[1178] in java-interest
Re: help with class loader
daemon@ATHENA.MIT.EDU (Chuck McManis)
Fri Aug 25 15:01:35 1995
Date: Fri, 25 Aug 1995 09:07:21 -0700
From: cmcmanis@scndprsn.Eng.Sun.COM (Chuck McManis)
To: ekim@nyquist.bellcore.com
Cc: java-interest@java.Eng.Sun.COM, jkanerva@scndprsn.Eng.Sun.COM
Class loader secrets:
#1) After calling defineClass() you _must_ call resolveClass() before you
hand the class back to the person loading it.
#2) You must use the system versions of class Class and Object or it won't
be able to cast your objects into something runnable. Typical class loader
flow is:
loadClass(String name, boolean resolve) {
Class c = Class.forName(name);
if (c == null) {
c = (Class)(classHashTable.get(name));
if (c != null)
return c;
}
... load class some other way ...
c = defineClass(...);
if (resolve)
resolveClass(c)
classHashTable.put(name, c);
return c;
}
#3) To invoke this new class you need to define an interface loaded on the
"system" side of the class loader (for example
public interface fooable {
void foo();
}
and then cast your new object to a fooable and invoke foo. like so:
c = tstLoader.loadClass("my.class", true);
Object x = c.newInstance();
((fooable) c).foo();
Of course you can do this in only one line:
(fooable)(c.newInstance()).foo();
#4) Classes loaded through your class loader will be run through the code
verifier. (This is a good way to see how good the code verifier is!)
That also means that if you compile them with the -O switch they may not
verify (the compiler inlines accesses to private data which the verifier
will complain about).
That's about it. Once you've got a working class loader you have to be aware however
that a lot of the system classes key off the fact that there is a class loader on
the stack and assume you are an applet. So if your newly loaded class tries to open
a file for example it will be constrained to the read path and write path that HotJava
uses. This will change somewhat in Beta.
--Chuck
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