Develop Reference

Why some jar file not build?

<dependency>
<groupId>oracle</groupId>
<artifactId>ojdbc6</artifactId>
<version>11.2.0.3</version>
</dependency>
This dependency not be downloaded. What am I doing?

This has already been covered on another question, but there's new information.
Oracle Maven Repository
Starting in 2016, Oracle started posting its drivers to a secure Oracle Maven repository, and they post the instructions for how to use the repository on their site. The process looks like this:
Register for the site, if needed.
Create a Maven Master password for encryption, if needed.
mvn -emp [YOUR MASTER PASSWORD]
Add the Oracle server to your Maven settings.xml (~/.m2/settings.xml).
<servers>
<server>
<id>maven.oracle.com </id>
<username>YOUR ORACLE USERNAME</username>
<password>YOUR ORACLE PASSWORD</password>
<configuration>
<basicAuthScope>
<host>ANY </host>
<port>ANY </port>
<realm>OAM 11g </realm>
</basicAuthScope>
<httpConfiguration>
<all>
<params>
<property>
<name>http.protocol.allow-circular-redirects </name>
<value>%b,true </value>
</property>
</params>
</all>
</httpConfiguration>
</configuration>
</server>
</servers>
Encrypt the Oracle password using Maven:
mvn -ep [YOUR ORACLE PASSWORD]
Add repositories to your Maven POM or settings.xml:
<repositories>
<repository>
<id>maven.oracle.com</id>
<name>oracle-maven-repo</name>
<url>https://maven.oracle.com</url>
<layout>default</layout>
<releases>
<enabled>true</enabled>
<updatePolicy>always</updatePolicy>
</releases>
</repository>
</repositories>
<pluginRepositories>
<pluginRepository>
<id>maven.oracle.com</id>
<name>oracle-maven-repo</name>
<url>https://maven.oracle.com</url>
<layout>default</layout>
<releases>
<enabled>true</enabled>
<updatePolicy>always</updatePolicy>
</releases>
</pluginRepository>
</pluginRepositories>
After you do this, Maven will be able to resolve the dependencies.
Maven Central
As of September 2019, Oracle has started posting its jars to Maven Central. If you can use those versions, then you can just update your dependency:
<dependency>
<groupId>com.oracle.ojdbc</groupId>
<artifactId>ojdbc8</artifactId>
<version>19.3.0.0</version>
</dependency>

Oracle drivers can't be installed using maven, as stated by the author of this blog post:
Due to Oracle license restrictions, the Oracle JDBC driver is not available in the public Maven repository. To use the Oracle JDBC driver with Maven, you have to download and install it into your Maven local repository manually.
You have to download the correct drivers manually and then you can take one of two possible paths, namely:
Install the driver in your local maven repository
Define your dependency indicating the path to the jar file
Here a some examples of both cases:
Local maven repository
Download and extract the driver jar in your file system,
Then execute the following command
mvn install:install-file -Dfile=path/to/your/ojdbc6.jar -DgroupId=com.oracle
-DartifactId=ojdbc6 -Dversion=11.2.0.3 -Dpackaging=jar
Finally adjust your dependency, please notice that I have used a different groupId
<dependency>
<groupId>com.oracle</groupId>
<artifactId>ojdbc6</artifactId>
<version>11.2.0.3</version>
</dependency>
Indicating the location of the jar file
This is quite simple and straight to the point, just put the jar file in some folder and indicate the path in your dependency, you can even use any project path but you will have to be careful with possible legal issues about the driver license or distribution rules; so I recommend you using the first option.
<dependency>
<groupId>com.oracle</groupId>
<artifactId>ojdbc6</artifactId>
<version>6</version>
<scope>system</scope>
<systemPath>${project.basedir}/lib/ojdbc6.jar</systemPath>
</dependency>
As this process needs to be repeated in every development machine, is worth to mention that you should leave some form of documentation indicating the need of this driver and the necessary steps to install it.
Regards

Yes, Oracle JDBC drivers will be available on central maven. 19.3 version is currently available and other versions will follow soon. Refer to this blog for more details. Also, check out the Maven repo to browse the available jars.

