Hi, I have a question about how changing the system class loader by setting "java.system.class.loader" interacts with the module system. I couldn't find a lot of information on this topic but if it has been discussed before please point me to the corresponding place.
Traditionally, setting "java.system.class.loader" to the name of a class loader will instruct the JVM to use that class loader as the system class loader which will be used to load the application main class. But this class loader has to be loaded itself by a class loader. So the JVM defines its own "default system class loader" which will be used to load the custom system class loader and which will act as delegation parent for it [1]. By writing a custom class loader which intercepts calls to loadClass() and defining it as the system class loader by setting "java.system.class.loader" it can be easily verified that the JVM will load the applications main class with that custom class loader. This behaviour changes however if we use the new module syntax "-m <module>/<class>" to start an application. In that case, the main application class will always be loaded by the default system class loader, no matter if we define "java.system.class.loader" or not. >From what I've found by looking into the sources, this is because the application module will be added to the boot layer and associated with the default system class loader early in the JVM boot process (i.e. "initPhase2") when only classes from java.base can be loaded and the custom class loader is therefore not available. The system class loader is only initialized later in "initiPhase3" at which it seems to be already to late to associate it with the application module (is this really the case?). My question now is if this is an inherent property of the module system or merely an implementation detail? I.e. would it be possible to put the application module into its own layer and initialize that only later when the custom system class loader will be available? I think this would be relatively easy if the custom class loader can be found on the class path. If the custom class loader is in a module itself, that module would have to be in the boot layer to make it accessible to the default system class loader. The current API documentation of the system class loader [1] mentions that the system class loader "is typically the class loader used to start the application". Shouldn't that be updated to mention that this will not be the case for modular applications? Thank you and best regards, Volker [1] https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/ClassLoader.html#getSystemClassLoader()
