Maven中${}的来龙去脉

it2026-03-31 7

一、pom文件中${}是干什么的

1、我们知道一般情况下在配置文件中使用${}都是在xxx.xml中然后相应的具体属性配置在xxx.properties之中。

如在在applicationContext.xml文件中配置数据连接池：

applicationContext.xml

<xml> <propertyname="dataSource"> <beanclass="com.mchange.v2.c3p0.ComboPooledDataSource"> <propertyname="jdbcUrl"value="${jdbcUrl}"></property> <propertyname="driverClass"value="${driverClass}"></property> <propertyname="user"value="${user}"></property> <propertyname="password"value="${password}"></property>  <xml/>

这个xml中就使用了${}的功能。它相当于Java中调用了变量名一样。

在本例子中，这些变量来自另外一个properties文件。该properties为： jdbc.properties（主要用来存放JDBC连接数据库的一些配置，方便下次修改，而不需要在applicationContext.xml文件中修改。）

jdbc.properties:

jdbcUrl=jdbc:mysql://localhost:3306/oa driverClass=com.mysql.jdbc.Driver user=root password=123456

而且还需要在xxx.xml中引入properties文件，一般情况下，如果你只有一个applicationContext.xml配置文件而已的话，那么只需要在applicationContext.xml文件中添加一行：

<context:property-placeholder location="classpath:jdbc.properties"/>

2、SpringEL表达式取值一般是${var}方式取值, 见于application.properties和@Value("${var}")中

3、maven的pom.xml文件中也有类似的取值表达式, 也是通过${var}的方式取值然而: 它们并不是一个东西!

EL表达式适用于配置文件及代码中的注解maven的占位符取值表达式默认仅仅适用于pom.xml文件中我们的需求大多是想打通二者的交流,如何做?此时filtering就派上用场了 maven的占位符解析表达式的使用场合默认只在pom文件范围内活动如果想扩大它的活动范围,就必须指定需要扩大到哪些文件,在pom文件中指定filtering=true.然后maven的占位符解析表达式就可以用于它里面的表达式解析了 <build> <resources> <resource> <directory>${project.basedir}/src/main/resources</directory> <filtering>true</filtering> </resource> </resources> </build>

如果你的项目继承了spring-boot-starter-parent, 如果想在application.properties中使用maven的占位符以获取maven的参数,此时就不能用${},因为可能会与EL表达式冲突,此时parent依赖将maven占位符表达式默认改为@var@的形式. 所以只能用@var@替代${var} 4、总结

Spring EL表达式和MAVEN的占位符表达式长得一样,但两者默认进水不犯河水,不能再Spring的范围内取maven的参数

filtering的作用就是打通两者的连接, 让井水犯河水, 具体来说是让Spring的范围内能取到maven的参数

filtering的使用要配合resource一起使用: 前者开启打通连接,后者指定打通的范围.

二、Maven内置三大特性中的属性

${}变量的读取这个功能是Maven提供的，属于 "maven properties 资源过滤

Maven内置了三大特性：属性、Profile和资源过滤来支持构建的灵活性。

1、先看我们关心得问题。“为什么在xxx.properties,xxx.xml 中能使用${} 来访问pom文件的属性” 这是因为Maven 提供的资源过滤的功能

默认情况下，Maven属性只有在POM中才会被解析。资源过滤就是指让Maven属性在资源文件(src/main/resources、src/test/resources)中也能被解析。

在POM中添加下面的配置便可以开启资源过滤

<build> <resources> <resource> <directory>${project.basedir}/src/main/resources</directory> <filtering>true</filtering> </resource> </resources> <testResources> <testResource> <directory>${project.basedir}/src/test/resources</directory> <filtering>true</filtering> </testResource> </testResources> </build>

从上面的配置中可以看出，我们其实可以配置多个主资源目录和多个测试资源目录。

Maven除了可以对主资源目录、测试资源目录过滤外，还能对Web项目的资源目录(如css、js目录)进行过滤。这时需要对maven-war-plugin插件进行配置

<plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-war-plugin</artifactId> <version>2.1-beta-1</version> <configuration> <webResources> <resource> <filtering>true</filtering> <directory>src/main/webapp</directory> <includes> <include>**/*.css</include> <include>**/*.js</include> </includes> </resource> </webResources> </configuration> </plugin>

注意这个${} 只能引用Maven中的属性

2、Maven的属性：事实上有六种类型的Maven属性

内置属性：主要有两个常用内置属性

${basedir}表示项目根目录，即包含pom.xml文件的目录; ${version}表示项目版本。

POM属性：pom中对应元素的值。

例如${project.artifactId}对应了<project><artifactId>元素的值。

常用的POM属性包括：

${project.build.sourceDirectory}:项目的主源码目录，默认为src/main/java/. ${project.build.testSourceDirectory}:项目的测试源码目录，默认为/src/test/java/. ${project.build.directory}:项目构建输出目录，默认为target/. ${project.build.outputDirectory}:项目主代码编译输出目录，默认为target/classes/. ${project.build.testOutputDirectory}:项目测试代码编译输出目录，默认为target/testclasses/. ${project.groupId}:项目的groupId. ${project.artifactId}:项目的artifactId. ${project.version}:项目的version,于${version}等价 ${project.build.finalName}:项目打包输出文件的名称，默认为 ${project.artifactId}${project.version}.

