Maven会沿着父子层次向上赎ͼŒç›´åˆ°æ‰‘Öˆ°ä¸€ä¸ªæ‹¥æœ‰dependencyManagement元素的项目，然后它就会ä‹É用在˜q™ä¸ªdependencyManagement元素中指定的版本受÷€?/p>

<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.sonatype.mavenbook</groupId>
  <artifactId>a-parent</artifactId>
  <version>1.0.0</version>
  
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>5.1.2</version>
      </dependency>
      
    <dependencies>
  </dependencyManagement>

然后åQŒåœ¨å­é¡¹ç›®ä¸­åQŒä½ å¯ä»¥ä½¿ç”¨å¦‚下的依赖XMLæ·ÕdŠ ä¸€ä¸ªå¯¹MySQL Java Connector的依赖：

<project>
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>org.sonatype.mavenbook</groupId>
    <artifactId>a-parent</artifactId>
    <version>1.0.0</version>
  </parent>
  <artifactId>project-a</artifactId>
  
  <dependencies>
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
    </dependency>
  </dependencies>
</project>

如果mysql-connector-java在顶层父™å¹ç›®ä¸­è¢«ä½œäؓ一个依赖列出，˜q™ä¸ªå±‚次中的所有项目都ž®†å¼•ç”¨è¯¥ä¾èµ–。ä‹É用dependencyManagement能让你统一òq‰™›†ä¸­åŒ–依赖版本的管理，而不用添加那些会被所有子™å¹ç›®¾l§æ‰¿çš„依赖ã€?/p>

解决˜q™ä¸ªé—®é¢˜çš„方法就是ä‹Éç”?lt;scope>标签åQŒå¦‚ä¸?/p>

˜q™é‡Œçš„意思是è¯ß_¼Œ¾~–译的时候用到servlet-apiå’Œjsp-apiåQŒä½†åœ¨æ‰“包的时候不用这两个依赖ã€?/p>

在maven的官方中是这æ äh˜q°çš„

Dependency Scope
Dependency scope is used to limit the transitivity of a depedency, and also to affect the classpath used for various build tasks.

There are 6 scopes available:

compile
This is the default scope, used if none is specified. Compile dependencies are available in all classpaths of a project. Furthermore, those dependencies are propagated to dependent projects.
provided
This is much like compile, but indicates you expect the JDK or a container to provide the dependency at runtime. For example, when building a web application for the Java Enterprise Edition, you would set the dependency on the Servlet API and related Java EE APIs to scope provided because the web container provides those classes. This scope is only available on the compilation and test classpath, and is not transitive.
runtime
This scope indicates that the dependency is not required for compilation, but is for execution. It is in the runtime and test classpaths, but not the compile classpath.
test
This scope indicates that the dependency is not required for normal use of the application, and is only available for the test compilation and execution phases.
system
This scope is similar to provided except that you have to provide the JAR which contains it explicitly. The artifact is always available and is not looked up in a repository.
import (only available in Maven 2.0.9 or later)
This scope is only used on a dependency of type pom in the <dependencyManagement> section. It indicates that the specified POM should be replaced with the dependencies in that POM's <dependencyManagement> section. Since they are replaced, dependencies with a scope of import do not actually participate in limiting the transitivity of a dependency.
如果你这样做了，但ä‹É用eclipse+tomcat做测试的时候发现servlet-api˜q˜æ˜¯è¢«æ‰“包到lib下面了，你要把maven插äšg中的WTP也安装一下，问题应该ž®Þp§£å†³äº†ã€?br />

    private static final char PATH_SEPARATOR = '/';

    private static final char GROUP_SEPARATOR = '.';

    private static final char ARTIFACT_SEPARATOR = '-';

    public String pathOf( Artifact artifact )
    {
        ArtifactHandler artifactHandler = artifact.getArtifactHandler();

        StringBuilder path = new StringBuilder( 128 );

        path.append( formatAsDirectory( artifact.getGroupId() ) ).append( PATH_SEPARATOR );
        path.append( artifact.getArtifactId() ).append( PATH_SEPARATOR );
        path.append( artifact.getBaseVersion() ).append( PATH_SEPARATOR );
        path.append( artifact.getArtifactId() ).append( ARTIFACT_SEPARATOR ).append( artifact.getVersion() );

        if ( artifact.hasClassifier() )
        {
            path.append( ARTIFACT_SEPARATOR ).append( artifact.getClassifier() );
        }

        if ( artifactHandler.getExtension() != null && artifactHandler.getExtension().length() > 0 )
        {
            path.append( GROUP_SEPARATOR ).append( artifactHandler.getExtension() );
        }

        return path.toString();
}

    private String formatAsDirectory( String directory )
    {
        return directory.replace( GROUP_SEPARATOR, PATH_SEPARATOR );
    }
??

