I have some classes, with a custom annotation, that shouldn't be instantiated (abstract class, and it's just a subcomponents for a real beans). But on top of this classes, on runtime, on context initialization phase, I want to put extra beans into application context.
So, basically I need to scan classpath, process results, and introduce new beans into curent application context.
It seems that spring-mvc, spring-tasks and spring-integration are doing this (I tried to learn it from sources - no luck)
I found that I can create my own BeanFactoryPostProcessor, scan classpath and call registerSingleton for my custom bean. But I'm not sure that it's a good way for introducing new beans (seems that it's used only for postprocess of exising beans only). And I believe there are some Spring internal tools that I may reuse to simplify process.
What is a conventional way to introduce extra beans on Spring context initialization?
Your observation is actually correct, BeanFactoryPostProcessor is one of the two ways Spring provides a mechanism to modify the bean definition/instances before making them available to the application(the other is using BeanPostProcessors)
You can absolutely use BeanFactoryPostProcessors to add/modify bean definitions, here is one sample from Spring Integration codebase that adds a errorChannel if not explicitly specified by a user, you can probably use a similar code for registering your new beans:
RootBeanDefinition errorChannelDef = new RootBeanDefinition();
errorChannelDef.setBeanClassName(IntegrationNamespaceUtils.BASE_PACKAGE
+ ".channel.PublishSubscribeChannel");
BeanDefinitionHolder errorChannelHolder = new BeanDefinitionHolder(errorChannelDef,
IntegrationContextUtils.ERROR_CHANNEL_BEAN_NAME);
BeanDefinitionReaderUtils.registerBeanDefinition(errorChannelHolder, registry);
There are at least two ways to include custom annotated classes as bean definitions:
Annotate the custom annotation with #Component
Add a include filter with type annotation to <context:component-scan/>
for example:
<context:component-scan base-package="org.example">
<context:include-filter type="annotation" expression="org.example.Annotation"/>
</context:component-scan>
Then you can use a BeanPostProcessor to instantiate them, for example:
public class CustomAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
#Override
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
if (beanClass.isAnnotationPresent(org.example.Annotation.class)) {
Object bean = createBeanInstance();
...
return bean:
}
return null;
}
}
Or use a BeanFactoryPostProcessor to process the ScannedGenericBeanDefinitions.
See AnnotationConfigUtils.registerAnnotationConfigProcessors() for sample code of internal Spring annotation postprocessors.
Related
In my ApplicationContext I have several Beans being created the same style. So I have a lot of dublicated code writing a FactoryBean for each of this beans. Those beans have a common ground, implementing all one special interface.
I would like to move all that bean creation to one factory. That one would have to provide a methode like this
<T extends CommonInterface> T createInstance(Class<T> clazz);
There I could implement all the instantiation necessary to create one of my special beans.
My implementation would be called by spring for
#Autowired
private MyCommonInterfaceImplementation impl;
in that way
createInstance(MyCommonInterfaceImplementation.class)
So far I looked at BeanFactory and FactoryBean, both seem not to be I'm searching for.
Any suggestions?
why not use #bean
#Bean
public MyCommonInterfaceImplementation getMyCommonInterfaceImplementation(){
return MyBeanFactory.createInstance(MyCommonInterfaceImplementation.class);
}
//should autowire here
#Autowired
private MyCommonInterfaceImplementation impl;
Basically you need the #Bean annotation on a "factory" only if you need some special handling during the creation of a bean.
If everything can be #Autowired, either by setters, fields, or one constructor, and nothing else needs to be done on a bean during initialization, you can simply declare the annotation #Component on each implementation of your interface. This works as long as you have component scanning active inside your application. The result will be that for each component spring will create a bean which you can use.
I'm writing this on a mobile so showing code is not the best. Just follow some tutorial on #ComponentScan, or if you need, let me know and I can augment this answer with an example.
As of Spring 4.3 you no longer have to annotate your bean classes and you can let them be instantiated via a componentscan.
#Configuration
#ComponentScan(
value = "some.package.path",
includeFilters = {
#Filter(type = ASSIGNABLE_TYPE, value = {
MyClass1.class,
MyClass2.class,
MyClass3.class
})
})
This actually creates beans for the three classes listed there. The example should work without filters as well (everything in the package becomes a bean). This works as long as the classes have a single constructor that can be used for autowiring. I don't think it is possible to filter for all implementations of a particular interface and then register a bean.
To do that, you might do something with a ContextListener and e.g. use reflection to find out what classes to instantiate and then use context.autowire(..) to inject any dependencies from your context. A bit hacky but it might work.
#Override
public void onApplicationEvent(ContextRefreshedEvent event) {
ApplicationContext context = event.getApplicationContext();
MyClass bean
= context
.getAutowireCapableBeanFactory()
.autowire(MyClass.class, Autowire.BY_NAME.value(), true);
...
}
That still leaves the problem of how to get the bean registered in the context of course.
