getClass().getClassLoader() is null, why?

Citing the API doc:

Some implementations may use null to represent the bootstrap class loader. This method will return null in such implementations if this class was loaded by the bootstrap class loader.

This is how it works . Whenever JVM try to load any class it's checks below conditions.

If Class is loaded from Bootstrap ClassPath i.e; jdk\jre\lib\rt.jar , BootStrap ClassLoader will be called.

If Class is loaded from Extension Classpath i.e; jdk\jre\lib\ext*.jar , Extension ClassLoader will be called.

If Class is loaded from Application ClassPath i.e; as specified in Environment Variable , Application ClassLoader is called .

Since Bootstrap ClassLoader is not implemented in java , it's either implemented in c or c++ so there is no reference for it that's why it returns null . But Extension and Application class Loader is written in java so you will get the reference as sun.misc.Launcher$ExtClassLoader@someHexValue and sun.misc.Launcher$AppClassLoader@someHexValue .

So, if you do something like this System.out.println(String.class.getClassLoader()) you will get null since this class is been called by BootStrap ClassLoader, On the other hand if you do the same thing for a class in Ext or App Class path you will get $ExtClassLoader@someHexValue and sun.misc.Launcher$AppClassLoader@someHexValue respectively .

This method will return null in such implementations if this class was loaded by the bootstrap class loader.

https://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Class.html#getClassLoader()