Monthly Archives: December 2025

Quickly Inspect your Java Application with JStall

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Welcome to the last blog post of the year. Last week, I discussed the limitations of custom JFR events. This week, I’ll also be covering a profiling-related topic and showcasing a tiny tool called JStall.

I hope I’m not the only one who sometimes wonders: “What is my Java application doing right now?” When you don’t see any output. Yes, you could perform a simple thread dump via jstack, but it is hard to understand which threads are actually consuming CPU and making any sort of progress. This is where my tiny tool called JStall comes in:

JStall is a small command-line tool for one-shot inspection of running JVMs using thread dumps and short, on-demand profiling. The tool essentially takes multiple thread dumps of your application and uses the per-thread cpu-time information to find the most CPU-time-consuming Java threads.

First, download the JStall executable from the GitHub releases page. Let us then start by finding the currently running JVMs:

> ./jstall
Usage: jstall <command> <pid|files> [options]

Available commands:
  status     - Show overall status (deadlocks + most active threads)
  deadlock   - Check for deadlocks
  most-work  - Show threads doing the most work
  flame      - Generate flame graph
  threads    - List all threads

Available JVMs:
  7153 ./jstall
  1223 <unknown>
  8136 ./renaissance-gpl-0.16.0.jar
  6138 org.jetbrains.idea.maven.server.RemoteMavenServer36
  5597 DeadlockDemo
  49294 com.intellij.idea.Main

This provides us with a list of options for the main status command, as well as a list of JVM processes and their corresponding main classes. Let’s start checking for deadlocking:

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Don’t use Arrays or other Complex Types in Custom JFR Events

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JDK Flight Recorder (JFR) provides support for custom events as a profiler. Around two years ago, I wrote a blog post on this very topic: Custom JFR Events: A Short Introduction. These custom events are beneficial because they enable us to record additional project-specific information alongside the standard JFR events, all in the same file. We can then view and process this information with the JFR tools. You can freely specify these events in Java.

There is only one tiny problem nobody talks about: Array support (and, in more general, the support of complex types).

Take the following event:

class ArrayEvent extends Event {
    
    @Label("String Array")
    String[] stringArray;
    
    @Label("Int Array")
    int[] intArray;
    
    @Label("Long Array")
    long[] longArray;
    
    @Label("Non array field")
    String nonArrayField = "default";
}

What would you expect to happen when we create the event using the following code?

ArrayEvent event = new ArrayEvent();
event.stringArray = new String[]{"one", "two", "three"};
event.intArray = new int[]{1, 2, 3, 4, 5};
event.longArray = new long[]{100L, 200L, 300L};
event.commit();
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Creating a Gridfinity Chocolate Advent Calendar

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This week is a bit different as I’m working on a fun year-end blog and doing my regular work, so I’ll share with you how to create an advent calendar:

Normal Advent calendars are boring: so let’s make our own! We’ll combine all our favorite technologies: Gridfinity (for the grid system), 3D printing (for the grid), vacuum molding (for the chocolate), laser cutting (for the frame), and automated paper cutting to create advent calendars that are both beautiful and functional.

Along the way, we’ll cover practical food safety considerations and show how these techniques come together to produce something tasty, nerdy, and gift-worthy:

You can mind the positive for the chocolate mold on MakerWorld:

I also created a 2×2 Gridfinity grid that fits RitterSport:

You can learn how to create QR codes out of chocolate in another video of mine:

I usually post computer/Java-related blog posts, but I hope you also liked this blog post, in which I shared my hobbies.