ContactServiceImpl ContactDAO interface is injected. Shouldn't we supposed to inject class instead of an interface?
When your code is dependent on interface and its implementation is injected by Spring, your code becomes decoupled with the implementation. This has an advantage that now you can swap in a different implementation without having to change the code that makes use of interface.
Spring is smart. It will find the implementation of the interface and inject it appropriately (or a proxy thereof.)
You should be programming to interfaces, not implementations.