Strange NullPointerException in maven-resources-pl

About twice a day we get a NullPointerException in our build:

[06:44:23]: [ERROR] Failed to execute goal org.apache.maven.plugins:maven-resources-plugin:2.5:resources (default-resources) on project spring-lib: Execution default-resources of goal org.apache.maven.plugins:maven-resources-plugin:2.5:resources failed. NullPointerException -> [Help 1]
[06:44:23]: org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.apache.maven.plugins:maven-resources-plugin:2.5:resources (default-resources) on project spring-lib: Execution default-resources of goal org.apache.maven.plugins:maven-resources-plugin:2.5:resources failed.
[06:44:23]: at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:225)
[06:44:23]: at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
[06:44:23]: at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
[06:44:23]: at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:84)
[06:44:23]: at org.apache.maven.lifecycle.internal.LifecycleThreadedBuilder$1.call(LifecycleThreadedBuilder.java:167)
[06:44:23]: at org.apache.maven.lifecycle.internal.LifecycleThreadedBuilder$1.call(LifecycleThreadedBuilder.java:164)
[06:44:23]: at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
[06:44:23]: at java.util.concurrent.FutureTask.run(FutureTask.java:138)
[06:44:23]: at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
[06:44:23]: at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
[06:44:23]: at java.util.concurrent.FutureTask.run(FutureTask.java:138)
[06:44:23]: at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
[06:44:23]: at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
[06:44:23]: at java.lang.Thread.run(Thread.java:619)
[06:44:23]: Caused by: org.apache.maven.plugin.PluginExecutionException: Execution default-resources of goal org.apache.maven.plugins:maven-resources-plugin:2.5:resources failed.
[06:44:23]: at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:110)
[06:44:23]: at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:209)
[06:44:23]: ... 13 more
[06:44:23]: Caused by: java.lang.NullPointerException
[06:44:23]: at java.util.ArrayList.<init>(ArrayList.java:131)
[06:44:23]: at org.apache.maven.shared.filtering.DefaultMavenResourcesFiltering.filteredFileExtension(DefaultMavenResourcesFiltering.java:115)
[06:44:23]: at org.apache.maven.shared.filtering.DefaultMavenResourcesFiltering.filterResources(DefaultMavenResourcesFiltering.java:264)
[06:44:23]: at org.apache.maven.plugin.resources.ResourcesMojo.execute(ResourcesMojo.java:310)
[06:44:23]: at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:101)
[06:44:23]: ... 14 more

This comes at random times, and the next build doesn't fail.

I've googled for this failure and found nothing. I have this gut feeling that this may be caused by a concurrency issue in the maven-resources-plugin.

We got this error with maven-resources-plugin 2.4.3, and now with 2.5. Our maven version is 3.0.3.

We execute the build on TeamCity with the following parameters:

Goals: install
Additional Maven command line parameters: -T 2C -e -P!releasex,integration 
-Dmaven.test.failure.ignore=true -Dmaven.test.error.ignore=true 
-Dmaven.test.haltafterfailure=false -Dmaven.junit.timeout=1000000
-DwarProject.packaging=jar

Any sort of help for solving this issue is appreciated.

Try to run maven with -X and analyze the debug output. This might help. At least for me it does very often.

Edit: This is the problematic spot:

113  public boolean filteredFileExtension( String fileName, List userNonFilteredFileExtensions )
114  {
115      List nonFilteredFileExtensions = new ArrayList( getDefaultNonFilteredFileExtensions() );

getDefault.. returns null. Did you change the nonFilteredExts somehow? Because they are populated by default here:

63  // ------------------------------------------------
64  //  Plexus lifecycle
65  // ------------------------------------------------
66  public void initialize()
67      throws InitializationException
68  {
69      // jpg,jpeg,gif,bmp,png
70      this.defaultNonFilteredFileExtensions = new ArrayList( 5 );
71      this.defaultNonFilteredFileExtensions.add( "jpg" );
72      this.defaultNonFilteredFileExtensions.add( "jpeg" );
73      this.defaultNonFilteredFileExtensions.add( "gif" );
74      this.defaultNonFilteredFileExtensions.add( "bmp" );
75      this.defaultNonFilteredFileExtensions.add( "png" );
76  }