为什么Maven每次下载maven-metadata.xml？

Question

以下是我在尝试使用maven构建Web应用程序时，我的互联网连接处于flanky状态时通常会收到的错误。

我的问题是，为什么每次当同一个应用程序已经建成之前，maven总是需要下载。

什么可能是错误的，我的配置，使Maven每次下载？

下面是一个错误，我得到当我尝试建立离线：

[INFO] ------------------------------------------------------------------------ 
[INFO] Building mywebapp 1.0-SNAPSHOT 
[INFO] ------------------------------------------------------------------------ 
Downloading: https://raw.github.com/pagecrumb/mungo/mvn-repo/com/pagecrumb/mungo/0.0.1-SNAPSHOT/maven-metadata.xml 

[WARNING] Could not transfer metadata com.mywebapp:mungo:0.0.1-SNAPSHOT/maven-metadata.xml 
from/to mungo-mvn-repo (https://raw.github.com/pagecrumb/mungo/mvn-repo/): raw.github.com 
[INFO] 
[INFO] --- maven-war-plugin:2.1.1:war (default-cli) @ mywebapp --- 
[INFO] Packaging webapp 
[INFO] Assembling webapp [mywebapp] in [D:\workspace\web\target\mywebapp-1.0-SNAPSHOT] 
[INFO] Processing war project 
[INFO] Copying webapp resources [D:\workspace\web\src\main\webapp] 
[INFO] Webapp assembled in [1237 msecs] 
[INFO] Building war: D:\workspace\web\target\mywebapp-1.0-SNAPSHOT.war 
[WARNING] Warning: selected war files include a WEB-INF/web.xml which will be ignored 
(webxml attribute is missing from war task, 
or ignoreWebxml attribute is specified as 'true') 
[INFO]                   
[INFO] ------------------------------------------------------------------------ 
[INFO] Building com.mywebapp [com.mywebapp] 0.0.1-SNAPSHOT 
[INFO] ------------------------------------------------------------------------ 
Downloading: http://repo.maven.apache.org/maven2/org/apache/maven/plugins/maven-release-plugin/2.1/maven-release-plugin-2.1.pom 

[WARNING] Failed to retrieve plugin descriptor for org.apache.maven.plugins:maven-release-plugin:2.1: Plugin org.apache.maven.plugins:maven-release-plugin:2.1 or one of its dependencies could not be resolved: Failed to read artifact descriptor for org.apache.maven.plugins:maven-release-plugin:jar:2.1 
Downloading: http://download.java.net/maven/2/org/apache/maven/plugins/maven-metadata.xml 
Downloading: http://download.java.net/maven/2/org/codehaus/mojo/maven-metadata.xml 

397/397 B 

Downloaded: http://download.java.net/maven/2/org/codehaus/mojo/maven-metadata.xml (397 B at 0.0 KB/sec) 
[WARNING] Failure to transfer org.apache.maven.plugins:maven-war-plugin/maven-metadata.xml from http://download.java.net/maven/2 was cached in the local repository, resolution will not be reattempted until the update interval of maven2-repository.dev.java.net has elapsed or updates are forced. Original error: Could not transfer metadata org.apache.maven.plugins:maven-war-plugin/maven-metadata.xml from/to maven2-repository.dev.java.net (http://download.java.net/maven/2): download.java.net 
[INFO] 
[INFO] --- maven-war-plugin:2.3:war (default-cli) @ mywebapp-build --- 
[INFO] Packaging webapp 
[INFO] Assembling webapp [mywebapp-build] in [D:\workspace\target\mywebapp-build-0.0.1-SNAPSHOT] 
[INFO] Processing war project 
[INFO] Webapp assembled in [15 msecs] 
[INFO] Building war: D:\workspace\target\mywebapp-build-0.0.1-SNAPSHOT.war 
[INFO] ------------------------------------------------------------------------ 
[INFO] Reactor Summary: 
[INFO] 
[INFO] mywebapp ..................................... SUCCESS [27.999s] 
[INFO] com.mywebapp [com.mywebapp] ..................... FAILURE [1:00.406s] 
[INFO] ------------------------------------------------------------------------ 
[INFO] BUILD FAILURE 
[INFO] ------------------------------------------------------------------------ 
[INFO] Total time: 1:41.409s 
[INFO] Finished at: Tue May 07 22:13:38 SGT 2013 
[INFO] Final Memory: 11M/28M 
[INFO] ------------------------------------------------------------------------ 
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-war-plugin:2.3:war 
(default-cli) on project mywebapp-build: Error assembling WAR: webxml attribute is required (or pre-existing WEB-INF/web.xml if executing in update mode) 

Answer 1

查看您的settings.xml（或者可能是您项目的父母或母公司POM）<repositories>元素。它看起来像下面。

<repositories> 
    <repository> 
     <id>central</id> 
     <url>http://gotoNexus</url> 
     <snapshots> 
      <enabled>true</enabled> 
      <updatePolicy>always</updatePolicy> 
     </snapshots> 
     <releases> 
      <enabled>true</enabled> 
      <updatePolicy>daily</updatePolicy> 
     </releases> 
    </repository> 
</repositories> 

注意<updatePolicy>元素。这个例子告诉Maven在任何时候联系远程仓库（在我的例子中为Nexus，如果你没有使用自己的远程仓库，Maven Central），Maven需要在构建过程中检索快照工件，检查是否有更新的副本。元数据是必需的。如果有更新的副本，Maven会将其下载到您当地的回购站点。

在示例中，对于发布版本，策略是daily，因此它会在您第一次构建当天的过程中进行检查。 never也是一个有效的选项，如Maven settings docs中所述。

插件分开解决。您也可以为这些配置存储库，并根据需要使用不同的更新策略。其他人提到了-o选项。如果使用它，Maven将以“脱机”模式运行。它知道它只有一个本地回购，并且无论您使用哪种更新策略，它都不会联系远程回购来刷新工件。

Answer 2

我还没有研究，当Maven做哪些查找，但要获得稳定的和可重复的版本，我强烈建议不要直接访问Maven Respositories，而是使用Maven Repository Manager（如Nexus）。

这里是教程如何建立你的设置文件：

http://books.sonatype.com/nexus-book/reference/maven-sect-single-group.html

http://maven.apache.org/repository-management.html

Answer 3

我想，因为你因此触发相关元数据的下载，以便没有指定插件版本得到最后一个。

否则，您是否尝试使用-o强制本地回购使用？

Answer 4

可能使用标志-o,--offline "Work offline"来防止这种情况。

像这样：

maven compile -o