Packaging war with new gwt maven plugin (having migrated from default maven plugin)

I've stated using Thomas Broyer's gwt maven plugin as it allows me to run gwt 2.8-rc2. I've got it running with the codeserver fine and with minimum effort.
However now I'm trying to figure out how to use it to do a full compile and package.
Simply running maven install (I expected this to work as it does work with the default) does not actually run the gwt compile.
Then it talks about various packaging formats etc and I'm not sure why these are necessary?
I assume someone has got this plugin packaging the war and has also migrated from the original plugin...
This is my plugin config - I am using skipModule as I've already got a module configured the way the other plugin expects.
<plugin>
<groupId>net.ltgt.gwt.maven</groupId>
<artifactId>gwt-maven-plugin</artifactId>
<version>1.0-rc-6</version>
<extensions>true</extensions>
<configuration>
<moduleName>com.afrozaar.ashes.web.AshesWeb-safari</moduleName>
<skipModule>true</skipModule>
<style>DETAILED</style>
<!-- <logLevel>DEBUG</logLevel> -->
<classpathScope>compile+runtime</classpathScope>
</configuration>
<dependencies>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-user</artifactId>
<version>2.8.0-rc2</version>
</dependency>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-dev</artifactId>
<version>2.8.0-rc2</version>
</dependency>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-servlet</artifactId>
<version>2.8.0-rc2</version>
</dependency>
</dependencies>
</plugin>

You're missing "executions" in your plugin configuration to run the compile goal (works the same as with the CodeHaus plugin).
My plugin works better when you separate client and server code into distinct Maven modules, which is why this setup is not clearly documented (because I actively discourage it). You can have a look at the samples in the GWT git repository to find examples similar to your case though.
BTW, I believe you can use rc2 with the CodeHaus plugin rc1; that's probably why you added those dependencies, which are useless with my plugin.
See also https://tbroyer.github.io/gwt-maven-plugin/migrating.html

Maven2 plugin dependency

I have a troubles with plugin dependecies.
I want to use "proguard-maven-plugin", but by default this plugin use proguard 4.3. Proguard 4.3 don't support jdk 7.
To fix my problem i'm just need to use proguard 4.6+. Buuut, last version in central repo is 4.4. I can manualy download proguard 4.6+ from proguard repo, but how i can include it to plugin?
I have my own nexus repo, and i put proguard 4.8 there. How can i load dependecies for "proguard-maven-plugin" from my repo?
i did as written there: http://www.sonatype.com/people/2008/04/how-to-override-a-plugins-dependency-in-maven/, but maven looking proguard 4.8 in central repo. How can i force maven search in my own repo?
Sorry my terrible English, i hope you understand me.

You have to edit the file .m2/settings.xml in your home folder (and the home folder of every user who runs the Maven job). There you have to add your Nexus as a repository as described here. Basically the config looks like this:
<settings>
...
<mirrors>
<mirror>
<id>mynexus</id>
<name>My Nexus</name>
<url>http://mynexusurl</url>
<mirrorOf>*</mirrorOf>
</mirror>
</mirrors>
...
</settings>
Additionally you have to configure your Nexus to mirror the central repo.

Maybe this will also help, I don't use it, but this is the form of the config for changing a dependency
<plugin>
<artifactId>proguard-maven-plugin</artifactId>
<version>2.0.4</version>
<dependencies>
<dependency>
<groupId>net.sf.proguard</groupId>
<artifactId>proguard</artifactId>
<version>4.6</version>
</dependency>
</dependencies>
</plugin>
Also an ant-run example
<plugin>
<artifactId>maven-antrun-plugin</artifactId>
<version>1.7</version>
<dependencies>
<dependency>
<groupId>ant-contrib</groupId>
<artifactId>ant-contrib</artifactId>
<version>1.0b2</version>
<exclusions>
<exclusion>
<groupId>ant</groupId>
<artifactId>ant</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>org.apache.ant</groupId>
<artifactId>ant-nodeps</artifactId>
<version>1.8.1</version>
</dependency>
</dependencies>
</plugin>
Note also! for maven2 - issues resolved maven3 http://jira.codehaus.org/browse/MNG-1323
For multi module reactor builds dependencies for plugin resolved in first use of the plugin. If your dependency isn't being downloaded in reactor build but works fine in single module then you may need to include it in an earlier project - easiest done by adding to pluginManagement of shared parent

Maven: best way of linking custom external JAR to my project?

