Why does the JVM require warmup?

Question

I understand that in the Java virtual machine (JVM), warmup is potentially required as Java loads classes using a lazy loading process and as such you want to ensure that th

Answer 1

Which parts of the code should you warmup?

There is no answer to this question in general. It depends entirely on your application.

Even if I warmup some parts of the code, how long does it remain warm (assuming this term only means how long your class objects remain in-memory)?

Objects remain in memory for as long as your program has a reference to them, absent any special weak-reference use or something similar. Learning about when your program "has a reference" to something can be a little more obscure than you might think at first glance, but it is the basis for memory management in Java and worth the effort.

How does it help if I have objects which need to be created each time I receive an event.

This is entirely dependent on the application. There is no answer in general.

I encourage you to study and work with Java to understand things like classloading, memory management, and performance monitoring. It takes some amount of time to instantiate an object, in general it takes more time to load a class (which, of course, is usually done far less often). Usually, once a class is loaded, it stays in memory for the life of the program -- this is the sort of thing that you should understand, not just get an answer to.

There are also techniques to learn if you don't know them already. Some programs use "pools" of objects, instantiated before they're actually needed, then handed off to do processing once the need arises. This allows a time-critical portion of the program to avoid the time spent instantiating during the time-critical period. The pools maintain a collection of objects (10? 100? 1000? 10000?), and instantiate more if needed, etc. But managing the pools is a significant programming effort, and, of course, you occupy memory with the objects in the pools.

It would be entirely possible to use up enough memory to trigger garbage collection much more often, and SLOW THE SYSTEM YOU WERE INTENDING TO SPEED UP. This is why you need to understand how it works, not just "get an answer".

Another consideration -- by far most of the effort put into making programs faster is wasted, as in not needed. Without extensive experience with the application being considered, and/or measurement of the system, you simply do not know where (or whether) optimization will even be noticeable. System/program design to avoid pathological cases of slowness ARE useful, and don't take nearly the time and effort of 'optimization'. Most of the time it is all any of us need.

-- edit -- add just-in-time compilation to the list of things to study and understand.