I am trying to understand the javax.inject package and I am not clear what the javax.inject.Named annotation is supposed to be used for. The Javadoc does not explain the the idea behind it.
Javadoc is at http://download.oracle.com/javaee/6/api/javax/inject/Named.html
I am using Spring 3.0 to write some sample programs, by putting #Named on a bean it seems to add it to the bean factory but the Javadoc description is so light I can't tell if that is the standard behavior or Spring specific behavior.
My questions are:
What is the difference between #Named and #Qualifier
How are you supposed to tell the Runtime system a class should be injectable in other classes what's the annotation for that? The equivalent of #Component in Spring?
Update 1 there is an excellent explanation of #Named and #Qualifier at Nice article about #Named and #Qualifier https://dzone.com/articles/java-ee6-cdi-named-components thanks #xmedeko for linking to it the comment below.
Use #Named to differentiate between different objects of the same type bound in the same scope.
#Named("maxWaitTime")
public long maxWaitTimeMs;
#Named("minWaitTime")
public long minWaitTimeMs;
Without the #Named qualifier, the injector would not know which long to bind to which variable.
If you want to create annotations that act like #Named, use the #Qualifier annotation when creating them.
If you look at #Named, it is itself annotated with #Qualifier.
#Inject instead of Spring’s #Autowired to inject a bean.
#Named instead of Spring’s #Component to declare a bean.
Those JSR-330 standard annotations are scanned and retrieved the same way as Spring annotation (as long as the following jar is in your classpath)
Regarding #2, according to the JSR-330 spec:
This package provides dependency
injection annotations that enable
portable classes, but it leaves
external dependency configuration up
to the injector implementation.
So it's up to the provider to determine which objects are available for injection. In the case of Spring it is all Spring beans. And any class annotated with JSR-330 annotations are automatically added as Spring beans when using an AnnotationConfigApplicationContext.
The primary role of the #Named annotation is to define a bean for the purpose of resolving EL statements within the application, usually through JSF EL resolvers. Injection can be performed using names but this was not how injection in CDI was meant to work since CDI gives us a much richer way to express injection points and the beans to be injected into them.
Related
I have a entity class called Customer, I am using this entity object in another class to set the data. When I use this object below like
#Autowired
Customer customer
Spring is complaining that please configure the bean in your classes.
Can we use auto wiring with entity objects?
You can only autowire only those beans whose life-cycle are managed by Spring IoC container.
These beans are defined in xml form with </bean> tag, or with some special annotations like #Bean, #Component, #Service, #Repository etc.
On the other hand,
in simple terms, entities are some java objects that you will need to create, update by yourself according to your business logic and save/update/remove them in/from DB. Their life-cycle cannot be managed by Spring IoC container.
So, you should never feel like you need to autowire an entity if you are doing it right!
In fact, Spring support #Autowire only for Spring Beans. A java class becomes Spring Bean only when it is created by Spring, otherwise it is not.
A workaround might be to annotate your class with #Configurable but you would have to use AspectJ
Please look in the Spring documentations on how to use #Configurable
Also, I wonder why you would autowire an entity class ?
I would warn you not to mix Spring Bean and JPA entities in one class/usecase because:
Spring Beans are instantiated and managed by Spring
Entities are managed by JPA provider
If you mean JPAs #Entity-annotation, Spring is simply telling you, that there isn't a bean in its context.
On startup/runtime classes in the application will be scanned and each class annotated with spring annotations like #Component, #Service etc. will be instantiated as beans and put into a global context (Spring applicationcontext).
This context is then used to lookup and inject those beans into other beans when #Autowired is found during scanning.
Opposed to this, #Entity is used during the creation of the Persistence-Context of JPA (as far as I remember) which isn't aware of Spring and it's context.
Most of the solutions to make both contexts aware of each other a mostly a little bit hacky.
I made a simple xhtml page for upload file by using <h:inputFile> component. And everything works fine. And in managed bean I used dependency injection for the Logger. I used a factory class and createLogger() method in, to enable injection for object of Logger class.
Everything is ok but, nothing works without a #Model annotation in managed bean.
Can somebody explain the meaning of the #Model annotation.
I can not find on internet proper explanation. A founded explanation of other annotation like as #Session, #Request, #Application etc.
What does the #Model annotation do?
Hej vmaric,
#Model == #RequestScoped + #Named
It exposes the Backing Bean directly to your JSF 2 or JSP and its context will be destroyed after the end of the servlet request.
So it should not be used for entities.
Here is a hint from the Weld Reference Guide:
Notice the controller bean is request-scoped and named. Since this combination is so common in web applications, there's a built-in annotation for it in CDI that we could have used as a shorthand. When the (stereotype) annotation #Model is declared on a class, it creates a request-scoped and named bean.
I am going through some blogs on SpringSource and in one of the blogs, author is using #Inject and I suppose he can also use #Autowired.
Here is the piece of code:
#Inject private CustomerOrderService customerOrderService;
I am not sure about the difference between #Inject and #Autowired and would appreciate it if someone explained their difference and which one to use under what situation?
Assuming here you're referring to the javax.inject.Inject annotation. #Inject is part of the Java CDI (Contexts and Dependency Injection) standard introduced in Java EE 6 (JSR-299), read more. Spring has chosen to support using the #Inject annotation synonymously with their own #Autowired annotation.
So, to answer your question, #Autowired is Spring's own annotation. #Inject is part of a Java technology called CDI that defines a standard for dependency injection similar to Spring. In a Spring application, the two annotations works the same way as Spring has decided to support some JSR-299 annotations in addition to their own.
Here is a blog post that compares #Resource, #Inject, and #Autowired, and appears to do a pretty comprehensive job.
From the link:
With the exception of test 2 & 7 the configuration and outcomes were
identical. When I looked under the hood I determined that the
‘#Autowired’ and ‘#Inject’ annotation behave identically. Both of
these annotations use the ‘AutowiredAnnotationBeanPostProcessor’ to
inject dependencies. ‘#Autowired’ and ‘#Inject’ can be used
interchangeable to inject Spring beans. However the ‘#Resource’
annotation uses the ‘CommonAnnotationBeanPostProcessor’ to inject
dependencies. Even though they use different post processor classes
they all behave nearly identically. Below is a summary of their
execution paths.
Tests 2 and 7 that the author references are 'injection by field name' and 'an attempt at resolving a bean using a bad qualifier', respectively.
The Conclusion should give you all the information you need.
To handle the situation in which there is no wiring, beans are available with #Autowired required attribute set to false.
But when using #Inject, the Provider interface works with the bean which means that the bean is not injected directly but with the Provider.
The key difference(noticed when reading the Spring Docs) between #Autowired and #Inject is that, #Autowired has the 'required' attribute while the #Inject has no 'required' attribute.
As of Spring 3.0, Spring offers support for JSR-330 dependency injection annotations (#Inject, #Named, #Singleton).
There is a separate section in the Spring documentation about them, including comparisons to their Spring equivalents.
Better use #Inject all the time. Because it is java configuration approach(provided by sun) which makes our application agnostic to the framework. So if you spring also your classes will work.
If you use #Autowired it will works only with spring because #Autowired is spring provided annotation.
#Autowired annotation is defined in the Spring framework.
#Inject annotation is a standard annotation, which is defined in the standard "Dependency Injection for Java" (JSR-330). Spring (since the version 3.0) supports the generalized model of dependency injection which is defined in the standard JSR-330. (Google Guice frameworks and Picocontainer framework also support this model).
With #Inject can be injected the reference to the implementation of the Provider interface, which allows injecting the deferred references.
Annotations #Inject and #Autowired- is almost complete analogies. As well as #Autowired annotation, #Inject annotation can be used for automatic binding properties, methods, and constructors.
In contrast to #Autowired annotation, #Inject annotation has no required attribute. Therefore, if the dependencies will not be found - will be thrown an exception.
