Log4j2 custom plugins - annotation processing with Maven Assembly Plugin

Question

I am not very familiar with Maven, I started using it just yesterday, but I like it. In my project I use Log4j2 library for logging and because of insuffici

Answer 1

Another solution is mentioned in Log4j 2 issue 673. Use maven shade plugin with specific transformer instead of maven assembly plugin.

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-shade-plugin</artifactId>
  <version>2.4.3</version>
  <configuration>
    <transformers>
      <transformer implementation="com.github.edwgiz.mavenShadePlugin.log4j2CacheTransformer.PluginsCacheFileTransformer" />
    </transformers>
  </configuration>
  <dependencies>
    <dependency>
      <groupId>com.github.edwgiz</groupId>
      <artifactId>maven-shade-plugin.log4j2-cachefile-transformer</artifactId>
      <version>2.6.1</version>
    </dependency>
  </dependencies>
  <executions>
    <execution>
      <id>make-assembly</id>
      <phase>package</phase>
      <goals>
        <goal>shade</goal>
      </goals>
    </execution>
  </executions>
</plugin>

If I understand it accurately, transformer creates Log4j2Plugins.dat file by correctly merging Log4j2Plugins.dat from all dependencies and the main jar, i.e. all plugins will be included.

Answer 2

I think the problem stems from packaging the dependencies into the jar. Doing a quick dive into the code, it looks like the plugin processor overwrites the plugin dat file for each set of plugins it handles. My guess is that during the packaging process, your custom plugins are processed and written to the dat file, and then overwritten when your log4j dependency is processed for inclusion in the package. There may be better solutions, but off the top of my head I would suggest that you do one of the following:

1. Do not package the dependencies into your jar. Just package your project and then include the dependencies on your classpath when you execute. Even if you want to package everything in a single portable jar, doing this would allow you to at least confirm if your plugins are being overwritten, or if there is something else wrong.

2. Create a separate project for your custom plugins, package it separately from your main project, and then include the resulting jar as a dependency. As with option 1, make sure that you do not include the log4j jars in this package. Once you have created your custom plugin jar, you can package it along with the other dependencies in your main jar and it should work fine since your custom plugin jar will have its own plugin dat file.

Good luck!