Saturday, August 4, 2018

Stack Trace got eaten up in the log

During the production troubleshooting today, we found out in the log that NPE was logged occasionally, but without detailed stack trace. We've made sure that we always log the stack trace in our code. So, what was going on?

After searching back in our log history (we have a log aggregation service that keeps log history), we found there were NPEs with the full stack trace, then after some time, the stack trace was gone.

In this end, this turns out to be caused by a Hotspot JIT compiler optimization, when a particular exception keeps being thrown up to a threshold, the same exception stack trace will not be filled anymore due to performance impact of throwing exceptions.

The solution is to turn off this optimization by providing this JVM option:



1. Stackoverflow post on "NullPointerException in Java with no StackTrace"
2. A more detailed analysis with a demo app

Friday, August 3, 2018

scheduled task silently died with ScheduledThreadPoolExecutor

We had a very annoying bug lately where a scheduled job silently stopped executing without our knowledge. After debugging and carefully reading Javadoc again, it turns out that the task throws an Exception thus suppressing the subsequent executions of the task.

Javadoc for ScheduledExecutorService.scheduleWithFixedDelay (scheduleAtFixedRate has the same issue):

ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
                                          long initialDelay,
                                          long delay,
                                          TimeUnit unit)
Creates and executes a periodic action that becomes enabled first after the given initial delay, and subsequently with the given delay between the termination of one execution and the commencement of the next. If any execution of the task encounters an exception, subsequent executions are suppressed. Otherwise, the task will only terminate via cancellation or termination of the executor.

So, the quick fix is to add a try-catch block around the task execution logic to catch Exception. I have created a simple demo app to show this issue and how the fix works with some analysis why it happens:

The lesson learned is that we need to read doc more carefully and read source code to understand the library behavior closely.

Saturday, February 3, 2018

Interview Preparation for Tech Companies

For a candidate, to find a job in tech industry is not easy. From the hiring company's point of view, it is even more challenging to find qualifying candidates. In a very short process of phone interview and onsite interviews, it is very difficult to properly judge a candidate. Companies today mostly rely on short algorithmic or system design questions, here are some resources that I found quite useful and hopefully will be helpful to you as well.


Elements of Programming Interviews: Java version and Python version

Free Online Resources

5-star Comprehensive Tech Interview Handbook

Assorted Notes from

System Design Gitbook by soulmachine (Online, PDF and epub)

Leetcode Algorithmic Questions and Solutions Gitbook by soulmachine (Online, PDF and epub)

Top 10 Algorithms for Coding Interview from programmingcreek, also Leetcode grouped by type

Cracking the coding interview questions and answers by Hawstein (in C++, Chinese)

Top 10 algorithms in Interview Questions from GeeksforGeeks (a very good resource, also groups questions by type, e.g. dynamic programming, etc.)


Massive Technical Interviews Tips (very good coverage of system design problems)

Learn for Master
Leetcode questions organized by companies

There are also lots of video tutorials that gives more in-depth explanation how to solve problems. One good example is Huahua's Tech Road, it has the latest collection of Leetcode problems as well. If you search on Youtube, there are tons of similar resources as well.

Online Judge and Test Sites

- Leetcode
- Lintcode
- Interview Cake
- Hacker Rank
- TopCoder
- Kaggle (for data science and machine learning)