This article is about techniques which I have used to debug codeBases of various kinds, such as:

  1. CodeBase with high concurrent nature.
  2. CodeBase with a lot of proprietary (unsupported) libraries.
  3. CodeBase with a lot of deprecated/unwanted code.
  4. CodeBase with memory leaks.
  5. CodeBase where every JVM can talk to every other JVM.

So let’s look at them one by one.

CodeBase with high concurrent nature.

It may happen that for serving a request, JVM uses many threads for eg:

Req -> tomcatThread-1 -> executorThread-2 -> BizThread-3->…`

Let say, we find that exception is coming in BizThread-3. Now as a debugger, we want to understand the Request flow. But the stacktrace will not be able to provide the complete request flow (for example, what happened in executorThread-2 and what happened in tomcatThread-1 and so on).

Technique 1.1: Write a custom java-agent which will be used to effectively add log.debug() to the start and end of every method of certain java packages. This will give us some insight into what all is getting called.

Technique 1.2: In certain frameworks, if supported, use AOP to proxy all methods and effectively add log.debug().

CodeBase with a lot of proprietary (unsupported) libraries.

Sometimes we find ourself in a situation where, after hours of debugging, we nail the problem that xyz-gov-secret library is misbehaving and this library is now unsupported.

Technique 2.1: Roll up your sleeves and install eclipse-decompiler and dive into the code base.

CodeBase with a lot of deprecated/unwanted code.

This is a classic one: we sometimes find ourselves in a method of 500+ lines with tons of deprecated if-else. Now, how do we figure out what is the code flow for a particular call, which if-else are going to use, and which is the dead code?

Technique 3.1: We can use a tool called jacoco agent. It collects the execution details during runtime and can color-code the code in eclipse.
Basically, it is the same tool, generally used in analyzing code coverage by JUnit Test.

CodeBase with memory leaks.

Every developer has a day when, in their local system, all goes good, in production OutOfMemory :(

Technique 4.1: JVM provides techniques to capture heap dumps in case of outOfMemory.

Add the following as an argument while starting the JVM
-XX:+HeapDumpOnOutOfMemoryError . This will capture the heap dump and put it in a file, which can be used to analyze what is eating up the memory.

Technique 4.2: You can also take the heap dump/thread dump of a running JVM using jProfiler/Jvisiualvm.

CodeBase where every JVM can talk to every other JVM.

When you are thrown into a spaghetti distributed environment, it becomes difficult to track down the request flow.

Technique 5.1: You can use tools like Wireshark. Wireshark captures the network data and represents it in a nice UI. You can then view the HTTP request/response flowing through the system

Honorable mentions

Technique 6.1: In a single-threaded environment, intentionally insert
try catch in order to quickly know the stacktrace.

try {
	throw new RuntimeException(); 
} catch(Exception e){

Technique 6.2: Using eclipse breakpoint or using conditional breakpoint.

Technique 6.3:

Motivation of article: Team Learning/Knowledge Sharing.