blog/how-the-jvm-executes-java-code

[giscus[bot]]

blog/how-the-jvm-executes-java-code

This blog post describes in details what happens when executing a Java application. After reading this post, you will understand the execution lifecycle of a Java application and the activities performed by the JVM during the execution phase.

https://www.cesarsotovalero.net/blog/how-the-jvm-executes-java-code.html

[apedano]

Hi César, first of all thank you for the nice and clear article. One remark though: in the class unloading paragraph you write that the "class or interface" are unloaded, which as far as I know is correct. Your code example, using the WeakReference shows that the "instance" has been garbage collected and not the class itself. I think this is a bit misleading.

[cesarsotovalero]

Hi @apedano, thanks for the remark. You're right! The JVM loads the binary format of the class (i.e., the .class file) and not an instantiated object. The instantiation of objects from the class happens separately and subsequently during the program's execution. On the other hand, the JVM unloads a class to free the memory associated with the Class object representing that class. After a class has been unloaded, it can be loaded again by another ClassLoader, which treats it as a completely new class. In typical standalone applications, class unloading is less common because the system ClassLoader is usually active for the lifetime of the application, and hence the classes it loads are not unloaded.

Therefore, the LargeClass is an example of object garbage collection, not class unloading. It demonstrates how a large object can be garbage collected when it is no longer reachable, but this is different from unloading the LargeClass class itself. I've clarified this in the article and added an example of a potential class unloading scenario. You can look at the changes here: 783d3f6.