该pathOf()æ–ÒŽ³•çš„目的是æ ÒŽ®æž„äšg信息生成其在仓库中的路径。在阅读本段代码之前åQŒè¯»è€…可以先回顾一下上一章Maven坐标的相兛_†…宏V€‚这里，我们æ ÒŽ®ä¸€ä¸ªå®žé™…的例子来分析èµ\径的生成åQŒè€ƒè™‘˜q™æ ·ä¸€ä¸ªæž„ä»Óž¼šgroupId=org.testng、artifactId=testng、version=5.8、classifier=jdk15、packaging=jaråQŒå…¶å¯¹åº”çš„èµ\径按如下步骤生成åQ?/p>

        首先åŸÞZºŽæž„äšgçš„groupId准备路径åQŒformatAsDirectory()ž®†groupId中的句点分隔½W¦è{换成路径分隔½W¦ï¼Œè¯¥ä¾‹ä¸­ï¼ŒgroupId org.testngž®×ƒ¼šè¢«è{换成org/testngåQŒä¹‹åŽå†åŠ ä¸€ä¸ªèµ\径分隔符斜杠åQŒé‚£ä¹ˆorg.testngž®±æˆä¸ÞZº†org/testng/ã€?
åŸÞZºŽæž„äšgçš„artifactId准备路径åQŒä¹Ÿž®±æ˜¯åœ¨å‰é¢çš„基础上加上artifactId以及一个èµ\径分隔符åQŒè¯¥ä¾‹ä¸­çš„artifactId为testngåQŒé‚£ä¹ˆåœ¨˜q™ä¸€æ­¥è¿‡åŽèµ\径就成äؓ了org/testng/testng/ã€?
        接着使用版本信息åQŒåœ¨å‰é¢çš„基¼‹€ä¸ŠåŠ ä¸Šversionå’Œèµ\径分隔符åQŒè¯¥ä¾‹ä¸­ç‰ˆæœ¬æ˜?.8åQŒé‚£ä¹ˆèµ\径就成äؓ了org/testng/tesgng/5.8/ã€?
˜q™ä¸€æ­¥å†ä¾æ¬¡åŠ ä¸ŠartifactIdåQŒæž„件分隔符˜qžå­—åøP¼Œä»¥åŠversionåQŒäºŽæ˜¯æž„建的路径ž®±å˜æˆäº†org/testng/testng/5.8/testng-5.8。读者可能会注意到这里ä‹É用了artifactId.getVersion()åQŒè€Œä¸Šä¸€æ­¥ç”¨çš„是artifactId.getBaseVersion()åQŒversionå’ŒbaseVersion的区别在本章讨论SNAPSHOT的时候会具体阐述ã€?
紧接着如果构äšg有classifieråQŒå°±åŠ ä¸Šæž„äšg分隔½W¦å’ŒclassifieråQŒè¯¥ä¾‹ä¸­æž„äšgçš„classifier是jdk15åQŒé‚£ä¹ˆèµ\径就变成org/testng/testng/5.8/testng-5.8-jdk5ã€?
最后第‹‚€æŸ¥æž„件的extensionåQŒè‹¥extension存在åQŒåˆ™åŠ ä¸Šå¥ç‚¹åˆ†éš”½W¦å’ŒextensionåQŒä»Žä»£ç ä¸­å¯ä»¥çœ‹åˆŽÍ¼Œextension是从artifactHandler而非artifact获取åQŒartifactHandler是由™å¹ç›®çš„packaging军_®šçš„，因此可以è¯ß_¼Œpackaging军_®šäº†æž„件的扩展名，该例的packaging是的jaråQŒå› æ­¤æœ€¾lˆçš„路径为org/testng/testng/5.8/testng-5.8-jdk5.jarã€?
        到这里笔者（包括读者你åQ‰éƒ½åº”该感谢Maven开源社区，正是ç”׃ºŽMaven的所有源代码都是开攄¡š„åQŒæˆ‘们才能仔¾l†å¾—深入到其内部工作的所有细节ã€?
        ç”׃ºŽMaven仓库是基于简单文件系¾lŸå­˜å‚¨çš„åQŒçŽ°åœ¨æˆ‘们又理解了其存储方式åQŒå› æ­¤å½“遇到一些与仓库相关的问题时åQŒå¯ä»¥å¾ˆæ–¹ä¾¿çš„查扄¡›¸å…Ïx–‡ä»Óž¼Œæ–¹ä¾¿å®šä½é—®é¢˜ã€‚例如当Maven无法获得™å¹ç›®å£°æ˜Žçš„依赖时åQŒå¯ä»¥ç®€å•è¯¥ä¾èµ–对应的文件在仓库中是否存在，如果不存在，是否有其它版本可用，½{‰ç­‰ã€?