问题描述
以下是我尝试使用maven构建Web应用程序时，我的互联网连接通常会出现的错误。
Below is the error I usually get when my internet connection is flanky when trying to build a web application with maven.
我的问题是，为什么maven总是必须每次下载早期构建相同的应用程序时下载。
My question is that, why does maven always have to download every time when the same app has been built earlier.
我的配置中每次都能下载maven会出现什么问题？
What could be wrong in my configuration that makes maven to download every time?
以下是我尝试离线构建时遇到的错误：
Below is an error I get when I try to build offline:
[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)
推荐答案
查看 settings.xml （或者，可能是您项目的父级或公司父级POM）< repositories> 元素。它看起来如下所示。
Look in your settings.xml (or, possibly your project's parent or corporate parent POM) for the <repositories> element. It will look something like the below.
<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>
请注意< updatePolicy> 元素。该示例告诉Maven在构建期间任何时候需要检索快照工件时检查远程仓库（在我的情况下为Nexus，Maven Central，如果您没有使用自己的远程仓库），检查是否有更新的副本。这需要元数据。如果有更新的副本，Maven会将其下载到您的本地仓库。
Note the <updatePolicy> element. The example tells Maven to contact the remote repo (Nexus in my case, Maven Central if you're not using your own remote repo) any time Maven needs to retrieve a snapshot artifact during a build, checking to see if there's a newer copy. The metadata is required for this. If there is a newer copy Maven downloads it to your local repo.
在示例中，对于版本，策略为每天 因此它将在您第一次构建当天进行检查。 never 也是一个有效选项，如中所述。
In the example, for releases, the policy is daily so it will check during your first build of the day. never is also a valid option, as described in Maven settings docs.
插件是单独解决的。您也可以为这些存储库配置存储库，如果需要，可以使用不同的更新策略。
Plugins are resolved separately. You may have repositories configured for those as well, with different update policies if desired.
<pluginRepositories>
<pluginRepository>
<id>central</id>
<url>http://gotoNexus</url>
<snapshots>
<enabled>true</enabled>
<updatePolicy>daily</updatePolicy>
</snapshots>
<releases>
<enabled>true</enabled>
<updatePolicy>never</updatePolicy>
</releases>
</pluginRepository>
</pluginRepositories>
其他人提到 -o 选项。如果您使用它，Maven将以离线模式运行。它知道它只有一个本地仓库，无论你使用什么更新策略，它都不会联系远程仓库来刷新工件。
Someone else mentioned the -o option. If you use that, Maven runs in "offline" mode. It knows it has a local repo only, and it won't contact the remote repo to refresh the artifacts no matter what update policies you use.
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