It's my first couple of days learning Maven and I'm still struggling with the basics. I have an external .jar file (not available in the public repos) that I need to reference in my project and I'm trying to figure out what my best option is.
It's a small scale project without a central repository for libraries, so it has to be either a local repository (somehow added to source control, don't know if it's supposed to work that way?) or the .jar needs to be stored on disk outside of any formal repository.
1) What's my best option for adding the .jar file to my project's references with maven given that I want both the project and the library to be in source control?
2) I still can't seem to have Eclipse see the dependency. I manually added it to the section of the pom, and it shows up fine in the Dependencies list in m2eclipse. mvn compile and mvn package both succeed, but running the program results in:
Exception in thread "main" java.lang.Error: Unresolved compilation problems:
LibraryStuff cannot be resolved to a type
This is after editing the POM as:
<dependency>
<groupId>stuff</groupId>
<artifactId>library</artifactId>
<version>1.0</version>
<systemPath>${lib.location}/MyLibrary.jar</systemPath>
<scope>system</scope>
</dependency>
Should I be executing mvn install:install-file even thought I already have the pom.xml edited as above?
Thanks!

You can create an In Project Repository, so you don't have to run mvn install:install-file every time you work on a new computer
<repository>
<id>in-project</id>
<name>In Project Repo</name>
<url>file://${project.basedir}/libs</url>
</repository>
<dependency>
<groupId>dropbox</groupId>
<artifactId>dropbox-sdk</artifactId>
<version>1.3.1</version>
</dependency>
/groupId/artifactId/version/artifactId-verion.jar
detail read this blog post
https://web.archive.org/web/20121026021311/charlie.cu.cc/2012/06/how-add-external-libraries-maven

I think you should use mvn install:install-file to populate your local repository with the library jars then you should change the scope from system to compile.
If you are starting with maven I suggest to use maven directly not IDE plugins as it adds an extra layer of complexity.
As for the error, do you put the required jars on your classpath? If you are using types from the library, you need to have access to it in the runtime as well. This has nothing to do with maven itself.
I don't understand why you want to put the library to source control - it is for sources code not binary jars.

This can be easily achieved by using the <scope> element nested inside <dependency> element.
For example:
<dependencies>
<dependency>
<groupId>ldapjdk</groupId>
<artifactId>ldapjdk</artifactId>
<scope>system</scope>
<version>1.0</version>
<systemPath>${basedir}\src\lib\ldapjdk.jar</systemPath>
</dependency>
</dependencies>
Reference: http://www.tutorialspoint.com/maven/maven_external_dependencies.htm

The Maven manual says to do this:
mvn install:install-file -Dfile=non-maven-proj.jar -DgroupId=some.group -DartifactId=non-maven-proj -Dversion=1 -Dpackaging=jar

update We have since just installed our own Nexus server, much easier and cleaner.
At our company we had some jars that we some jars that were common but were not hosted in any maven repositories, nor did we want to have them in local storage.
We created a very simple mvn (public) repo on Github (but you can host it on any server or locally):
note that this is only ideal for managing a few rarely chaning jar files
Create repo on GitHub:
https://github.com/<user_name>/mvn-repo/
Add Repository in pom.xml
(Make note that the full path raw file will be a bit different than the repo name)
<repository>
<id>project-common</id>
<name>Project Common</name>
<url>https://github.com/<user_name>/mvn-repo/raw/master/</url>
</repository>
Add dependency to host (Github or private server)
a. All you need to know is that files are stored in the pattern mentioned by #glitch
/groupId/artifactId/version/artifactId-version.jar
b. On your host create the folders to match this pattern.
i.e if you have a jar file named service-sdk-0.0.1.jar, create the folder service-sdk/service-sdk/0.0.1/ and place the jar file service-sdk-0.0.1.jar into it.
c. Test it by trying to download the jar from a browser (in our case: https://github.com/<user_name>/mvn-repo/raw/master/service-sdk/service-sdk/0.0.1/service-sdk-0.0.1.jar
Add dependency to your pom.xml file:
<dependency>
<groupId>service-sdk</groupId>
<artifactId>service-sdk</artifactId>
<version>0.0.1</version>
</dependency>
Enjoy

