Develop Reference

Maven how to automate installation of dependencies?

I have 3 projects, A->B->C in that dependency order. Currently everytime I make a change to B or C I have to go to the directory and do a mvn clean install in order to install it into the local repository. It is troublesome if I have to do this every time the projects updates.
How can I do it such that every time I do a mvn clean package on A, it will automatically build and install my dependent projects B and C into the local repository?

Create a parent project for all your projects A,B,C and then add all your child project on the parent pom.xml file something like this
<modules>
<module>A</module>
<module>B</module>
<module>C</module>
</modules>
Its called maven multi module project mentioned by #khmarbaise
Here are some example for this
How do I create a multi-module project in Eclipse?
Maven Multi module tutorial
Guide to Working with Multiple Modules
By use of multi module project you will get plenty of benefits like
Anytime you can add any new project with all of the current project
Separation of project is good for code cleanup
You can build Single project or You can build all project in one go.
Duplicacy of jar can be easily ignore .
Maven take care of the build order for you.
One single Jenkins job to build everything.
Plenty of other benefits.But remember if there will some pros then cons also there,its totally now what you want to use .

You can follow the solution I provided to the question Maven 2 Projects, since it is the pattern I usually use when building project with a certain complexity.
Summarizing you would have to create a main Maven project which has three submodules, say master, platform and parent.
The main project has simply the order in which the other projects will be evaluated by Maven
The master pom contains the list of project to be built and their order (aka Reactor order)
The platform pom contains all information about your platform, like JDK version, maven plugin versions, encoding and so on.
The parent pom has the platform pom as a parent and contains all global GAV information about the dependencies you are going to use in your project (Spring, CXF, junit, log4j etc.)

Is Maven a good solution for my Application

I'm developing on an java application that consists on a main-application that is loading extensions (jars) at runtime. Each extension is a java project.
all jars have the main application in class path and other libraries. The manifest of the main application also has dependencies in the manifest classpath, e.g. sqldriver, poi,log4j etc.
Everytime I change on of the lib, i have to set all classpaths of the projects. Also if i want to build all the jars, i have to export each project once.
So I thought maybe is Maven a good solution for me. So I've installed m2eclipse and converted the projects to maven projects.
But for now i have for each projekt an own pom.xml and i also have to build all projects once.
I'm new to Maven and searching for best practises for my problem. So I'll thank you for your help and advice

Make all your projects modules of one parent pom project. This way you can build them all at the same time.

You can use a hierarchy for your pom files. Here's an older question (similar to yours) that has a great example in the answers:
Maven: POM modules and submodules hierarchy
Basically this structure:
<project>
<parent>...
</parent>
<artifactId>module</artifactId>
<packaging>pom</packaging>
<name>Module</name>
<modules>
<module>submodule</module>
</modules>
(...)
</project>
in a 'root' pom.xml file, besides the actual submodule pom.xml files.

Unless you really, really need OSGi , Maven is great. If you're doing OSGi maven is less great.
M2eclipse however is less helpful, and in my experience only leads to confusing headaches.
How you should build your projects depends on a few things. I agree that the submodule approach described in the other answers is best-practice, and if all your sub-module candidates are related (for instance, each represents a tier in a n-tiered application), in the same SCM repository, and the interfaces change often, and the versions are co-dependant then by all means, you should do that.
If however your submodules are stand-alone and don't have a lot of transitive dependencies, particularly if they are in separate SCM repositories, they are independently versioned, and you have a little spare hardware for a build server (like say Hudson) and a Maven2 artifactory (like Sonatype Nexus), you could just keep them as seperate projects, and let maven handle the rest. This way you avoid having to re-build everything because you made a small change in one of the submodules. You can just build that one.

Grouping of Dependencies in Maven 3

I am using maven 3 for building my huge multi module project. I have nearly 80 projects and many dependencies are used by many project thus makes me to add the same dependency entry in all my projects. In case if i am updating/moving any of my dependency to its latter or earlier version I need to update in all other projects which uses that dependency which is quite impossible.More over it is not possible to create a parent pom for set of projects which has same dependency.
Is there any plugin just to map as below in a pom of parent project
<dependency>somegroup:somefact:anyV<dependency>
<projects>
<project>somegroup:somefact:anyV</project>
</projects>
or is it possible to implement a plugin on my own </br>
or whether maven provide any other way to do this in its style.

Maven has a concept of importing dependencies. The documentation says that this feature is for cases where extending a base POM is not possible. I have not used this feature so I'm not sure how it works, might be what you are looking for though.

Applying Maven to a project

I've been asked to apply Maven to a project. After browsing a dozen sites it appears that it's quite vast and I'm not familiar as I'd like with similar tools like Ant. Why is it used/preferred and what does it offer over a standard Eclipse project? Also, how could it be added to an existing project?

Why is it used/preferred and what does
it offer over a standard Eclipse
project?
It is a build tool which can build your project without the need for an IDE like Eclipse. It can create a jar or war or other artifacts from the source, performing a bunch of steps like compilation, running unit tests, etc.
Where maven scores over ant is in managing third-party dependencies and in convention over configuration (which mean less lines of build script if you follow convention).
Also, how could it be added to an
existing project?
You start by creating a new maven project, following the step here.
Place it in the root folder of your project
If your source and resource files do not follow maven folder convention, update maven properties suitably referring to this documentation.
Run mvn package
It will fail if it needs any third party dependencies, which you can add as specified in the doc
With some trial and error, you should have your project running with maven, possibly, much quicker than if you were to set up the same with ant.
Others are already provided sufficient resources to read more about maven.

