I have seen many examples of Spring boot wherein we declare an interface which extends crudrepository and declare the CRUUD methods. Post which in Service CLass we autowire the interface and invoke the crud methods from Service class.
Since we are not implementing this interface(which extends CrudRepo in example PersonRepo ) anywhere ,how come spring instantaties it ?
Ex :
interface PersonRepo extends CRUDRepository {
// method declaration
}
#Service
class PersonService{
#Autowire
PersonRepo perrepo;
void insert(){
perrepo.methodname();
}
}
Spring introduces dynamic proxy.
In fact it scans package and get all interfaces which extend Crud Repository interface. For each of them a Dynamic Proxy instance is created. On each call the proxy checks which method is called and uses appropriate annotation. The proxies are used as beans so can be autowired.
Related
I've looked through various documents and questions about it, but I haven't been able to find a clear answer.
I don't know how an interface that extends JpaRepository can be registered as a bean, and why it doesn't need the #Repository annotation.
In Spring, an interface cannot register a bean with that type without an implementation.
So I tried experimenting like JpaRepository myself, but it didn't work.
// JpaRepository
#NoRepositoryBean
public interface WackRepository {
}
// SimpleJpaRepository
#Repository
public class WackRepositoryImpl implements WackRepository {
}
public interface HelloRepository extends WackRepository {
}
#RestController
#RequiredArgsConstructor
public class HelloController {
private final HelloRepository helloRepository;
}
Parameter 0 of constructor in com.example.demo.HelloController required a bean of type 'com.example.demo.HelloRepository' that could not be found
HelloController, of course, has no implementation, so it is not registered as a bean and throws an exception.
Spring Data detects extensions of the Repository Interface.
In Spring Boot the interfaces extending Repository are found automatically without Spring Boot or if the interfaces are not below the SpringBootApplication in the package hierarchy you have to configure the packages:
#EnableJpaRepositories("com.acme.repositories")
Source: https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#repositories.create-instances
I am using Spring Boot with Spring Data JPA, there is only one #SpringBootApplication. And I have also a repository classes, for example:
package com.so;
public interface SORepository {
//methods
}
And impl
#Repository("qualifier")
#Transactional(readOnly = true)
public class SORepositoryImpl implements SORepository {
//methods
}
The proplem is, when I start the application, I get following error:
Parameter 0 of constructor in com.so.SomeComponent required a single bean, but 2 were found:
- qualifier: defined in file [path\to\SORepositoryImpl.class]
- soRepositoryImpl: defined in file [path\to\SORepositoryImpl.class]
So, as you see, somehow 2 beans of one repository class are created. How can I fix this?
You can use Spring Data JPA methods having created Proxy element and than inject it into public class SORepositoryImpl:
public interface Proxy() extends JpaRepository<Element, Long>{
Element saveElement (Element element); //ore other methods if you want}
And than:
#Repository
#Transactional(readOnly = true)
public class SORepositoryImpl implements SORepository {
#Autowired
private Proxy proxy;
//end realisation of methods from interface SORepository
}
Try taking the #Repository annotation off the SORepositoryImpl class
e.g.
#Transactional(readOnly = true)
public class SORepositoryImpl implements SORepository {
//methods
}
The error message is implying you have two beans, one named "qualifier" and one named "soRepositoryImpl", which is probably in a Config class.
I guess you should share your SomeComponent class supposing you have no extra configuration class/xml. My take is that you are injecting as 'soRepositoryImpl' there where you have defined as 'qualifier'. Having two options them. I would say to just remove the annotation parameter 'qualifier' and it should work.
Moreover, unless you want do specify an custom DAO implementation you can avoid #Repository at all (That's an annotation you use to make it injectable for your services). You can just create an interface extending Spring interface and define methods for queries.
For example:
public interface PersonRepository extends Repository<User, Long> {
List<Person> findByEmailAddressAndLastname(EmailAddress emailAddress, String lastname);
Then you can just inject it in your services/controller directly.
private final PersonRepository personRepository;
public PersonController(final PersonRepository personRepository) {
this.personRepository = personRepository;
}
check samples:
https://spring.io/guides/gs/accessing-data-jpa/
http://docs.spring.io/spring-data/data-commons/docs/1.6.1.RELEASE/reference/html/repositories.html
OK, I've found the issue.
I just couldn't understand, how Spring creates the second bean (soRepositoryImpl), because I've never told it, neither explicitly nor in config classes. But I figured out that the second bean us created during the instantiation of my another SORepository (which is in the different package com.another and which extends JpaRepository).
So, when Spring tries to resolve all dependencies of com.another.SORepository it somehow finds my com.so.SORepositoryImpl (which has nothing familiar with com.another.SORepository - not extending\implementing, not jpa stuff, only similar names!).
