title: Usage of Maven pom tag
body: <p>I hava a question about Maven pom.xml.</p>
  
  <p>I add a <em>dependency</em> in pom.xml like: </p>
  
  <pre><code>&lt;dependencies&gt;
      &lt;dependency&gt;
              &lt;groupId&gt;org.opendaylight.controller&lt;/groupId&gt;
              &lt;artifactId&gt;features-restconf&lt;/artifactId&gt;
              &lt;version&gt;1.2.1-Lithium-SR1&lt;/version&gt;
              &lt;classifier&gt;features&lt;/classifier&gt;
              &lt;type&gt;xml&lt;/type&gt;
              &lt;scope&gt;runtime&lt;/scope&gt;
      &lt;/dependency&gt;
  &lt;/dependencies&gt;
  </code></pre>
  
  <p>I can not understand the usage of &lt; classifier > and &lt; type >.</p>
  
  <p>My questions are: </p>
  
  <blockquote>
    <ol>
    <li>What is the meaning of <strong>classifier</strong> and <strong>type</strong></li>
    <li>What is the meaning of <strong>scope</strong>(runtime) while the type is xml. I guess xml means maven needs a xml file, but what is it to do with
    <strong>runtime</strong>? I think <strong>runtime</strong> is always associated with "added to classpath", but why xml?</li>
    </ol>
  </blockquote>
  
  <p>If I comment on &lt; type > I got error like: </p>
  
  <blockquote>
    <p>[ERROR] Failed to execute goal on project test: Could not resolve
    dependencies for project com.ruan:test:jar:1.0-SNAPSHOT: Failure to
    find
    org.opendaylight.controller:features-restconf:jar:features:1.2.1-Lithium-SR1
    in <a href="http://nexus.opendaylight.org/content/repositories/public/" rel="nofollow">http://nexus.opendaylight.org/content/repositories/public/</a> was
    cached in the local repository, resolution will not be reattempted
    until the update interval of opendaylight-mirror has elapsed or
    updates are forced -> [Help 1]</p>
  </blockquote>
  
  <p>If I comment on &lt; classifier > I got error like: </p>
  
  <blockquote>
    <p>[ERROR] Failed to execute goal on project test: Could not resolve
    dependencies for project com.ruan:test:jar:1.0-SNAPSHOT: Failure to
    find
    org.opendaylight.controller:features-restconf:xml:1.2.1-Lithium-SR1 in
    <a href="http://nexus.opendaylight.org/content/repositories/public/" rel="nofollow">http://nexus.opendaylight.org/content/repositories/public/</a> was cached
    in the local repository, resolution will not be reattempted until the
    update interval of opendaylight-mirror has elapsed or updates are
    forced -> [Help 1]</p>
  </blockquote>
  
  <p>If I run it correctly, I got the directory like: </p>
  
  <pre><code>haoruan:~/.m2/repository/org/opendaylight/controller/features-restconf $ cd 1.2.1-Lithium-SR1/
  total 96
  -rw-r--r--  1 haoruan  staff   264B Oct 29 13:58 _remote.repositories
  -rw-r--r--  1 haoruan  staff   397B Oct 29 13:38 features-restconf-1.2.1-Lithium-SR1-features.jar.lastUpdated
  -rw-r--r--  1 haoruan  staff   9.0K Oct 29 13:58 features-restconf-1.2.1-Lithium-SR1-features.xml
  -rw-r--r--  1 haoruan  staff    40B Oct 29 13:58 features-restconf-1.2.1-Lithium-SR1-features.xml.sha1
  -rw-r--r--  1 haoruan  staff    12K Oct 29 13:38 features-restconf-1.2.1-Lithium-SR1.pom
  -rw-r--r--  1 haoruan  staff    40B Oct 29 13:38 features-restconf-1.2.1-Lithium-SR1.pom.sha1
  -rw-r--r--  1 haoruan  staff   397B Oct 29 13:39 features-restconf-1.2.1-Lithium-SR1.xml.lastUpdated
  </code></pre>