I suggest to start reading here:
http://www.sonatype.com/books/mvnref-book/reference/public-book.html

Maven is a great tool when you know how to use it. Maven (at core) is a dependency manager.
You include in your pom.xml (similar in function to the build.xml from Ant) all the librairies your project depends on (example : apache commons) along with their version and Maven get them directly from a repository (by default, the central maven repository)
Then you do not have to manually install any jar to make your project work. All is downloaded and cached on your local machine. You can even create an enterprise repository where you put all the jars needed by your company
Maven uses the concept of artifacts which are pre-built library projects with their own dependencies
To mavenize a project, you'll have to write a pom.xml describing your project (examples are numerous), get rid of your libs directory (or whatever classpath you described under Eclipse) and add all your dependencies to your pom.xml
You could also check Mavenizer for a first-start
But Maven is a lot more what i've just said. Read the docs, read poms from librairies and you'll get used to it quickly ;-)

If you use the M2Eclipse plugin from Sonatype, it's just a matter of right clicking the project in the package explorer and choosing Enable Dependency Management in the Maven menu. You are also advised to adjust the directories that contain the sources to the Maven standard directory layout but if you absolutely can't, you can configure that later.
Apart from that: Well, look for tutorials and documentation (for example there is the free book Better builds with Maven. Maven is very complex (yes, I don't think it is simple) and very powerful.

Migrating from ant to maven in Netbeans

Our software is written in Java and comprise many (7) projects.
These projects are Netbeans ant projects.
I'm considering to converting them to maven2.
Where can I find some hints for doing such thing?

Don't read that book. It will only make you confused. Read this book instead: "Maven - The definitive guide" http://www.sonatype.com/books/maven-book/reference/ .
Also, the maven site has a lot of information, but the structure is terrible so you'll need to use google to navigate in it.
Here is my suggestion:
Do this by hand, not with "automagic" "help" from the IDE. Maven integration doesn't work that good yet, not in any IDE.
Make sure you program project is divided into modules under a common umbrella module, so that each module produces a single binary artifact (jar, war,...) possibly accompanied by the javadoc of the source code behind that artifact, a zip with the source code etc. The basic principle is that each module produces a single artifact, containing all the non-test-code under that module. You can do this while the project is still built by ant.
Each module should conform to the standard maven directory layout. The build destination is under [module]/target/[output-type, e.g. "classes"]. The source code is under [module]/src/main/[src-type e.g. "java"] and [module]/test/[src-type]. The artifact consists of all the code under src/main, and none of the code under src/test, as it built to the target directories. You can do this while the is still built by ant.
Start by transforming the sub-module that has no dependencies on other modules in the project.
Now you can create the parent maven module pom.xml with artifact type "pom", consisting of one of the modules below. Make a child module for the first submodule (the one with only external dependencies), using the umbrella module as "parent". Remember that you need to specify version for the parent. Remember to add the child module as a "module" in the parent too. Always use ${project.version} as version in the child modules when you create multi-module projects like this. All modules under a parent must be released simultaneously in a single operation, and if you use this setting maven will make sure the version fields stay the same across all modules and gets updated everywhere during the release. This may make it difficult to re-use the existing numbering scheme, but that doesn't matter. You are never going to run out of version numbers anyway.
Add the necessary dependencies, and make sure you can build the parent and the child module together using the command "mvn clean install" from the parent module.
Proceed with the rest of the modules the same way. Dependencies to other modules under the same parent project should also use ${project.version} as the "version" they are depending on, meaning "the same version as this". NOTE THAT in order to build, the module you are depending on must be built using "mvn install", so that it gets deployed to you local (computer) repository. Otherwise the depending module will not be able to find the classes. There are NO source-code dependencies between modules in maven, only dependencies to built and packed versions installed in local and remote repositories. This can be very confusing if you come from ant-projects. Build from the root module until you get comfortable with this. It takes two days.
Don't use maven integration in IDEs. It is a bad idea. Use "mvn idea:idea" or "mvn eclipse:eclipse" to set up your workspace as a non-maven ordinary IDE project. The inter-module dependencies mechanisms in maven and the IDE aren't identical and will never be. Also, if you have several mavenized projects with dependencies in between, you want to have several of these in your workspace with dependencies set up between. You can do this with mvn idea:idea / eclipse:eclipse if you create a separate maven project file called "workspace.xml" (or whatever) in the same directory as parent module, set up as a multi-module project containing modules "." and "../otherproject" (only one-way reference here, no parent ref back). If you run "mvn idea:idea / eclipse:eclipse -f workspace.xml" you get a workspace with all these modules linked together. No IDE integration lets you do that. This sound like a lot of extra work, but the workspace.xml-file is really small. It doesn't have to contain all that dependency stuff and all that, only the reference to the modules you want to bind together in your IDE.

I did a succeful migration of NetBeans Ant project to Maven project using the instruccions by Joseph Mocker here: http://forums.netbeans.org/ptopic55953.html
I cite the important part:
close the project
rename the build.xml, nbproject files/folders to something so NB won't recognize them.
close and restart NB (so any memory cache knowledge of the project is gone)
copy in an empty pom from some other project.
open the project back up in NB (NB should now identify it as a maven project)
rearrange the files to follow the maven way (™)

This won't be an easy task since Maven2 expects the files to be organized in a specific way. Anyway Better Builds with Maven is a free book that should get you started. It will help you understand Maven and it also has a chapter on migration.

I discovered that the migration is not necessary. The real requirements that I need was automatic download of dependencies (libraries).
This is also achieved by Ivy which nonetheless uses maven repositories.
I solved converting project from ant to ant+ivy with IvyBeans.

I have built a script to migrate Ant builds to Maven. You can find more information here:
https://github.com/ewhauser/ant2maven
It won't help you with fixing your directory structure and or any additional Ant tasks, but it removes a lot of the tedious steps to get started.