You might also be able to adapt the answer to this SO question on how to add beans programmatically.
Finally the best approach I've found is using a ConfigurationClassPostProcessor. As example I've used https://github.com/rinoto/spring-auto-mock
But, since it is quite complicated and "too much magic" to create beans from nothing, we decided to explicitly create those beans via #Bean.
Thanks for your answers.
I'm trying to get a better understanding of the #Autowired annotations component scanning, but all the examples I found so far use context.getBean(..) at some point to get at least one Bean to start with.
I also read that doing that is considered bad practice , but I can't seem to find any information on how to do it without context.getBean(..)
Could somebody please enlighten me with an example and information on how to do this ?
Define your bean in xml and use
<context:component-scan base-package="com" />
<mvc:annotation-driven />
Bean def
<bean id="processEngine" class="com.processEngine">
<property name="processEngineConfiguration" ref="processEngineConfiguration" />
</bean>
now you can get bean as following
#Autowired
private ProcessEngine processEngine;
how it works
spring scans the bean's recipes either from xml or java configuration. then spring creates a beanDefinitions which are 'loaded' into BeanFactory. BeanFactory triggers a set of BeanPostProcessors (BPP) which are scanning classes for particular annotations like Autowired/Resource/PostProcessor and etc. and do appropriate actions. in case when your class contains #Autowired annotation, AutowiredAnnotationBeanPostProcessor would auto wire required field (dependencies), and when creation of an object is done and all BPP worked out, object is ready to be used by the app, from this point your code can get 'ready' objects from container.
there are some cases when you would need to access this beans from the code which is out of spring's control and not managed by container. in order to do so, you would need to get the ApplicationContext (container) and call #getBean specifying either name or type. using applicationContext directly is not a good practice because there are some problems that you can come to, f.ex. id of a bean might be changed and if you refer to bean by id then NPE would be thrown.
configuration
there are several approaches to configure spring to scan the classes for finding bean recipes. one would be defining component-scan, in this case classes which are located in the path that you've set and having any of valid spring annotations like #Component, #Service, #Repository, #Controller (for web container) would be considered. another way would be specifying each bean separately using <bean> or #Bean.
examples.
if you want to create a web app then you should see DispatcherServlet with ContextLoaderListener classes. this classes would boot your app and load everything according to configuration. f.ex. here,
but if you want to create a desktop app, then you would end up with something like this
From time to time (usually when not using Spring Boot), I use something along the lines of the following code:
public static <T> T autowire(ApplicationContext ctx, T bean) {
ctx.getAutowireCapableBeanFactory().autowireBean(bean);
return bean;
}
In my main, I create an instance of the main application class that contains a few #Autowired annotations for the main services / entry points to my Spring application.
With Spring it is possible to inject a list of beans by the interface class like:
#Component
public class Service {
#Autowire
private List<InterfaceType> implementingBeans;
...
}
All defined beans that implement this interface will be present in this List.
The annotation based approach is not possible for me, as the Service class is in a module that must not have spring dependencies.
I need to use this mechanism from outside via xml configuration.
<bean id="service" class="...Service">
<property name="implementingBeans">
??? tell spring to create a list bean that resolves all beans of the interfaceType.
</property>
</bean>
Does anyone know how to solve this?
EDIT: Additionally, there are more than one spring applications that use this service. So the best solution would be to handle this szenario completely via xml configuration. I can then copy the xml parts to all spriong applications that need this.
I want to avoid having a kind of initializer bean that gets the service injected and must then be copied to all spring applications.
Kind regards.
An XML-only solution would simply have you declare a <bean> of the "external" type and provide an autowire value of "byType".
Controls whether bean properties are "autowired". This is an
automagical process in which bean references don't need to be coded
explicitly in the XML bean definition file, but rather the Spring
container works out dependencies.
[...]
"byType" Autowiring if there is exactly one bean of the property type in the container. If there is more than one, a fatal error is
raised, and you cannot use byType autowiring for that bean. If there
is none, nothing special happens.
The explanation is a little confusing in that we expect multiple InterfaceType beans, but the actual field is of type List and Spring will be able to dynamically instantiate one and add all the InterfaceType beans to it, then inject it.
Your XML would simply look like
<bean id="service" class="...Service" autowire="byType">
</bean>
My original suggested solution made use of SpEL.
In the module that does have Spring dependencies, create a DTO
#Component(value = "beanDTO")
public class BeanDTO {
#Autowire
private List<InterfaceType> implementingBeans;
public List<InterfaceType> getImplementingBeans() {
return implementingBeans;
}
}
and then use SpEL to retrieve the value of implementingBeans from the beanDTO bean.
<bean id="service" depends-on="beanDTO" class="...Service">
<property name="implementingBeans" value="{beanDTO.implementingBeans}" />
</bean>
Spring will create the BeanTDO bean, inject all the beans that are of type InterfaceType. It will then create the service bean and set its property from beanDTO's implementingBeans property.
Following comments on question:
In an effort to be more JSR 330 compliant, Spring has introduced support for Java EE's javax.inject package. You can now annotate your injection targets with #javax.inject.Inject instead of #Autowired. Similarly, you can use #Named instead of #Component. The documentation has more details.
I'm writing a service registry class. This service registry will scan packages for annotated classes and then populate some internal map. What I need then, is to be able to query for services (by String name) using some method (let's say Object get(String name)). This method will then search internal map for a service with such name and returns instance.
What I'm doing right now, is having this ServiceRegistryBean implement ApplicationContextAware and BeanDefinitionRegistryPostProcessor and a list of Strings (package names) given on construct.
Then, as soon as the bean is constructed, registry post processor kicks in (see note) and the registry class adds the service classes as new beans (singleton, lazy loaded) to the spring bean registry. Then, getting the service instance is as simple as requesting a bean from context, returning singleton instance.
My question is: is there a better way in Spring to do this? I've looked into bean factories, but it does not seem to me the same. Support for auto-wiring and DI in service instances is essential, that's why I want Spring to instantiate it. Also, I like the idea of Spring taking care of singletons.
Note: I've found out, that when I inline the bean creation in <constructor-arg> (that is, bean is not named and is just an instance passed as constructor argument of other bean - in my case, I'm passing registry as a parameter to other constructor), BeanDefinitionRegistryPostProcessor interface methods (namely public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry)) is not called at all! I'm quite sure, it is some behavior of Spring I don't know about, but I was not able to find proper documentation for the post-processor.
Thank you for any hints or insights!
Scanning for custom annotations it's already supported, you only need to add a include-filter to <context:component-scan> tag, ie
<context:component-scan base-package="org.example">
<context:include-filter type="annotation" expression="some.Annotation"/>
</context:component-scan>
see http://static.springsource.org/spring/docs/current/spring-framework-reference/html/beans.html#beans-scanning-filters
If you turn on default-lazy-init I suppose that the DI Container is ready to use as Service Locator Registry.
About the note, only root bean definitions are taken into account when looking for BeanFactoryPostProcessors, inner beans are ignored.
Usually, scanning and registering beans is done by BeanDefinitionParsers instead because you known when the beans are registered and beans are visible for tools, like STS Spring Bean Explorer, but using a BeanDefinitionRegistryPostProcessor is correct. The interface ensures that beans are defined before other BeanFactoryPostProcessors run.
I want to inject a plain java object using Spring programmatically without using any xml configuration. Want to inject fields/methods annotated with tags Like #EJB, #PostConstruct etc. Is that possible? Thanks!
Creating an ApplicationContext without XML (using AnnotationConfigApplicationContext)
With AnnotationConfigApplicationContext, you don't need any XML at all. You create the Application context programatically and either
a) manually register annotated classes
appContext.register( MyTypeA.class,
MyTypeB.class,
MyTypeC.class );
b) or scan the classpath for annotated classes
appContext.scan(
"com.mycompany.myproject.mypackagea",
"com.mycompany.myproject.mypackageb"
)
If you use one of the convenience constructors
AnnotationConfigApplicationContext(Class<?> ... annotatedClasses)
or
AnnotationConfigApplicationContext(String ... basePackages)
the context is created and refreshed automatically, otherwise you need to call the refresh() method manually after adding the classes or packages.
Autowiring existing non-Spring beans (using AutowireCapableBeanFactory)
For autowiring an existing bean I think the preferred idiom is to use
appContext.getAutowireCapableBeanFactory().autowireBean(existingBean)
Or, if you want more control, use
appContext.getAutowireCapableBeanFactory()
.autowireBeanProperties(
existingBean,
autowireMode,
// e.g. AutowireCapableBeanFactory.AUTOWIRE_BY_TYPE
dependencyCheck
)
For further reference, see
ApplicationContext.getAutowireCapeableBeanFactory()
AutowireCapeableBeanFactory.autowireBean(Object existingBean)
AutowireCapeableBeanFactory.autowireBeanProperties(Object
existingBean, int autowireMode,
boolean dependencyCheck)
Yes, you can annotate any POJO with #Component, #Service, #Controller, or #Respository (depending on its responsibilities), and it becomes a spring bean. You just have to put this line into the applicationContext.xml:
<context:component-scan base-package="org.yourbasepackage" />
You can also use #PostConstruct and #PreDestroy instead of the xml init-method and destroy-method.
Update: There is a project called spring-javaconfig. Parts of it have become part of the core spring and you can see more details here. In short, it allows you to use java classes instead of xml for configuring your beans.
The documentation around annotation-based Spring configuration can be found here. You will need a minimal amount of XML, so that Spring knows to look for annotation-based configuration:
<context:component-scan base-package=".." />
Once that is done, you can use the #Autowired annotation to set up your beans.