Can "context: componentscan" scan custom annotations? If so, where does it store the scanned beans in application context after scanning? How can I access the results?
We register beans or components in XML configuration file. So Spring can detect those beans, components. Spring also support to auto scan, detect and instantiate beans from pre-defined project package via the annotation. So we don't have to declare in the configuration anymore. For example:
<context:component-scan base-package="abc.controller, abc.service" />
in your Controller or Service, you just have to add annotation like:
#Controller
public class SampleController
#Service
public class SampleService
Spring will know that your SampleController and SamplerService and you can use it as you want.
Some detail here: http://docs.spring.io/spring-javaconfig/docs/1.0.0.M4/reference/html/ch06s02.html
Related
I and my friend were discussed about #ComponentScan and #Import. Which one is better?
We have 2 different ideas.
#ComponentScan: Easy to use, import all beans from the component
scan.
#Import: You need to know what component you want to use, no need to scan all.
How about your idea? Which one is better for you to use?
Thanks!
#Import is used to import Java configuration classes marked with #Configuration/#Component typically. So if you have a bean inside this component, Spring will load it into Application Context. You can just put the name of the component or class and Spring will pull it up for you.
However, by using #ComponentScan, you tell the application which packages to scan for java classes are annotated with #Configuration/#Component (or any of #Component's sub-annotations like #Service or #Repository etc) and load all of them up in Application Context so they can be autowired when required. If there are inner instances that need to be populated, Spring will take care of it.
You can read more about #Import and #ComponentScan on their respective doc pages.
This page explains pretty well the difference.
#ComponentScan scans and searches for any beans inside packages/classes specified under basePackageClasses or basePackages options, whichever is configured.
This option also allows you to filter some classes that you do not want to be included in search.
#Import is like clubbing one java configuration into another.
eg:
#Configuration
#ComponentScan(basePackages="com.stackoverflow")
public class Dbconfig {
#Bean
public Datasource dSource(){
return new Datasource()
}
}
#Configuration
#Import(Dbconfig.class)
#ComponentScan(basePackages="org.hellospring")
public class AppConfig {
...// beans
}
So here, if we check AppConfig class,
it will include all beans registered in Dbconfig configuration class including inside of package com.stackoverflow
+
It will include all beans inside AppConfig class and beans under package org.hellospring
Good day, guys. I have a question about autowiring services into my classes when using Springboot. All of the examples I have seen on the Internet as well as in the Springboot specification do something of the like (taking an excerpt from the Springboot version 1.5.7 specification):
package com.example.service;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
#Service
public class DatabaseAccountService implements AccountService {
private final RiskAssessor riskAssessor;
#Autowired
public DatabaseAccountService(RiskAssessor riskAssessor) {
this.riskAssessor = riskAssessor;
}
// ...
}
This is a class that injects a property through its constructor, by means of #Autowiring the constructor. Another form is to #Autowire the property like this:
#Autowired
private final RiskAssessor riskAssessor
But, where I work, for these two methods to work, I have been told that I need to use this method:
applicationContext.getAutowireCapableBeanFactory().autowireBean(Object.class)
They have told me that I need this in order for the #Autowired annotation to work.
Now my question to you is: why is there no simple annotation that allows the #Autowire to function correctly? (Something like #AutowiredClass). The above method is too verbose and hard to remember, so surely there must be a better way to make #Autowired work on classes in order to inject services, just like we do in Grails where we just say def someService and it is automatically injected.
If you want properly use #Autowired in your spring-boot application, you must do next steps:
Add #SpringBootApplicationto your main class
Add #Service or #Component annotation to class you want inject
Use one of two ways that you describe in question, to autowire
If you don't have any wiered package structure and the main class package includes all other classes you want spring to instantiate (directly or in the subpackages) a simple annotation #ComponentScan on your main class will help you save all those boiler plate code. Then spring will do the magic, it will go and scan the package(and subpackages) and look for classes annotated with #Service, #Component etc and instantiate it.
Even better, use #SpringBootApplication in your main class, this will cover #Configuration as well. If it is a green field project , I would encourage to start from start.spring.io - a template generation/scaffolding tool for spring
Now my question to you is: why is there no simple annotation that allows the #Autowire to function correctly?
There is: #SpringBootApplication
If you put this at the root of your application (file that contains the main class) and as long as your services are at the same package or a sub-package, Spring will auto-discover, instantiate, and inject the proper classes.
There's an example in this walk-through: REST Service with Spring Boot
As described in that page:
#SpringBootApplication is a convenience annotation that adds all of the following:
#Configuration tags the class as a source of bean definitions for the application context.
#EnableAutoConfiguration tells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings.
#ComponentScan tells Spring to look for other components, configurations, and services in the hello package, allowing it to find the controllers.
You need to annotate the implementation of RestService as a #Service or #Component so Spring would pick it up.
#Service
public class MyRiskAssessorImpl implements RiskAssessor {
///
}
#Autowired almost works out of the box. Just do your component scanning of the class you want to autowire and you are done. Just make sure your main class (or main configuration class) uses #ComponentScan("{com.example.app}") or #SpringBootApplication (main class). The docs explain this stuff pretty good
I have this interface:
public interface liteRepository extends CrudRepository<liteEntity, Long>, JpaSpecificationExecutor<liteEntity> {...}
It works, all is well.
