I have declared following list using spring util namespace in my spring configuration file:
<util:list id="childList">
<ref bean="child1"/>
<ref bean="child2"/>
<ref bean="child3"/>
</util:list>
where all reference bean are marked with #Componant annotation and their respective beans are creating. But whenever I am trying to Autowired any beans property like:
#Component
public class ListTest{
#Autowired
#Qualifier("childList")
private List<IParent> list;
public List<IParent> getList() {
return list;
}
}
Gives exception as: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'listTest': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private java.util.List com.spring3.componentScanFilterTest.ListTest.list; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No matching bean of type [com.spring3.componentScanFilterTest.IParent] found for dependency [collection of com.spring3.componentScanFilterTest.IParent]: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {#org.springframework.beans.factory.annotation.Autowired(required=true), #org.springframework.beans.factory.annotation.Qualifier(value=childList)}
But instead of #Autowired and #Qualifier if I use as:
#Resource(name="childList")
It works. Why? As per my understanding #Autowired is used to autowire the property matching by type and #Qualifier is used to select any one bean from multiple ambiguous beans.
Please explain.
Spring docs says.
As a specific consequence of this semantic difference, beans that are themselves defined as a collection or map type cannot injected through #Autowired, because type matching is not properly applicable to them. Use #Resource for such beans, referring to the specific collection or map bean by unique name.
Hope this clear your doubt.
Type matching is not properly applicable to beans which defined as collection.
If you intend to express annotation-driven injection by name, do not primarily use #Autowired - even if is technically capable of referring to a bean name through #Qualifier values. Instead, prefer the JSR-250 #Resource annotation which is semantically defined to identify a specific target component by its unique name, with the declared type being irrelevant for the matching process.
As a specific consequence of this semantic difference, beans which are themselves defined as a collection or map type cannot be injected via #Autowired since type matching is not properly applicable to them. Use #Resource for such beans, referring to the specific collection/map bean by unique name.
Here:
http://docs.spring.io/spring/docs/2.5.x/reference/beans.html#beans-autowired-annotation-qualifiers
You are trying to get list of all beans of type Parent that have the qualifier "childList".
I'm wondering if I add a #Value annotation on a property, the class who contains this property cannot be used by another one with a different value, Example :
MyClassUtil.java had
#Value("${some.value}")
private int _myProperty;
And of course there is one module.properties who contain :
some.value=10
Another class ClassA.java wants to use this class with value 10. Ok, no problem.
But another class ClassB.java wants to use this class but with another value : 20. I cannot do this if I'm not mistaken.
Because before #Value era, I could declare two beans in the moduleContext.xml without any problem.
So, is #Value pushes you to do some strong coupling ?
You are right that the annotation configuration can not be instance specific. It is important to understand the concept of bean definitions in bean factory.
Manual bean definition:
Single <bean> element in your XML config leads to a single bean definition. Multiple <bean> mean multiple definitions (regardless of a bean type).
Single #Bean method within #Configuration class leads to a single bean definition. Multiple #Bean methods mean multiple definitions (regardless of a bean type).
However when using component scan, classes annotated with #Component-like annotations are auto-registered as a single bean definition. There is no way you can register bean multiple times via component scan.
Similarly, annotation configurations (#Value, #Autowired, etc.) are type-wide. Your bean instances are always augmented and processed with the same effect (e.g. injecting the same value). There is no way you can alter annotation processing behaviour from instance to instance.
Is this tight coupling? It is not in its general understanding - bean factory (Spring) is still free to inject whatever it thinks is suitable. However it is more of a service lookup pattern. This simplifies your life when working with domain specific singletons. And most beans in an application context tend to be singletons, many of them domain specific (controllers, services, DAOs). Framework singletons (non-project specific reusable classes) should never use annotation based configuration - in this scope, it is an unwanted tight coupling.
If you need different bean instances, you should not use annotation configuration and define your beans manually.
Is there any way to inject bean dependencies make configurable except from factory pattern?
I have 3 class implement same interface and have 3 bean definition. I want to change that beans using in other class? For excample is it possile to read bean name form conf file and use it as varible?
Yes, you can go with #Qualifier annotation. As you have 3 classes which implement same interface, name those classes with different names and use #Qualifier annotation.
Spring documentation says : autowiring by type may lead to multiple candidates, it is often necessary to have more control over the selection process. One way to accomplish this is with Spring's #Qualifier annotation.
I am using
<context:component-scan base-package="com.package.dao"/> (Spring 3)
and in my controllers with #Autowired will retrieve automatically the DAO I need. That's perfect and works.
The problem is when I need more than one DAO manager.
How can I do?
I tried with #Qualifier but I am not sure about the name of the dao-manager. className doesn't work
someone can help me?
Thank you!
