Whenever spring boot application runs it sees the #SpringBootApplication annotation and runs the #ComponentScan which scans the classes with the annotations such as #Component etc and makes the object in the container.I have a little bit of idea that it might be using reflections internally to create the objects but I am not able to connect all the dots .I want to know what exact information does #AutoConfiguration gives to the container that it is able to get all the information of all the component annotated clases of any component ?
P.s. I have edited the wrong question
First of all #AutoConfiguration is responsible for setting up the default configurations for a Spring boot application depending on the dependencies we have added in the pom.xml.
The thing you are looking for is #ComponentScan which is what performs the component class scanning within the default package and all of its sub packages. It does this by going through each class in the package and looking for the #Component,#Service or #Repository annotations. If any of them is present then the container adds this as a bean.
#SpringBootApplication internally has the #ComponentScan annotation added to it along with two others.
Related
Why do we need META-INF/spring.factories when create starters for Spring Boot applications? What if omit it at all or leave empty?
Doesn't the target application's #SpringBootApplication which is
a combination of three annotations #Configuration (used for Java-based
configuration), #ComponentScan (used for component scanning), and
#EnableAutoConfiguration
scan everything and find all beans from all the starters with no help of META-INF/spring.factories?
Component Scanning would scan the packages that you give it. You could technically tell it to scan all the packages of your dependencies, too, and it would start loading up any beans defined in them. If you don’t specify any packages to scan, then Spring will use the base package where the annotation is applied, which would very likely not include beans defined in any dependency libs.
There’s another layer to this- a lot of the libraries you use may be using annotations like “#AutoConfigureBefore” to give spring instructions on the order of bean creation. Component Scanning will not respect that, which could result in some weird behaviors if some dependency tries to override a bean from another which is annotated with #ConditionalOnMissingBean (I.e. create this bean only if it doesn’t exist.) You could easily end up with name collision issues where that bean actually gets created first, and then the override bean is created, too.
So the answer seems to be no. You need spring.factories.
Doesn't the target application's #SpringBootApplication scan everything...
No, it doesn't scan everything because if it was it could take a lot of time and resources. Think about it in a way that in order to understand whether the file with an extension *.class contains a bean (something annotated with #Component for example) it needs at least to read a class and analyze the byte code or even load it into memory to check the annotation by reflection.
So if your application's root package is in com.sample.app (the package with the class annotated with #SpringBootApplication), then spring boot by convention scans only the beans in this package and the packages beneath it. This means that it won't scan any thirdparties (assuming they won't be placed in com.sample.app anyway).
Now Its true that you can change the rules of component scanning, but again, you don't want to scan everything for performance reasons at least.
So Autoconfiguration modules (technically implemented with META-INF/spring.factories can specify an additional place (classes annotated with #Configuration) that spring boot will load despite the fact that they are not placed under the packages of your application
or, in other words, they do not obey the default component scanning rules.
In addition, spring.factories file allows to specify much more than auto configuration rules, you can specify environment post processors there for example, and other stuff that can be useful for your application, probably mostly beneficial at the level of application infrastructure, but still.
Is it possible to let #ComponentScan work on whole Spring Boot project? Now when I create a new package, I need to add it into #ComponenScan annotation otherwise beans won't be found. If it's possible to allow to search everywhere in project can it cause performance problems or any other issues?
Example:
first component StorageRoom\backend\src\main\java\com.storeage (here is main class with #ComponentScan annotation)
second component StorageRoom\backend\src\main\java\security
third component StorageRoom\backend\src\main\java\component3
#ComponentScan without arguments tells Spring to scan the current package and all of its sub-packages.
So for your problem, I would recommend that you should have your main class at the root of the application and then annotate it with #ComponentScan there.
Something like here -
StorageRoom\backend\src\main\java\SpringBootApplicationMain.java
Find more here: https://www.baeldung.com/spring-component-scanning
What is the entry point of a spring boot application?
While going through a Spring Boot application code, all that it says is there is a code
public static void main having - SpringApplication.run(Application.class, args)
Example - SpringBoot2RestServiceApplication.java .