Don't use systemPath. Contrary to what people have said here, you can put an external jar in a folder under your checked-out project directory and haven Maven find it like other dependencies. Here are two crucial steps:
Use "mvn install:install-file" with -DlocalRepositoryPath.
Configure a repository to point to that path in your POM.
It is fairly straightforward and you can find a step-by-step example here:
http://randomizedsort.blogspot.com/2011/10/configuring-maven-to-use-local-library.html

If you meet the same problem and you are using spring-boot v1.4+, you can do it in this way.
There is an includeSystemScope that you can use to add system-scope dependencies to the jar.
e.g.
I'm using oracle driver into my project.
<dependency>
<groupId>com.oracle</groupId>
<artifactId>ojdbc14</artifactId>
<version>10.2.0.3.0</version>
<scope>system</scope>
<systemPath>${project.basedir}/src/main/resources/extra-jars/ojdbc14-10.2.0.3.0.jar</systemPath>
</dependency>
then make includeSystemScope=true to include the jar into path /BOOT-INF/lib/**
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
<configuration>
<includeSystemScope>true</includeSystemScope>
</configuration>
</plugin>
and exclude from resource to avoid duplicated include, the jar is fat enought~
<build>
<testSourceDirectory>src/test/java</testSourceDirectory>
<resources>
<resource>
<directory>src/main/resources</directory>
<excludes>
<exclude>**/*.jar</exclude>
</excludes>
</resource>
</resources>
</build>
Good luck!

Maven way to add non maven jars to maven project
Maven Project and non maven jars
Add the maven install plugins in your build section
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-install-plugin</artifactId>
<version>${version.maven-install-plugin}</version>
<executions>
<execution>
<id>install-external-non-maven1-jar</id>
<phase>clean</phase>
<configuration>
<repositoryLayout>default</repositoryLayout>
<groupId>jar1.group</groupId>
<artifactId>non-maven1</artifactId>
<version>${version.non-maven1}</version>
<file>${project.basedir}/libs/non-maven1.jar</file>
<packaging>jar</packaging>
<generatePom>true</generatePom>
</configuration>
<goals>
<goal>install-file</goal>
</goals>
</execution>
<execution>
<id>install-external-non-maven2-jar</id>
<phase>clean</phase>
<configuration>
<repositoryLayout>default</repositoryLayout>
<groupId>jar2.group</groupId>
<artifactId>non-maven2</artifactId>
<version>${version.non-maven2}</version>
<file>${project.basedir}/libs/non-maven2.jar</file>
<packaging>jar</packaging>
<generatePom>true</generatePom>
</configuration>
<goals>
<goal>install-file</goal>
</goals>
</execution>
<execution>
<id>install-external-non-maven3-jar</id>
<phase>clean</phase>
<configuration>
<repositoryLayout>default</repositoryLayout>
<groupId>jar3.group</groupId>
<artifactId>non-maven3</artifactId>
<version>${version.non-maven3}</version>
<file>${project.basedir}/libs/non-maven3.jar</file>
<packaging>jar</packaging>
<generatePom>true</generatePom>
</configuration>
<goals>
<goal>install-file</goal>
</goals>
</execution>
</executions>
</plugin>
Add the dependency
<dependencies>
<dependency>
<groupId>jar1.group</groupId>
<artifactId>non-maven1</artifactId>
<version>${version.non-maven1}</version>
</dependency>
<dependency>
<groupId>jar2.group</groupId>
<artifactId>non-maven2</artifactId>
<version>${version.non-maven2}</version>
</dependency>
<dependency>
<groupId>jar3.group</groupId>
<artifactId>non-maven3</artifactId>
<version>${version.non-maven3}</version>
</dependency>
</dependencies>
References Note I am the owner of the blog

Change your systemPath.
<dependency>
<groupId>stuff</groupId>
<artifactId>library</artifactId>
<version>1.0</version>
<systemPath>${project.basedir}/MyLibrary.jar</systemPath>
<scope>system</scope>
</dependency>

The pom.xml is going to look at your local repository to try and find the dependency that matches your artifact.
Also you shouldn't really be using the system scope or systemPath attributes, these are normally reserved for things that are in the JDK and not the JRE
See this question for how to install maven artifacts.

