Observability of the Java Virtual Machine

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The JVM is one of the most observable runtimes. It provides us lots of tools for troubleshooting a JVM application in production. 1. Thread observability Threads are how the JVM actually does work. When something is wrong in production, the symptom is almost always a thread: stopped, blocked, leaking etc. Thread dumps work on any JVM with no  instrumentation, no agents, no restarts. <Example project link with /threaddump endpoint>         // (1) Deadlock — two threads grab the same pair of locks in opposite order.         new Thread(() -> grab(LOCK_A, LOCK_B), "deadlock-A-then-B").start();         new Thread(() -> grab(LOCK_B, LOCK_A), "deadlock-B-then-A").start(); http://localhost:8080/actuator/threaddump To list the JVMS, we can use the command below. PS C:\observe-jvm> jps -lv 25296 jdk.jcmd/sun.tools.jps.Jps -Dapplication.home=C:\Program Files\Microsoft\jdk-21.0.3.9-hotspot -Xms8m -Djdk.module.main=...
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Sharpening Regex Skills

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I needed to revisit regexes and found this sites useful:

https://regexone.com

https://regex101.com/
(Regex debugger is cool)

Below is a useful summary:


Groups:

Parsing XML.. Following code creates 2 groups, then references to group 1.

         boolean matchFound = false;
         Pattern r = Pattern.compile("<(.+)>([^<]+)</\\1>");
         Matcher m = r.matcher(line);

          while (m.find()) {
             System.out.println(m.group(2));
             matchFound = true;
         }
         
         if ( ! matchFound) {
             System.out.println("None");
         }


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