爲什麼Maven每次下載maven-metadata.xml？

Question

以下是我在嘗試使用maven構建Web應用程序時，我的互聯網連接處於flanky狀態時通常會收到的錯誤。

我的問題是，爲什麼每次當同一個應用程序已經建成之前，maven總是需要下載。

什麼可能是錯誤的，我的配置，使Maven每次下載？

下面是一個錯誤，我得到當我嘗試建立離線：

[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（或者可能是您項目的父母或母公司POM）<repositories>元素。它看起來像下面。

<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。這個例子告訴Maven在任何時候聯繫遠程倉庫（在我的例子中爲Nexus，如果你沒有使用自己的遠程倉庫，Maven Central），Maven需要在構建過程中檢索快照工件，檢查是否有更新的副本。元數據是必需的。如果有更新的副本，Maven會將其下載到您當地的回購站點。

在示例中，對於發佈版本，策略是daily，因此它會在您第一次構建當天的過程中進行檢查。 never也是一個有效的選項，如Maven settings docs中所述。

插件分開解決。您也可以爲這些配置存儲庫，並根據需要使用不同的更新策略。其他人提到了-o選項。如果使用它，Maven將以「脫機」模式運行。它知道它只有一個本地回購，並且無論您使用哪種更新策略，它都不會聯繫遠程回購來刷新工件。

Answer 2

我還沒有研究，當Maven做哪些查找，但要獲得穩定的和可重複的版本，我強烈建議不要直接訪問Maven Respositories，而是使用Maven Repository Manager（如Nexus）。

這裏是教程如何建立你的設置文件：

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

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

Answer 3

我想，因爲你因此觸發相關元數據的下載，以便沒有指定插件版本得到最後一個。

否則，您是否嘗試使用-o強制本地回購使用？

Answer 4

可能使用標誌-o,--offline "Work offline"來防止這種情況。

像這樣：

maven compile -o