Maven初用问题一（依赖下载问题）

  在Maven项目pom.xml中为项目添加依赖，如下所示：

<dependency>
          <groupId>org.apache.hadoop</groupId>
          <artifactId>hadoop-core</artifactId>
          <version>0.20.2-cdh3u3</version>
      </dependency>

  使用eclipse->Run as Maven install时，出现以下错误：

[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building loogn 0.0.1-SNAPSHOT
[INFO] ------------------------------------------------------------------------
Downloading:
[WARNING] The POM for org.apache.hadoop:hadoop-core:jar:0.20.2-cdh3u3 is missing, no dependency information available
Downloading:
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 3.784s
[INFO] Finished at: Wed Jul 18 21:54:34 CST 2012
[INFO] Final Memory: 5M/55M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal on project loogn: Could not resolve dependencies for project com.loogn.hadoop:loogn:jar:0.0.1-SNAPSHOT: Could not find artifact org.apache.hadoop:hadoop-core:jar:0.20.2-cdh3u3 in central () -> [Help 1]
[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]

  在第16行中就已经将问题说的很清楚了，是因为指定的版本在maven2库中并没有，所以通过  搜索hadoop-core包含了几个版本，所以将前面的pom.xml文件将版本号改成其下有的就okay了。

  但是就算是这样，其实如果单纯地使用eclipse编译，还是会有可能编译失败，所以这时需要在命令行下，进入项目根目录，使用 mvn eclipse:eclipse编译，然后在eclipse workspace中刷新一下，这样依赖就下载完成了。

  以上是一种解决方法，还有一种就是可以通过添加自己的maven本地库，将hadoop-core-0.20.2-cdh3u3.jar下载好放置其中，然后编译也能通过。