Develop Reference

Can maven plugin update project dependencies?

Is it possible to update or add dependencies to project during initialization/compilation via maven plugin? I've tried ti my Mojo:
Dependency dep = new Dependency();
dep.setGroupId("example");
dep.setArtifactId("sample");
dep.setVersion("4.3");
dep.setScope("compile");
project.getModel().addDependency(dep);
getLog().info("Dependency count:" + project.getDependencies().size())
project obtained via #Parameter as ${project}. But running sample project with this plugin with goals validate dependency:tree shows that plugin mojo was executed however dependency tree is empty - added dependency is lost. Is project just readable? I couldn't find any documentation about this so far.
I know you can update project dependencies in gradle like this via custom gradle plugin.

Dependencies are resolved at the beginning of the Maven run.
So when you run a plugin, dependencies have already been resolved.
The question that remains: What do you want to achieve with your approach? Which problem do you want to solve?

Export maven as jar file with all dependencies

I have my own java library created as maven project and has some dependencies included in pom.xml
I want to export project as jar and include it into others maven projects.
The problem is that I need to copy all dependencies from pom.xml of my library into maven projects where is imported my library to make it to work.
How to export my library to not be necessary to copy dependencies of my library.

That is easy to do; the central feature of Maven is that it manages the project dependencies for you.
You need to mvn install your project from the command line; that will install the jar and the pom files to your local repository.
You can then include your library as a Maven dependency in other Maven based projects; Maven will resolve the (transitive) dependencies for your project.

Normally you don't need to list all the dependencies in the project that imports your library. Maven should fetch them for you. What you need to do is to declare dependencies in your library.
Make sure you declare correct types of dependencies. Here is more info. In your case you need to make sure that dependencies you want to copy to the downstream projects are marked as 'compile'
There are tools that make 'Fat' jars by copying all dependencies inside. But they are mostly used to build the final project such as a deployable WAR file or a desktop app. Not in case of the libraries

Import Maven Project as Dependency into Gradle Project

I went through this link to import a gradle project as dependency into another gradle project. Is there a way to include a maven project as dependency into a gradle project?

If that Maven project is built somewhere else and deployed to a Maven repository, you can specify the artifact it produces as a simple compile dependency. If this Maven project is somehow a subproject of a Gradle multi-project build, I suppose you could hack it to work by simply ignoring the Maven POM file and perhaps adding a build.gradle to that project.

To use the solution described on the link that you provided - both projects must be gradle and included in gradle settings. Therefore you can use project closure to compile and depend on the project without building it explicitly.
I am not aware of any way to do this with maven project. I understand you use some maven plugins that you dont want to rewrite in gradle as simply can not find any equivalents etc. Often had that problem.
In this scenario I would suggest to build maven project and depend on a built jar in your gradle project.
Otherwise you could probably amend sourcesets in your gradle project to include maven classes. But I think it would be to complicated.
If I would be you I would turn it into gradle and try to replicate what you had using maven or just build the artifact and depend on it in dependencies closure.
Gradle is not that new anymore and there are many plugins that are superseding old good maven stuff.

Install Maven for two or more projects with one pom.xml

I'm new to maven..
I have three different projects in the same workspace and I want to install Maven in the three projects with only one pom.xml in order to avoid doing it one at a time and manually, just run the maven install in Project 1 and automatically have installed maven in the other two projects.
I don't know if there's a maven plugin that do this task?
Anyone has good solution for this, a really need help with this!
Regards

create a multi-module project: maven aggregation

You could create a multi-module project. Here is the quickest way for me to do it with Idea: How to create an empty multi module Maven project?

Maven is designed for a pom.xml for each project. You can save time by having a multi module Pom pointing to all three modules so you can use maven install there. The easiest for a workspace with multiple projects is to have it at the root of the workspace.

Import jar from ant ivy project to maven

I am working on an ant project and i want to use the jar created by it , in my maven project.
The ant project employs ivy to manage its dependency.
Is there any easy way to do this if possible without changing any code on their side.

It sounds like you want to be able to publish artefacts generated by your ivy-based project to a maven repo so they can be resolved by the mvn-based project.
Try http://draconianoverlord.com/2010/07/18/publishing-to-maven-repos-with-ivy.html
Once the ivy-based project has published its artefacts to the mvn repo, then the mvn-based project should be able to resolve the published artefacts.