Almost to the day, a year ago, I published my blog post called Level-up your Java Debugging Skills with on-demand Debugging. In this blog post, I wrote about multiple rarely known and rarely used features of the Java debugging agent, including the onjcmd feature. To quote my own blog post:

JCmd triggered debugging

There are often cases where the code that you want to debug is executed later in your program’s run or after a specific issue appears. So don’t waste time running the debugging session from the start of your program, but use the onjcmd=y option to tell the JDWP agent to wait with the debugging session till it is triggered via jcmd.

A similar feature long existed in the SAPJVM. In 2019 Christoph Langer from SAP decided to add it to the OpenJDK, where it was implemented in JDK 12 and has been there ever since.

The alternative to using this feature is to start the debugging session at the beginning and only connect to the JDWP agent when you want to start debugging. But this was, for a time, significantly slower than using the onjcmd feature (source):