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Maven connection to IDE

I am using Maven to use Postrgres SQL driver. Besides I am using InteliJ IDEA Ultimatre Edition, and, as I understood, Maven is included in Ultimate version initially. Correct me - all I need, is to set dependencies, and connect PostrgeSQL to Java. I am not oblige to Download Maven (except required Dependecie of course, I mean Maven as framework)? Thanks a lot!

When you are creating a new project, choose Maven. After the project is created, you will receive an empty Maven project structure with the pom.xml and a script mvnw of Maven Wrapper, which you can use (instead of mvn) to build your app.
Just add dependencies to the pom.xml and build.
The Maven Wrapper will do the work for you - download Maven into the project subdirectory and use it.

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.

How do I include a gradle composite build into a multi project build?

I have a project that is a library (ProjectLib) and is used in many other projects. I have another project which is a multi project build (MultiProject) with a few sub projects (SubProj1, SubProj2, CoreProj). Some of these sub projects depend on the library project.
Normally I have the sub projects that depend on the library have the library specified in the dependency block of each of their build scripts and it fetches a built version of my library.
Sometimes I have to develop something in my library for this multi project and while doing this I would like to include the library as a composite build so that I can make changes and see the effect in the multi project build.
I have tried adding the path to my library in the settings.gradle of the root project using 'includeBuild' but this only half works.
What I tried is this:
MultiProject settings.gradle
include "SubProj1", "SubProj2", "CoreProj"
includeBuild "../ProjectLib"
SubProj1 and SubProj2 build.gradle
dependencies {
implementation project(":CoreProj")
implementation "com.myCompany:ProjectLib:1.0.0"
}
The build file for the CoreProj doesn't depend on the ProjectLib.
My ProjectLib normally builds to a private repo which is fetched by gradle and so typically version 1.0.0 would be included from this repo. What I would like to happen is that instead of fetching this version of the library, gradle instead includes the project in my local directory so that it has my latest changes without me having to build and release the library to the repo.
What I am getting at the moment is that the ProjectLib is being included in my IDE (I am using IntelliJ) but I get the following warning:
org.gradle.api.artifacts.UnknownConfigurationException: Configuration with name 'default' not found.
This warning appears twice for my MultiProject and the once each for SubProj1 and SubProj2. This also breaks up my project structure in my IDE so that it looks like only CoreProj is included in the multi project build MultiProject.
I am using gradle 5.5.1

Manage multi module project

Current setup:
Maven
Spring Boot
Angular
12 maven project + 1 Pom project
Git
Jenkin
Currently i have separate 13 project.
Every time if i have to setup on another machine then i have to clone each project individually.
Also i did the Jenkin pipeline setup for auto build.
Looking for
Once i clone the single pom/main project , all other project should get cloned
If i take pull of the main project then all other projects code should get updated
Child project should get head revision every time i start the jenkin
build.
Want to keep separate repo for each sub projects so that it can be reuse in other project as i have multiple requirements.
Solution that i have thought of:
Maven Child projects
Git Sub modules
Issues facing in above solution
1. Maven Child projects
--> If we are using maven child projects then we are literally copying the sub projects in pom/main project.
In this case i cannot reuse the project. Every time i have to clone the project to use it.
For big project with number of team member working, merging issue will come.
2. Git Sub modules
--> This is best solution for me but when we create sub module it always point to commit not the head revision.
Every time i have to manually pull the latest code push it to parent project.
This approach we can not use in Jenkins.
Is there best approach which can resolve all the issue.

You can add git sub-modules by specifying what modules to be added from other repositories. A .gitmodules file will be created on root that will contain list of all submodules and other preferences of yours.
Creating git submodules for projects in different repositories
There are set of commands available that you can use to sync up the git module and all it's sub-modules.
Set of steps that you could take is:
Add all submodules in project using git submodule add [ssh or https path] e.g. git submodule add https://github.com/test
Add other submodules and push the changes
Fetch submodule content by running
git submodule init
git submodule update
or alternatively run git clone --recurse-submodules https://github.com/chaconinc/MainProject to get sub-modules as well
I would recommend that you read through this link and ensure your requirement is fulfilled by this.

Java to Maven project conversion related details

I am having a java project with a ant build file, using this ant file i create an ejb of the project and deploy it on the jboss server.
Now I am planning to use maven and convert this existing project which consist of nearly 28-30 jar's in its class path(jars related to ejb3, hibernate, jboss, etc).
I can easily do it using eclipse i.e right click project goto maven and click Conver to Maven.
A pom.xml is generated and the MavenClassPath Container is also added to the project.
Now I want to know how to get rid of those 28-30 jar's present in the lib folder of the project and in the classpath. i.e. I want my pom.xml handle all the dependencies.
Does Maven provide any mechanism to achieve this goal while converting the project or I have to add all of these jar dependencies one by one manually in the pom.xml file.
The intention of doing this is I want to have common maven remote repository where the jars will be stored and each developer machine will point to it through their maven project.
Thanks

I think you're after a repository manager like Nexus (I use Nexus, it seems to be the most popular http://nexus.sonatype.org/ ).
Nexus can be used as:
A proxy repository (for Maven Central, etc)
A repository for your own releases.
Nexus provides user management for your developers to release builds into the repo.
Developers will then point their Maven settings.xml file to your Nexus repository, and all their dependencies will come from here (Nexus will cache them).
I'm afraid you will have to configure the dependencies individually, but that is a good thing, because you should pay attention to what version ranges you are interested in for each dependency.
Any jars which can't be found in Maven Central, etc, you can add to your own Nexus repository .
Ofcourse there are alternatives to Nexus, but I haven't used any.
HTH

The most important thing i can recommend is to use a Maven Repository Manager (Nexus, Artifactory or Achiva or other..).
Second your pom conversion via Eclipse shows me that you are not using an up-to-date Eclipse nor an up-to-date Maven Plugin for Eclipse. The best thing would be use Eclipse-Indigo (m2e is the newest and greatest).
Furthermore you have to go through all your jar's and add them step by step to you pom (dependencies) and see if your project can be compiled. This should be checked on command line not inside Eclipse.
After you got a working pom.xml file put it into your version control and check if you can remove some of your added dependencies based on transitive dependencies. After that you can finally delete your lib folder.