I'm wondering whether it's possible to autowire a class annotated with #Configuration into any annotated class (#Component, #SpringBootTest, etc)? I know it's possible between #Configuration classes, but I can't find a straight answer on if it's possible outside #Configuration classes (both test and non-test classes). Sample code to illustrate what I mean below:
#Configuration
public class Config {
//Anything in here
}
//any non-Configuration annotation
public class Autowired {
#Autowire
Config config; //like this
}
If you are looking at the source code of #Configuration annotation you can see it's annotated itself with #Component, making annotated classes with it also Spring components, so making them available for injection.
#Target({ElementType.TYPE})
#Retention(RetentionPolicy.RUNTIME)
#Documented
#Component
public #interface Configuration {
Yes. you can autowire configuration class as #configuration annotation contains #component
Related
#Import annotation in Spring is used in order to group configurations.
I know that the standard syntax for this annotation looks like this:
#Configuration
#Import({ Manager.class, Programmer.class })
class WorkerConfiguration {
}
But I am wondering is it possible to use #Import annotation to import a group of those annotations outside of a configuration file(maybe in the main file).
Example:
#SpringBootApplication
#Import({ Manager.class, Programmer.class })
public class App{
public static void main(String[] args) {
SpringApplication.run(App.class, args);
}
}
Import Javadoc :
Indicates one or more component classes to import — typically
#Configuration classes.
#Import is often used in the context of a class annotated with #Configuration class to include some declared beans in #Configuration in another one. But it also works with composite annotations that contains among other annotations the #Configuration one.
And in Spring Boot it turns out that several annotations include #Configuration:
For example #SpringBootApplication that you ask to is also composed (among other things) of a #Configuration annotation :
Indicates a configuration class that declares one or more #Bean
methods and also triggers auto-configuration and component scanning.
This is a convenience annotation that is equivalent to declaring
#Configuration, #EnableAutoConfiguration and #ComponentScan.
So yes what you want to do is valid.
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 am creating tests with #WebMvcTest annotation and found that if I have a #ComponentScan annotation in the application class it will break the expected behavior of the tests.
According to the WebMvcTest javadoc:
Using this annotation will disable full auto-configuration and instead apply only configuration relevant to MVC tests (i.e. #Controller, #ControllerAdvice, #JsonComponent Filter, WebMvcConfigurer and HandlerMethodArgumentResolver beans but not #Component, #Service or #Repository beans)."
The problem is that with #ComponentScan it is instantiating beans annotated with #Service. If instead of #ComponentScan I specify the scan base packages in the #SpringBootApplication annotation everything works as expected.
Another problem happens when I specify the controller class in the #WebMvcTest annotation. When there is a #ComponentScan annotation in the application class it will load all controllers instead of loading only the specified one.
Is this a bug in Spring Boot?
I want to use #ComponentScan because of the excludeFilters attribute which is not available in the #SpringBootApplication annotation.
A workaround I have found is to create a separate class with #Configuration annotation and move the #ComponentScan there.
Found the reason for this odd behavior.
This is the declaration of the #SpringBootApplication annotation:
#Target(ElementType.TYPE)
#Retention(RetentionPolicy.RUNTIME)
#Documented
#Inherited
#SpringBootConfiguration
#EnableAutoConfiguration
#ComponentScan(excludeFilters = {
#Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
#Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public #interface SpringBootApplication {
As you can see the #ComponentScan annotation inside the #SpringBootApplication has the excludedFilters attribute specified.
When I added the #ComponentScan annotation directly in my application class I did not specify the default excludedFilters and that is why the behavior was different.
#ComponentScan creates beans using both #Configuration and #Component. Both these annotations work fine when swapped. What is the difference then?
#Configuration Indicates that a class declares one or more #Bean
methods and may be processed by the Spring container to generate bean
definitions and service requests for those beans at runtime
#Component Indicates that an annotated class is a "component". Such
classes are considered as candidates for auto-detection when using
annotation-based configuration and classpath scanning.
