Google App Engine - Maven plugin not found - eclipse

Question

I want to run maven in my app engine application. However, I get this exception:
Error resolving version for plugin
'org.apache.maven.plugins:gwt-maven-plugin' from the repositories
[local (C:\Users\user.m2\repository), central
(http://repo.maven.apache.org/maven2)]: Plugin not found in any plugin
repository
Thats my maven file:
<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/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>test</groupId>
<artifactId>test</artifactId>
<version>0.0.1-SNAPSHOT</version>
<build>
<sourceDirectory>src</sourceDirectory>
<resources>
<resource>
<directory>src</directory>
<excludes>
<exclude>**/*.java</exclude>
</excludes>
</resource>
</resources>
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>2.3.2</version>
<configuration>
<source>1.7</source>
<target>1.7</target>
</configuration>
</plugin>
<plugin>
<artifactId>gwt-maven-plugin</artifactId>
<configuration>
<webappDirectory>/test/war/WEB-INF/classes</webappDirectory>
</configuration>
</plugin>
</plugins>
</build>
<repositories>
<repository>
<id>wso2</id>
<url>http://dist.wso2.org/maven2/</url>
</repository>
<repository>
<id>maven-repository-1</id>
<url>http://repo1.maven.org/maven2/</url>
</repository>
</repositories>
<dependencies>
<dependency>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-servlet</artifactId>
<version>2.5.1</version>
<scope>runtime</scope>
</dependency>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-user</artifactId>
<version>2.5.1</version>
<scope>provided</scope>
</dependency>
<groupId>commons-logging</groupId>
<artifactId>commons-logging</artifactId>
<version>1.1</version>
<type>jar</type>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.12</version>
<type>jar</type>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.webharvest.wso2</groupId>
<artifactId>webharvest-core</artifactId>
<version>1.0.0.wso2v1</version>
<type>jar</type>
<scope>compile</scope>
</dependency>
<!-- web harvest pom doesn't track dependencies well -->
<dependency>
<groupId>net.sf.saxon</groupId>
<artifactId>saxon-xom</artifactId>
<version>8.7</version>
</dependency>
<dependency>
<groupId>org.htmlcleaner</groupId>
<artifactId>htmlcleaner</artifactId>
<version>1.55</version>
</dependency>
<dependency>
<groupId>bsh</groupId>
<artifactId>bsh</artifactId>
<version>1.3.0</version>
</dependency>
<dependency>
<groupId>commons-httpclient</groupId>
<artifactId>commons-httpclient</artifactId>
<version>3.1</version>
</dependency>
<dependency>
<!-- jsoup HTML parser library # http://jsoup.org/ -->
<groupId>org.jsoup</groupId>
<artifactId>jsoup</artifactId>
<version>1.7.2</version>
</dependency>
</dependencies>
</project>
What should I add to make it run?
UPDATE
I am using this maven plugin because I get:
The output directory for the project should be set to
/test/war/WEB-INF/classes
UPDATE 2
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>gwt-maven-plugin</artifactId>
<version>2.5.1</version>
<configuration>
<webappDirectory>/test/war/WEB-INF/classes</webappDirectory>
</configuration>
</plugin>
My exception is the same:
The output directory for the project should be set to
/test/war/WEB-INF/classes
My maven file:
<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/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>test</groupId>
<artifactId>test</artifactId>
<version>0.0.1-SNAPSHOT</version>
<build>
<sourceDirectory>src</sourceDirectory>
<resources>
<resource>
<directory>src</directory>
<excludes>
<exclude>**/*.java</exclude>
</excludes>
</resource>
</resources>
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>2.3.2</version>
<configuration>
<source>1.7</source>
<target>1.7</target>
</configuration>
</plugin>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>gwt-maven-plugin</artifactId>
<version>2.5.1</version>
<configuration>
<webappDirectory>/test/war/WEB-INF/classes</webappDirectory>
</configuration>
</plugin>
</plugins>
</build>
<repositories>
<repository>
<id>wso2</id>
<url>http://dist.wso2.org/maven2/</url>
</repository>
<repository>
<id>maven-repository-1</id>
<url>http://repo1.maven.org/maven2/</url>
</repository>
</repositories>
<dependencies>
<dependency>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-servlet</artifactId>
<version>2.5.1</version>
<scope>runtime</scope>
</dependency>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-user</artifactId>
<version>2.5.1</version>
<scope>provided</scope>
</dependency>
<groupId>commons-logging</groupId>
<artifactId>commons-logging</artifactId>
<version>1.1</version>
<type>jar</type>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.12</version>
<type>jar</type>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.webharvest.wso2</groupId>
<artifactId>webharvest-core</artifactId>
<version>1.0.0.wso2v1</version>
<type>jar</type>
<scope>compile</scope>
</dependency>
<!-- web harvest pom doesn't track dependencies well -->
<dependency>
<groupId>net.sf.saxon</groupId>
<artifactId>saxon-xom</artifactId>
<version>8.7</version>
</dependency>
<dependency>
<groupId>org.htmlcleaner</groupId>
<artifactId>htmlcleaner</artifactId>
<version>1.55</version>
</dependency>
<dependency>
<groupId>bsh</groupId>
<artifactId>bsh</artifactId>
<version>1.3.0</version>
</dependency>
<dependency>
<groupId>commons-httpclient</groupId>
<artifactId>commons-httpclient</artifactId>
<version>3.1</version>
</dependency>
<dependency>
<!-- jsoup HTML parser library # http://jsoup.org/ -->
<groupId>org.jsoup</groupId>
<artifactId>jsoup</artifactId>
<version>1.7.2</version>
</dependency>
<dependency>
<groupId>org.codehaus.mojo</groupId>
<artifactId>gwt-maven-plugin</artifactId>
<version>2.5.1</version>

