I have the following classes:
#ComponentScan
public class CDPlayerConfig {
#Autowired
private static CompactDisc cd;
public static void main(String[] args) {
ApplicationContext context = new AnnotationConfigApplicationContext(CDPlayerConfig.class);
CompactDisc cd = context.getBean(CompactDisc.class);
System.out.println(cd);
}
}
public interface CompactDisc {
void play();
}
#Component
public class SgtPeppers implements CompactDisc {
public void play() {
System.out.println("play song....");
}
}
When I run the class CDPlayerConfig, the program runs successfully. However, if I remove the ComponentScan annotation to CompactDisc interface or SgtPeppers I get the following error:
Exception in thread "main"
org.springframework.beans.factory.NoSuchBeanDefinitionException: No
qualifying bean of type 'CompactDisc' available
I think that ComponentScan annotation marks the package where Spring looks for beans. CDPlayerConfig, CompactDisc and SgtPeppers are all placed in the same package, so allegedly moving ComponentScan annotation from one class to another should not make a difference.
If so, why do I get an error?
For #ComponentScan to work you have to "tell" spring where to search, or it must find it with help of other, already loaded, #ComponentScan annotated class (your class must be then annotated also with #Component, #Configuration etc. so it could be found).
In your case, you register application context in the first line of main method - you have specified there to load CDPlayerConfig.class which is #ComponentScan annotated so now spring can automatically find other beans in the package:
ApplicationContext context =
new AnnotationConfigApplicationContext(CDPlayerConfig.class);
If you want to move #ComponentScan to another class, you have to change class registered in AnnotationConfigApplicationContext to some #ComponentScan annotated class:
SgtPeppers:
#Component
#ComponentScan
public class SgtPeppers implements CompactDisc {
(...)
Main in CDPlayerConfig:
ApplicationContext context =
new AnnotationConfigApplicationContext(SgtPeppers.class);
Note you should register context from concrete classes (not interfaces).
Also, above sample would work even without #ComponentScan annotation on SgtPeppers, but then beans defined in other classes from the package wouldn't be found.
Related
I read inside many articles that #ComponentScan should be placed with #Configuration on top of a class. Here some references:
we use the #ComponentScan annotation along with #Configuration
annotation to specify the packages that we want to be scanned
(https://www.baeldung.com/spring-component-scanning)
#ComponentScan(basePackages = "com.zetcode") #Configuration public
class Application { ... } (http://zetcode.com/spring/componentscan)
The #ComponentScan annotation is used with the #Configuration
annotation to tell Spring the packages to scan for annotated
components. (https://dzone.com/articles/spring-component-scan)
I was curious to try if without #Configuration an exception would have been thrown. Surprisingly everything works fine even without #Configuration. Here the code:
#ComponentScan
public class AppConfig {
public static void main(String[] args) {
ApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
for (String beanDefinitionName : context.getBeanDefinitionNames()) {
System.out.println(beanDefinitionName);
}
}
}
I had just one sample bean which got printed.
#Component
public class Car {
}
This was the output of the main method:
org.springframework.context.annotation.internalConfigurationAnnotationProcessor
org.springframework.context.annotation.internalAutowiredAnnotationProcessor
org.springframework.context.annotation.internalCommonAnnotationProcessor
org.springframework.context.event.internalEventListenerProcessor
org.springframework.context.event.internalEventListenerFactory
appConfig
car
Why does it work? and why do they tell to use it with configuration? was it an old requirement?
And even more surprisingly appConfig becomes a bean, even if it does not have any particular annotation such as #Configuration or #Component. So does that mean that anything that gets put as argument of new AnnotationConfigApplicationContext() gets turned into a bean no matter what annotation does it has or has not?
I probably miss some core spring behavior which would justify this. Any idea?
You are still using #Configuration together with #ComponentScan implicitly. By pasing the AppConfig.class as param to the context, it considers it configuration. That would explain the bean created for it as well
I know that #ComponentScan with #Configuration tell Spring where to look for beans.
#ComponentScan
#Configuration
public class MyApp{
...
}
What I do not understand is on which class I have to put these two annotations. Should they stay on the class containing the main method?
Like this
#ComponentScan
#Configuration
public class MyApp{
public static void main(String[] args) {
...
}
}
Or they can stay on whatever class of the application?
The question comes from the fact that Spring has to know the location of #ComponentScan... or is there an automatic way of detection of the #ComponentScan annotation which Spring is performing under the hood?
Hope to have explained myself!
You can put it wherever you want (I usually put mine in com.domain.project-name.config) and just specify the directories it should scan, for example if you want it to scan everything in project use
#ComponentScan("com.domain.project-name")
#Configuration
public class Config {
...
By default, ComponentScan scans all the annotated classes at the current directory level and below.
#Configuration annotation tells the Spring container that the class contains Spring bean configuration.
