Maven dependency conflicts demo

Problem demo

Two maven projects (guavam1 and guavam2) use two different and incompatible versions of Guava (versions 27 and 15, respectively).

Used separately the two projects work:

$  mvn -DskipTests package exec:java -am -pl guavam1
...
[INFO] --- exec-maven-plugin:1.6.0:java (default-cli) @ guavam1 ---
Optional.absent()

(it prints Optional.fromJavaUtil(java.util.Optional.empty()).toString())

$  mvn -DskipTests package exec:java -am -pl guavam2
...
[INFO] --- exec-maven-plugin:1.6.0:java (default-cli) @ guavam2 ---
Not null

(this is MapConstraints.notNull().toString())

Projects guavam12 and guavam12r depend on these two previous projects and try to invoke both test methods. The difference between the integrator projects is the order of the dependencies: guavam12r has guavam2 before guavam1.

The output of the integrator projects:

$ mvn -DskipTests package exec:java -am -pl guavam12
...
[INFO] --- exec-maven-plugin:1.6.0:java (default-cli) @ guavam12 ---
App1 test: Optional.absent()
App2 test: java.lang.NoClassDefFoundError: com/google/common/collect/MapConstraints

$ mvn -DskipTests package exec:java -am -pl guavam12r
...
[INFO] --- exec-maven-plugin:1.6.0:java (default-cli) @ guavam12r ---
App1 test: java.lang.NoSuchMethodError: com.google.common.base.Optional.fromJavaUtil(Ljava/util/Optional;)Lcom/google/common/base/Optional;
App2 test: Not null

Depending on the order in which the dependencies are declared, one of the tests fails because there can be only one Guava library on the classpath.

Solution attempt

Projects guavab1 and guavab2 are bundles that embed the previous Maven projects together with their dependencies. Project guavab12 uses Eclipse Concierge to load both bundles and then invokes the tests. Output:

$ mvn -DskipTests package exec:exec -am -pl guavab12
...
[INFO] --- exec-maven-plugin:1.6.0:exec (default-cli) @ guavab12 ---
---------------------------------------------------------
  Concierge OSGi 5.1.0 on Mac OS X 10.13.6 starting ... (default) startlevel=3
---------------------------------------------------------
---------------------------------------------------------
  Framework started in 0.0 seconds.
---------------------------------------------------------
user.dir=/Users/ckoncz/work/git/hwx/maven-dependency-conflict/guavab12
[com.ck.guavab1-0.0.1.SNAPSHOT] says: Optional.absent()
[com.ck.guavab2-0.0.1.SNAPSHOT] says: Not null

The Guava libraries now coexist in the JVM.