Note that all of the example that use
<repository>...</respository>
require outer
<repositories>...</repositories>
enclosing tags. It's not clear from some of the examples.

The best solution here is to install a repository: Nexus or Artifactory. If gives you a place to put things like this, and further it speeds things up by caching your stuff from the outside.
If the thing you are dealing with is open source, you might also consider putting in into central.
See the guide.

With Eclipse Oxygen you can do the below things:
Place your libraries in WEB-INF/lib
Project -> Configure Build Path -> Add Library -> Web App Library
Maven will take them when installing the project.

If the external jar is created by a Maven project only then you can copy the entire project on your system and run a
mvn install
in the project directory. This will add the jar into .m2 directory which is local maven repository.
Now you can add the
<dependency>
<groupId>copy-from-the=maven-pom-of-existing-project</groupId>
<artifactId>copy-from-the=maven-pom-of-existing-project</artifactId>
<version>copy-from-the=maven-pom-of-existing-project</version>
</dependency>
This will ensure that you
mvn exec:java
works. If you use suggested here
<scope>system</scope>
Then you will have to add classes individually while using executing through command line.
You can add the external jars by the following command described here
mvn install:install-file -Dfile=<path-to-file> -DgroupId=<group-id> \
-DartifactId=<artifact-id> -Dversion=<version> -Dpackaging=<packaging>

The most efficient and cleanest way I have found to deal with this problem is by using Github Packages
Create a simple empty public/private repository on GitHub as per your requirement whether you want your external jar to be publicly hosted or not.
Run below maven command to deploy you external jar in above created github repository
mvn deploy:deploy-file \
-DgroupId= your-group-id \
-DartifactId= your-artifact-id \
-Dversion= 1.0.0 -Dpackaging= jar -Dfile= path-to-file \
-DrepositoryId= id-to-map-on-server-section-of-settings.xml \
-Durl=https://maven.pkg.github.com/github-username/github-reponame-created-in-above-step
Above command will deploy you external jar in GitHub repository mentioned in -Durl=.
You can refer this link on How to deploy dependencies as GitHub Packages GitHub
Package Deployment Tutorial
After that you can add the dependency using groupId,artifactId and version mentioned in above step in maven pom.xml and run mvn install
Maven will fetch the dependency of external jar from GitHub Packages registry and provide in your maven project.
For this to work you will also need to configure you maven's settings.xml to fetch from GitHub Package registry.

Maven GWT 2.0 and Eclipse

Does anyone know of a good guide for creating a project with the new 2.0 release of GWT using maven and eclipse? I am running into a lot of problems getting them to play nicely together.
For what it's worth, I can create a gwt project using the maven eclipse plugin which works fine, but porting it to maven doesn't work (so a guide for this would be great).
Likewise, I can use the maven plugin (gwt-maven-plugin), but when I import it to eclipse (import -> materialize maven projects), it does not get recognised as a GWT project...
Thanks

