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Publish jar to maven local and use this dependency in root project

Update
I would like to use external library that is not available on maven repository.
Currently, I have subproject called libs and within CLIPSJNI.jar
In Gradle.build of root project I declare dependency like this
dependencies {
implementation files('/libs/library.jar')
}
but in order to run java application I also have to add file with .jnilib.
My question is it possible to publish to maven local this jar and reuse this dependency. So that .jnilib is redundant.
Initial question was resolve. If you wanna use CLIPSJNI in java application there are only 2 steps you should follow, which are described in comments down below.

Gradle Plugin dependency

What is the exact dependency I need to develop a Gradle Plugin in Java? Ideally I would like to get it from a well-known repository such as Maven Central or similar.
I have a Maven project with a core functionality and I just added two extra plugins, one for Ant, one for Maven. They are already tested and working; easy! Now, I wanted to add a third module for a Gradle plugin to make this functionality also available from any Gradle project.
However, I can't find the exact dependencies I need to develop a Gradle plugin.
The Gradle docs (such as https://docs.gradle.org/current/userguide/java_gradle_plugin.html) are not very well written to say the least. They mention:
the gradleAPI() dependency
or the java-gradle-plugin dependency
But they are quite unclear... no group, no version (really?).
If anyone can enlighten me to where I can get these dependencies from, I would be very thankful.

Gradle's public and internal APIs, aka gradleApi(), are bundled with the Gradle distribution and not independently published and therefore not easily consumable by Maven builds. There's the pending epic #1156 (Ensure plugin cross-version compatibility by allowing a user to depend on gradlePublicApi()) that might help here.
Since Gradle plugins are best to be built with Gradle, a pragmatic solution is to invoke the Gradle build from Maven and attach the produced artifact to the Maven build. Andres Almiray (aalmiray) once described this in the blog post Running Gradle Inside Maven (Web Archive Link). He describes the following high level steps:
Create a new Maven module (e.g. gradle-plugin) and add attach it to the parent POM
In the POM of gradle-plugin add a dependency to your core module. Use the maven-dependency-plugin to store dependencies to the Maven build folder, e.g. target/dependencies.
Create the build.gradle, add a Maven repository that points to target/dependencies (step 2) and let it depend on the core module as well as gradleApi(). Implement the Gradle plugin.
Use the exec-maven-plugin to invoke the Gradle build.
Use the maven-resources-plugin to copy the Gradle built plugin jars to the standard Maven build folder.
Use the build-helper-maven-plugin to attach the copied jars to the Maven build.
Sample project to be found here (gradle-in-maven).

https://docs.gradle.org/current/userguide/custom_plugins.html#sec:custom_plugins_standalone_project
In here it is mentioned that it is gradleApi() and I know that this works (from experience). The localGroovy() on that page is only needed if your plugin code uses groovy (does not apply if you only use groovy in the build.gradle of your plugin).
java-gradle-plugin is a library that makes it a bit simpler to make plugins, it is not required though. I personally prefer using gradleApi only.
EDIT:
It appears I've misunderstood the question. Here are the steps to get gradleApi jar:
Create a Gradle project with your desired Gradle version.
Add implementation gradleApi() dependency.
Import/run the project once.
Go to your .gradle folder (located in home folder in Linux-based operating systems).
Open caches folder
Open the version folder you want, e.g. 6.0.1
Open generated-gradle-jars folder.
Copy the jar to wherever you want and use it.
For me the 6.0.1 jar is at ~/.gradle/caches/6.0.1/generated-gradle-jars/gradle-api-6.0.1.jar
Please note that I have not tested this, I know the jar is there but I haven't tried using it.

