Java: How to Remove Unused Dependencies from a Maven pom.xml

The Maven Dependency Plugin will help, especially the dependency:analyze goal:
dependency:analyze analyzes the dependencies of this project and determines which are: used and declared; used and undeclared; unused and declared.
Another thing that might help to do some cleanup is the Dependency Convergence report from the Maven Project Info Reports Plugin.
Let's use maven-depedency-plugin to find obvious (and easy) stuff to fix:

1. Duplicated Dependencies

mvn dependency:analyze-duplicate
Just remove what's duplicated and that's it.

2. Used and declared; used and undeclared; unused and declared

mvn dependency:analyze
This will take a while and may warn you about a lot of depedency errors.
You will be able to remove dependencies that are not used anymore and add dependencies that are directly used.
Beware: This might show some false-positives.

Bring the big guns

1. Multiple versions of the same depedency

This is the harder, as maven doesn't have a plugin to find them easily.
So, I used a little bit of bash trickery to do the job for me:
mvn dependency:list -Dsort=true |
grep "^\[INFO\] " |
awk '{print $2}' |
cut -f1-4 -d: |
sort |
uniq |
cut -f1-3 -d: |
uniq -c |
grep -v '^ *1 '
Line-by-line, explained version:
mvn dependency:list -Dsort=true | # list all deps
grep "^\[INFO\] " | # grep for the deps list only
awk '{print $2}' | # remove the INFO prefix
cut -f1-4 -d: | # removes the dep scope
sort | # sort (duh)
uniq | # remove duplicates
cut -f1-3 -d: | # removes the version
uniq -c | # count line groups
grep -v '^ *1 ' # grep groups that repeat
You'll end up with a list like this:
2 com.foo:foo-modules-commons:jar
3 com.fasterxml.jackson.core:jackson-annotations:jar
3 com.fasterxml.jackson.core:jackson-core:jar
2 com.fasterxml.jackson.core:jackson-databind:jar
2 com.squareup.okio:okio:jar
2 commons-collections:commons-collections:jar
2 commons-fileupload:commons-fileupload:jar
2 joda-time:joda-time:jar
2 org.mapstruct:mapstruct:jar
Now, for each of these deps, you'll have to run:
mvn dependency:tree -Dincludes=DEP
as in
mvn dependency:tree -Dincludes=org.mapstruct:mapstruct
This will show you all the places this dependency is being used, so all you need to do is fix it.

2. Duplicated Classes

This is something that I have seen a lot in Apache Commons libraries.
As far as I know, the easiest way to find these problems is by using Jboss' Tattletale Maven Plugin.
While the plugin seems to be abandoned, it still works. Just add it to your parent pom and run it to get the reports.
These kind of problems may require manually removing .class files from jars, excluding dependencies or, sometimes, just ignoring some of them.

Preventive actions

Act when the everything went to shit already usually is harder than avoiding small mistakes.
So, my tips are:
  • Give more attention to it in code reviews;
  • Run maven-dependency-plugin and tattletale-maven in the build for every pull request to block problematic changes;
  • You can tweak the script to find dependencies with multiple versions to exit 1 in thoses cases, and just add it to the build too.
That's it. Hope it helps!

Reference:

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