引用jar版本错误

[INFO] Scanning for projects... 
[ERROR] The build could not read 1 project -> [Help 1] 
[ERROR]   
[ERROR]   The project cas_client_sso:cas_app2:0.0.1-SNAPSHOT (D:\java\workSpace\cas_app2\pom.xml) has 7 errors 
[ERROR]     'dependencies.dependency.version' for org.springframework:spring-context:jar must be a valid version but is '${spring.version}'. @ line 28, column 13 
[ERROR]     'dependencies.dependency.version' for org.springframework:spring-web:jar must be a valid version but is '${spring.version}'. @ line 34, column 13 
[ERROR]     'dependencies.dependency.version' for org.springframework:spring-orm:jar must be a valid version but is '${spring.version}'. @ line 39, column 13 
[ERROR]     'dependencies.dependency.version' for org.springframework:spring-test:jar must be a valid version but is '${spring.version}'. @ line 45, column 13 
[ERROR]     'dependencies.dependency.version' for org.slf4j:slf4j-api:jar must be a valid version but is '${slf4j.version}'. @ line 93, column 13 
[ERROR]     'dependencies.dependency.version' for ch.qos.logback:logback-core:jar must be a valid version but is '${logback.version}'. @ line 99, column 13 
[ERROR]     'dependencies.dependency.version' for ch.qos.logback:logback-classic:jar must be a valid version but is '${logback.version}'. @ line 106, column 13 
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch. 
[ERROR] Re-run Maven using the -X switch to enable full debug logging. 
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles: 
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/ProjectBuildingException

问题缘由，maven不知道你引用的jar包的版本，导致无法获取这个jar，所以你必须指定一个版本号，${spring.version}为引用一个变量，必须在pom.xml中声明，上面错误是我从一个已有项目中直接copy过来导致，初次接触maven不懂这些，还需要把以下配置copy过来.

        <properties>
        <slf4j.version>1.7.5</slf4j.version>
        <logback.version>1.0.13</logback.version>
        <spring.version>3.2.3.RELEASE</spring.version>
        <jersey.version>2.2</jersey.version>
    </properties>