Looking for resource to help understand how Spring boot works [closed] - document

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I am learning Spring boot recently, there are too much magic in it. I find most of the resource, include the official website, is focused on how to use it, not explain how it works. Could anyone share such resources?
UPDATE:
Maybe my description cause some confusion(close votes), I am actually looking for answer to questions like below:
What will happen when boot find "org.springframework.boot:spring-boot-starter-web" in pom.xml? Does it add Tomcat lib jar into my classpath? Will web.xml be generated? (documents said boot do not generate code)
What will happen when boot find "org.springframework.boot:spring-boot-starter-data-jpa", will persistence.xml be generated? How boot create a repository instance based on the database driver dependency in pom.xml?
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Answer 1

Spring Boot does not generate any code at all, all configuration is performed programmatically. So for example, there won't be a web.xml file that configures your servlets, instead the servlet 3.0 API will be used to register them in code.
The starter POMs provide a convenient way to add jars to your classpath, but they do not themselves do anything. For example, spring-boot-starter-web will add Tomcat and Spring MVC jars, spring-boot-starter-data-jpa will add Hibernate and Spring Data JPA etc.
Once you have relevant jars on your classpath, Spring Boot will attempt to auto-configure things for you. When it starts it consults META-INF/spring.factories files to find potential configuration (here is an example).
Each configuration is tried in turn to see if it can be applied. Configurations use #Condition annotations to limit when they apply. For example, the HibernateJpaAutoConfiguration only applies when certain Hibernate classes can be found (see here). Auto-configurations are just typical Spring #Configuration classes, so you can take a look at the source code to see what is actually happening.
The reference documentation (and specifically Appendix B) might also help.

Answer 2

Based upon my intial understanding my answer is given below.
Basically, Spring Boot is helping in providing dependencies based upon your requirements.
We don't need to manage pom.xml.
We just need to include correct base project of Spring Boot.
Rest all of things can be done without Spring Boot as well, like all configuration with Annotation only.
Below given link may help you more.
http://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/html/index.html

Answer 3

Did a simple experiment with the jars inclusion:
First run the application without the "spring-boot-starter-web" jar, the "spring-boot" application will be standalone application.
Now add the "spring-boot-starter-web", the "spring-boot" application will starts the "tomcat" container.
2 things here, based on my observation.
The pom.xml is declaration to what jars to be added.
Once the SpringBoot application started, first it will check the "Auto-configurations", once the "spring-boot-starter-web" is in the classpath, this will start the tomcat automatically.
For any more information, we need to run the "spring-boot" application in --debug option.
This will publish the "AUTO-CONFIGURATION REPORT" which will be helpful.