SpringBoot dependency management source code analysis

1. Dependency management

Ⅰ. Why don’t you need to specify the version when importing some dependencies?

The dependencies we added when we created the project did not help us specify the version number, so how does SpringBoot specify the version account?

Now let’s peel off SpringBoot’s parent dependency starter

<!--	SpringBoot父项目依赖管理-->
	<parent>
		<artifactId>spring-boot-parent</artifactId>
		<groupId>org.springframework.boot</groupId>
		<version>2.2.9.RELEASE</version>
		<relativePath/>			<!-- 查看本地仓库的源码 -->
	</parent>

1.1 The work of the parent dependency starter

The mouse pointer stays on spring -boot-parentUpCtrl left mouse buttonClick in, let’s take a look at what this parent dependency launcher is responsible for:

<parent>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-dependencies</artifactId>
   <version>${revision}</version>
   <relativePath>../spring-boot-dependencies</relativePath>
</parent>

First go in and find: this parent dependency launcher There is a dad, as above! We will analyze this in depth later! ! !

Then continue to read

<properties>
   <main.basedir>${basedir}/../..</main.basedir>
   <disable.checks>false</disable.checks>
   <git.url>https://github.com/spring-projects/spring-boot</git.url>
   <git.connection>scm:git:git://github.com/spring-projects/spring-boot.git</git.connection>
   <git.developerConnection>scm:git:ssh://git@github.com/spring-projects/spring-boot.git</git.developerConnection>
   <java.version>1.8</java.version>
   <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
   <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
   <maven.version>3.5.4</maven.version>
   <maven-resolver.version>1.1.1</maven-resolver.version>
   <spock.version>1.3-groovy-2.5</spock.version>
   <spring-asciidoctor-extensions.version>0.4.0.RELEASE</spring-asciidoctor-extensions.version>
   <spring-doc-resources.version>0.1.3.RELEASE</spring-doc-resources.version>
   <spring-doc-resources.url>https://repo.spring.io/release/io/spring/docresources/spring-doc-resources/${spring-doc-resources.version}/spring-doc-resources-${spring-doc-resources.version}.zip</spring-doc-resources.url>
   <testcontainers.version>1.12.5</testcontainers.version>
   <testng.version>6.14.3</testng.version>
</properties>

I see a property declaration here, declaring the jdk version, encoding format, packaged and compiled version, etc.; but this is just a statement! ! !

Let’s continue

This part is SpringBoot’s plug-in management

tips:

When importing plug-ins, since the management module plug-in here requires a specific version number, there is no need to consider plug-in version control.

1.2 Answer to the question

We Ctrl right-click , and as soon as we enter, we will see a series of version numbers:

The version numbers of many third-party frameworks are declared here. This corresponds to the version number of 2.2.9.RELEASE. If the SpringBoot version is changed, the version here will also change. But as mentioned before, is just a statement! ! !

We then

The reason why the project does not need to import the version number must be

tips:

spring-boot-starter-parent realizes the dependency management of SpringBoot by inheriting spring-boot-dependencies, so my Springboot project already has configurations such as version locking after inheriting spring-boot-starter-parent. Yes, this is why there is no need to write version numbers for partial dependencies in Spring Boot projects.

Ⅱ. Where does the JAR package that the project depends on come from?

Spring-Boot-Starter-Parent Parent Dependency Starter The main function is to carry out unified version management, so where does the JAR package that the project depends on come from?

2.1 Analyze the source code

We use < ;> For a breakthrough, we still click with the Ctrl left mouse button. In spring-boot-starter-web, we can clearly see a series of dependency management:

<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-json</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-tomcat</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-validation</artifactId>
			<exclusions>
				<exclusion>
					<groupId>org.apache.tomcat.embed</groupId>
					<artifactId>tomcat-embed-el</artifactId>
				</exclusion>
			</exclusions>
		</dependency>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-web</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-webmvc</artifactId>
		</dependency>
	</dependencies>

We only introduce it in pom.xml Spring-boot-starter-web is a dependency (that is, it directly relies on the web dependency starter), and it relies on tomcat, springmvc, etc., which means that dependency transfer is carried out in this process, and we When you click on any dependency in the web dependency launcher, you will find that there are still many dependencies in it. Take tomcat as an example:

still contains everything needed to start tomcat. rely.

2.2 Answer to the question

At this point we can know that the main function of the spring-boot-starter-web dependency starter is to package all the dependencies of the Web development scenario (based on dependency delivery method, load the corresponding jar package in the current warehouse).

Just like this, when the spring-boot-stater-web dependency starter is introduced in pom. Dependency files, etc. Of course, the version numbers of these introduced dependency files are still managed uniformly by the spring-boot-starter-parent parent dependency (this is also an important manifestation of SpringBoot conventions are greater than configuration) Spring Boot provides In addition to the Web dependency launcher introduced above, it also provides related dependencies for many other development scenarios.

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