#Configuration is meta-annotated with #Component, therefore
#Configuration classes are candidates for component scanning
You can see more here:
http://docs.spring.io/spring-framework/docs/4.0.4.RELEASE/javadoc-api/org/springframework/context/annotation/Configuration.html
A #Configuration is also a #Component, but a #Component cannot act like a #Configuration.
Actually answer is not complete, is it true that:
#Component Indicates that an annotated class is a "component". Such
classes are considered as candidates for auto-detection when using
annotation-based configuration and classpath scanning.
But you do can create i.e MyConfiguration.java class then stereotype with #Component and add #Beans declaration to it. In this way it will looks as a configuration, main difference is that when annotated class with #Configuration #Bean annotated methods are proxy using CGLIB which made in code calls after the first one to return bean from context instead of execute method again and create another instance as happens when using #Component with #Bean
There is a very subtle difference between them. Let me provide a very quick outlook to this.
Consider the below scenario:
#Configuration
public class MyConfig {
#Bean
public ServiceA aService(){
return new ServiceA();
}
#Bean
public ServiceB bService(){
return new ServiceB(aService());
}
}
Note that ServiceB bean has a dependecy on ServiceA and this is not autowired. Instead, the way it's written implies that a new instance is created, which is not actually created by Spring. You, the programmer, did it with the new keyword instead.
So, if we do use #Configuration, then it uses CGLIB proxying, and in this situation it creates a singleton bean managed by the Spring context. If you invoke it multiple times, it returns the same bean that was created by Spring - sort of autowiring effect.
Whereas if you use #Component, it won't do this proxying and will simply return a new instance every time the method is invoked, instead of providing the Spring managed instance. (Remember that a Spring bean is something that is managed by the Spring container, and, as a developer, it's your job is to pull them in, e.g. with #Autowired.
The same #Component effect can be achieved with #Configuration(proxyEnabled= false) (This is also referred to as bean light mode processing). So, in light mode, you would end up doing something like this:
#Configuration(proxyEnabled = false) // Lite mode, same effect as #Component
public class MyConfig {
#Bean
public ServiceA aService() {
return new ServiceA();
}
#Autowired
#Bean
public ServiceB bService(ServiceA aServiceBean){
return new ServiceB(aServiceBean);
}
}
Refer here for a more elaborate explanation
Hope that helps! Happy Coding!
#Configuration - It is like beans.xml but Java-based bean configuration. It means class annotated with this annotation is the place where beans are configured and will be a candidate for auto-detection. In this class, methods are annotated with #Bean which return an object of the class.
Example:
#Configuration
public class ConfigClass {
#Bean
public UserClass getObject() {
return new UserClass();
}
}
#Component - You cannot autowire (#Autowired) any class if it is not marked with #Component. It means when you want to autowire any class using annotation that class should be annotated with #Component.
Example:
#Component
public class A { .... }
public class B {
#Autowired
A a;
.....
.....
}
Spring Document for reference:
https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/context/annotation/Configuration.html
#Component is imported by default with #Configuration. controllers, service, and repostory are children components (along with Configuration). They are also candidate for auto-detection.
I am extending on #reus's answer.
#Configuration Indicates that a class declares one or more #Bean methods and may be processed by the Spring container to generate bean definitions and service requests for those beans at runtime.
If you look at the #Configuration class, you will see that it is meta-annotated with #Component.
#Target(value=TYPE)
#Retention(value=RUNTIME)
#Documented
#Component
public #interface Configuration
#Bean is enables us to define the dependency in any way we like, this is why the #Bean annotation goes above a methods and we manually create a bean object and return it from that method. #Component enables us to define a dependency quickly, that is why #Component goes above classes. We only inject it wherever we need.
Collectively these 3 points says that- to quickly define a bean, we can annotate the class with #Component. To define a bean as we like (support custom requirements), we can write the bean definition using #Bean inside a #Configuration annotated class.
Apart from the differences highlighted by reos.
The reason why #Configuration cannot be replaced by #Component is as below:
The difference is in how the inter bean dependency is handled.
Refer the link for a detailed explanation with example:
Difference between Configuration and Component