I recently study <<Jboss Administration and Development 3.2>> about classloader.I change the org.jboss.chap2.ex0.ExCtx class as below:
public class ExCtx
{
public static void main(String[] args) throws Exception
{
String filepath="D:/java_study/jboss_040811/j0.jar";

File jar0 = new File(filepath);
URL[] cp0 = {jar0.toURL()};
URLClassLoader ucl0 = new URLClassLoader(cp0);
Thread.currentThread().setContextClassLoader(ucl0);
Class objClass = ucl0.loadClass("org.jboss.chap2.ex0.ExObj");
Object value = objClass.newInstance();

// Load ExCtx from j0.jar using URLClassLoader instance 1
File jar1 = new File(filepath);
URL[] cp1 = {jar1.toURL()};
URLClassLoader ucl1 = new URLClassLoader(cp1);
Thread.currentThread().setContextClassLoader(ucl1);
Class ctxClass2 = ucl1.loadClass("org.jboss.chap2.ex0.ExCtx");
Object ctx2 = ctxClass2.newInstance();

try
{
// Invoke ExCtx[UCL1].useValue(value=ExObj[UCL0]) via reflection
Class[] types = {Object.class};
Method useValue = ctxClass2.getMethod("useValue", types);
Object[] margs = {value};
useValue.invoke(ctx2, margs);
}
catch(Exception e)
{

throw e;
}
}
}

and change the org.jboss.chap2.ex0.ExCtx class as below:
public class ExCtx
{
ExObj value;

public ExCtx() throws IOException
{
value = new ExObj();
}
public Object getValue()
{
return value;
}
public void useValue(Object obj) throws Exception
{
System.out.println("********************");

ExObj ex = (ExObj) obj;
}
void pkgUseValue(Object obj) throws Exception
{

}
}

other classes i don't change.when I run the program,I think the running program whill throw some exceptions,but the program run right and donesn't throw exception.why?