Well it seems like a Spring bug to me, because it doesn't check the real inheritance of dependent classes of repositories, only name + Impl (even in different package) suits for him.
The only thing that I should do is to rename `com.so.SORepositoryImpl and that it, no 2 beans anymore.
Thanks everyone for answers!
lets say I have code like this:
#Repository
public class Foo{
}
#Service
public class Boo{
#Autowired
private Foo foo;
}
so now what here are we calling bean? Bean is the object of Foo type of refrence "foo" BUT are Boo class annotated as Service and Foo as Repository ALSO beans? Ihve been using spring for a while now but this basic question makes me feel bad for not knowing...
In the context of Spring, A bean is a spring managed object. Here spring managed means an object created, initialised, managed, destroyed by Spring IoC container.
Whenever we mark a class with #Component, Spring IOC container will create object for your class and manage it, Whenever we can simply get it from ApplicationContext, or access it using #Autowired/#Resource/#Inject annotations
We can also use #Controller, #Repository, #Service, #ControllerAdvice, #Configuration,#Aspect in place of #Component to tell more specifically that our class is a service or a repository or an aspect etc.
We can also use #Bean annotation to create a bean from method return value
#Configuration
public class SolrConfig {
#Value("${spring.data.solr.host}") String solrUrl;
#Bean
public SolrServer solrServer() {
return new HttpSolrServer(solrUrl);
}
#Bean(name = "solrTemplate")
public SolrTemplate solrTemplate() {
return new SolrTemplate(new HttpSolrServer(solrUrl), RULE_ENGINE_CORE);
}
}
All of your application components (#Component, #Service, #Repository, #Controller etc.) will be automatically registered as Spring Beans
http://docs.spring.io/autorepo/docs/spring-boot/current/reference/html/using-boot-spring-beans-and-dependency-injection.html
Defining Beans can be thought of as replacing the keyword new.
Further information can be found here which might be helpful for understanding Beans in Spring.
I'm developing an application using Spring. I need to use the #Service annotation. I have ServiceI and ServiceImpl such that ServiceImpl implements ServiceI. I'm confused here as to where should I keep the #Service annotation.
Should I annotate the interface or the implementation with #Service? What are the differences between these two approaches?
I never put #Component (or #Service, ...) at an interface, because this make the interface useless. Let me explain why.
claim 1: If you have an interface then you want to use that interface for the injection point type.
claim 2: The purpose of an interface is that it define a contract that can been implemented by several implementations. On the other side you have your injection point (#Autowired). Having just one interface and only one class that implement it, is (IMHO) useless, and violates YAGNI.
fact: When you put:
#Component (or #Service, ...) at an interface,
have multiple classes that implements it,
at least two classes become Spring Beans, and
have an injection point that use the interface for type based injection,
then you will get and NoUniqueBeanDefinitionException
(or you have a very special configurations setup, with Environment, Profiles or Qualifiers ...)
Conclusion: If you use #Component (or #Service, ...) at an interface then you must violate at least one of the two clains. Therefore I think it is not useful (except some rare scenarios) to put #Component at interface level.
Spring-Data-JPA Repository interfaces are something complete different
Basically annotations like #Service, #Repository, #Component, etc. they all serve the same purpose:
auto-detection when using annotation-based configuration and classpath
scanning.
From my experience I am always using #Service annotation on the interfaces or abstract classes and annotations like #Component and #Repository for their implementation. #Component annotation I am using on those classes which serves basic purposes, simple Spring beans, nothing more. #Repository annotation I am using in the DAO layer, for e.g. if I have to communicate to the database, have some transactions, etc.
So I would suggest to annotate your interface with the #Service and other layers depending on the functionality.
I used #Component, #Service, #Controller and #Repository annotations only on the implementation classes and not on the interface. But #Autowired annotation with Interfaces still worked for me. If there's only one implementation of your interface Spring component scan automatically finds it with just #Autowired annotation. In case you have multiple implementations, you will need to use the #Qualifier annotation along with #Autowired to inject the correct implementation at the injection point.
1. #Service on Interfaces
#Service
public interface AuthenticationService {
boolean authenticate(String username, String password);
}
Normally, that's fine, but there's a drawback. By putting Spring's #Service on interfaces, we create an extra dependency and couple our interfaces with an outside library.
Next, to test the autodetection of our new service beans, let's create an implementation of our AuthenticationService:
public class InMemoryAuthenticationService implements AuthenticationService {
#Override
public boolean authenticate(String username, String password) {
//...
}
}
We should pay attention that our new implementation, InMemoryAuthenticationService, doesn't have the #Service annotation on it. We left #Service only on the interface, AuthenticationService.
So, let's run our Spring context with the help of a basic Spring Boot setup:
#SpringBootApplication
public class AuthApplication {
#Autowired
private AuthenticationService authService;
public static void main(String[] args) {
SpringApplication.run(AuthApplication.class, args);
}
}
When we run our app, we may get the infamous NoSuchBeanDefinitionException, and the Spring context fails to start.
Therefore, placing #Service on interfaces isn't enough for the auto-detection of Spring components.
2. #Service on Abstract Classes
Using the #Service annotation on abstract classes isn't common.
We'll start by defining an abstract class from scratch and putting the #Service annotation on it:
#Service
public abstract class AbstractAuthenticationService {
public boolean authenticate(String username, String password) {
return false;
}
}
Next, we extend AbstractAuthenticationService to create a concrete implementation without annotating it:
public class LdapAuthenticationService extends AbstractAuthenticationService {
#Override
public boolean authenticate(String username, String password) {
//...
}
}
Accordingly, we also update our AuthApplication, to inject the new service class:
#SpringBootApplication
public class AuthApplication {
#Autowired
private AbstractAuthenticationService authService;
public static void main(String[] args) {
SpringApplication.run(AuthApplication.class, args);
}
}
After we run our AuthApplication, the Spring context doesn't start. It ends up with the same NoSuchBeanDefinitionException exception again.
So, using #Service annotation on abstract classes doesn't have any effect in Spring.
3. #Service on Concrete Classes
Contrary to what we've seen above, it's quite a common practice to annotate the implementation classes instead of abstract classes or interfaces.
In this way, our goal is mostly to tell Spring this class is going to be a #Component and mark it with a special stereotype, which is #Service in our case.
Therefore, Spring will autodetect those classes from the classpath and automatically define them as managed beans.
So, let's put #Service on our concrete service classes this time around. We'll have one class that implements our interface and a second that extends the abstract class that we defined previously:
#Service
public class InMemoryAuthenticationService implements AuthenticationService {
#Override
public boolean authenticate(String username, String password) {
//...
}
}
#Service
public class LdapAuthenticationService extends AbstractAuthenticationService {
#Override
public boolean authenticate(String username, String password) {
//...
}
}
We should take notice here that our AbstractAuthenticationService doesn't implement the AuthenticationService here. Hence, we can test them independently.
Finally, we add both of our service classes into the AuthApplication and give it a try:
#SpringBootApplication
public class AuthApplication {
#Autowired
private AuthenticationService inMemoryAuthService;
#Autowired
private AbstractAuthenticationService ldapAuthService;
public static void main(String[] args) {
SpringApplication.run(AuthApplication.class, args);
}
}
Our final test gives us a successful result, and the Spring context boots up with no exceptions. Both of the services are automatically registered as beans.
You might have a look at this page for the other explanations.
Pros of putting annotation on #Service is that it gives a hint that it is a service. I don't know if any implementing class will by default inherit this annoation.
Con side is that you are coupling your interface with a specific framework i.e. Spring, by using spring specific annotation.
As interfaces are supposed to be decoupled from implementation, I would not suggest using any framework specific Annotations or object part of your interface.
I would put #Service on your class but put the name of the interface as a parameter to the annotation e.g.
interface ServiceOne {}
#Service("ServiceOne")
class ServiceOneImpl implements ServiceOne{}
By doing that you get all the benefits and can still inject the interface but get the class
#Autowired
private ServiceOne serviceOne;
So your interface is not tied to spring framework and you can change the class at any time and not have to update all your injection points.
So if I wanted to change the implementation class I could just annotate the new class and remove from the first but that's all that is required to be changed. If you inject the class you could have a lot of work when ever you want to change the impl class.
One benefit of spring is to easily switch Service (or other) implementation.
For this, you need to annotate on the interface and declare variable like this :
#Autowired
private MyInterface myVariable;
and not :
#Autowired
private MyClassImplementationWhichImplementsMyInterface myVariable;
Like the first case, you can activate which implementation to inject from the moment it is unique (only one class implements the interface).
In the second case, you need to refactor all your code (the new class implementation has another name).
As a consequence, the annotation needs to be on the interface as much as possible. Furthermore, JDK proxies are well suited for this : they are created and instantiated at application startup because runtime type is known by advance, contrary to CGlib proxies.
interface MyService {}
#Service
class MyServiceImpl implements MyService{}
#Autowired
private MyService myService;
My testing result on spring-boot 2.7.4 is:
Adding #Service ONLY to interface doesn't create spring bean named MyService. It will error on Autowired.
#Service will need to be added to implementation class to create bean com.*.service.impl.MyServiceImpl $$EnhancerBySpringCGLIB$$9140ae19 Spring will wire it to private MyService myService;
There are 5 annotations which could be used for making spring beans. List in below of answers.
