[MENFORCER-459] Avoid NullPointerException where there is no plugins

[turbanoff]

When I debugged build of my project I noticed that maven-enforcer-plugin generates lots of NPEs with this stack trace:

  at java.util.ArrayList.addAll(ArrayList.java:702)
  at org.apache.maven.plugins.enforcer.RequirePluginVersions.getReportingPlugins(RequirePluginVersions.java:1062)
  at org.apache.maven.plugins.enforcer.RequirePluginVersions.getAllPluginEntries(RequirePluginVersions.java:970)
  at org.apache.maven.plugins.enforcer.RequirePluginVersions.execute(RequirePluginVersions.java:237)
  at org.apache.maven.plugins.enforcer.EnforceMojo.execute(EnforceMojo.java:199)
  - locked <0x1eb6> (a org.apache.maven.plugins.enforcer.RequirePluginVersions)
  at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:137)
  at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute(MojoExecutor.java:301)
  at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:211)
  at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:165)
  at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:157)
  at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:121)
  at org.apache.maven.lifecycle.internal.builder.multithreaded.MultiThreadedBuilder$1.call(MultiThreadedBuilder.java:210)
  at org.apache.maven.lifecycle.internal.builder.multithreaded.MultiThreadedBuilder$1.call(MultiThreadedBuilder.java:195)
  at java.util.concurrent.FutureTask.run(FutureTask.java:264)
  at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
  at java.util.concurrent.FutureTask.run(FutureTask.java:264)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
  at java.lang.Thread.run(Thread.java:829)

Exceptions shouldn't be used for non-exceptional flow.
Also it's considered bad practice to return null from a method which returns collection. Empty collection should be returned instead.
I checked all current usages of this method. All of them use this pattern:

        try
        {
            //...
            plugins.addAll( PluginWrapper.addAll( /*..*/ );
        }
        catch ( NullPointerException e )
        {
            // guess there are no plugins here.
        }

So, it's safe to change return value to emptyList() - nothing will be added to plugins in either case.

• I have run the integration tests successfully (mvn -Prun-its clean verify).
• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

[slawekjaranowski]

Thanks for fix
I assume that we can also remove catch ( NullPointerException e ) patterns 😄

Please resolve conflicts, and create an issue for tracking.

[turbanoff]

Created issue https://issues.apache.org/jira/browse/MENFORCER-459