SpringBoot的Starter机制

Starter机制介绍

我们知道SpringBoot自己在“后台”帮我们配置了很多原本需要我们手动去的东西,至于这个“后台”是啥,就是Starter机制。我们先来看一看常用的Starter启动器,他们都是以spring-boot-starter开头明名的,至于spring-boot这个前缀则被Spring Boot官方保留。

image.png

至于Starter机制是如何实现上文说的“自动变速箱”功能,简单说就是通过条件注解来实现的,SpringBoot通过条件注解确定需要加载哪些组件,读取哪些配置,还有一些控制加载顺序的注解。这里我们先来看一些常用的条件注解和控制加载顺序的注解:

image.png

依赖配置文件

这里以一个简单的SpringBoot中的依赖配置文件做说明,只加入最基础的配置。

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <packaging>pom</packaging>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.1.RELEASE</version>
    </parent>

    <groupId>com.david</groupId>
    <artifactId>SpringBootDemo</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <!--web功能的起步依赖-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <!--@ConfiguaritionProperties的执行器的配置-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-configuration-processor</artifactId>
            <optional>true</optional>
        </dependency>
    </dependencies>

</project>

可以看到有如下几个依赖需要加入:

1)spring-boot-starter-parent,这个是所有SpringBoot工程都需要加入的依赖项,可以说这个是一个“根依赖”,必须要加入,注意这里指定的版本号也会约束其他依赖项,相当于一个最上层的版本配置,这里指定的就是我们常说的SpringBoot的版本。

2)spring-boot-starter-web,这个是Web工程的依赖项,所有的Web程序都要加。

3)spring-boot-configuration-processor,这个是ConfiguaritionProperties的配置,有了它我们的Spring项目中的Jar就会包含上很多meta-data,这些元数据会在开发的时候通过idea给我们很多输入的提示。

spring-boot-starter-parent的作用

可以说这是SpringBoot项目中最重要的配置文件,这里看下关于他的几个核心部分,首先关于它的描述是这样的:

<description>
     Parent pom providing dependency and plugin management for applications built with Maven
</description>

大意是说这个文件是一个提供了通过maven编译的应用的依赖和插件管理一个“父配置文件”。嗯,就理解是管理基础依赖和插件的就可以了。

看他的resource标签,是这样的:

<resources>
    <resource>
        <filtering>true</filtering>
        <directory>${basedir}/src/main/resources</directory>
        <includes>
            <include>**/application*.yml</include>
            <include>**/application*.yaml</include>
            <include>**/application*.properties</include>
        </includes>
    </resource>
    <resource>
        <directory>${basedir}/src/main/resources</directory>
        <excludes>
            <exclude>**/application*.yml</exclude>
            <exclude>**/application*.yaml</exclude>
            <exclude>**/application*.properties</exclude>
        </excludes>
    </resource>
</resources>

这里指定了资源的导入和导出路径,什么意思?就是你的配置文件配在对应的目录下,框架都会去扫描到读取的,按上面的配置会读src/main/resources目录下的配置文件的内容。比如我们在这里放了如下的配置文件:

image.png

这样程序启动的会去读application.properties和application.yml文件里面的配置的。这里要说一下,上面的配置加载是有顺序的,先加yml,yaml ,后加properties里面的配置,这也决定了properties文文件配置的优先级是要高于yaml文件的,或者说相同参数的配置,properties里面的参数会覆盖yaml里面的。

最后看到最上面还有一个parent标签,是这样描述的,这里描述了spring-boot-starter-parent的父依赖配置spring-boot-dependencies:

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-dependencies</artifactId>
    <version>2.0.1.RELEASE</version>
    <relativePath>../../spring-boot-dependencies</relativePath>
</parent>

同样的我们看一看spring-boot-dependencies的描述文件,看看里面的properties标签,是这样:

<properties>
    <activemq.version>5.15.3</activemq.version>
    <antlr2.version>2.7.7</antlr2.version>
    <appengine-sdk.version>1.9.63</appengine-sdk.version>
    <artemis.version>2.4.0</artemis.version>
    <aspectj.version>1.8.13</aspectj.version>
    <assertj.version>3.9.1</assertj.version>
    <atomikos.version>4.0.6</atomikos.version>
    <bitronix.version>2.1.4</bitronix.version>
    <build-helper-maven-plugin.version>3.0.0</build-helper-maven-plugin.version>
    <byte-buddy.version>1.7.11</byte-buddy.version>
    <caffeine.version>2.6.2</caffeine.version>
    <cassandra-driver.version>3.4.0</cassandra-driver.version>
    <classmate.version>1.3.4</classmate.version>
    <commons-codec.version>1.11</commons-codec.version>
    <commons-dbcp2.version>2.2.0</commons-dbcp2.version>
    <commons-lang3.version>3.7</commons-lang3.version>
    <commons-pool.version>1.6</commons-pool.version>
    <commons-pool2.version>2.5.0</commons-pool2.version>
    <couchbase-cache-client.version>2.1.0</couchbase-cache-client.version>
    <couchbase-client.version>2.5.7</couchbase-client.version>
    <derby.version>10.14.1.0</derby.version>
    <dom4j.version>1.6.1</dom4j.version>
    <dropwizard-metrics.version>3.2.6</dropwizard-metrics.version>
    <ehcache.version>2.10.4</ehcache.version>
    <ehcache3.version>3.5.2</ehcache3.version>
    <elasticsearch.version>5.6.8</elasticsearch.version>
    <embedded-mongo.version>2.0.3</embedded-mongo.version>
    <exec-maven-plugin.version>1.5.0</exec-maven-plugin.version>
    <flatten-maven-plugin.version>1.0.0</flatten-maven-plugin.version>
    <flyway.version>5.0.7</flyway.version>
    <freemarker.version>2.3.28</freemarker.version>
    <git-commit-id-plugin.version>2.2.3</git-commit-id-plugin.version>
    <glassfish-el.version>3.0.0</glassfish-el.version>
    <groovy.version>2.4.15</groovy.version>
    <gson.version>2.8.2</gson.version>
    <h2.version>1.4.197</h2.version>
    <hamcrest.version>1.3</hamcrest.version>
    <hazelcast.version>3.9.3</hazelcast.version>
    <hazelcast-hibernate5.version>1.2.3</hazelcast-hibernate5.version>
    <hibernate.version>5.2.16.Final</hibernate.version>
    <hibernate-validator.version>6.0.9.Final</hibernate-validator.version>
    <hikaricp.version>2.7.8</hikaricp.version>
    <hsqldb.version>2.4.0</hsqldb.version>
    <htmlunit.version>2.29</htmlunit.version>
    <httpasyncclient.version>4.1.3</httpasyncclient.version>
    <httpclient.version>4.5.5</httpclient.version>
    <httpcore.version>4.4.9</httpcore.version>
    <infinispan.version>9.1.7.Final</infinispan.version>
    <influxdb-java.version>2.9</influxdb-java.version>
    <jackson.version>2.9.5</jackson.version>
    <janino.version>3.0.8</janino.version>
    <javax-annotation.version>1.3.2</javax-annotation.version>
    <javax-cache.version>1.1.0</javax-cache.version>
    <javax-jaxb.version>2.3.0</javax-jaxb.version>
    <javax-jms.version>2.0.1</javax-jms.version>
    <javax-json.version>1.1.2</javax-json.version>
    <javax-jsonb.version>1.0</javax-jsonb.version>
    <javax-mail.version>1.6.1</javax-mail.version>
    <javax-money.version>1.0.1</javax-money.version>
    <javax-transaction.version>1.2</javax-transaction.version>
    <javax-validation.version>2.0.1.Final</javax-validation.version>
    <jaxen.version>1.1.6</jaxen.version>
    <jaybird.version>3.0.3</jaybird.version>
    <jboss-logging.version>3.3.2.Final</jboss-logging.version>
    <jboss-transaction-spi.version>7.6.0.Final</jboss-transaction-spi.version>
    <jdom2.version>2.0.6</jdom2.version>
    <jedis.version>2.9.0</jedis.version>
    <jersey.version>2.26</jersey.version>
    <jest.version>5.3.3</jest.version>
    <jetty.version>9.4.9.v20180320</jetty.version>
    <jetty-el.version>8.5.24.2</jetty-el.version>
    <jetty-jsp.version>2.2.0.v201112011158</jetty-jsp.version>
    <jmustache.version>1.14</jmustache.version>
    <jna.version>4.5.1</jna.version>
    <joda-time.version>2.9.9</joda-time.version>
    <johnzon-jsonb.version>1.1.7</johnzon-jsonb.version>
    <jolokia.version>1.5.0</jolokia.version>
    <jooq.version>3.10.6</jooq.version>
    <jsonassert.version>1.5.0</jsonassert.version>
    <json-path.version>2.4.0</json-path.version>
    <jstl.version>1.2</jstl.version>
    <jtds.version>1.3.1</jtds.version>
    <junit.version>4.12</junit.version>
    <junit-jupiter.version>5.1.0</junit-jupiter.version>
    <junit-platform.version>1.1.0</junit-platform.version>
    <kotlin.version>1.2.31</kotlin.version>
    <lettuce.version>5.0.3.RELEASE</lettuce.version>
    <liquibase.version>3.5.5</liquibase.version>
    <log4j2.version>2.10.0</log4j2.version>
    <logback.version>1.2.3</logback.version>
    <lombok.version>1.16.20</lombok.version>
    <mariadb.version>2.2.3</mariadb.version>
    <maven-antrun-plugin.version>1.8</maven-antrun-plugin.version>
    <maven-assembly-plugin.version>3.1.0</maven-assembly-plugin.version>
