Maven的pom文件里，类似于这种版本号${spring.version} 是什么意思？

统一管理版本号

<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>

<groupId>tk.liaohongdong</groupId>

<artifactId>myResume</artifactId>

<version>0.0.1-SNAPSHOT</version>

<!-- 定义版本号 -->

<properties>

<junit.version>4.12</junit.version>

<commons.lang.version>2.6</commons.lang.version>

<commons.io.version>2.5</commons.io.version>

<commons.net.version>3.5</commons.net.version>

<commons.fileupload.version>1.3.1</commons.fileupload.version>

<jackson.databind.version>2.8.5</jackson.databind.version>

<httpclient.version>4.5.2</httpclient.version>

<joda.time.version>2.9.6</joda.time.version>

<slf4j.log4j12.version>1.7.21</slf4j.log4j12.version>

<mysql.version>5.1.40</mysql.version>

<mybatis.version>3.4.1</mybatis.version>

<mybatis.spring.version>1.3.0</mybatis.spring.version>

<pagehelper.version>4.2.1</pagehelper.version>

<druid.version>1.0.27</druid.version>

<spring.version>4.3.4.RELEASE</spring.version>

<jstl.version>1.2</jstl.version>

<servlet.api.version>2.5</servlet.api.version>

<jsp.api.version>2.2</jsp.api.version>

<jedis.version>2.8.2</jedis.version>

<solr.solrj.version>6.2.1</solr.solrj.version>

<httpcore.version>4.4.5</httpcore.version>

</properties>

<dependencies>

<!-- 单元测试工具 -->

<!-- https://mvnrepository.com/artifact/junit/junit -->

<dependency>

<groupId>junit</groupId>

<artifactId>junit</artifactId>

<version>${junit.version}</version>

</dependency>

<!-- Json解析工具 -->

<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind -->

<dependency>

<groupId>com.fasterxml.jackson.core</groupId>

<artifactId>jackson-databind</artifactId>

<version>${jackson.databind.version}</version>

</dependency>

<!-- 日期处理工具 -->

<!-- https://mvnrepository.com/artifact/joda-time/joda-time -->

<dependency>

<groupId>joda-time</groupId>

<artifactId>joda-time</artifactId>

<version>${joda.time.version}</version>

</dependency>

<!-- 日志处理 -->

<!-- https://mvnrepository.com/artifact/org.slf4j/slf4j-log4j12 -->

<dependency>

<groupId>org.slf4j</groupId>

<artifactId>slf4j-log4j12</artifactId>

<version>${slf4j.log4j12.version}</version>

</dependency>

<!-- Redis客户端 -->

<!-- https://mvnrepository.com/artifact/redis.clients/jedis -->

<!-- <dependency>

<groupId>redis.clients</groupId>

<artifactId>jedis</artifactId>

<version>${jedis.version}</version>

</dependency> -->

<!-- solr客户端 -->

<!-- https://mvnrepository.com/artifact/org.apache.solr/solr-solrj -->

<!-- <dependency>

<groupId>org.apache.solr</groupId>

<artifactId>solr-solrj</artifactId>

<version>${solr.solrj.version}</version>

</dependency> -->







<!-- apache 工具 -->

<!-- https://mvnrepository.com/artifact/commons-lang/commons-lang -->

<dependency>

<groupId>commons-lang</groupId>

<artifactId>commons-lang</artifactId>

<version>${commons.lang.version}</version>

</dependency>

<!-- https://mvnrepository.com/artifact/org.apache.httpcomponents/httpcore -->

<dependency>

<groupId>org.apache.httpcomponents</groupId>

<artifactId>httpcore</artifactId>

<version>${httpcore.version}</version>

</dependency>

<!-- https://mvnrepository.com/artifact/commons-io/commons-io -->

<dependency>

<groupId>commons-io</groupId>

<artifactId>commons-io</artifactId>

<version>${commons.io.version}</version>

</dependency>

<!-- https://mvnrepository.com/artifact/commons-net/commons-net -->

<dependency>

<groupId>commons-net</groupId>

<artifactId>commons-net</artifactId>

<version>${commons.net.version}</version>

</dependency>

<!-- https://mvnrepository.com/artifact/commons-fileupload/commons-fileupload -->