自定义属性：在pom中元素下自定义的Maven属性。例如

<project> <properties> <my.prop>hello</my.prop> </properties> </project>

Settings属性：与POM属性同理。如${settings.localRepository}指向用户本地仓库的地址。

Java系统属性：所有Java系统属性都可以使用Maven属性引用，例如${user.home}指向了用户目录。可以通过命令行mvn help:system查看所有的Java系统属性

环境变量属性：所有环境变量都可以使用以env.开头的Maven属性引用。例如${env.JAVA_HOME}指代了JAVA_HOME环境变量的值。也可以通过命令行mvn help:system查看所有环境变量。

三、pom文件配置解释

<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/4.0.0http://maven.apache.org/maven-v4_0_0.xsd">  <parent>  <artifactId/>  <groupId/>  <version/>  <relativePath/> </parent>  <modelVersion>4.0.0</modelVersion>  <groupId>asia.banseon</groupId>  <artifactId>banseon-maven2</artifactId>  <packaging>jar</packaging>  <version>1.0-SNAPSHOT</version>  <name>banseon-maven</name>  <url>http://www.baidu.com/banseon</url>  <description>A maven project to study maven.</description>  <prerequisites>  <maven/> </prerequisites>  <issueManagement>  <system>jira</system>  <url>http://jira.baidu.com/banseon</url> </issueManagement>  <ciManagement>  <system/>  <url/>  <notifiers>  <notifier>  <type/>  <sendOnError/>  <sendOnFailure/>  <sendOnSuccess/>  <sendOnWarning/>  <address/>  <configuration/> </notifier> </notifiers> </ciManagement>  <inceptionYear/>  <mailingLists>  <mailingList>  <name>Demo</name>  <post>banseon@126.com</post>  <subscribe>banseon@126.com</subscribe>  <unsubscribe>banseon@126.com</unsubscribe>  <archive>http:/hi.baidu.com/banseon/demo/dev/</archive> </mailingList> </mailingLists>  <developers>  <developer>  <id>HELLO WORLD</id>  <name>banseon</name>  <email>banseon@126.com</email>  <url/>  <roles> <role>Project Manager</role> <role>Architect</role> </roles>  <organization>demo</organization>  <organizationUrl>http://hi.baidu.com/banseon</organizationUrl>  <properties> <dept>No</dept> </properties>  <timezone>-5</timezone> </developer> </developers>  <contributors>  <contributor> <name/><email/><url/><organization/><organizationUrl/><roles/><timezone/><properties/> </contributor> </contributors>  <licenses>  <license>  <name>Apache 2</name>  <url>http://www.baidu.com/banseon/LICENSE-2.0.txt</url>  <distribution>repo</distribution>  <comments>A business-friendly OSS license</comments> </license> </licenses>  <scm>  <connection> scm:svn:http://svn.baidu.com/banseon/maven/banseon/banseon-maven2-trunk(dao-trunk) </connection>  <developerConnection> scm:svn:http://svn.baidu.com/banseon/maven/banseon/dao-trunk </developerConnection>  <tag/>  <url>http://svn.baidu.com/banseon</url> </scm>  <organization>  <name>demo</name>  <url>http://www.baidu.com/banseon</url> </organization>  <build>  <sourceDirectory/>  <scriptSourceDirectory/>  <testSourceDirectory/>  <outputDirectory/>  <testOutputDirectory/>  <extensions>  <extension>  <groupId/>  <artifactId/>  <version/> </extension> </extensions>  <defaultGoal/>  <resources>  <resource>  <targetPath/>  <filtering/>  <directory/>  <includes/>  <excludes/> </resource> </resources>  <testResources>  <testResource> <targetPath/><filtering/><directory/><includes/><excludes/> </testResource> </testResources>  <directory/>  <finalName/>  <filters/>  <pluginManagement>  <plugins>  <plugin>  <groupId/>  <artifactId/>  <version/>  <extensions/>  <executions>  <execution>  <id/>  <phase/>  <goals/>  <inherited/>  <configuration/> </execution> </executions>  <dependencies>  <dependency> ...... </dependency> </dependencies>  <inherited/>  <configuration/> </plugin> </plugins> </pluginManagement>  <plugins>  <plugin> <groupId/><artifactId/><version/><extensions/> <executions> <execution> <id/><phase/><goals/><inherited/><configuration/> </execution> </executions> <dependencies>  <dependency> ...... </dependency> </dependencies> <goals/><inherited/><configuration/> </plugin> </plugins> </build>  <profiles>  <profile>  <id/>  <activation>  <activeByDefault/>  <jdk/>  <os>  <name>Windows XP</name>  <family>Windows</family>  <arch>x86</arch>  <version>5.1.2600</version> </os>  <property>  <name>mavenVersion</name>  <value>2.0.3</value> </property>  <file>  <exists>/usr/local/hudson/hudson-home/jobs/maven-guide-zh-to-production/workspace/</exists>  <missing>/usr/local/hudson/hudson-home/jobs/maven-guide-zh-to-production/workspace/</missing> </file> </activation>  <build> <defaultGoal/> <resources> <resource> <targetPath/><filtering/><directory/><includes/><excludes/> </resource> </resources> <testResources> <testResource> <targetPath/><filtering/><directory/><includes/><excludes/> </testResource> </testResources> <directory/><finalName/><filters/> <pluginManagement> <plugins>  <plugin> <groupId/><artifactId/><version/><extensions/> <executions> <execution> <id/><phase/><goals/><inherited/><configuration/> </execution> </executions> <dependencies>  <dependency> ...... </dependency> </dependencies> <goals/><inherited/><configuration/> </plugin> </plugins> </pluginManagement> <plugins>  <plugin> <groupId/><artifactId/><version/><extensions/> <executions> <execution> <id/><phase/><goals/><inherited/><configuration/> </execution> </executions> <dependencies>  <dependency> ...... </dependency> </dependencies> <goals/><inherited/><configuration/> </plugin> </plugins> </build>  <modules/>  <repositories>  <repository> <releases> <enabled/><updatePolicy/><checksumPolicy/> </releases> <snapshots> <enabled/><updatePolicy/><checksumPolicy/> </snapshots> <id/><name/><url/><layout/> </repository> </repositories>  <pluginRepositories>  <pluginRepository> <releases> <enabled/><updatePolicy/><checksumPolicy/> </releases> <snapshots> <enabled/><updatePolicy/><checksumPolicy/> </snapshots> <id/><name/><url/><layout/> </pluginRepository> </pluginRepositories>  <dependencies>  <dependency> ...... </dependency> </dependencies>  <reports/>  <reporting> ...... </reporting>  <dependencyManagement> <dependencies>  <dependency> ...... </dependency> </dependencies> </dependencyManagement>  <distributionManagement> ...... </distributionManagement>  <properties/> </profile> </profiles>  <modules/>  <repositories>  <repository>  <releases>  <enabled/>  <updatePolicy/>  <checksumPolicy/> </releases>  <snapshots> <enabled/><updatePolicy/><checksumPolicy/> </snapshots>  <id>banseon-repository-proxy</id>  <name>banseon-repository-proxy</name>  <url>http://192.168.1.169:9999/repository/</url>  <layout>default</layout> </repository> </repositories>  <pluginRepositories>  <pluginRepository> ...... </pluginRepository> </pluginRepositories>  <dependencies> <dependency>  <groupId>org.apache.maven</groupId>  <artifactId>maven-artifact</artifactId>  <version>3.8.1</version>  <type>jar</type>  <classifier></classifier>  <scope>test</scope>  <systemPath></systemPath>  <exclusions> <exclusion> <artifactId>spring-core</artifactId> <groupId>org.springframework</groupId> </exclusion> </exclusions>  <optional>true</optional> </dependency> </dependencies>  <reports></reports>  <reporting>  <excludeDefaults/>  <outputDirectory/>  <plugins>  <plugin>  <groupId/>  <artifactId/>  <version/>  <inherited/>  <configuration/>  <reportSets>  <reportSet>  <id/>  <configuration/>  <inherited/>  <reports/> </reportSet> </reportSets> </plugin> </plugins> </reporting>  <dependencyManagement> <dependencies>  <dependency> ...... </dependency> </dependencies> </dependencyManagement>  <distributionManagement>  <repository>  <uniqueVersion/> <id>banseon-maven2</id> <name>banseon maven2</name> <url>file://${basedir}/target/deploy</url> <layout/> </repository>  <snapshotRepository> <uniqueVersion/> <id>banseon-maven2</id> <name>Banseon-maven2 Snapshot Repository</name> <url>scp://svn.baidu.com/banseon:/usr/local/maven-snapshot</url> <layout/> </snapshotRepository>  <site>  <id>banseon-site</id>  <name>business api website</name>  <url> scp://svn.baidu.com/banseon:/var/www/localhost/banseon-web </url> </site>  <downloadUrl/>  <relocation>  <groupId/>  <artifactId/>  <version/>  <message/> </relocation>  <status/> </distributionManagement>  <properties/> </project>

转载： 1、Maven项目为什么能使用${}来引用pom文件中的属性 2、maven中filtering的使用 3、Maven配置文件详情

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