Question

(This question is asked on Maven User mailing list too)

I have recently faced a strange problem, that I cannot even able to judge the cause or source of problem. It will be great if someone can give me some direction:

(The story may be a bit long)

I am using Nexus 1.8.0 as our company’s repository manager. I use it as proxy of external repo, and hosting our own repository.
There are many repositories in Nexus. I have one repository group (let’s call it PUBLIC) which groups all public repositories, including maven central, codehaus etc.
There is another repository group (let’s call it EXT) which we put 3rd party artifacts.

In our project, we used org.codehaus.mojo:native2ascii-maven-plugin.
Due to a bug at that time, instead of using the publicly available org.codehaus.mojo:native2ascii-maven-plugin:1.0-alpha-1, I have fixed the bug and deploy it to our EXT repository, and called it org.codehaus.mojo:native2ascii-maven-plugin:1.0-alpha-1.1 (i.e. used a new version number 1.0-alpha-1.1 instead of 1.0-alpha-1)

This have been running fine for several years.

However recently a new developer tries to get the code and build, using Maven 2.2.1. Strange things happened: the build failed. By inspecting result of mvn -X clean install, it states that POM of native2ascii-maven-plugin:1.0-alpha-1.1 cannot be downloaded from PUBLIC, therefore it will use a default emtpy POM, which cause the build problem.

By inspecting the local repository, I found that only the JAR of native2ascii-maven-plugin:1.0-alpha-1.1 was downloaded. I am sure that there is no native2ascii-maven-plugin:1.0-alpha-1.1 in PUBLIC repository, and the SHA of the JAR matches with native2ascii-maven-plugin:1.0-alpha-1.1 in EXT. It seems that, Maven is capable to download the JAR correctly from EXT repo, but when it tries to download the POM afterwards, Maven mistakenly think that it should be downloaded from PUBLIC. Because PUBLIC do not contains 1.0-alpha-1.1, Maven assume there is no POM.

I have EXT repo defined before PUBLIC in my settings.xml. What even more strange is, I tried to block accessing in Nexus for native2ascii-maven-plugin from PUBLIC. Maven, instead of getting the POM from repository EXT, it get from central directly. At last I add PUBLIC as mirror for central, and Maven can build correctly, because EXT is the the only repo that contains native2ascii-maven-plugin. Maven seems tries to download the POM from every repository else which contains native2ascii-maven-plugin in despite of the version number, except from EXT

I simply cannot understand why this will happen. This have been used for years, and it used to be fine even several weeks before (I have other new developers, who can correctly download the plugin, several weeks ago). May anyone guide me the possible cause of the problem? I have neither changed anything in my repo, nor changed version of Maven. Why Maven’s “download” behavior suddenly changed?

Answer 1

At last I managed to find out the “cause” of the problem. It is due to my fault, combined with still-unknown behavior of Maven. I add this as an answer to ease future reference for other people.

They key problem is that I missed plugin version for this specific project (I did put corresponding pluginManagement for other projects, and other plugins for this project… I wonder how come I made this mistake this time)

The way to reproduce the problem:

1. A separate repository to store the plugin (in my case, org.codehaus.mojo:native2ascii-maven-plugin:1.0-alpha-1.1)

2. In project POM, add plugin, without version. For example,

   <plugins>
       <plugin>
           <groupId>org.codehaus.mojo</groupId>
           <artifactId>native2ascii-maven-plugin</artifactId>
       <plugin>
   </plugins>
   

3. in settings.xml, avoid defining mirrors (i.e. the settings.xml contains list of repositories and pluginRepositories only)

With such setup, first purge the local repository. Then build the project. After build, inspect the directory in local repository for that plugin (in my case .m2\repository\org\codehaus\mojo\native2ascii-maven-plugin\1.0-alpha-1.1), you will find only the JAR presents, without corresponding POM. (Caused by Maven successfully get the plugin JAR corresponding to the pluginRepositories in settings.xml, but trying to get the POM from a weird location)

With the same setup, put the version in project POM, clean up the local repo, and build again. Everything is fine now.

The reason for work fine even for a recently clean CI environment, is probably due to other “correct” project made the plugin downloaded correctly, which can be used by this “incorrect” project. A periodic purge in local repository in CI won’t necessary help much on this too because for that many projects, the chance is always very high for other “correct” project build earlier than that “incorrect” project.

The reason behind such behavior of Maven is still unknown, but at least in a “correct” POM (with plugin version correctly declared), Maven works fine. I will raise this as a issue for Maven though.