Do you really need an interface? If you are going to have one implementation for each service interface, just avoid it, use only class. Of course, if you don't have RMI or when interface proxy is required.
#Repository - use for injecting your dao layer classes.
#Service - use for injecting your service layer classes. In service layer also you might need to use #Transactional annotation for db transaction management.
#Controller - use for your frontend layer controllers, such as JSF managed beans injecting as spring beans.
#RestController - use for spring rest controllers, this would help you to avoid every time to put #ResponseBody and #RequestBody annotations in your rest methods.
#Component - use it in any other case when you need to Inject spring bean which is not controller, service, or dao class
To put it simply:
#Service is a Stereotype annotation for the service layer.
#Repository is a Stereotype annotation for the persistence layer.
#Component is a generic stereotype annotation used to tell Spring to create an instance of the object in the Application Context. It's possible to
define any name for the instance, the default is the class name as camel case.
I'm currently working on a Spring MVC project in which I integrated Hibernate. The pure Spring MVC part (DispatcherServlet + request mapping) works fine. Now, the problem I have to cope with is quite strange : I've read "Java Persistence with Hibernate" and I am trying to design my persistence layer in a similar way than explained in the book. That is, I've designed it in two parallel hierarchies : one for implementation classes and a second for the interfaces.
So, I have an abstract class named GenericDaoImpl, that implements the GenericDao interface. Then I have a concrete class named AdvertisementDaoImpl, that extends GenericDaoImpl and that implements the AdvertisementDao interface (which extends GenericDao).
Then, in a service bean (class marked with #Service), I'll have my dao class autowired.
Here's my problem :
autowiring a DAO class that implements an interface but does not extends my abstract GenericDaoImpl class : OK
autowiring my AdvertisementDaoImpl that implements the AdvertisementDao interface and extends my abstract GenericDaoImpl class : leads to bean initialization exception.
The abstract class I have at the top of my DAO hierarchy handles all the boilerplate code for common CRUD methods. So, I definitely want to keep it.
Does anyone have an explanation about that?
Here's an excerpt of code :
public abstract class GenericDaoImpl <T, ID extends Serializable> implements BeanPostProcessor, GenericDao<T, ID>{
#Autowired(required=true)
private SessionFactory sessionFactory;
private Session currentSession;
private Class<T> persistentClass;
...
}
#Repository
public class AdvertisementDaoImpl extends GenericDaoImpl<Advertisement, Long> implements AdvertisementDao {
...
public List<Advertisement> listAdvertisementByType(AdvertisementType advertisementType, Class<? extends Good> type) {
return null;
}
}
#Service
public class AdvertisementServiceImpl implements AdvertisementService{
#Autowired(required=false)
private AdvertisementDao advertisementDao;
public List<Advertisement> listAllAdvertisements() {
return null;
}
}
Here's the most relevant part of the stacktrace (at least, I guess it is):
nested exception is
org.springframework.beans.factory.BeanCreationException: Could not
autowire field: be.glimmo.service.AdvertisementService
be.glimmo.controller.HomeController.advertisementService; nested
exception is java.lang.IllegalArgumentException: Can not set
be.glimmo.service.AdvertisementService field
be.glimmo.controller.HomeController.advertisementService to
be.glimmo.dao.AdvertisementDaoImpl
And here's my Spring configuration (link to pastebin.com) :
I believe you should use proxy-target-class in your transaction management configuration:
<tx:annotation-driven transaction-manager="transactionManagerForHibernate"
proxy-target-class="true" />
The symptoms of the problem you describe match the ones mentioned in Spring Transaction Management (look for Table 10.2) and AOP proxying with Spring:
If the target object to be proxied implements at least one interface
then a JDK dynamic proxy will be used. All of the interfaces
implemented by the target type will be proxied. If the target object
does not implement any interfaces then a CGLIB proxy will be created.
So, when CGLIB is not there by default, you have all the methods coming from implemented interfaces but you will miss proxying of the methods that come from super classes in the hierarchy and that's why you get an exception on this.
After a few more tests, it turns out that the problem was caused by my abstract GenericDaoImpl class implementing BeanPostProcessor interface : for some reason, the methods from this interface were executed not only at this bean instantiation but at every bean intantiation.
Given that, within my BeanPostProcessor hook methods, I was retrieving generics parameterized types, when these methods were executed against classes that are not in my DAO hierarchy, they would end up yielding runtime Exceptions (more specifically, ClassCastException).
So, to solve this issue, I had my GenericDaoImpl class not implement BeanPostProcessor interface anymore and I moved the body of the hook method in the empty contructor.