Maven-Shade-Plugin 介绍

在我们像集群中提交任务的时候，常常因为app的jar包与Hadoop CLASSPATH中jar包冲突导致了No such method错误等等。
为了解决这一问，采用Maven-shade-plugin,这个插件的主要作用是将打包过程中内部的jar包重新命名为新的名称，防止由于引用不同版本导致的问题，例如：

<project>
  ...
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-shade-plugin</artifactId>
        <version>3.0.0</version>
        <executions>
          <execution>
            <phase>package</phase>
            <goals>
              <goal>shade</goal>
            </goals>
            <configuration>
              <relocations>
                <relocation>
                  <pattern>org.codehaus.plexus.util</pattern>
                  <shadedPattern>org.shaded.plexus.util</shadedPattern>
                  <excludes>
                    <exclude>org.codehaus.plexus.util.xml.Xpp3Dom</exclude>
                    <exclude>org.codehaus.plexus.util.xml.pull.*</exclude>
                  </excludes>
                </relocation>
              </relocations>
            </configuration>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
  ...
</project>

How to solve “.Git Directory Could Not Be Found! Please Specify a Valid [dotGitDirectory] in Your pom.xml” Problem

When i modify the source code of presto, and execute mvn package, something wrong happened, the stack trace is as follow:

.Git Directory Could Not Be Found! Please Specify a Valid [dotGitDirectory] in Your pom.xml

After google the problem, i find a simple way to solve it. Add this plugin to your pom.xml and everything is ok.

            <plugin>
              <groupId>pl.project13.maven</groupId>
              <artifactId>git-commit-id-plugin</artifactId>
              <version>2.1.15</version>
              <configuration>
                <failOnNoGitDirectory>false</failOnNoGitDirectory>
              </configuration>
            </plugin>

Hadoop trunk maven编译问题

 从Hadoop trunk git库中pull下来代码后，需要进行编译，但是编译过程中报以下错误：

[ERROR] Plugin org.apache.hadoop:hadoop-maven-plugins:3.0.0-SNAPSHOT or one of its dependencies could not be resolved: Could not find artifact org.apache.hadoop:hadoop-maven-plugins:jar:3.0.0-SNAPSHOT -> [Help 1]

       解决方法很简单：

       $ cd hadoop-maven-plugins

       $ mvn install

MAVEN org.eclipse.m2e:lifecycle-mapping问题

      最近在编译Presto时候遇到了一个MAVEN的问题，MAVEN报以下错误：

       [WARNING] The POM for org.eclipse.m2e:lifecycle-mapping:jar:1.0.0 is missing, no dependency information available

       [WARNING] Failed to retrieve plugin descriptor for org.eclipse.m2e:lifecycle-mapping:1.0.0: Plugin org.eclipse.m2e:lifecycle-mapping:1.0.0 or one of its dependencies could not be resolved: Failed to read artifact descriptor for org.eclipse.m2e:lifecycle-mapping:jar:1.0.0

      google以下，发现了以下的解决方法：

      1、checkout https://github.com/mfriedenhagen/dummy-lifecycle-mapping-plugin

      2、解压，mvn clean install

      3、再去编译自己的工程，okay，解决

Maven加入新的source问题

    最近将之前做的hadoop-raid移植到2.4.0上面，因为之前的RPC都是通过POJO，而现在2.x的RPC都是通过PB(参见我之前写过的Blog)，所以需要将新加入的Protocol进行PB的改写。

    其中写好proto文件后，需要用hadoop写好的hadoop-maven-plugin插件将proto文件生成java文件。

 <plugin>
        <groupId>org.apache.hadoop</groupId>
        <artifactId>hadoop-maven-plugins</artifactId>
        <executions>
          <execution>
            <id>compile-protoc-raid</id>
            <phase>generate-sources</phase>
            <goals>
              <goal>protoc</goal>
            </goals>
            <configuration>
              <protocVersion>${protobuf.version}</protocVersion>
              <protocCommand>${protoc.path}</protocCommand>
              <imports>
                <param>${basedir}/../../hadoop-common-project/hadoop-common/src/main/proto</param>
                <param>${basedir}/src/main/proto</param>
              </imports>
              <source>
                <directory>${basedir}/src/main/proto</directory>
                <includes>
                  <include>RaidProtocol.proto</include>
                </includes>
              </source>
              <output>${project.build.directory}/generated-sources/java</output>
            </configuration>
          </execution>
        </executions>
      </plugin>

    同时需要将生成的java文件加入ClassPath中，采用的是org.codehaus.mojo.build-helper-maven-plugin插件。

      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>build-helper-maven-plugin</artifactId>
        <executions>
          <execution>
            <id>add-raid-generated-sources-directory</id>
            <phase>generate-sources</phase>
            <goals>
              <goal>add-source</goal>
            </goals>
            <configuration>
              <sources>
                <source>${project.build.directory}/generated-sources/java</source>
              </sources>
            </configuration>
          </execution>
        </executions>
      </plugin>

    但是使用后能够在terminal中顺利编译，而eclipse却显示没有加入这个classpath。

    最后想到还是用maven命令生成一遍，mvn clean generate-sources eclipse:eclipse

就解决问题了。

Maven out of memory 解决方法

    有时候使用maven的时候，如果采用默认的jvm参数会报oom错误，这个所有java程序员都知道为什么，那修改方法只要加入maven jvm参数就可以了。方法是在环境变量中，加入以下

    export MAVEN_OPTS=”-Xms1024m -Xmx2000m”