How to avoid Jar conflicts (Jar Hell) with container libraries

What is Jar hell?

Jar hell or classpath hell or dependency hell is a term used to describe all the various ways in which the classloading process can end up not working. It ends with this list of the different circles of JAR hell: unexpressed dependencies, transitive dependencies shadowing, version conflicts, complex class loading.

Simple solution

Overwrite jar file in container by your libraries. Simply, copy your libraries to folder container/lib or some kind like that.

Several pitfalls may occur:
• We have to do it every freaking time we deploy the project.
• Depending on the container, we have to know how to do it properly.
• If we get the bad luck that the version we prefer to use is not compatible with the container, we may get more problem at runtime.

Use maven-shade-plugin

A better way is shading them. maven-shade-plugin  provides the capability to package the artifact in an uber-jar, including its dependencies and to shade – i.e. rename – the packages of some of the dependencies.

 

How to do

For example, we have Google Guice  lib is conflicting with Container

Step 1

Create a new maven project, for example repacked and add conflict dependency.

 

</pre>
&lt;project&gt;
 &lt;groupId&gt;com.com.javacoffie&lt;/groupId&gt;
 &lt;artifactId&gt;repacked&lt;/artifactId&gt;

 &lt;version&gt;4.0&lt;/version&gt;

&lt;dependencies&gt;

 &lt;dependency&gt;
 &lt;groupId&gt;com.google.inject&lt;/groupId&gt;
 &lt;artifactId&gt;guice&lt;/artifactId&gt;
 &lt;version&gt;4.0&lt;/version&gt;
 &lt;/dependency&gt;
 &lt;/dependencies&gt;
&lt;/project&gt;
<pre>

 

Step 2 Configure shade plugin

</pre>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-shade-plugin</artifactId>
<version>2.4.3</version>
<configuration>
<artifactSet>
<includes>
<include>com.google.inject:guice</include>
<include>com.google.guava:guava</include>
</includes>
</artifactSet>
</configuration>
<executions>
<execution>
<phase>package</phase>
<goals>
<goal>shade</goal>
</goals>
<configuration>
<relocations>
-->
<relocation>
<pattern>com.google</pattern>
<shadedPattern>com.example.shaded.google</shadedPattern>
</relocation>
<relocation>
<pattern>com.google.thirdparty.publicsuffix</pattern>
<shadedPattern>com.axonivy.shaded.google.thirdparty.publicsuffix</shadedPattern>
</relocation>
</relocations>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>
<pre>

Step 4

Execute command

mvn clean install.

Step 5

Add new antifact as dependency to your project

<dependency>
<groupId>com.javacoffie</groupId>
<artifactId>repacked</artifactId>
<version>4.0</version>
</dependency>

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