minor improvements

bcel-builder/src/main/java/org/aspectj/apache/bcel/classfile/LocalVariableTable.java

 		return buf.toString();
 	}
 
-	@Override
-	public Object clone() throws CloneNotSupportedException {
-		return super.clone();
+	/**
+	 * Returns copy of this attribute using same packed state. Used in unit tests.
+	 */
+	public synchronized LocalVariableTable copyFromPackedState() {
+		if (!isInPackedState) throw new IllegalStateException("No in packed state");
+		try {
+			return new LocalVariableTable(nameIndex, length, new DataInputStream(new ByteArrayInputStream(data)), getConstantPool());
+		} catch (IOException e) {
+			throw new RuntimeException("Failed to unpack clone", e);
+		}
 	}
 
 	/**

bcel-builder/src/test/java/org/aspectj/apache/bcel/classfile/tests/LocalVariableTableConcurrencyTest.java

  * http://www.eclipse.org/legal/epl-v10.html
  *
  * Contributors:
- *     Andy Clement -     initial implementation
+ *     Dmitry Mikhaylov - initial implementation
  * ******************************************************************/
 
 package org.aspectj.apache.bcel.classfile.tests;
 import org.aspectj.apache.bcel.classfile.JavaClass;
 import org.aspectj.apache.bcel.classfile.LocalVariableTable;
 import org.aspectj.apache.bcel.classfile.Method;
-import org.aspectj.apache.bcel.classfile.tests.BcelTestCase;
 
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 
 	protected void setUp() throws Exception {
 		super.setUp();
+
 		for (int i = 0; i < nThreads; i++) workers[i] = Executors.newSingleThreadExecutor();
 
 		JavaClass clazz = getClassFromJar("SimpleGenericsProgram");
-
 		Method mainMethod = getMethod(clazz,"main");
 		Code codeAttr = (Code) findAttribute("Code",mainMethod.getAttributes());
-
-		reference =
-				(LocalVariableTable) findAttribute("LocalVariableTable",codeAttr.getAttributes());
-	}
-
-	private LocalVariableTable createReferenceCopy() {
-		try {
-			return (LocalVariableTable) reference.clone();
-		} catch (CloneNotSupportedException e) {
-			throw new RuntimeException("Faileed to clone LocalVariableTable", e);
-		}
+		reference = (LocalVariableTable) findAttribute("LocalVariableTable",codeAttr.getAttributes());
 	}
 
+	/**
+	 * Try to hit concurrency bug in org.aspectj.apache.bcel.classfile.LocalVariableTable.unpack().
+	 * We do so by running unpack() on same instance with multiple threads, and artificially
+	 * delaying all threads but first so that they enter unpack() the moment first thread is about to leave it.
+	 *
+	 * Since this test relies on empirically obtained access pattern and number of iterations,
+	 * it never has 100% probability of hitting the bug. If it fails - there is certainly a bug.
+	 * If it passes, it could mean anything - fully correct code or slightly changed execution order preventing
+	 * threads to collide at problematic location.
+	 *
+	 * As such, it is not really good for unit testing.
+	 */
 	public void testLocalVariableTableAttributeConcurrency() throws RuntimeException, InterruptedException {
 		final AtomicReference<RuntimeException> error = new AtomicReference<>();
-		for (int i = 0; i < 100000; i++) {
-			LocalVariableTable sharedInstance = createReferenceCopy();
+		for (int i = 0; i < 10000; i++) {
+			LocalVariableTable sharedInstance = reference.copyFromPackedState();
 			CountDownLatch preStart = new CountDownLatch(nThreads);
 			Semaphore start = new Semaphore(0);
 			CountDownLatch finish = new CountDownLatch(nThreads);
 				workers[j].execute(() -> {
 					preStart.countDown();
 					start.acquireUninterruptibly();
-					// trying to trigger concurrent unpacking - one tread should enter unpack() when other is about to leave it
-					if (needsDelay) createReferenceCopy().getTableLength();
+					// trying to trigger concurrent unpacking bug - one tread should enter unpack() when other is about to leave it
+					if (needsDelay) reference.copyFromPackedState().getTableLength();
 					try {
 						sharedInstance.getTableLength();
 					}