Re: ClassLoader not loading recompiled classes

 "Aryeh M. Friedman" <>
Tue, 02 Oct 2007 09:04:37 -0000
On Oct 2, 7:56 am, Silvio Bierman <> wrote:

Aryeh M. Friedman wrote:

On Oct 2, 6:02 am, Daniel Pitts <> wrote:

On Oct 1, 10:53 pm, "Aryeh M. Friedman" <>

ClassLoader does not update class on recompile:
Script started on Tue Oct 2 01:45:20 200> cat
public class Main
        public static void main(String[] args)
                throws Throwable
                while(true) {
                        ClassLoader loader=ClassLoader.getSystemClassLoader();
                        Class klass=loader.loadClass("MyClass");
                        MyClass mc=(MyClass) klass.newInstance();
                        System.out.println("hit any key to reload/rerun MyClass");


public class MyClass
        public MyClass()
                System.out.println("not hi there");

javac *.java
java Main

not hi there
hit any key to reload/rerun MyClass
not hi there
hit any key to reload/rerun MyClass
Suspended> cat foo
public class MyClass
        public MyClass()
                System.out.println("foo on you");

mv foo

java Main
not hi there
hit any key to reload/rerun MyClass
^C> exit
Script done on Tue Oct 2 01:47:02 200

Right, a ClassLoader will not re-load a class. You will have to
instantiate a new class loader to do so.
ClassLoader.loadClass will first look for already loaded classes. It
will not re-load the class into the JVM.

Its generally difficult to get dynamic class behavior from Java. You
aren't able to unload a class, and load a different version of it.
Also, and already loaded classes that refer to that other class will
only be able to refer to one instance of it, not one from one class
loader, and then another from another class loader.

Since I am relativally naive with class loaders how do I create a new
instance of the system class loader?

You don't. You can write your own class loaders if you want and can
implement any loading behavior you think is suitable.

Beware that class-reloading introduces all kinds of behavior that is
counter-intuitive. Multiple instances of what appears to be the same
static variable thereby also breaking singleton patterns in your code
(which are broken already, but that is a completely different matter) is
only one example.

Please start by telling us what the real problem is that you need to
solve. Going the class-loader way is probably among the worst solutions
for your problem.

I wrote a gui based unit testing framework and it loads the top level
test suites from a text box with there names in it (initially
populated from command line)... I do not want to have to close/reopen
the app when I rewrite/recompile some code under test.


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