Writing in the front

Originally wanted to write an article on how to customize the standard Spring Boot Starter, but in order to better understand some of the design concepts and key points of Starter, so some details are extracted separately in advance to explain

In Maven POM.xml, you’ll often see dependencies like the following:

<dependency>
  <groupId>sample.ProjectA</groupId>
  <artifactId>Project-A</artifactId>
  <version>1.0</version>
  <scope>compile</scope>
  <optional>true</optional> 
</dependency>
Copy the code

What does
true
 mean?

The mystery of the optional keyword

As usual, draw a picture to illustrate the problem:

Project C uses two classes from Project A (OptionalFeatureAClass) and project B (OptionalFeatureBClass). If Project C does not rely on packageA and packageB, the compilation will fail.

Project D relies on Project C, but classes (OptionalFeatureAClass) and classes (OptionalFeatureBClass) are optional features for Project D, So to ensure that the final WAR/EJB package does not contain unnecessary dependencies, use
to declare that the current dependencies are optional and will not be inherited by other projects by default (just as final classes in Java cannot be inherited by other classes).

What if Project D does need to use OptionalFeatureAClass in Project C? We need to explicitly declare project A dependencies in the pom.xml of project D.

Project D requires OptionalFeatureAClass of Project A, so you need to explicitly add A dependency to Project A in Project D’s POP.xml file

It is easy to understand why Maven designed the optional keyword. Suppose a Project on database persistence (Project C) is designed to accommodate more types of database persistence design. For example, Mysql persistence design (Project A) and Oracle persistence design (Project B), when our Project (Project D) will use the persistence design of Project C, It is impossible to import both mysql and Oracle drivers, so we will specify one explicitly

The actual case

In the Spring-boot-Actuator POP. XML file, more than 20 dependencies are optional

Because Spring Boot cannot pack unnecessary dependencies into your final JAR package, the final JAR package generated by projects using Spring Boot does not include these 20 dependent jars. If you need one, add it explicitly to your project

In the following article, we will use this strategy to customize the Spring Boot Starter, because the Starter is a role similar to Project C, which contains specific functions for other projects.

Reverse application

If Project C does not include
true
, Project D also needs to rely on Project C, but only uses the classes of Project A. Maven also has a solution, using the exclusion keyword, which you can learn from the previous code:

<dependencies>
    <dependency>
      <groupId>top.dayarch.demo</groupId>
      <artifactId>Project-C</artifactId>
      <exclusions>
        <exclusion>
          <groupId>top.dayarch.demo</groupId>
          <artifactId>Project-B</artifactId>
        </exclusion>
      </exclusions> 
    </dependency>
</dependencies>
Copy the code

conclusion

Now, when you’re designing functional dependencies in the future, you should know how to design dependencies, and I recommend optional, to put it simply, you can design dependencies for any dish you want, you can just “grab CAI Ming” yourself, Next we create a custom starter…… that emulates the official standard Blog access back to normal, welcome to exchange

Soul asking

  1. Gradle has a lot of build tools for kids. Do you know what Gradle says?
  2. Custom starter, do you know what the official standard starter structure is?

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