#vulnerable code 
                                @Test
    public void testReadWriteAsyncSingleClient() 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();
                }
            }

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

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

            // open ledger
            lh = bkc.openLedger(ledgerId, digestType, ledgerPassword);
            LOG.debug("Number of entries written: " + (lh.getLastAddConfirmed() + 1));
            assertTrue("Verifying number of entries written", lh.getLastAddConfirmed() == (numEntriesToWrite - 1));

            // read entries
            lh.asyncReadEntries(0, numEntriesToWrite - 1, this, sync);

            synchronized (sync) {
                while (sync.value == false) {
                    sync.wait();
                }
            }

            LOG.debug("*** READ COMPLETE ***");

            // at this point, Enumeration<LedgerEntry> ls is filled with the returned
            // values
            int i = 0;
            while (ls.hasMoreElements()) {
                ByteBuffer origbb = ByteBuffer.wrap(entries.get(i));
                Integer origEntry = origbb.getInt();
                byte[] entry = ls.nextElement().getEntry();
                ByteBuffer result = ByteBuffer.wrap(entry);
                LOG.debug("Length of result: " + result.capacity());
                LOG.debug("Original entry: " + origEntry);

                Integer retrEntry = result.getInt();
                LOG.debug("Retrieved entry: " + retrEntry);
                assertTrue("Checking entry " + i + " for equality", origEntry.equals(retrEntry));
                assertTrue("Checking entry " + i + " for size", entry.length == entriesSize.get(i).intValue());
                i++;
            }
            assertTrue("Checking number of read entries", i == numEntriesToWrite);

            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 testReadWriteAsyncSingleClient() 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());
            }

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

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

            // open ledger
            lh = bkc.openLedger(ledgerId, digestType, ledgerPassword);
            LOG.debug("Number of entries written: " + (lh.getLastAddConfirmed() + 1));
            assertTrue("Verifying number of entries written", lh.getLastAddConfirmed() == (numEntriesToWrite - 1));

            // read entries
            lh.asyncReadEntries(0, numEntriesToWrite - 1, this, sync);

            synchronized (sync) {
                while (sync.value == false) {
                    sync.wait();
                }
                assertEquals("Error reading", BKException.Code.OK, sync.getReturnCode());
            }

            LOG.debug("*** READ COMPLETE ***");

            // at this point, Enumeration<LedgerEntry> ls is filled with the returned
            // values
            int i = 0;
            Enumeration<LedgerEntry> ls = sync.getLedgerEntries();
            while (ls.hasMoreElements()) {
                ByteBuffer origbb = ByteBuffer.wrap(entries.get(i));
                Integer origEntry = origbb.getInt();
                byte[] entry = ls.nextElement().getEntry();
                ByteBuffer result = ByteBuffer.wrap(entry);
                LOG.debug("Length of result: " + result.capacity());
                LOG.debug("Original entry: " + origEntry);

                Integer retrEntry = result.getInt();
                LOG.debug("Retrieved entry: " + retrEntry);
                assertTrue("Checking entry " + i + " for equality", origEntry.equals(retrEntry));
                assertTrue("Checking entry " + i + " for size", entry.length == entriesSize.get(i).intValue());
                i++;
            }
            assertTrue("Checking number of read entries", i == numEntriesToWrite);

            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");
        }
    }