For example, have a class like as follows.
First XService service in class A is not null but second XService service in AmountValidator is null.I get NullPointerException I try to create bean new it works and then I get same exception when call AmountValidateService outsideRestService in XService.
How can I use XService everywhere that I use #Autowired annotation.
My main class:
#Service
class A extends AbstractA implements IA {
#Autowired
XService service; //first autowired definition. code go to check() method. service not null now.
public doSometing(){
validator.check();
service.methodA();
super.AbstractMethod();
}
}
Validator class used in class A :
class Validator<T> implements IValidator<T> {
public void check(){
rule.check(); // rule have a implements IValidator eg: amountValidator, dateValidator class
}
}
AmountValidator added to rule in class Validator.
#Component
class AmountValidator implements IValidator<T>{
#Autowired
XService service; // code comes here service is null. same service class mentioned above class A.
#Override
public void check(){
service.validateAmount(); // nullPointerException.
}
}
My main Service
#Component
class XService {
#Autowired
AmountValidateService outsideRestService;
public validateAmount(){
outsideRestService.validate(); // nullPointer when create XService with the `New` keyword
}
}
You have an error cause you are trying to create components/beans/services yourself. As i mentioned in comment when you create components yourself it - #Autowired doesn't work - thats you've got NPE
All classes annotated with #Component, #Service are considered special classes which are instantiated by Spring automatically via DI, instantiating them with new defeats the purpose of DI.
These special classes are named Spring Beans.
Every time the application starts, the framework instances all Spring Beans, and all #Autowired fields are injected by Spring automatically. But the Spring Beans must be defined somewhere in the class path. Else you will receive a NoSuchBeanDefinitionException
As an attempt to answer the question, since I don't have a stack trace nor all the Spring Bean definitions:
When you instantiate XService using new XService() your new instance will not actually initialize the field AmountValidateService outsideRestService, effectively leaving it as null.
You may set the field yourself but as I mentioned earlier, it defeats the purpose of DI
Your question is not complex, it is incomplete.
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.
In a Spring MVC proyect I'm using Spring Data in the persistence layer so I have a bunch of repositories to access the data. I also have a layer for services so I have things like UserService or AuthorityService that use that repositories.
The problem is that I've been asked to create an interface to be able to change the implementation of the persistence layer (using DAOs for example) without have to touch a single line in the services. How can I specify in an Autowired of that interface what implementation to use? I´m using Java based config and I don't see how to inject it.
I also have a problem with the name of these new interfaces. Normally I would use a name like UserService but Spring use Service for the service layer so, What name is suitable for this type of interface?
You could mark the new implementation of the DAO as #Primary. Indicates that a bean should be given preference when multiple candidates are qualified to autowire a single-valued dependency. If exactly one 'primary' bean exists among the candidates, it will be the autowired value.
#Component
public class FooService {
private FooRepository fooRepository;
#Autowired
public FooService(FooRepository fooRepository) {
this.fooRepository = fooRepository;
}
}
#Component
public class JdbcFooRepository {
public JdbcFooService(DataSource dataSource) {
// ...
}
}
#Primary
#Component
public class HibernateFooRepository {
public HibernateFooService(SessionFactory sessionFactory) {
// ...
}
}
Because HibernateFooRepository is marked with #Primary, it will be injected preferentially over the jdbc-based variant assuming both are present as beans within the same Spring application context, which is often the case when component-scanning is applied liberally.
This annotation is semantically equivalent to the element's primary attribute in Spring XML.
I didn't completely follow your second question.
I have this HTTP listener subclass
public class MigificSessionListener implements HttpSessionListener {
#Autowired
#Qualifier("notificationThread")
private NotificationThread notificationThread;
#Override
public void sessionDestroyed(HttpSessionEvent hse) {
// here notificationThread value is null
}
}
Value of notificationThread inside sessionDestroyed() is null.
How can i autowire sessionDestroyed inside this class ?
Your MigificSessionListener in not in your spring conext, spring even do not know it exists.
You can use WebApplicationContextUtils to get your spring context from ServletContext
WebApplicationContextUtils.getWebApplicationContext(sessionEvent.getSession().getServletContext())
You can enable Spring AOP with #EnableSpringConfigured and annotate your class with #Configurable. This let spring manage instances which are created outside the spring context with new. You will also need to enable either load-time weaving or compile-time weaving. This is documented in 9.8.1 Using AspectJ to dependency inject domain objects with Spring.
#Configuration
#EnableSpringConfigured
public class AppConfig {
}
#Configurable
public class MigificSessionListener implements HttpSessionListener {
#Autowired
#Qualifier("notificationThread")
private NotificationThread notificationThread;
//...
}
Convert your non-Spring managed class MigificSessionListener into a Spring-managed one by annotating it with #Configurable.
For this annotation to be recognised you need <context:spring-configured/> in your Spring XML config or #EnableSpringConfigured if you are using Spring Java config.
The #Autowired or injection of other dependencies will then succeed.
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.