Fix a Maven Dependency Conflict

2019 Jan 4

The Dependency Conflict Cannot be Resolved

A project I work on is using both “com.google.sitebricks:sitebricks”, a now inactively under development web framework, and “org.drools:drools-compiler” as its dependencies, both of which then depend on “org.mvel:mvel2”. Maven used to find a version of mvel2 to satisfy both of the two dependencies.

However, when the “drools-compiler” dependency is upgraded to a newest version (“7.13.0.Final”), something unfortunate happens. The “drools-compiler:7.13.0.Final” uses a mvel2 version, which is incompatible with the one sitebricks uses. The “drools-compiler” uses some new APIs from the newer version of mvel2, unfortunately, that version of mvel2 deletes some APIs “sitebricks” uses. In this case, Maven cannot resolve the dependency conflict easily since there is NO version of mvel2 to satisfy “drools-compiler:7.13.0.Final” and “sitebricks:*”.

A Fix

Use Maven Shade Plugin to package sitebricks and its mvel2 dependency into a “shaded jar”, and also use this plugin to “relocate” mvel2 classes inside the “shaded jar”. “Relocation” here is to move mvel2 classes from package “org.mvel2” to some other package like “org.shaded.mvel2”, and to also modify bytecode of some sitebricks classes, which refers mvel2 classes, correspondingly, so that these “mvel2 classes” do not conflict with those used by “drools-compiler”.

Create a Maven project/module for the “shaded jar”.

<artifactId>sitebricks-shaded</artifactId>
<packaging>jar</packaging>
<build>
  <plugins>
    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-shade-plugin</artifactId>
      <version>3.2.1</version>
      <executions>
        <execution>
          <phase>package</phase>
          <goals>
            <goal>shade</goal>
          </goals>
          <configuration>
            <finalName>sitebricks-shaded-${project.version}</finalName>
            <relocations>
              <relocation>
                <pattern>org.mvel2</pattern>
                <shadedPattern>org.shaded.mvel2</shadedPattern>
              </relocation>
            </relocations>
            <artifactSet>
              <includes>
                <include>org.mvel:mvel2</include>
                <include>com.google.sitebricks:*</include>
              </includes>
            </artifactSet>
          </configuration>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>
<dependencies>
  <dependency>
    <groupId>com.google.sitebricks</groupId>
    <artifactId>sitebricks</artifactId>
  </dependency>
</dependencies>

In the project, use the “sitebricks-shaded” as a dependency instead.

<dependency>
  <groupId>com.foo.bar</groupId>
  <artifactId>sitebricks-shaded</artifactId>
  <version>${project.version}</version>
</dependency>
<dependency>
  <groupId>org.drools</groupId>
  <artifactId>drools-compiler</artifactId>
  <version>${drools.version}</version>
</dependency>

And one more step.

maven

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