EDIT: I've updated my answer with additional steps provided by the OP. Credits to the OP for the details.
I just broke my Eclipse setup trying to install the latest version of the Google Plugin for Eclipse (for GWT 2.0) so I can't confirm everything but, let's assume the following prerequisites are fulfilled:
Eclipse 3.5
Google Plugin for Eclipse (installed from http://dl.google.com/eclipse/plugin/3.5, see instructions)
m2eclipse Plugin for Eclipse (installed from http://m2eclipse.sonatype.org/update)
Did you try to:
Create a new project from Eclipse (New > Other... then select Maven Project and choose the gwt-maven-plugin archetype).
Edit the generated pom.xml, update the gwt.version property to 2.0.0 (which has been released in the central repo), add the Codehaus Snapshot repository and set the gwt-maven-plugin version to 1.2-SNAPSHOT (the version 1.2 isn't released in central, this should happen soon) 1.2 (which has been released in central too).
Add a <runTarget> to the gwt-maven-plugin configuration as documented in Using the Google Eclipse Plugin.
Configure the maven-war-plugin as documented in the page mentioned in the previous step.
Manually enable GWT on the project from project preference by setting the Use Google Web Toolkit checkbox This step is unnecessary since you'll be building/running with a Maven run configuration, not the GWT Plugin for Eclipse.
This is how my pom.xml actually looks like:
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
<!--
GWT-Maven archetype generated POM
-->
<modelVersion>4.0.0</modelVersion>
<groupId>com.mycompany.demo</groupId>
<artifactId>my-gwtapp</artifactId>
<packaging>war</packaging>
<version>0.0.1-SNAPSHOT</version>
<name>gwt-maven-archetype-project</name>
<properties>
<!-- convenience to define GWT version in one place -->
<gwt.version>2.0.0</gwt.version>
<!-- tell the compiler we can use 1.5 -->
<maven.compiler.source>1.5</maven.compiler.source>
<maven.compiler.target>1.5</maven.compiler.target>
</properties>
<dependencies>
<!-- GWT dependencies (from central repo) -->
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-servlet</artifactId>
<version>${gwt.version}</version>
<scope>runtime</scope>
</dependency>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-user</artifactId>
<version>${gwt.version}</version>
<scope>provided</scope>
</dependency>
<!-- test -->
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.4</version>
<scope>test</scope>
</dependency>
</dependencies>
<build>
<outputDirectory>war/WEB-INF/classes</outputDirectory>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>gwt-maven-plugin</artifactId>
<version>1.2</version>
<executions>
<execution>
<goals>
<goal>compile</goal>
<goal>generateAsync</goal>
<goal>test</goal>
</goals>
</execution>
</executions>
<configuration>
<runTarget>com.mycompany.demo.gwt.Application/Application.html</runTarget>
</configuration>
</plugin>
<!--
If you want to use the target/web.xml file mergewebxml produces,
tell the war plugin to use it.
Also, exclude what you want from the final artifact here.
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-war-plugin</artifactId>
<configuration>
<webXml>target/web.xml</webXml>
<warSourceExcludes>.gwt-tmp/**</warSourceExcludes>
</configuration>
</plugin>
-->
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-war-plugin</artifactId>
<version>2.0.2</version>
<configuration>
<warSourceDirectory>war</warSourceDirectory>
<webXml>src/main/webapp/WEB-INF/web.xml</webXml>
</configuration>
</plugin>
</plugins>
</build>
</project>
Run the gwt:eclipse goal (using m2eclipse Maven2 > build...) to setup your environment and create the launch configuration for your GWT modules.
Run gwt:compile gwt:run to compile and run a GWT module in the GWT Hosted mode.

You can run the following command to generate a Maven GWT project:
webAppCreator -maven -noant -out
For more information:
GWT webappcreator creating a Maven project: the source attachment does not contain the source for the file URLClassPath.class

Just in case. If you use Google GIN in your project you should add compile goal before gwt:compile. So the whole sequence would be:
compile gwt:compile gwt:run
You can read explanation here:
http://code.google.com/p/google-gin/wiki/GinTutorial#Compilation

An annoying problem with GWT and m2eclipse:
GWT Development Mode requires all JARs
to be placed in WEB-INF/lib. It's
especially painful when programmers
use m2eclipse, which provides its own
Eclipse classpath container.
http://code.google.com/p/google-web-toolkit/issues/detail?id=5693
good news, the workaround is working for me

very useful thread
#Pascal: thank you (I donot have enough reputations to comment on others posts; so here I am posting on what is working for me).
I needed 2.5.1 GWT (not 2.6, the latest) working with maven and eclipse (because sencha GXT is not supported for 2.6 yet). Tried following without luck
1)tried few archetypes with in eclipse to generate the project
2)modify pom file (based on above discussion) to change versions to 2.5.1
Following worked for me http://mojo.codehaus.org/gwt-maven-plugin/user-guide/archetype.html
mvn archetype:generate -DarchetypeGroupId=org.codehaus.mojo -DarchetypeArtifactId=gwt-maven-plugin -DarchetypeVersion=2.5.1
mvn gwt:eclipse
mvn gwt:run