Develop Reference

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

Download a Maven artifact and install into a specific local repository (not .m2)

I have several local Maven repositories besides the one located in ~/.m2 directory and I want to simplify the process of installing new artifacts into them from Maven central.
So far I couldn't find a way to tell mvn dependency:get that dependency should be put into a specific local repository.
I did manage to find a way to install a given downloaded artifact using mvn install:install-file -DlocalRepositoryPath=, but I want to be able to get and put dependencies into a specific repository with as few manual steps as possible.
Before you ask why can't I just configure my project to use Maven central directly here is the answer: the project uses Gradle and I do not own its build script (i.e. I can't modify it). The project build script is written to work with several distinct repositories having the same base URI that I fortunately can change using build.properties file. So my idea is to have several local maven repositories in the same root directory and trick the build script to use them.

You can use the maven.repo.local property:
mvn dependency:get -Dmaven.repo.local=/path/to/localrepo

Running a Maven project with many dependencies

I'm hacking on a Maven-based project with a lot of dependencies; the project is normally meant to be developed in Eclipse but I need to work on it from the command line.
How to build+execute the project in a sane way? Something like mvn run, but of course Maven is not meant for running Java projects (for some reason).
The problem is specifying all the dependencies on java's commandline, I don't even know how to autogenerate that. I can currently deal with it using the assembly:single maven plugin (using the jar-with-dependencies descriptor) which will package the dependencies to a single .jar for me.
However, there really is a lot of dependencies and the assembly phase can take about two minutes, greatly disrupting my hack-test cycles so I'm looking for other ways to run the project with minimum build overhead - any recommendations, please?
Note: One possibility is running it in Eclipse once and capturing the java commandline. However, that's just a one-time hack, not a general solution in case I change pom.xml later or come to another project from the suite without Eclipse access anymore.

Have a look at the maven exec plugin
mvn exec:java -Dexec.mainClass="com.example.Main"
if you do this frequently, you can of course configure it via plugin configuration.

Regarding finding out project dependencies - you can use maven dependency plugin
http://maven.apache.org/plugins/maven-dependency-plugin/list-mojo.html
If you want to put them into file it'd be smth like
mvn dependency:list > dependencies.txt

See this question: How can I create an executable JAR with dependencies using Maven?. You can use the dependency-plugin to generate all dependencies in a separate directory before the package phase and then include that in the classpath of the manifest.

I see three solution to this:
onejar-maven-plugin - faster than assemlby with jar-with-dependencies descriptor
With onejar-maven-plugin, you'll (...) get a nice clean super jar with the dependency jars inside.
Spring Boot Maven Plugin - but this is dedicated to Spring projects
Maven Assembly Plugin with custom descriptor. This custom descriptor should grab all dependencies into lib folder, maven-jar-plugin should set Class-Path in Manifest.fm according to this new location. After this you can simply execute your program or zip your jar with lib folder as distribution.
After this is possible to run your program on your computer or any other with one command:
java -jar myjar.jar

How to add external jar files to maven

How do I add external jar files to maven web app project using Eclipse M2 Plugin?

The most correct way is probably to install them to the repo.
However, for an alternate, see the top answer to:
Can I add jars to maven 2 build classpath without installing them?

You want to use mvn install:install-file. See here for the instructions in the Maven guide:
http://maven.apache.org/guides/mini/guide-3rd-party-jars-local.html

Maven and m2e - build jar with dependencies information

We are using Maven and m2e tools for our development and today we encountered a problem.
One of our projects is small library that is required for other projects, so we packaged it into jar file and put in our private Maven repository.
For now, all of the jars that we put in this repository didn't have any external dependencies, but this library I mentioned uses some external jars.
Now, when I add information about this jar to other poms, this jar is downloaded from our private repository but Maven doesn't download dependencies needed by this jar.
I am wondering if I need to use some special target/add something to my pom.xml file that will inform Maven to include information about dependencies needed by this artifact.
EDIT:
Here is the workflow I perform when I upload jar to our private repository:
1.I generate jar file from Eclipse using m2e
2.mvn install:install-file -DgroupId=<your_group_name> -DartifactId=<your_artifact_name> -Dversion=<snapshot> -Dfile=<path_to_your_jar_file> -Dpackaging=jar -DgeneratePom=true -DcreateChecksum=true
3.I copy folder created in my local repository to remote repository

If your small library is a maven project as you state, there should be no reason to have eclipse build the jar and then use maven to install it and then manually copy to the remote repo. Instead you should use m2e to run the deploy goal:
mvn deploy
That will cause the jar to get built and then install it directly into your local maven repo then deploy it to the remote repo.
In eclipse this can be accomplished by right clicking your project, choosing Run As -> Maven Build... then in the run configuration window for Goals input type deploy then click Run. After this has been done once, you can just use Run As -> Maven Build to run the same config again.

I see you use -DgeneratePom=true during the installation of the jar file. What you need to do is create a pom.xml for your artifact. In the pom.xml, you can specify the dependencies that your jar file requires. When executing the install:install plugin goal, you use -DpomFile=pom.xml instead.

The best way to do this is to run mvn deploy
You have to setup the distribution repository to your private artifact manager (nexus or artifactory) in your settings.xml
see this for more details