See in the spring documentation:
#Autowired is fundamentally about type-driven injection with optional semantic qualifiers. This means that qualifier values, even when using the bean name fallback, always have narrowing semantics within the set of type matches; they do not semantically express a reference to a unique bean id
...
If you intend to express annotation-driven injection by name, do not primarily use #Autowired - even if is technically capable of referring to a bean name through #Qualifier values. Instead, prefer the JSR-250 #Resource annotation which is semantically defined to identify a specific target component by its unique name, with the declared type being irrelevant for the matching process.
Use #Resource instead, see here.
I'm reviewing some Spring code, and I see a few bean defs that do not have an id or a name.
The person who did it is not around to ask.
The application is working fine.
I am not familiar what this necessarily means.
Anybody know if this means anything in particular?
Some beans are not required to be accessed by other beans in the context file or programmatically. So as mentioned by JacobM, they don't require an id or name as they're not referenced.
Such an example would be a PropertyPlaceholderConfigurer, which reads a property file, then allows for runtime property replacement in the context definition.
The example definition would be
<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="location" value="myapp.properties" />
</bean>
The JavaDoc provides further documentation on this object, but further on in the file you can reference properties from your file by just using the standard template replace placeholder ${...}.
One possibility is that you can define a bean in place, and so you don't need an id since you don't need to refer to it from anywhere else. Say I have a Foo object that takes a Bar property:
<bean id="foo" class="Foo">
<property name="bar">
<bean class="Bar">
</property>
</bean>
The Bar bean doesn't need a name because it's only used to set that one property.
Check the possibility of auto-wiring. An other bean could reference the unnamed bean by having the autowire property set to byType.
This is just a guess. Without a concrete example, I can't say any more.
The id and name attributes are optional and are used to reference the bean definition from other definitions. Look at the official Spring documentation for more detail.
take a look at https://docs.spring.io/spring/docs/4.3.12.RELEASE/spring-framework-reference/htmlsingle/#beans-beanname it says,
You are not required to supply a name or id for a bean. If no name or id is supplied explicitly, the container generates a unique name for that bean. However, if you want to refer to that bean by name, through the use of the ref element or Service Locator style lookup, you must provide a name. Motivations for not supplying a name are related to using inner beans and autowiring collaborators.
Also, Beans such as BeanPostProcessor, BeanFactoryPostProcessor and PropertyPlaceholderConfigurer are automatically detected by application context and typically won't have name
If you consider any spring bean, Spring mandates it to have an identifier. In case, if you have not provided any identifier (via id or name attribute) to a bean in your configuration, you won't run into exceptions. Spring will manage such situation by assigning a default identifier. it has BeanNameGenerator to assign default name. <bean class="com.package.name.TestBean"> will be named as "com.package.name.TestBean" #Bean kind of beans will have its method name as bean name
thus, in your code, for some reason, you can skip naming few beans and the code still works if you are accessing those beans with their default name
credits : https://www.javacodegeeks.com/2013/02/spring-bean-names.html
Beans without id or name can still be referenced by the class name. Spring names those beans automatically using the class name and if there is more than one bean of the same class it appends a number to them.
Anonymous beans are usually defined inside a property tag, but if they're just there maybe there's autowiring configured in some other beans.
Anyway, I think adding a name or id to those beans won't break your application.
As a couple of people mentioned above, not all bean-grabbing is based on name/ID; some of it is based on type. For example, there is a method
BeanFactoryUtils.beansOfTypeIncludingAncestors(...)
that grabs all the beans of some given type. This is used for example by the Spring Web MVC DispatcherServlet (among many other places) to discover beans by type, such as HandlerMappings, HandlerAdapters, HandlerExceptionResolvers and so forth. Contrast this with cases where the bean must have a specific well-known name/ID to be found, such as the LocaleResolver (ID must be "localeResolver" or it won't be found) and ThemeResolver (ID must be "themeResolver" or it won't be found).
Beans defined without name and ID can be accessed with a generated ID (full package name and class name), for example:
bean defined as
<bean class="pl.finsys.initOrder.TestBeanImpl">
can be accessed by
TestBean bean = (TestBean) ctx.getBean("pl.finsys.initOrder.TestBeanImpl");
//Bean Cfg File without Bean id
<bean class="com.ds.DemoBean">
<property name="msg" value="Hello"/>
</bean>
//We can Access
Object obj=factory.getBean("com.ds.DemoBe
its not a mandatory to provide java Bean Id..If we are not providing Bean Id,our Container Provides the Default Been Id.Default Bean Id look like as
"(Package Name).(Bean Class Name)#N"where N=0,1,2,......etc.
It seems there is a subtle difference between unnamed and named bean behaviour. If you have an XML config file imported twice, each named bean will be created only once, but an unnamed bean will be created as many times as its definition is included. When trying to autowire such a bean by type, it leads to errors like this:
No qualifying bean of type [your.class.Name] is defined: expected single matching bean but found 4