Re: Class loading and the new keyword.

 chrislewis <>
Fri, 06 Jul 2007 03:04:05 -0000
On Jul 5, 4:23 pm, Roedy Green <>

On Thu, 05 Jul 2007 14:43:16 -0000, chrislewis
<> wrote, quoted or indirectly quoted someone
who said :

I read through the pdf and while helpful it didn't really touch on my
specific misunderstandings. Particularly, where/how the runtime finds
classes requested via new, and then if classes loaded by a child class
loader are at all available to classes loaded by the parent. I dont
think that part is true, but I'm seeking confirmation.


Read up on how you write your own, and look at the how the default one

A ClassLoader can get the text of the java class file in any way it
pleases, e.g. from a database, file, compose it on the fly, from jars
... It can have dynamic classpaths, or none. Exactly how it finds
the class bytes is totally up to it.
It then feeds the byte array to a standard method
java.lang.ClassLoader.defineClass that turns it into internal machine
Roedy Green Canadian Mind Products
The Java Glossary

Thanks for the input. I understand the idea behind a custom class
loader and defineClass. As an example I'm writing a simple test case
that, given a jar file on the cmd line, will load all .class files
into the virtual machine. The idea is basically:

1) open jar
2) enumerate jar entries
3) for each non-directory entry that ends in .class, load it

My test loader overrides no methods, and provides an addJar method for
adding jar files. addjar calls loadJarClasses:

    protected void loadJarClasses(JarFile jf) {
        List<String> loaded = new ArrayList<String>();
        List<String> notLoaded = new ArrayList<String>();
        for(Enumeration<JarEntry> jEntries = jf.entries();
jEntries.hasMoreElements();) {
            JarEntry je = jEntries.nextElement();
            String jeName = je.getName();
            if(! je.isDirectory() && jeName.endsWith(".class")) {
                String fullName = jeName.substring(0, jeName.length() -
".class".length()).replace('/', '.');
                try {
                    InputStream in = jf.getInputStream(je);
                    byte[] cbytes = new byte[ (int)je.getSize() /*in.available()*/];
                    int bread =;
                    defineClass(fullName, cbytes, 0, cbytes.length);
                } catch (Throwable e) {

        for(String s : notLoaded) {
            System.out.println("JarClassLoader.loadJarClasses() -- " + s);

        System.out.println("\nJarClassLoader.loadJarClasses() -- loaded " +
loaded.size() + ", failed on " + notLoaded.size());

You'll notice i've added some debugging. The method is supposed to
load all classes in a jar when called, but as it happens, only some
get loaded. For example, when I supply js-1.6R5.jar as the jar to load
(mozilla rhino's jar), 188 classes load and 64 fail to load, with
mysterious java.lang.NoClassDefFoundError errors. The errors provide
no message save for the path name of the class and a stack trace. Any
idea as to what may be going on? I realize my example is simplistic
(currently thats the point), but I may be overlooking something.
There's really no other code that does anything useful so I didn't
include the rest.

thanks again for your input

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