    <maven-clean-plugin.version>3.0.0</maven-clean-plugin.version>
    <maven-compiler-plugin.version>3.7.0</maven-compiler-plugin.version>
    <maven-dependency-plugin.version>3.0.2</maven-dependency-plugin.version>
    <maven-deploy-plugin.version>2.8.2</maven-deploy-plugin.version>
    <maven-eclipse-plugin.version>2.10</maven-eclipse-plugin.version>
    <maven-enforcer-plugin.version>3.0.0-M1</maven-enforcer-plugin.version>
    <maven-failsafe-plugin.version>2.21.0</maven-failsafe-plugin.version>
    <maven-help-plugin.version>2.2</maven-help-plugin.version>
    <maven-install-plugin.version>2.5.2</maven-install-plugin.version>
    <maven-invoker-plugin.version>3.0.0</maven-invoker-plugin.version>
    <maven-jar-plugin.version>3.0.2</maven-jar-plugin.version>
    <maven-javadoc-plugin.version>3.0.0</maven-javadoc-plugin.version>
    <maven-resources-plugin.version>3.0.1</maven-resources-plugin.version>
    <maven-shade-plugin.version>2.4.3</maven-shade-plugin.version>
    <maven-site-plugin.version>3.6</maven-site-plugin.version>
    <maven-source-plugin.version>3.0.1</maven-source-plugin.version>
    <maven-surefire-plugin.version>2.21.0</maven-surefire-plugin.version>
    <maven-war-plugin.version>3.1.0</maven-war-plugin.version>
    <micrometer.version>1.0.3</micrometer.version>
    <mockito.version>2.15.0</mockito.version>
    <mongodb.version>3.6.3</mongodb.version>
    <mongo-driver-reactivestreams.version>1.7.1</mongo-driver-reactivestreams.version>
    <mssql-jdbc.version>6.2.2.jre8</mssql-jdbc.version>
    <mysql.version>5.1.46</mysql.version>
    <narayana.version>5.8.0.Final</narayana.version>
    <nekohtml.version>1.9.22</nekohtml.version>
    <neo4j-ogm.version>3.1.0</neo4j-ogm.version>
    <netty.version>4.1.23.Final</netty.version>
    <nio-multipart-parser.version>1.1.0</nio-multipart-parser.version>
    <postgresql.version>42.2.2</postgresql.version>
    <quartz.version>2.3.0</quartz.version>
    <querydsl.version>4.1.4</querydsl.version>
    <rabbit-amqp-client.version>5.1.2</rabbit-amqp-client.version>
    <reactive-streams.version>1.0.2</reactive-streams.version>
    <reactor-bom.version>Bismuth-SR8</reactor-bom.version>
    <rest-assured.version>3.0.7</rest-assured.version>
    <rxjava.version>1.3.8</rxjava.version>
    <rxjava2.version>2.1.12</rxjava2.version>
    <rxjava-adapter.version>1.2.1</rxjava-adapter.version>
    <selenium.version>3.9.1</selenium.version>
    <selenium-htmlunit.version>2.29.3</selenium-htmlunit.version>
    <sendgrid.version>4.1.2</sendgrid.version>
    <servlet-api.version>3.1.0</servlet-api.version>
    <simple-json.version>1.1.1</simple-json.version>
    <slf4j.version>1.7.25</slf4j.version>
    <snakeyaml.version>1.19</snakeyaml.version>
    <solr.version>6.6.3</solr.version>
    <spring.version>5.0.5.RELEASE</spring.version>
    <spring-amqp.version>2.0.3.RELEASE</spring-amqp.version>
    <spring-batch.version>4.0.1.RELEASE</spring-batch.version>
    <spring-cloud-connectors.version>2.0.1.RELEASE</spring-cloud-connectors.version>
    <spring-data-releasetrain.version>Kay-SR6</spring-data-releasetrain.version>
    <spring-hateoas.version>0.24.0.RELEASE</spring-hateoas.version>
    <spring-integration.version>5.0.4.RELEASE</spring-integration.version>
    <spring-kafka.version>2.1.5.RELEASE</spring-kafka.version>
    <spring-ldap.version>2.3.2.RELEASE</spring-ldap.version>
    <spring-plugin.version>1.2.0.RELEASE</spring-plugin.version>
    <spring-restdocs.version>2.0.1.RELEASE</spring-restdocs.version>
    <spring-retry.version>1.2.2.RELEASE</spring-retry.version>
    <spring-security.version>5.0.4.RELEASE</spring-security.version>
    <spring-session-bom.version>Apple-SR1</spring-session-bom.version>
    <spring-ws.version>3.0.1.RELEASE</spring-ws.version>
    <sqlite-jdbc.version>3.21.0.1</sqlite-jdbc.version>
    <statsd-client.version>3.1.0</statsd-client.version>
    <sun-mail.version>1.6.1</sun-mail.version>
    <thymeleaf.version>3.0.9.RELEASE</thymeleaf.version>
    <thymeleaf-extras-data-attribute.version>2.0.1</thymeleaf-extras-data-attribute.version>
    <thymeleaf-extras-java8time.version>3.0.1.RELEASE</thymeleaf-extras-java8time.version>
    <thymeleaf-extras-springsecurity4.version>3.0.2.RELEASE</thymeleaf-extras-springsecurity4.version>
    <thymeleaf-layout-dialect.version>2.3.0</thymeleaf-layout-dialect.version>
    <tomcat.version>8.5.29</tomcat.version>
    <unboundid-ldapsdk.version>4.0.5</unboundid-ldapsdk.version>
    <undertow.version>1.4.23.Final</undertow.version>
    <versions-maven-plugin.version>2.3</versions-maven-plugin.version>
    <webjars-hal-browser.version>3325375</webjars-hal-browser.version>
    <webjars-locator-core.version>0.35</webjars-locator-core.version>
    <wsdl4j.version>1.6.3</wsdl4j.version>
    <xml-apis.version>1.4.01</xml-apis.version>
    <xml-maven-plugin.version>1.0.1</xml-maven-plugin.version>
    <xmlunit2.version>2.5.1</xmlunit2.version>
</properties>