<dependency>

<groupId>commons-fileupload</groupId>

<artifactId>commons-fileupload</artifactId>

<version>${commons.fileupload.version}</version>

</dependency>

<!-- https://mvnrepository.com/artifact/org.apache.httpcomponents/httpclient -->

<dependency>

<groupId>org.apache.httpcomponents</groupId>

<artifactId>httpclient</artifactId>

<version>${httpclient.version}</version>

</dependency>





<!-- Mybatis -->

<!-- https://mvnrepository.com/artifact/org.mybatis/mybatis -->

<!-- <dependency>

<groupId>org.mybatis</groupId>

<artifactId>mybatis</artifactId>

<version>${mybatis.version}</version>

</dependency> -->

<!-- https://mvnrepository.com/artifact/org.mybatis/mybatis-spring -->

<!-- <dependency>

<groupId>org.mybatis</groupId>

<artifactId>mybatis-spring</artifactId>

<version>${mybatis.spring.version}</version>

</dependency> -->

<!-- 分页工具 -->

<!-- https://mvnrepository.com/artifact/com.github.pagehelper/pagehelper -->

<!-- <dependency>

<groupId>com.github.pagehelper</groupId>

<artifactId>pagehelper</artifactId>

<version>${pagehelper.version}</version>

</dependency> -->





<!-- 数据库 -->

<!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->

<dependency>

<groupId>mysql</groupId>

<artifactId>mysql-connector-java</artifactId>

<version>${mysql.version}</version>

</dependency>

<!--连接池 -->

<!-- https://mvnrepository.com/artifact/com.alibaba/druid -->

<dependency>

<groupId>com.alibaba</groupId>

<artifactId>druid</artifactId>

<version>${druid.version}</version>

</dependency>





<!-- spring -->

<!-- https://mvnrepository.com/artifact/org.springframework/spring-context -->

<dependency>

<groupId>org.springframework</groupId>

<artifactId>spring-context</artifactId>

<version>${spring.version}</version>

</dependency>

<!-- https://mvnrepository.com/artifact/org.springframework/spring-webmvc -->

<dependency>

<groupId>org.springframework</groupId>

<artifactId>spring-webmvc</artifactId>

<version>${spring.version}</version>

</dependency>

<!-- https://mvnrepository.com/artifact/org.springframework/spring-beans -->

<dependency>

<groupId>org.springframework</groupId>

<artifactId>spring-beans</artifactId>

<version>${spring.version}</version>

</dependency>

<!-- https://mvnrepository.com/artifact/org.springframework/spring-jdbc -->

<dependency>

<groupId>org.springframework</groupId>

<artifactId>spring-jdbc</artifactId>

<version>${spring.version}</version>

</dependency>

<!-- https://mvnrepository.com/artifact/org.springframework/spring-aspects -->

<dependency>

<groupId>org.springframework</groupId>

<artifactId>spring-aspects</artifactId>

<version>${spring.version}</version>

</dependency>






<!-- jsp -->

<!-- https://mvnrepository.com/artifact/javax.servlet/jstl -->

<dependency>

<groupId>javax.servlet</groupId>

<artifactId>jstl</artifactId>

<version>${jstl.version}</version>

</dependency>

<!-- https://mvnrepository.com/artifact/javax.servlet/servlet-api -->

<dependency>

<groupId>javax.servlet</groupId>

<artifactId>servlet-api</artifactId>

<version>${servlet.api.version}</version>

</dependency>

<!-- https://mvnrepository.com/artifact/javax.servlet.jsp/jsp-api -->

<dependency>

<groupId>javax.servlet.jsp</groupId>

<artifactId>jsp-api</artifactId>

<version>${jsp.api.version}</version>

</dependency>



</dependencies>

</project>

这样，下面就可以使用变量的是形式来管理这个版本号