However, intellij does not register this class as a spring component. If I annotate this interface with #Component, then intellij recognizes this as a spring bean and I can #Autowire this repository in my integration tests.
My code still works after annotation, but I'm not confident that I am not messing with things that I should not be messing with.
Question:
Is there any harm in adding the #Component annotation to this interface?
The only thing that #Component annotation means is that the class is eligible for becoming a Spring bean during Spring's component-scan.
So, if you want it to be a Spring bean and you did not define it as a Spring bean anywhere else, you can safely add the #Component annotation.
Of course, this will only work if you have the actual component scan configured somewhere(for, example <context:component-scan base-package="..."> in some Spring config file), which I am assuming you already heave, since the bean is properly getting autowired after you add the annotation.
I am new to Spring framework.While reading dependency injection i found out two ways to inject beans anotationbased and xml based.
In xmlBased it is quite simple that you define one bean inside your application context xml file.
eg.
<bean id="wild" class="com.javapapers.spring.ioc.Wolf" />
<bean id="zoo" class="com.javapapers.spring.ioc.Zoo">
<property name="wild" ref="wild" />
</bean>
but in Annotation based configuration we just have to write
<context:component-scan base-package="com.javapapers.spring.ioc" />
I want to know how it will load "wild" and "zoo" .
Does it means it will load all beans or only specific which is written under #Service annotation..???
I also want to know how it is loaded ..??? is all beans gets initialized when application is loaded..??
Thanks ...!!
When you use <context:component-scan base-package="com.javapapers.spring.ioc" />, spring will instanciate all classes that are in the "com.javapapers.spring.ioc" package and have one of this annotation :
#Service
#Controller
#Repository
...
And yes, all beans gets initialized when you launch your application.
You can have more info in this page : here
Beans get initialized through an ApplicationContext, which is also a BeanFactory. With an XML configuration, you would need an implementation of that interface, ClassPathXmlApplicationContext. Your application needs to create such a class, register your XML file(s), and refresh the context. When that is done, Spring will start creating your beans by reading the configuration.
When it hits the <component-scan> element, Spring will scan your declared packages for any classes annotated with #Component or its specializations. From the docs:
In Spring 2.0 and later, the #Repository annotation is a marker for
any class that fulfills the role or stereotype (also known as Data
Access Object or DAO) of a repository. Among the uses of this marker
is the automatic translation of exceptions.
Spring 2.5 introduces further stereotype annotations: #Component,
#Service, and #Controller. #Component is a generic stereotype for any
Spring-managed component. #Repository, #Service, and #Controller are
specializations of #Component for more specific use cases, for
example, in the persistence, service, and presentation layers,
respectively.
Therefore, you can annotate your component classes with #Component,
but by annotating them with #Repository, #Service, or #Controller
instead, your classes are more properly suited for processing by tools
or associating with aspects. For example, these stereotype annotations
make ideal targets for pointcuts.
Thus, if you are choosing between using #Component or #Service for
your service layer, #Service is clearly the better choice. Similarly,
as stated above, #Repository is already supported as a marker for
automatic exception translation in your persistence layer.
When it finds those classes, it will create an instance of each of them.
As for how it does this, it's a little more complicated. The overall strategy is with reflection. However, because of your configuration, Spring will sometimes generate (java or cglib) proxies instead of clear instances so that it can add behavior.
All the steps are described in detail in the official documentation.
Will be loaded all beans annotated with
#Controller
#Component
#Service
#Repository
which are inside a packages com.javapapers.spring.ioc, and its subpackages.
I have a custom annotation that is declared as a Spring managed Service:
#Service
public #interface MyServiceAnnotation {
// my service url
String url();
}
The above declaration enables my services to be autowired as Spring Managed beans.
#MyServiceAnnotation(url="/path/serviceLocation")
public class SomeService {
doWork();
}
However, there are certain services that have their bean definitions in an applicationContext.xml. Adding the #MyServiceAnnotation to such beans makes them both Autowiring enabled, as well as inject dependecy through the xml file.
Due to issues related to legacy code, I don't want to remove the xml bean definitions and make them all autowired.
So, is there a way in which I could turn off autowiring in this case, and still use #MyServiceAnnotation? Ideally I would like to have the #Service annotation on MyServiceAnnotation, the existing services would still use the #MyServiceAnnotation but would get their dependencies injected based on the xml. All the new services would be autowired without the xml bean definitions.
One possible approach is to create NonSpringManagedMyServiceAnnotation that is same as MyServiceAnnotation, but without the #Service annotation on it. The downside of this is that I'd have to duplicate rest of the code from MyServiceAnnotation, which I don't want to.
This is one approach, may not be that optimal though. I am assuming you would have specified a component-scan tag in your xml to scan the classes having the Spring stereotype annotations, these tags support a exclude-filter sub-tag to ignore specific patterns. If the files that you specify follow a specific pattern(specific packages, specific names etc) then you may simply be able to specify this sub-tag to ignore classes holding your annotation.
<context:component-scan base-package="mypackage">
<context:exclude-filter type="regex" expression=".*ToBeIgnoredNaming"/>
</context:component-scan>
I think you should try to maintain separation of concern.
Adding #Service on your annotation makes it a de facto Spring service : since this is not the behavior you want, you may simply have a simple #MyAnnotation (with url property) on each services (legacy and new).
Then you add Spring's #Service annotation on each new service to enable bean registration by annotation.