How is the system classloader in java useful?

1
votes

I expect the system classloader to take over class loading for my application. So, if I run my program as,

java -Djava.system.class.loader=MyClassLoader Main

I expect Main to be loaded by MyClassLoader so that I can control class loading within my application. But that does not happen. Main is still loaded by the default system class loader (sun.misc.Launcher$AppClassLoader), as are other classes referenced from Main. However, calls to getSystemClassLoader() return an instance of MyClassLoader.

How is this useful? Wouldn't it make sense for Main to be loaded by my custom system class loader? How do I take control of class loading for my application? And who uses the getSystemClassLoader() value?

1
javaclassloader
Something has to load your class loader.... and that is the system classloader (which is magically inside the JVM). - Basile Starynkevitch
And how did you implement your classloader? Do you perhaps delegate to the parent? - Marko Topolnik
OK, so you didn't write it from scratch, but it's still your class. You must first disprove this hypothesis: Your classloader is being invoked, but it delegates to the parent classloader. - Marko Topolnik
Would it be possible for you to provide a minimal working example? - Axel
Please post the code of your ClassLoader - Aubin

1 Answers

1
votes

If you just subclassed URLClassLoader with no overriding, this is the documented behaviour you should expect:

The URLs will be searched in the order specified for classes and resources after first searching in the parent class loader.

If the parent class loader can load the class, then it does so, and the loader reported for that class will be the parent class loader. In order to revert this parent-first classloading policy that is the default for Java classloaders, you must override loadClass.