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.
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
It says here that
I would not recommend to use InitializingBean and DisposableBean
interface, because it will tight coupled your code to Spring
Does it make sense? I thought this would be just the opposite to tight-coupling.
Here the author means that if you let your application classes implement InitializingBean and DisposableBean interfaces (that are spring specific interfaces), then you are coupling your code with spring.
In future if spring renames these interfaces (unlikely though), or you stop using spring you will have to update your class code.
Instead if you use init-method and destroy-method attributes in your bean config, your class is independent of spring, i.e. there is no depedency of your class on spring specific classes.
Hope it helps.
I think the idea here is to not create any dependency on your code to spring annotations, see
import org.springframework.beans.factory.DisposableBean;
import org.springframework.beans.factory.InitializingBean;
When you explicitly declare this kind of dependency, you're coupling your code to spring jars.
In this other example - http://www.mkyong.com/spring/spring-init-method-and-destroy-method-example/ - it shows how you could use convention methods defined in the XML. No spring imports.
As in the title above, I am confused about pros cons between injecting applicationContext by directly #Autowired annnotation or implementing ApplicationContextAware interface in a singleton spring bean.
Which one do you prefer in which cases and why? Thanks.
Actually, both are bad. Both of them tie your application to the Spring framework, thus inverting the whole inversion-of-control concept. In an ideal world, your application should not be aware of being managed by an ApplicationContext at all.
Once you have chosen to violate this principle, it doesn't really matter how you do it. ApplicationContextAware is the legacy version that has been around at least since Version 2.0. #Autowired is a newer mechanism but they work in pretty much the same way. I'd probably go with ApplicationContextAware, because it semantically makes clear what it is about.
As #Sean Patrick Floyd says, the need of ApplicationContext is often due to a bad design. But sometimes you have no other option. In those cases I prefer the use of #Autowired because is the way I inject all other properties. So, if I use #Autowired for injecting MyRepository, why can't I use it for ApplicationContext or any other Spring bean?
I use Spring interfaces only for those things I can't do with annotations, for example BeanNameAware.
If you need to get a prototype in a singleton then you can use method injection. Basically, you create an abstract method that returns the object you need and spring will return the prototype everytime you call that method. You define the "lookup-method" in your spring config. Here are some links:
http://docs.spring.io/spring/docs/1.2.9/reference/beans.html#beans-factory-method-injection
http://java.dzone.com/articles/method-injection-spring
Since you are not extending any of the spring classes your application is always separated from the framework. Most of the cases you will not wanted to inject the ApplicationContext as it, but will need to inject the beans defined in the ApplicationContext.
The best case is always to stick to the bare minimum, until and unless you have any specific requirement and this is very simple with spring.
So either,
Annotate your beans and scan them in application context, then use #Autowire to wire them up.
Use application context to wire your bean expediencies(old xml style configs). You can use #Autowire with this approach also.
When you want to control the bean life cycle, you can read the API and customize it, but most of the time these general settings will do the job.
Here are some examples.
Spring Auto-Wiring Beans with #Autowired annotation
Spring Auto-Wiring Beans XML Style
Spring IoC container API Docs
There is no need to use ApplicationContext at all.
ObjectFactory
If you need to use prototype scoped beans in a singleton bean, inject an org.springframework.beans.factory.ObjectFactory.
For example using constructor injection:
#Service
class MyClass {
private ObjectFactory<MyDependency> myDependencyFactory;
public MyClass(ObjectFactory<MyDependency> prototypeFactory) {
myDependencyFactory = prototypeFactory;
}
}
Why
Now what's the benefit over using ApplicationContext ?
You can substitute this dependency (e.g. in a test) by simply passing a lambda (since ObjectFactory is a #FunctionalInterface) that returns a stubbed version of it.
While it is possible to stub the ApplicationContext, it is not clear in that case which beans will be looked up and need to be stubbed.
Does it makes sense in Spring to use #Autowired to wire directly to a concrete class and not to an interface (and make use of 'by type' autowiring)
If a class doesn't implements an interface wouldn't it be better to instantiate it via constructor or a factory (keeping things simple); rather than make it a Spring bean just for the heck of it.
Does it makes sense in Spring to use #Autowired to wire directly to a concrete class and not to an interface
Sure. The practice of autowiring is independent of what you're autowiring. It'll work with classes just as well as interfaces.
However, whether or not it's a good idea is debatable, although this is a more general question of whether you should always introduce an interface for a given class, rather than talk directly to the class type. The benefits include easier unit-testing and a cleaner design, at the expense of code clutter.
There's another good reason to autowire interface types rather than class types, which is that if Spring needs to generate a proxy object around the bean before injecting it, then if the bean's class defines any interfaces, then the proxy will implement those interfaces, and will not be type-compatible with the bean class itself. If you then try and autowire that bean by class type, it will fail. The easiest way to avoid this annoying scenario is to always autowire by interface type, that way it will lways work as you expect.
and make use of 'by type' autowiring
If you mean container-level byType autowiring, then you don't want to do that. It's the old Spring 1.x style of autowiring, and it's highly inflexibile (see limitations of autowiring).
Stick with #Autowired, it's much more flexible and easier to control.
If a class doesn't implements an interface wouldn't it be better to instantiate it via constructor or a factory (keeping things simple); rather than make it a Spring bean just for the heck of it.
The two questions are completely separate. An object should be made a Spring bean if you need Spring to control its dependencies and lifecycle, regardless of whether or not it implements interfaces. If you find that the object has no dependencies, and no meaningful interface, then perhaps there is no reason to make it a bean.
You will have to decide if hardwiring this dependency into other classes is ok. For instance, how many different classes are likely to require this dependency? If the answer is many then you will be creating many instances of this class where only one is required.
Also, what dependancies does this concrete class have? You will have to configure those inside the class that depends on it.
The object of dependency injection is to reduce the dependencies between classes and make your code loosely coupled.