log4j2 java.lang.NoClassDefFoundError: org/apache/

问题:

I am using log4j 2.3 in my java application. I added the dependency via maven.
When running the program in eclipse everything work fine, but when I package it with maven and try to run the jar I get the following error:

Exception in thread "main" java.lang.NoClassDefFoundError: org/apache logging/log4j/LogManager
    at main.myclass.<clinit>(myclass.java:11)
Caused by: java.lang.ClassNotFoundException: org.apache.logging.log4j.LogManager 


    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    ... 1 more

Why is it not able to find the class while running it from a jar?

Adding log4j 1.2 did not work either. The program is running fine in eclipse so there should be no missing dependency.

回答1:

When you are running your application jar from command line your dependent jar are not available at runtime. You need to include any of these two plugins to pom.xml so have your dependencies available at runtime.

Using: maven-shade-plugin

<plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-shade-plugin</artifactId>
        <version>2.4.1</version>
        <executions>
          <execution>
            <phase>package</phase>
            <goals>
              <goal>shade</goal>
            </goals>
            <configuration>
              <transformers>
                <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                  <mainClass>org.sonatype.haven.HavenCli</mainClass>
                </transformer>
              </transformers>
            </configuration>
          </execution>
        </executions>
      </plugin>

Using:maven-dependency-plugin

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-dependency-plugin</artifactId>
    <version>2.8</version>
    <executions>
        <execution>
             <id>copy-dependencies</id>
             <phase>package</phase>
             <goals>
                 <goal>copy-dependencies</goal>
             </goals>
             <configuration>
                 <outputDirectory>${project.build.directory}/lib</outputDirectory>
             </configuration>
        </execution>
     </executions>
</plugin>

When you will execute the mvn package it will generate uber jar / or copy the dependencies to outputDirectory. I will prefer maven-shade-plugin to generate one jar will all dependencies.