Fix #192: AbstractTripleSort has memory leak.

AbstractTriplesSort registers with the JVM to be informed when
the available memory gets full. However, it never unregistered
from the JVM so that the JVM kept referring the instances
effectively producing a memory leak.

This commit fixes this leak by unregistering the module from
the JVM when calling closeStream().

Author: cboehme

src/main/java/org/culturegraph/mf/stream/pipe/sort/AbstractTripleSort.java

@@ -173,6 +173,7 @@ public int compare(final SortedTripleFileFacade o1, final SortedTripleFileFacade
 				}
 			}
 		}
+		MemoryWarningSystem.removeListener(this);
 	}
 
 	protected void onFinished() {

src/test/java/org/culturegraph/mf/stream/pipe/sort/AbstractTripleSortTest.java

@@ -15,6 +15,11 @@
  */
 package org.culturegraph.mf.stream.pipe.sort;
 
+import static org.junit.Assert.assertTrue;
+
+import java.lang.ref.ReferenceQueue;
+import java.lang.ref.WeakReference;
+
 import org.culturegraph.mf.types.Triple;
 import org.junit.Before;
 import org.junit.Test;
@@ -49,4 +54,24 @@ public void shouldNotFailIfFlushingBeforeFirstRecord() {
 		tripleSort.closeStream();
 	}
 
+	/**
+	 * This test case may throw fail unexpectedly as it relies on the
+	 * garbage collector to run when calling {@code System.gc()}. This
+	 * is not guaranteed by the JVM.
+	 */
+	@Test
+	public void issue192ShouldUnregisterFromTheJVMToNotCauseMemoryLeak() {
+
+		// Get weak reference for checking whether the object was actually freed later:
+		final ReferenceQueue<AbstractTripleSort> refQueue = new ReferenceQueue<AbstractTripleSort>();
+		final WeakReference<AbstractTripleSort> weakRef = new WeakReference<AbstractTripleSort>(tripleSort, refQueue);
+
+		tripleSort.closeStream();
+		tripleSort = null;
+
+		System.gc();
+
+		assertTrue(weakRef.isEnqueued());
+	}
+
 }