There are also differences in the clarifications of the binding properties. If there is ambiguity in the choice of components for the injection the #Named qualifier should be added. In a similar situation for #Autowired annotation will be added #Qualifier qualifier (JSR-330 defines it's own #Qualifier annotation and via this qualifier annotation #Named is defined).
In addition to the above:
The default scope for #Autowired beans is Singleton whereas using JSR 330 #Inject annotation it is like Spring's prototype.
There is no equivalent of #Lazy in JSR 330 using #Inject.
There is no equivalent of #Value in JSR 330 using #Inject.
#Inject has no 'required' attribute
#Autowired(required=false)
By default the dependency injection for #Autowired must be fulfilled because the value of required attribute is true by default. We can change this behavior by using #Autowired(required=false). In this case if bean is not found for dependency injection, it will not through error.
Please have look at
https://www.concretepage.com/spring/spring-autowired-annotation#required-false
But #Inject doesn’t need (required=false) it will not through error if dependency is not found
The #Inject annotation is one of the JSR-330 annotations collection. This has Match by Type,Match by Qualifier, Match by Name execution paths.
These execution paths are valid for both setter and field injection.The behavior of #Autowired annotation is same as the #Inject annotation. The only difference is the #Autowired annotation is a part of the Spring framework. #Autowired annotation also has the above execution paths. So I recommend the #Autowired for your answer.
How does Wicket's #SpringBean annotation work? Does it use reflection at run time? Does it make the compiler inject some code? Or what?
#SpringBean works using Wicket's underlying Injector mechanism. When you instantiate a Wicket component, the constructor of Wicket's component base class introspects the class being instantiated, looking for the #SpringBean annotation. If the bean is found, then Wicket generates a proxy for the spring bean and injects it into the component's field. This is Wicket's equivalent of Spring's #Autowired annotation, the effect is similar.
It doesn't, however, have anything to do with Spring's own context/classpath scanning functionality (e.g. #Component), which is about auto-discovery of what is and isn't a bean, rather having anything to do with wiring.
The class marked with a #SpringBean annotation has to have one of:
A no-args constructor
A superclass with a no-args constructor
Implement an interface
An exception will be thrown if these conditions are not met as Wicket will not be able to proxy the class.
Spring uses the class loader and ASM at runtime to find all annotated classes.
You can configure where spring should search for beans:
<context:component-scan base-package="some.package.to.start.from"/>
This uses the ClassPathBeanDefinitionScanner internally which will use the PathMatchingResourcePatternResolver to find the classes and the ASM-based MetadataReader to read the annotations.
I created a custom sterotype #Action, and Spring has managed to detect it in the package scan I configured in the configurations.
The next step I would like to do is to tell Spring that all classes with #Action should be created with prototype, instead of Singleton.
My #Action interface is as follows:
#Target({ElementType.TYPE})
#Retention(RetentionPolicy.RUNTIME)
#Documented
#Component
public #interface Action {
}
I tried to mark it with #Scope("prototype") but that does not seem to help.
Is what I desire possible?
Kent
The context:component-scan can be configured with a custom scope-resolver, which implements org.springframework.context.annotation.ScopeMetadataResolver.
I created a custom scope-resolver that not only checks the bean for a #Scope annotation (with the default resolver of org.springframework.context.annotation.AnnotationScopeMetadataResolver), but looks up annotations of annotations too (recursively).
One thing to note though, that looking up annotations recursively might go into an endless loop, as java.lang.annotation.Documented is annotated with java.lang.annotation.Documented. Best to maintain a table that indicates which annotation has been looked up already.
Unfortunately not with spring 2.5.X. Your #Component-annotation describes the role of the bean while the scope is a separate axis, so a role and scope descriptor typically have to be applied separately to the implementation class. When viewed in this way it makes some sense (edit: at least it did so for a few seconds, anyway)
I don't know how this will change i spring 3.x, which is not too far away. There seems to be some room for improvement.