喔,好多似曾相识的面孔在这里都能见到,比如aspectj,caffeine, dom4j,hikaricp,junit,log4j2, mysql,tomcat等等一些,而且你也看到了,这里帮我们把版本控制也做好了,防止了原来在Spring中我们自己手动引入依赖的时候的经常会发生的版本冲突问题,所谓的约定优于配置在这里就是一个典型的体现场景。

spring-boot-starter-web的作用

这个配置是web应用的核心配置,有了他不用像Spring再像Spring一样去导SpringMVC相关的依赖了,这里看一下这个文件的内容。

同样的也有一个parent标签指定父依赖spring-boot-starters.

<parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starters</artifactId>
  <version>2.0.1.RELEASE</version>
</parent>

另外dependencies标签指定是一些依赖坐标的打包,包含json,tomcat等等,包含版本控制都做好了。

<dependencies>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter</artifactId>
    <version>2.0.1.RELEASE</version>
    <scope>compile</scope>
  </dependency>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-json</artifactId>
    <version>2.0.1.RELEASE</version>
    <scope>compile</scope>
  </dependency>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-tomcat</artifactId>
    <version>2.0.1.RELEASE</version>
    <scope>compile</scope>
  </dependency>
  <dependency>
    <groupId>org.hibernate.validator</groupId>
    <artifactId>hibernate-validator</artifactId>
    <version>6.0.9.Final</version>
    <scope>compile</scope>
  </dependency>
  <dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-web</artifactId>
    <version>5.0.5.RELEASE</version>
    <scope>compile</scope>
  </dependency>
  <dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-webmvc</artifactId>
    <version>5.0.5.RELEASE</version>
    <scope>compile</scope>
  </dependency>
</dependencies>

其中spring-boot-starter的依赖又包含了如下的dependencies,有spring-boot,自动配置,日志,注解,core等等的依赖:

<dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot</artifactId>
      <version>2.0.1.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-autoconfigure</artifactId>
      <version>2.0.1.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-logging</artifactId>
      <version>2.0.1.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>javax.annotation</groupId>
      <artifactId>javax.annotation-api</artifactId>
      <version>1.3.2</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-core</artifactId>
      <version>5.0.5.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.yaml</groupId>
      <artifactId>snakeyaml</artifactId>
      <version>1.19</version>
      <scope>runtime</scope>
    </dependency>
</dependencies>

以上可以看到SpringBoot基本上通过spring-boot-starter-parent和spring-boot-starter-web两个配置就把起步依赖需要导入的依赖项把我们需要的依赖给导完了,这也就是SpringBoot的starter机制在依赖配置层的最好体现,实现了所谓约定优于配置。

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