Why Debugging Is the Best Way to Master Framework Source Code

When faced with the question of whether learning programming requires pure understanding or memorizing every detail, the author recommends adopting a debugging mindset and writing small demo programs to verify concepts.

Instead of worrying about the implementation language of a framework—whether C, D, or any other—the author suggests running the entire component (e.g., Redis) on a local machine and stepping through its execution with a debugger. This approach removes the “secret” nature of the code.

Applying the same method to other systems, such as Tomcat’s request‑handling flow, Spring Boot’s auto‑configuration mechanism, and MyBatis’s SQL execution chain, reveals that placing breakpoints at critical points and debugging eliminates mystery.

Concrete demos illustrate the process:

To understand Spring’s circular‑dependency resolution, create three classes (A, B, C) where A injects B, B injects C, and C injects A, then run the application and debug into DefaultSingletonBeanRegistry → getSingleton.

To see when a thread‑pool’s rejection policy triggers, use the following code snippet:

ThreadPoolExecutor pool = new ThreadPoolExecutor(
    2, 3, 60, TimeUnit.SECONDS,
    new ArrayBlockingQueue<>(1)
);

Submitting five tasks causes the fifth task to invoke the rejection policy, demonstrating the policy’s behavior without needing to memorize abstract rules.

To explore HashMap’s concurrency issues, run two threads that simultaneously call put and debug into the resize method to observe how linked‑list operations during resizing can lead to data loss.

The key lesson is to shrink the problem to the smallest verifiable unit—often just a few classes and a handful of configuration lines—rather than building a full project. This minimal‑demo approach makes the underlying mechanisms clear and repeatable.