#ComponentScan annotation tells the Spring container that the annotated class to scan/searches for other annotations and components. You can also define package name to scan with the annotation like #ComponentScan("your.package.name") or you can give package/class names that need not be scanned.
Hence, you can put these annotations on any class that defines your bean configuration and could be required by spring container to parse and create objects for your entities/POJOs, services and DAOs.
To conclude, I would like to add #ComponentScan and other annotations are there for automatic detection. Else, you would need to define XMLs (that's what happens under the hood with annotations) for spring to read and perform these actions.
Using simple example. You can place #ComponentScan with #Configuration in any class which main method can scan.
Main class scans MyScan class which then scan for bean class.
package com.boot.spring;
#SpringBootApplication
#ComponentScan(basePackages = "com.boot.scan")
public class BootApplication {
public static void main(String[] args) {
ConfigurableApplicationContext ctx = SpringApplication.run(BootApplication.class, args);
System.out.println(ctx.getBean("demoBean"));
}
}
Bean class is in different package
package com.boot.bean;
#Service
public class DemoBean {
}
Now, bean class is discovered through DemoScan class
package com.boot.scan;
#ComponentScan(basePackages = "com.boot.bean")
#Configuration
public class DemoScan {
}
I want to configure in my #Configuration class bean which is already created by other library's autoconfiguration. I just need to change some fields in that class after it's being initialized.
But I can't find a proper way how to provide code block in #Configuration class and not using #Bean annotation. Is there an ideomatic way to do so in spring?
One way to do this:
#Configuration
class TestConfig {
#Autowired
private SomeBean someBean;
#PostConstruct
private void initSomeBean() {
// someBean.setProperty("qwe");
}
}
#PostConstruct annotation defines init-method, which is getting called after SomeBean is autowired. In this method you can adjust your bean
Do you want to import one Config in AnotherConfig on? It can be done via annotation placed on AnotherConfig:
import org.springframework.context.annotation.Import;
...
#Import(value = {Config.class})
public class AnotherConfig ... {}
I've created this custom test configuration:
#TestConfiguration
public static class RestTemplateTestConfiguration {
#Bean
#Primary
public static ApplicationDao applicationDao() {
ApplicationDao mock = Mockito.mock(ApplicationDao.class);
// some stuff code
return mock;
}
}
I've set a breakpoint on applicationDao, but it's never reached, and therefore mock is never injected.
EDIT
ApplicationDao is an #Repository annotated class:
#Repository
public interface ApplicationDao extends MongoRepository<Application, String> {
So, how could I override this #Repository annotated AplicationDao?
Currently, I'm getting this message when spring starts:
Skipping bean definition for [BeanMethod:name=applicationDao,declaringClass=net.gencat.transversal.espaidoc.functional.references.GroupReferencesTest$RestTemplateTestConfiguration]: a definition for bean 'applicationDao' already exists. This top-level bean definition is considered as an override.
Any ideas?
If your method applicationDao() is never called it means that your spring boot is not scanning the package where RestTemplateTestConfiguration is located.
The simplest solutions is to move the configuration under the same package (or its children) as the one that contains the class annotated with #SpringBootApplication.
OBS : This rule applies even though the configuration is in the test directory instead of main.
Another solution is to add #ComponentScan with the configuration package or to use #Import(RestTemplateTestConfiguration.class) at your spring boot test level.
SUGGESTION:
For your problem you can use:
#Mock
ApplicationDao applicationDao;
and if you have another service that uses this one to use:
#InjectMock
MyService myService;
This is based on https://spring.io/guides/gs/accessing-data-jpa/
I tried to move demo() in a different class in a different package (Application still on top of the filesystem hierarchy)
How do I make demo() run when i boot the project?
Application.java
package com.company.app
#SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class);
}
}
CommandLineRunner.java
package com.company.app.runner
public class Test {
#Bean
public CommandLineRunner demo() {
System.out.print("Run 1");
return (args) -> {
System.out.print("Run 2");
};
}
}
Add #Configuration to the Test class so that it is picked up when the classpath is scanned.
I haven't seen a Lambda for the CommandLineRunner before. Very nifty and saves having to create a class that specifically implements the CommandLineRunner.
Otherwise, you could implement CommandLineRunner to test and annotate as #Component.
#Component
public class ApplicationLoader implements CommandLineRunner {
#Override
public void run(String... strings) throws Exception {
System.out.print("Run 2");
}
}
* Update *
Answering the question in the comment with this update as I have more room to type...
#SpringBootApplication composes those other annotations as you indicated but those annotations are only applied to the specific class that it is defined on. The #Configuration annotation is telling Spring that the class defines beans that should be managed by the application context. The #ComponentScan tells spring to look through the classpath for classes that have specific annotations (e.g. #Component, #Service, #Configuration) and then act on those classes based on the type of annotation. the #EnableAutoConfiguration is the magic that loads appropriate beans based on the project dependencies (e.g. if mongo driver is on the classpath then create a MongoTemplate).