为什么Maven每次都下载maven-metadata.xml？

Question

Below is the error I usually get when my internet connection is flanky when trying to build a web application with maven.

My question is that, why does maven always have to download every time when the same app has been built earlier.

What could be wrong in my configuration that makes maven to download every time?

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)

Answer 1

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>

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.

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 .

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>

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.

Answer 2

It is possibly to use the flag -o,--offline "Work offline" to prevent that.

Like this:

maven compile -o

Answer 3

I suppose because you didn't specify plugin version so it triggers the download of associated metadata in order to get the last one.

Otherwise did you try to force local repo usage using -o ?

Answer 4

I haven't studied yet, when Maven does which look-up, but to get stable and reproducible builds, I strongly recommend not to access Maven Respositories directly but to use a Maven Repository Manager such as Nexus.

Here is the tutorial how to set up your settings file:

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

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

Answer 5

Maven does this because your dependency is in a SNAPSHOT version and maven has no way to detect any changes made to that snapshot version in the repository. Release your artifact and change the version in pom.xml to that version and maven will no longer fetch the metadata file.

Answer 6

In my case I use private repository mananger. While some other maven repository manangers are deprecated. It looks like gradle looks up my private artifacts from deprecated repository managers. Update or remove deprecated repository manangers fix the issue for me.