I'm trying to run some unit tests with Apache Maven. I hoped this would be as simple as running the test "goal". But when I did that, maven complained that it could not download some dependencies and thus can't run my tests. This sounds fine, except that I have no idea why it decided I need those dependencies; they are not in my pom.xml, and I doubt they're in my transitive dependencies either. (I'm not sure about that last part; they very well might be in my transitive dependencies.)

Luckily, maven has the perfect tool for this: dependency:tree will tell us exactly which dependency is getting pulled in by what. Except for the small problem that maven thinks to itself "in order to build the tree, I have to resolve the dependencies first" so it tries (and fails) to download those very same dependencies so that it can build the part of the tree that's under them.

So now I don't have a tree, and I have no idea how to proceed from here.

CodePudding user response：

How exactly would you think that maven could resolve transitive dependencies (= dependencies of dependencies) without resolving the dependencies first? Escpecially for the goal "test" also the dependency scope "test" has to be used, which is more then the default scope "compile".

You can use the goal dependency:go-offline to prepare for the offline mode. Maven downloads then all required dependencies. Find the detailed docs for that on https://maven.apache.org/plugins/maven-dependency-plugin/go-offline-mojo.html

You could also have a look at this answer to get another opinion on going online.

CodePudding user response：

The main problem is maven downloads dependencies by demand, you may just check that by triggering different lifecycle phases like mvn initialize, mvn validate, mvn compile, mvn package and checking what maven is trying to download. Sometimes it is possible to figure out project dependencies via analysing project object model (pom), sometimes it is not, especially when plugins define their own dependencies either implicitly or explicitly, some examples below:

• we may ask maven-dependency-plugin to download something via dependency:copy-dependencies
• exec-maven-plugin has similar functionality: Running Java programs with the exec goal
• maven-invoker-plugin may run poms which are part of project but not a part of reactor.

In short: neither maven plugin will able to download all required dependencies. The only "reliable" way to go offline is to run target goal and only then go offline, unfortunately even in this cases some weird things may happen, especially when you or dependency authors are using snapshot versions, version ranges, third-party repositories, etc (my own preference is to run maven with -llr flag to make it more reliable).