Using setter method. Using constructor. Mapping via index. Mapping via Type. Inner beans. Referring previously defined beans. While we were recapping what we have learned so far, Mr. XYZ has written so many more books. So he has defined many more beans Titles, Chapters and Books in the Spring bean configuration file. But Mr. XYZ was not perfect. He wants to make some changes in the some of the beans. And he has to go through the long bean definition file to find his bean of interest and change it as per his new need.
It is getting more difficult to maintain so many beans in a single configuration file. We have a solution for Mr. Let us categories the beans in some logical groups.
All the title beans in one group, all the chapter beans in second group and all the book beans in third logical group There can be other logical groupings as well. And thus we will have three bean definition files.
The following are the XML based multiple Spring definition file. This is an optional file because we can import the chapters and titles bean file in books. Assuming that Mr. XYZ has used java annotation based Spring definitions.
In this case. Again, this is optional file because we can import chapters and titles beans in BooksConfiguration file. The structure of BeansConfiguration. Note the class level annotation Import being used at the top of the class in addition to Configuration annotation. Let us also look at the content of other three java annotations based Spring beans configuration files.
A class level annotation Import being used to import title beans. This is required because chapter beans depend on title beans. Member level annotation Autowired being used for member variable chapter1Title and chapter2Title. Although the title beans has not been initialised, they will be automatically initialized by the Spring container on seeing the Autowired annotation. This ensures that all the Chapter beans are also imported by the Spring container.
Now, since ChaptersConfiguration already import TitlesConfiguration so there is no need to explicitly import the TitlesConfiguration. Although we have made the life of Mr. XYZ simple by modularising the bean definition file, he is still not happy with us. Every time he has to update the title or chapter content he has to refer the bean definition file which also contains the bean definitions.
What MR. XYZ is interested in is just the content title, chapter content etc. So he wants the content to be separated from the actual bean definitions. We have a solution for him. Let us revisit the titles. It contains the title value for book, and various chapters. Let us move these values into a property file named title. So our title. Now in the titles. In this article, I will be covering how exactly spring achieves this. Just to recap , the Spring framework consists of the Spring container.
The Spring Container is responsible for creating the objects required in an application and providing them to the necessary classes.
It uses Inversion of Control via Dependency Injection to achieve this. Let us now see how exactly this happens. In order for Spring to be able to wire up the application, it requires developers to provide the following:.
The Spring container is also known as the Spring Application Context. It is responsible for creating and managing the objects in an application. The following diagram illustrates how the Application context uses the beans and the configuration metadata to create a ready to use application:.
As explained earlier, the beans in a Spring application are the actual objects that need to be configured. Lightweight IoC containers tend to be lightweight, especially when compared to EJB containers, for example. This is beneficial for developing and deploying applications on computers with limited memory and CPU resources. Spring provides a consistent transaction management interface that can scale down to a local transaction using a single database, for example and scale up to global transactions using JTA, for example.
The Inversion of Control IoC is a general concept, and it can be expressed in many different ways. Dependency Injection is merely one concrete example of Inversion of Control. When writing a complex Java application, application classes should be as independent as possible of other Java classes to increase the possibility to reuse these classes and to test them independently of other classes while unit testing.
Dependency Injection helps in gluing these classes together and at the same time keeping them independent. What is dependency injection exactly? Let's look at these two words separately.
Here the dependency part translates into an association between two classes. Whenever you use the Value annotation, Spring will go through your hierarchical environment and look for the appropriate property - or throw an error message if such a property does not exist. Spring framework consists of even more convenience utilities than you have seen so far. To the contrary, they are all part of the Spring framework project. Either offered by the JDK or a third-party library. Spring framework always builds on top of these existing features.
See here for a code example. Everything related to configuring an email server url, username, password is abstracted away into the Spring specific MailSender class, that you can inject in any bean that wants to send emails. Spring offers convenience builders, like the MimeMessageHelper, to create multipart emails from, say, files as fast as possible.
Note, however, that detailed coverage of all these tools is impossible in the scope of this guide. Instead, have a look at the official documentation for a full list. It will be covered in more detail in future versions of this guide. If you have read this guide, you should understand by now that Spring Boot builds on top of Spring.
Spring offers you the ability to read in. The issue is, you have to write and configure all these individual pieces yourself. Spring Boot, on the other hand, takes these single pieces and bundles them up together.
Always and automatically look for application. Always booting up an embedded Tomcat so you can immediately see the results of writing your RestControllers. All, by running a main method in a Java class, which is annotated with the SpringBootApplication annotation. So, Spring Boot is all about taking the existing Spring framework parts, pre-configuring and packaging them up - with as little development work needed as possible. Spring Boot : Probably the most popular Spring project.
Spring Boot is an opinionated version of Spring Framework. Spring Batch : A library that helps you write good old batch jobs. Spring Cloud : A set of libraries that help your Spring project integrate easier with "the cloud" think: AWS or write microservices. Spring Security : A library that helps you secure, e.
Takeaway: All these libraries extend Spring Framework and build on top of its dependency injection core principles. Spring started out with XML configuration only.
You want to strive for homogeneity in your Spring configuration, i. Note that Dagger only offers dependency injection, with no additional convenience features. Guice offers dependency injection and other features like transaction management with the help of Guice Persist. If you have read this far, you should now have a pretty thorough understanding of what Spring framework is about.
If you have any questions or suggestions, drop me a mail at marco marcobehler. For hands-on practice, check out the Learning Spring exercise course.
Patricio "Pato" Moschcovich , for doing an amazing job proof-reading this article and pointing out a ton of tiny mistakes.
Leave your email below and I'll send you regular updates on the production progress of my "The Confident Spring Professional" course. Absolutely no spam, ever. I respect your email privacy. Unsubscribe anytime. I'll send you an update when I publish new guides. I'm MarcoBehler and I share everything I know about making awesome software through my guides , screencasts , talks and courses.
Follow me on Twitter to find out what I'm currently working on. What is Spring Framework? An Unorthodox Guide Last updated on August 09, -. Introduction The complexity of the Spring ecosystem A lot of companies are using Spring, but then you go to spring. The short answer :. What is a dependency? How to instantiate dependencies with new The naive approach would be to simply create a new DataSource through a constructor, every time you need one.
We use our newly created DataSource for the query. This is our newly extracted method, able to create new DataSources. How to 'manage' dependencies in a global Application class To accommodate these issues, you could think about writing a global Application class, that looks something like this:.
What is Inversion of Control? The findByX methods will then simply use that DataSource. Dependency Injection Containers Hence, the issue is that you, as a programmer are still actively constructing UserDAOs through their constructor and thus setting the DataSource dependency manually. The Confident Spring Professional If you want to easily get a deep and practical understanding of the entire Spring Ecosystem or simply refresh your Spring knowledge: I have written a course for you.
Interested in trying out the full first module? Learn More. What is an ApplicationContext? ApplicationContext ; import org. AnnotationConfigApplicationContext ; import javax.
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