#fixed code
                                @Test
    public void testDoReadSession() throws NoSuchFieldException, IllegalAccessException {
        Session nullSession = redisSessionDAO.doReadSession(null);
        assertThat(nullSession, is(nullValue()));

        RedisSessionDAO redisSessionDAO2 = new RedisSessionDAO();
        redisSessionDAO2.setRedisManager(redisManager);
        redisSessionDAO2.setKeyPrefix(testPrefix);
        redisSessionDAO2.setExpire(2);
        ThreadLocal sessionsInThread = mock(ThreadLocal.class);
        Map<Serializable, SessionInMemory> sessionMap = new HashMap<Serializable, SessionInMemory>();
        SessionInMemory sessionInMemory = new SessionInMemory();
        sessionInMemory.setSession(new FakeSession(1, "Billy"));
        sessionInMemory.setCreateTime(new Date());
        sessionMap.put("1", sessionInMemory);
        when(sessionsInThread.get()).thenReturn(sessionMap);
        TestUtils.setPrivateField(redisSessionDAO2, "sessionsInThread", sessionsInThread);
        FakeSession actualSession = (FakeSession)redisSessionDAO2.doReadSession("1");
        assertThat(actualSession.getId().toString(), is("1"));
        assertThat(actualSession.getName(), is("Billy"));
        verify(redisManager, times(0)).get(any((new byte[0]).getClass()));
    }
#vulnerable code 
                                @Test
    public void testDoReadSession() {
        Session actualSession = redisSessionDAO.doReadSession(testKey);
        assertThat(actualSession.getId().toString(), is("3"));
        redisSessionDAO.doReadSession(null);
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE