preload common pool - issue #198 (#279)

* enable to preload common pool
* move to java 8
* fix build - weird java version was upgrade to j8 but not animal sniffer

pom.xml

@@ -262,6 +262,7 @@
           </signature>
           <ignores>
             <ignore>java.lang.invoke.MethodHandle</ignore>
+            <ignore>java.util.concurrent.ForkJoinPool</ignore>
           </ignores>
         </configuration>
       </plugin>

src/main/java/org/codehaus/mojo/exec/ExecJavaMojo.java

@@ -16,6 +16,9 @@
 import java.util.List;
 import java.util.Properties;
 import java.util.Set;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.ForkJoinPool;
 
 import org.apache.maven.artifact.Artifact;
 import org.apache.maven.artifact.factory.ArtifactFactory;
@@ -68,6 +71,18 @@ public class ExecJavaMojo
     @Parameter( required = true, property = "exec.mainClass" )
     private String mainClass;
 
+
+    /**
+     * Forces the creation of fork join common pool to avoids the threads to be owned by the isolated thread group
+     * and prevent a proper shutdown.
+     * If set to zero the default parallelism is used to precreate all threads,
+     * if negative it is ignored else the value is the one used to create the fork join threads.
+     *
+     * @since 3.0.1
+     */
+    @Parameter( property = "exec.preloadCommonPool", defaultValue = "0" )
+    private int preloadCommonPool;
+
     /**
      * The class arguments.
      * 
@@ -234,6 +249,11 @@ public void execute()
             getLog().debug( msg );
         }
 
+        if ( preloadCommonPool >= 0 )
+        {
+            preloadCommonPool();
+        }
+
         IsolatedThreadGroup threadGroup = new IsolatedThreadGroup( mainClass /* name */ );
         Thread bootstrapThread = new Thread( threadGroup, new Runnable()
         {
@@ -338,6 +358,42 @@ public void run()
         registerSourceRoots();
     }
 
+    /**
+     * To avoid the exec:java to consider common pool threads leaked, let's pre-create them.
+     */
+    private void preloadCommonPool()
+    {
+        try
+        {
+            // ensure common pool exists in the jvm
+            final ExecutorService es = ForkJoinPool.commonPool();
+            final int max = preloadCommonPool > 0
+                    ? preloadCommonPool :
+                    ForkJoinPool.getCommonPoolParallelism();
+            final CountDownLatch preLoad = new CountDownLatch( 1 );
+            for ( int i = 0;
+                  i < max;
+                  i++ )
+            {
+                es.submit(() -> {
+                    try
+                    {
+                        preLoad.await();
+                    }
+                    catch ( InterruptedException e )
+                    {
+                        Thread.currentThread().interrupt();
+                    }
+                });
+            }
+            preLoad.countDown();
+        }
+        catch (final Exception e)
+        {
+            getLog().debug(e.getMessage() + ", skipping commonpool earger init");
+        }
+    }
+
     /**
      * a ThreadGroup to isolate execution and collect exceptions.
      */