But how to get to know what is the entry point, just by going through the code. Earlier, if we go through applicationContext.xml - example - applicationContext.xml, we could understand the flow.
Is there any way, or maybe a standard to follow to make this understanding self-explanatory?
My question was more of understanding the flow of the application than finding the main class. One option could be separating configurations(#Configuration) to a separate class having multiple #Bean annotations, this would help in finding all bean wirings at one place. Is there a standard that large projects use to make code flow understandable?
The easiest thing to do would probably be to search for #SpringBootApplication in your code.
But, a guaranteed way to get this information would be to build the Spring Boot JAR. From there, you can open the resulting JAR and find the main class defined in the manifest, under META-INF/MANIFEST.MF. You'll see it under the Start-Class attribute:
Start-Class: com.example.foo.Application
I think the OP is studying an existing Spring Boot application, and is asking how to locate any runner, such as Application Runners, Command Line Runners, MVC controllers, Rest controllers, etc.
I don't know if there is an easy way to locate those, unless they are grouped together in the original design.
It's a difficult problem to do programmatically, because threads can be launched outside of Spring, for example in a constructor or a #PostConstruct.
It would be nice though if there were IDE support to easily locate anything that gets launched by Spring Boot
search #SpringBootApplication annotation in your project, the class with #SpringBootApplication annotation will automatically do the component-scan for the sub packages.
if no #SpringBootApplication annotation found, search the class extending "SpringBootServletInitializer" which is also a starting point for the spring boot application
The Entry of any spring boot application has an annotation of #SpringBootApplication
I'm a newbie in Spring and have one question about inserting all controllers in another directory than an executable spring boot file.
This tutorial
I have to have GreetingController.java and Application.java in one directory because if not, then I can't run mvn spring:boot-run
Is there a way to have Application.java in /java directory and all controllers in /java/controllers? Can I create some config file to inform Application.java where all the files are?
Thank you
I am little bit confused. But first point - It is good to have all controllers in separate directory, it is best practise, but better is using /rest directory.
Second point - You have to define application context and enable few things (for example via annotation). Try to look here http://docs.spring.io/spring-boot/docs/current/reference/html/using-boot-structuring-your-code.html, here you can see annotations and basic structure.
For you, set this annotations to your Application.java:
#Configuration
#EnableAutoConfiguration
#ComponentScan
More information and where to write it you can find in link what I posted.
If you configure application context and you set EnableAutoconfiguration and ComponentScan, Spring will look up for all annotated components and your controllers (if you annotated you controller class by #Controller annotation.
According to the Spring docs, to enable autodetection of annotated controllers, you add component scanning to your configuration:
<context:component-scan base-package="org.springframework.samples.petclinic.web"/>
My question is, why is this necessary?
If a Controller has an annotation to already indicate what it is, shouldn't that be enough for Spring without component scanning?
How else would Spring find the classes? If you haven't told Spring to look in a certain class or package, those classes aren't going to get loaded, and Spring is never going to find them.
This is more a limitation of the java classloading model (if you can call it a limitation), then it is a limitation of Spring.
You only have to put the following in your configuration:
<context:annotation-config/>
If you only put the annotation on your class, the framework had to load all the classes to check if the annotation is present.
To minimize this overhead, you have to put the annotation-config tag in your configuration. That way the framework knows it has to check the classes from that configuration.
You can help the framework by specifying the package where your annotated classes are with the "base-package" attribute.
//EDIT//
This also explains the note in the documentation:
Note
<context:annotation-config/> only
looks for annotations on beans in the
same application context in which it
is defined. This means that, if you
put in a
WebApplicationContext for a
DispatcherServlet, it only checks for
#Autowired beans in your controllers,
and not your services.
Simply annotating a class with #Controller doesn't necessarily mean you want it to be part of your Spring context. Imagine if the class is part of another application, or part of a third party library, or a deprecated component of your system, etc. Just because it is on the classpath doesn't necessarily mean that you want it automatically instantiated as a bean in your Spring context.