Name: yyT116575			Date: 11/13/2000


Linux:
java version "1.3.0"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.0)
Java HotSpot(TM) Client VM (build 1.3.0, mixed mode)

AND

Linux:
java version "1.2.2"
Classic VM (build 1.2.2_006, green threads, nojit)

AND

Windows98:
java version "1.2"
Classic VM (build JDK-1.2-V, native threads)


I'm trying do dynamic updates of classes using an explicit URLClassLoader, but
if I put the classes in a jar file, the updates aren't picked up correctly.  If
I use a directory instead it works fine.  Using the Unix tool fuser I can see
that the JVM keeps the jar file open.  This is not the behavior I expect when
I create a URLClassLoader.


SOURCE:

// Interface for changable implementation.
public interface cInterface {
    public void doStuff();
}

// Implementation
public class cImpl implements cInterface {
    public void doStuff() {
        System.out.println("\t\tdoStuff!");
        //doMoreStuff();
        //doYetMoreStuff();
    }
    /*
    public void doMoreStuff() {
        System.out.println("\t\tdoMoreStuff!");
    }
    public void doYetMoreStuff() {
        System.out.println("\t\tdoYetMoreStuff!");
    }
    */
}

// Client program
import java.io.*;
import java.net.*;

public class jarTest {
    public static void main( String[] argv ) throws Exception {
        URL[] auJars;
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String s;

        while(true) {
            System.out.println("Hit enter.");
            s = br.readLine();

            try {
                auJars = new URL[1];
                auJars[0]=new URL("file:test.jar");
                            
                ClassLoader cl = new java.net.URLClassLoader(auJars);
                System.out.println("new Classloader:"+cl);

                cInterface ci = (cInterface)
                cl.loadClass("cImpl").newInstance();
                ci.doStuff();
            } catch( Exception e ) {
                e.printStackTrace();
            }
        }
    }
}

Procedure to reproduce:
$ javac *.java
$ jar cvf test.jar *.class
$ rm cImpl.class
$ java jarTest
Hit enter.

new Classloader:java.net.URLClassLoader@c3579d5b
                doStuff!
Hit enter.

Open another window and perform the following steps
in the same directory:
1.uncomment other functions in cImpl.java
2.$ javac cImpl.java ; jar cvf test.jar *.class; rm cImpl.class 

Switch back to the original window without restarting
jarTest and hit enter, a exception thrown:

new Classloader:java.net.URLClassLoader@caef9d5b
Exception in thread "main" java.lang.ClassFormatError: cImpl (Truncated class file)
        at java.lang.ClassLoader.defineClass0(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:474)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:106)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:247)
        at java.net.URLClassLoader.access$1(URLClassLoader.java:215)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:196)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:295)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
        at jarTest.main(jarTest.java:26)

The JDC Tech Tip:

http://developer.java.sun.com/developer/TechTips/2000/tt1027.html

...shows the general dynamic class update method.  What I found is simply
that it doesn't work with jars.

Jim

(Review ID: 112059) 
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