[4198] in java-interest
Re: (Fwd) More class loader info.
daemon@ATHENA.MIT.EDU (neudeck.arthur@ch.swissbank.com)
Wed Dec 13 11:30:47 1995
From: neudeck.arthur@ch.swissbank.com
Date: Wed, 13 Dec 1995 15:33:56 +0100
To: pambrose@weblogic.com
Cc: neudeca@ch.swissbank.com, java-interest@java.sun.com
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Hoi ...
> Here is a routine that I use to plugins for a server:
>
> public synchronized Class loadClass(String name, boolean resolve)
> throws ClassNotFoundException
> {
> try {
> System.out.println("PluginLoader loading: " + name);
>
> if (name.startsWith("weblogic.") ||
> name.startsWith("java.") ||
> name.startsWith("sun."))
> return findSystemClass(name);
>
> if (PluginLoader.classes.containsKey(name)) {
> System.out.println("Using cached value " + name);
> return (Class)PluginLoader.classes.get(name);
> }
>
> byte data[] = loadClassFromFile(name);
>
> Class cl = defineClass(data, 0, data.length);
>
> resolveClass(cl);
>
> PluginLoader.classes.put(name, cl);
> return cl;
> }
> catch (Exception e)
> throw new ClassNotFoundException(e.getClass().getName() + " - " + e.getMessage());
> }
> }
>
> You can see I am also saving the defs in a hashtable.
> If your code is doing all the above, then the only other
> thing I can think of is: do you have a null constructor
> explicitly defined for the class you are loading.
> Also, do you have the correct constructors for each of the
> parent classes of the lowest-level object being
> instantiated?
>
> Paul
Sorry for replying that late, but I had to try and try and try and ....
Fortunatley my research has come to a positive end: My own ClassLoader is
doing his work almost fine.
There have been several problems, that I discovered:
1.) You'll get an "IllegalAccessError" whenever you try to instantiate a non
"public class". Why is it that way? Don't ask me ....
2.) A big problem still existing is the following matter of fact:
A oncely loaded base class within your own ClassLoader will never be found
from the system class loader. You'll get a "ClassDefNotFoundError" or anything
else for this class.
Probable solutions: Perhaps you can set your own ClassLoader as the default
one that covers the default one and more.
But how???
If you or somebody else did find out any solutions or had to add something, you could
write me. I will collect any info and summarize.
Thanx again for your help.
Have a nice day,
Arthy
BTW: The final result for all those people who have
the same problems as I have/ had is attached.
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import java.util.Hashtable;
import java.io.File;
import java.io.IOException;
import java.io.FileNotFoundException;
import java.io.FileInputStream;
import java.util.Vector;
/* ******************************************************************** */
/* Classname: ManagerClassLoader */
/* ******************************************************************** */
/**
* This class extends the default mechanism for loading classes. This is
* is done via three different ways: First a cache will be searched wether
* the class already has been loaded or not. Second the default mechanism
* in the java environment will be used. If this didn't succeed, the own
* extension would be invoked. This extension is represented by a loading
* method that looks at a certain directory on the local file system.
*
* @version: 0.1, 27. Oct 1995
* @version: 0.2, 07. Dez 1995
* @version: 0.3, 13. Dez 1995
* @author: Arthur Neudeck, SYSTOR AG
*/
/* ******************************************************************** */
/* Created: 10/27/95 */
/* ******************************************************************** */
public final class ManagerClassLoader extends ClassLoader{
// ---------- -
// attributes -
// ---------- -
private TOLogging loggingManager; // default error out
private Hashtable classCache; // avoids double load of
// one class (=cache)
private Hashtable collocatedClasses; // (not used yet)
/* -------------------------------------------------------------------- */
/* Method: ManagerClassLoader() */
/* -------------------------------------------------------------------- */
/**
* default constructor.
*
* @param inLoggingManager default output for warnings,
* errors, infos and anything else
*/
/* -------------------------------------------------------------------- */
public ManagerClassLoader(TOLogging inLoggingManager){
super();
loggingManager = inLoggingManager;
classCache = new Hashtable();
collocatedClasses = new Hashtable();
}
/* -------------------------------------------------------------------- */
/* Method: loadClassDataFromFile() */
/* -------------------------------------------------------------------- */
/**
* this method loads the binary code of a class from the local file
* system. It looks at the "../to" directory.
*
* @param inClassName real name of the class
* @return the binary code as a an array of bytes
*/
/* -------------------------------------------------------------------- */
private byte[] loadClassDataFromFile(String inClassName)
throws FileNotFoundException{
// ---------------- -
// local attributes -
// ---------------- -
byte classData[];
long dataSize;
File classFile = new File("../to/"+inClassName+".class");
// ------------------- -
// file does not exist -
// ------------------- -
if(classFile.exists()==false) throw new FileNotFoundException();
// ---------------------------------------- -
// open/close file and read the byte stream -
// ---------------------------------------- -
try{
FileInputStream fileStream = new FileInputStream(inClassName);
classData = new byte[fileStream.available()];
fileStream.read(classData);
fileStream.close();
return classData;
// ---------- -
// read error -
// ---------- -
} catch (IOException readError){
loggingManager.logError(this,"Couldn't read data from <"
+inClassName+">");
return null;
}
}
/* -------------------------------------------------------------------- */
/* Method: loadClass() */
/* -------------------------------------------------------------------- */
/**
* method that has to be implemented because of abstract method in
* super class "ClassLoader". It returns the executable class.
*
* @param inClassName real name of the class
* @param resolve should super and contained classes
* immediately be loaded by this loader?
* @return the instantiateable class
*/
/* -------------------------------------------------------------------- */
public synchronized Class loadClass( String inClassName,
boolean resolve){
// ------------------------ -
// get the class from cache -
// ------------------------ -
Class newClass = (Class)classCache.get(inClassName);
// ------------------------------------------------------------ -
// load the class via default mechanism within java environment -
// ------------------------------------------------------------ -
loggingManager.logInfo("Loading object <"+className+">");
if( newClass == null){
try{
newClass = findSystemClass(inClassName);
classCache.put(inClassName,newClass);
className = new String(inClassName);
System.out.println("Default:"+inClassName);
// class not found will be unhandled because later the -
// own method will be called -
// ---------------------------------------------------- -
} catch (ClassNotFoundException noClassDefFound);
}//if
// --------------------------------- -
// load the class via own mechanism -
// --------------------------------- -
if(newClass == null){
try{
byte data[] = loadClassDataFromFile(inClassName);
newClass = defineClass(data,0,data.length);
classCache.put(inClassName,newClass);
if(resolve == true) resolveClass(newClass);
System.out.println("Mine:"+className);
}catch (FileNotFoundException e){
loggingManager.logError(this,"<"+inClassName+
"> is not a class ...");
// class that can not be loaded with this loader -
// --------------------------------------------- -
} catch (ClassFormatError unknownFormat){
loggingManager.logError(this,"<"+inClassName+
"> is not a class ...");
// ---------------------------- -
// catch any uncaught exception -
// ---------------------------- -
} catch (Exception anyException){
System.out.println(anyException);
}//catch
}//if
// ----------------------------------------------- -
// return either null or the instantiateable class -
// ----------------------------------------------- -
return newClass;
}
}
/* *** */
/* END */
/* *** */
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