#vulnerable code 
                                @Test
    public void testReadWriteAsyncLength() throws IOException {
        try {
            // Create a ledger
            lh = bkc.createLedger(digestType, ledgerPassword);
            // bkc.initMessageDigest("SHA1");
            ledgerId = lh.getId();
            LOG.info("Ledger ID: " + lh.getId());
            for (int i = 0; i < numEntriesToWrite; i++) {
                ByteBuffer entry = ByteBuffer.allocate(4);
                entry.putInt(rng.nextInt(maxInt));
                entry.position(0);

                entries.add(entry.array());
                entriesSize.add(entry.array().length);
                lh.asyncAddEntry(entry.array(), this, sync);
            }

            // wait for all entries to be acknowledged
            synchronized (sync) {
                while (sync.counter < numEntriesToWrite) {
                    LOG.debug("Entries counter = " + sync.counter);
                    sync.wait();
                }
            }
            long length = numEntriesToWrite * 4;
            assertTrue("Ledger length before closing: " + lh.getLength(), lh.getLength() == length);

            LOG.debug("*** WRITE COMPLETE ***");
            // close ledger
            lh.close();

            // *** WRITING PART COMPLETE // READ PART BEGINS ***

            // open ledger
            lh = bkc.openLedger(ledgerId, digestType, ledgerPassword);
            assertTrue("Ledger length after opening: " + lh.getLength(), lh.getLength() == length);


            lh.close();
        } catch (BKException e) {
            LOG.error("Test failed", e);
            fail("Test failed due to BookKeeper exception");
        } catch (InterruptedException e) {
            LOG.error("Test failed", e);
            fail("Test failed due to interruption");
        }
    } 
                            #location 20 
                            #vulnerability type THREAD_SAFETY_VIOLATION
#fixed code
                                @Test
    public void testReadWriteAsyncLength() throws IOException {
        SyncObj sync = new SyncObj();
        try {
            // Create a ledger
            lh = bkc.createLedger(digestType, ledgerPassword);
            // bkc.initMessageDigest("SHA1");
            ledgerId = lh.getId();
            LOG.info("Ledger ID: " + lh.getId());
            for (int i = 0; i < numEntriesToWrite; i++) {
                ByteBuffer entry = ByteBuffer.allocate(4);
                entry.putInt(rng.nextInt(maxInt));
                entry.position(0);

                entries.add(entry.array());
                entriesSize.add(entry.array().length);
                lh.asyncAddEntry(entry.array(), this, sync);
            }

            // wait for all entries to be acknowledged
            synchronized (sync) {
                while (sync.counter < numEntriesToWrite) {
                    LOG.debug("Entries counter = " + sync.counter);
                    sync.wait();
                }
                assertEquals("Error adding", BKException.Code.OK, sync.getReturnCode());
            }
            long length = numEntriesToWrite * 4;
            assertTrue("Ledger length before closing: " + lh.getLength(), lh.getLength() == length);

            LOG.debug("*** WRITE COMPLETE ***");
            // close ledger
            lh.close();

            // *** WRITING PART COMPLETE // READ PART BEGINS ***

            // open ledger
            lh = bkc.openLedger(ledgerId, digestType, ledgerPassword);
            assertTrue("Ledger length after opening: " + lh.getLength(), lh.getLength() == length);


            lh.close();
        } catch (BKException e) {
            LOG.error("Test failed", e);
            fail("Test failed due to BookKeeper exception");
        } catch (InterruptedException e) {
            LOG.error("Test failed", e);
            fail("Test failed due to interruption");
        }
    }