New instance of a modified class

[jp...@free.fr]

How to obtain a new instance of a class which was modified by BCEL?

When I execute this code, a exception "java.lang.NoSuchFieldError" appears.

The goal of  this code is to remove the field "libelle".

Have you an idea ?

thanks.


    ClassGen clazz =new ClassGen(Repository.lookupClass(GenerateurVapeur.class));

    // Get the field "libelle".
    Field prpReelle =  clazz.containsField("libelle");

    // Remove the field.	
    if(prpReelle!=null)
         clazz.removeField(prpReelle);
		
    JavaClass classeJava = clazz.getJavaClass();

    try{

      // Save the class
      dumpClass(classeJava,GenerateurVapeur.class);

      org.apache.bcel.util.ClassLoader cl = new
org.apache.bcel.util.ClassLoader(ClassLoader.getSystemClassLoader());

      Class newClass =  cl.loadClass(GenerateurVapeur.class.getName());
      
       // Try to get a new instance of "GenerateurVapeur"	
      GenerateurVapeur newInstance = (GenerateurVapeur)newClass.newInstance();

    }catch(Exception exp){
      exp.printStackTrace();
    }


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Re: New instance of a modified class

[Stephen Kolaroff <ch...@riflexo.com>]

Hi
Unfortunately, you have to override some methods of BCEL's class loader 
to get actually class instrumentation/generation on the fly. Even so, 
because of the JLS (or JVM spec, I am not sure right now), you can not 
have different byte code for classes with same name loaded with the same 
class loader. I am affraid, beacuse of this you can never execute:

      GenerateurVapeur newInstance = (GenerateurVapeur)newClass.newInstance();

in your code without ClassCastException (newClass' instances will be 
thought as different, unassignable to GenerateurVapeur; GenerateurVapeur 
already loaded by your context classloader several lines above). I must 
confess, I have never tried this, so if you have success, pls, inform me.

What I am doing when have to do what you want:
1. Obtain JavaClass from a repository without using any Class-es, just 
String names.
2. Mess with the class, and make it implement a useful interface.
3. Dump the class into a fake classloader or even on the disk.
4. Load the class and typecast to the useful interface
5. enjoy


jpicard@free.fr wrote:

>How to obtain a new instance of a class which was modified by BCEL?
>
>When I execute this code, a exception "java.lang.NoSuchFieldError" appears.
>
>The goal of  this code is to remove the field "libelle".
>
>Have you an idea ?
>
>thanks.
>
>
>    ClassGen clazz =new ClassGen(Repository.lookupClass(GenerateurVapeur.class));
>
>    // Get the field "libelle".
>    Field prpReelle =  clazz.containsField("libelle");
>
>    // Remove the field.	
>    if(prpReelle!=null)
>         clazz.removeField(prpReelle);
>		
>    JavaClass classeJava = clazz.getJavaClass();
>
>    try{
>
>      // Save the class
>      dumpClass(classeJava,GenerateurVapeur.class);
>
>      org.apache.bcel.util.ClassLoader cl = new
>org.apache.bcel.util.ClassLoader(ClassLoader.getSystemClassLoader());
>
>      Class newClass =  cl.loadClass(GenerateurVapeur.class.getName());
>      
>       // Try to get a new instance of "GenerateurVapeur"	
>      GenerateurVapeur newInstance = (GenerateurVapeur)newClass.newInstance();
>
>    }catch(Exception exp){
>      exp.printStackTrace();
>    }
>
>
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>To unsubscribe, e-mail: bcel-user-unsubscribe@jakarta.apache.org
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>  
>



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