Answer 1

You have not mentioned the group id or artifact id for your plugin and maven is trying to assume the groupid.
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>gwt-maven-plugin</artifactId>
<version>2.5.1</version>
</plugin>
References
You can find gwt-maven-plugin available at central maven repo.
Reference on how to use gwt-maven-plugin.
Browse through Google teams pom sample for GWT+GAE app.
NOTE - I tested your pom.xml in a sample project and found no errors.
<?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/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.insanity.example.gwt</groupId>
<artifactId>learning</artifactId>
<version>1.0-SNAPSHOT</version>
<build>
<sourceDirectory>src</sourceDirectory>
<resources>
<resource>
<directory>src</directory>
<excludes>
<exclude>**/*.java</exclude>
</excludes>
</resource>
</resources>
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>2.3.2</version>
<configuration>
<source>1.7</source>
<target>1.7</target>
</configuration>
</plugin>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>gwt-maven-plugin</artifactId>
<version>2.5.1</version>
<configuration>
<webappDirectory>/test/war/WEB-INF/classes</webappDirectory>
</configuration>
</plugin>
</plugins>
</build>
<repositories>
<repository>
<id>wso2</id>
<url>http://dist.wso2.org/maven2/</url>
</repository>
<repository>
<id>maven-repository-1</id>
<url>http://repo1.maven.org/maven2/</url>
</repository>
</repositories>
<dependencies>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-servlet</artifactId>
<version>2.5.1</version>
<scope>runtime</scope>
</dependency>
<dependency>
<groupId>com.google.gwt</groupId>
<artifactId>gwt-user</artifactId>
<version>2.5.1</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>commons-logging</groupId>
<artifactId>commons-logging</artifactId>
<version>1.1</version>
<type>jar</type>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.12</version>
<type>jar</type>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.webharvest.wso2</groupId>
<artifactId>webharvest-core</artifactId>
<version>1.0.0.wso2v1</version>
<type>jar</type>
<scope>compile</scope>
</dependency>
<!-- web harvest pom doesn't track dependencies well -->
<dependency>
<groupId>net.sf.saxon</groupId>
<artifactId>saxon-xom</artifactId>
<version>8.7</version>
</dependency>
<dependency>
<groupId>org.htmlcleaner</groupId>
<artifactId>htmlcleaner</artifactId>
<version>1.55</version>
</dependency>
<dependency>
<groupId>bsh</groupId>
<artifactId>bsh</artifactId>
<version>1.3.0</version>
</dependency>
<dependency>
<groupId>commons-httpclient</groupId>
<artifactId>commons-httpclient</artifactId>
<version>3.1</version>
</dependency>
<dependency>
<!-- jsoup HTML parser library # http://jsoup.org/ -->
<groupId>org.jsoup</groupId>
<artifactId>jsoup</artifactId>
<version>1.7.2</version>
</dependency>
</dependencies>
</project>