These are two bugs from my first engineering job, circa 2010-2011. I was maybe a year into my career, working on database drivers at a product development company. The problems themselves are not sophisticated. A staff engineer would solve them faster and communicate the fix better than I did. But the shape of the debugging, working with almost no information, tracing through code you did not write, reasoning about systems you cannot touch, is something I have used every year since. I think about these two bugs more than most of the harder problems I have worked on since.

The server that would not start

We were building a database driver and server component that ran on midframe and mainframe class machines. We were doing product development for a client, who sold the product to their customers. When something broke at a customer site, the information had to travel through several hops before it reached us. We had no direct access to the customer or their machines.

One customer reported that after a scheduled maintenance window, their server would not start. The stack trace said it could not open the configured port. They confirmed no other process was using the port. They had been running the same version with the same configuration for a while, and it had worked fine until the maintenance.

This was not widespread. Many other customers on the same version and configuration had no issues. Our internal test suite, which was thorough, did not reproduce it. All I had was a stack trace, knowledge of which JDK version they were running, and our source code.

I started walking through the JDK source, mapping the calls in the stack trace to actual code paths. Most of it was straightforward server socket creation. But buried in the trace was something that did not belong: file listing operations. The JVM was trying to enumerate files in a directory as part of opening a network port. That made no sense on the surface, so I kept pulling the thread.

The path I eventually traced was this: during server socket creation, the JVM's security framework initializes itself. Part of that initialization involves seeding SecureRandom, which needs a source of entropy. On this particular platform and JVM, the entropy source had a fallback chain. The preferred sources (hardware random generators, OS-level entropy devices) were apparently unavailable or insufficient, so it fell back to a filesystem-based approach: enumerate the contents of the temp directory and use file metadata (timestamps, sizes, inode numbers) as entropy.

The temp directory on this machine had accumulated a massive number of files. The JVM ran out of stack memory trying to enumerate all of them. The server socket never got created, and the error that surfaced was just "could not open port," which told you nothing about the actual cause.

I was pretty sure I was right, but I was also a junior engineer with a theory and no way to test it. I was excited and confident about what I had found, eager to know if it would actually work, but with no way to verify it myself given the number of hops between us and the actual machine. So I did something a team member rightly called me out for: I sent an email directly to the customer saying "delete your temp directory and try again," with no explanation of why.

Next morning, the customer had replied. It worked. The server came up. Everyone was pleased, but also curious, and also annoyed that I had sent the suggestion without walking anyone through my reasoning first. The lesson was not just about debugging. It was about communication: a correct answer delivered without context is hard to trust, hard to learn from, and hard for anyone else to build on. My team member was right to flag it.

The class that disappeared overnight

Same era, same company. We were doing work for a large enterprise hardware and software manufacturer that had grown through acquisitions. They had a rigorous process for scanning open-source code, and when legal flagged something as a licensing risk, the response was swift and blunt: remove it.

One morning our nightly build was failing. Nobody had committed anything. The build had run the previous night on the same code and passed. Same code, same dependencies, same configuration, and now a ClassNotFoundException.

I traced it and found that the class existed in the dependency JAR the day before but was gone now. The JAR file was still there, same name, same location. But the class had been physically deleted from inside it. The company's legal scanning process had flagged a specific class in Apache Commons, a map implementation backed by a red-black tree, as a licensing concern. Their policy was to remove the offending class from the JAR and put the JAR back. No notification to the teams that depended on it.

It was a weekend. We had a release deadline. Legal was not going to put the class back. That was not negotiable.

I looked at what the class actually was: a Map implementation. A specific implementation that the company had a licensing concern with, but at the end of the day, just a class that satisfied the Map interface. Our code did not depend on any behavior specific to that implementation. It just needed something that fulfilled the same contract and had the same fully-qualified class name, because that is how the Java classloader resolves dependencies.

So I created a new class in our own source tree with the exact same package and class name, backed by a readily available JDK implementation of the same interface. No licensing concern, no third-party code. I dropped it into our source, the classloader picked it up, the build passed, and we shipped on time.

We needed to verify that nothing depended on behavior specific to the original implementation. Nothing did. The original class had been chosen by whoever wrote the dependency, probably for performance characteristics in a context that no longer applied. Our usage just needed a map.

We eventually found a properly licensed replacement, but the shim bought us the weekend we needed. The whole fix was maybe 15 lines of code and a basic understanding of how Java class loading works.

What stuck with me

Neither of these bugs was in our code. The first was a JVM implementation detail interacting with filesystem state. The second was a legal process deleting code out from under a running build system. In both cases, the symptoms pointed nowhere useful. "Cannot open port" does not suggest "your temp directory has too many files." "ClassNotFoundException" does not suggest "legal deleted a class from a JAR."

What solved both was the same approach: read the actual stack trace, trace the code path through source you did not write, and keep asking "why would this code be running here?" until the answer stops being surprising. I was a junior engineer and these were not elegant solutions. The temp directory fix was a guess I got lucky on. The classloader shim was undergraduate-level OOP. But the debugging method, following the thread past the point where the error message stops being helpful, is something I have used on much harder problems since. The problems got bigger. The method did not change.