Using Gradle to include my own Java library in a project

Disclaimer: I'm very new to Gradle and Dependency Management. I tried reading the documentation but just couldn't get through the sheer amount of information. I also couldn't find anything useful to answer my question, so sorry if this has been answered before, I tried searching...
So my situation is as follows: I have one Java project that's supposed to give me a standardized way of using program configurations using JSON files. This project has a dependency on Gson. So far so good, I simply added compile 'com.google.code.gson:gson:2.6.2' to that projects dependencies and all's fine, the library shows up as External Library in Idea, and I can use it and stuff.
Now I want to use that project in other projects to make use of the configuration stuff. And I can not for the life of me figure out how to add the project or the library jar to other projects using Gradle.
I tried things like copying the library jar to the libs folder of the projects to use it in and adding compile files('./libs/myLibrary-0.0.1.jar') to the dependencies list, or adding the jar as a library via the Project Structure thing in Idea. None of these methods worked, and I'm at my wits end.
Any help would be appreciated.

If you or your company have a central binary repository, such as artifactory. Then you should set up publishing your jar there.
But since you haven't mentioned a central repository, I'll assume that you don't have one, and are simply trying to get your dependency to work on a single machine. In that case, what I suggest doing is this:
Add the maven-publish plugin to your dependency project:
apply plugin: 'maven-publish'
Also make sure that you define the group, version and name variables of your project (see here). You'll need them later. Then add a publishing definition that will tell maven-publish to publish all classes:
publishing {
publications {
mavenJava(MavenPublication) {
from components.java
}
}
}
Using these settings you should now be able to run the publishToMavenLocal task. Do it. If successful, the dependency jar should now be in your local maven repository (~/.m2/repository)
Now, add mavenLocal as a repository in the project that needs the dependency:
repositories {
mavenLocal()
}
(you might want to add additional repositories here, such as mavenCentral())
Also add your jar's group, name, and version just like your gson dependency:
compile 'yourgrou:yourname:yourversion.
Gradle should now be able to fetch the dependency from the local maven repo.

You have couple of options. First and easy is to build your base project and available in your local maven repository and use it. To make your project available is your local maven repo, use maven plugin. In your build.gradle file, add the following.
apply plugin: 'maven'
Now use gradle clean build install to publish the jar to your local repo. Remember that install task is the one actually put your jar into your local.Then head over to your other project which depends on this one and tell it to look into your local maven repo by adding mavenLocal to the repositories.
repositories {
mavenCentral()
mavenLocal()
}
Another option is, if you are using centralized repo in your company, you can publish your base jar and use it in the other project. Check out the documentation.

IntelliJ Artifact Auto-Import Maven Dependencies?

When creating an artifact in IntelliJ IDEA and adding dependencies to my pom.xml, the artifact dependencies are not updated. If I then export the JAR file using my artifact configuration assuming everything is fine, I will get a class not found exception since the dependencies were not fully exported.
Is there a way to make IntelliJ automatically use all dependencies for exporting? I understand that this is not always desired behavior when using multiple artifacts (to cut down on the file size) but mostly when only using one artifact it is preferred to not having to delete and recreate the artifact configuration frequently.

This looks like a job for Shade plugin

Just import jar manually to project library
Project>Open Module Setting >Libraries>"+" sign in middle tab ( add new project library - from Maven) > type Your file and add it

Dependencies of a repackaged jar not working

I have the following dependency in my gradle file.
compile 'org.A:A:1.0'
which automatically pulls in
'org.B:B:1.0'
and many other jars which it depends on.
But,my project requires repackaged A.jar (let's call it A*.jar which I installed in a local maven repository as custom version).
So now I change the dependency as below
compile 'org.A:A:custom'
which doesn't pull in any of the dependencies mentioned in the pom.xml file present inside the A.
jar file (which it would, had it been org.A:A:1.0)
My questions are:
1) Based on what does the statement compile org.A:A:1.0 pull other jars ? Is it pom.xml file present inside the jar?
2) What are the changes required if I want to automatically pull in both 'org.B:B:custom' and regular versions of other jars which are dependee of org.A:A:1.0

Maven will read the pom file for the artifact it resolves as well. In there the dependencies are found and resolved.
You simply need to also upload the pom of A*.jar and modify the version of it accordingly to A* - that should already do the trick.