Replacing inititial value of static final fields

[Matthias Nuessler <m....@web.de>]

Hi,

I was trying to use BCEL to replace the initial value of static final fields of existing classes. It seemed to work fine, but when the fields are referenced from other classes they seem to contain the old values, eventhough the values in the changed class itself are updated to the new value. Is there any way using BCEL to change these constant values in that way? Maybe I'm not aware of certain problems regarding the constant pool?

Here is what I did. Let's say, in a class named "Hello" I have a field like this:

public static final String MESSAGE = "Hello";

Now I want to read in the class file and change the initial value:

ClassParser parser = new ClassParser("Hello.class");
JavaClass javaClass = parser.parse();
ClassGen cg = new ClassGen(javaClass);
ConstantPool cp = javaClass.getConstantPool();
ConstantPoolGen pg = new ConstantPoolGen(cp);

Field[] fields = classGen.getFields();
for (int i=0; i<fields.length; i++) {
if (fields[i].getName().equals("MESSAGE")) {
FieldGen fg = new FieldGen(ACC_PUBLIC | ACC_STATIC | ACC_FINAL, Type.STRING, "MESSAGE", pg)
fg.setInitialValue("New Message");
cg.removeField(fields[i]);
cg.addField(fg.getFielD());
}
}
JavaClass changedClass = cg.getJavaClass();
changedClass.setConstantPool(pg.getFinalConstantPool());
changedClass.dump("Hello_.class");

Thanks
Matt
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Re: Replacing inititial value of static final fields

[Andrew Huntwork <as...@huntwork.net>]

static final values tend to get inlined where they are used by the
compiler.  so:

class Foo {
static final int BAR = 4;
int getBar() { return BAR; }
}

gets compiled to bytecode equivalent to

class Foo {
static final BAR = 4;
int getBar() { return 4; }
}

(specifically:
int getBar();
  Signature: ()I
  Code:
   0:   iconst_4
   1:   ireturn
)

so you're completely screwed.  there's no way to distinguish the inlined 4
from any other 4 that might have appeared literally in the source code.

On 12/13/05, Matthias Nuessler <m....@web.de> wrote:
>
> Hi,
>
> I was trying to use BCEL to replace the initial value of static final
> fields of existing classes. It seemed to work fine, but when the fields are
> referenced from other classes they seem to contain the old values,
> eventhough the values in the changed class itself are updated to the new
> value. Is there any way using BCEL to change these constant values in that
> way? Maybe I'm not aware of certain problems regarding the constant pool?
>
> Here is what I did. Let's say, in a class named "Hello" I have a field
> like this:
>
> public static final String MESSAGE = "Hello";
>
> Now I want to read in the class file and change the initial value:
>
> ClassParser parser = new ClassParser("Hello.class");
> JavaClass javaClass = parser.parse();
> ClassGen cg = new ClassGen(javaClass);
> ConstantPool cp = javaClass.getConstantPool();
> ConstantPoolGen pg = new ConstantPoolGen(cp);
>
> Field[] fields = classGen.getFields();
> for (int i=0; i<fields.length; i++) {
> if (fields[i].getName().equals("MESSAGE")) {
> FieldGen fg = new FieldGen(ACC_PUBLIC | ACC_STATIC | ACC_FINAL,
> Type.STRING, "MESSAGE", pg)
> fg.setInitialValue("New Message");
> cg.removeField(fields[i]);
> cg.addField(fg.getFielD());
> }
> }
> JavaClass changedClass = cg.getJavaClass();
> changedClass.setConstantPool(pg.getFinalConstantPool());
> changedClass.dump("Hello_.class");
>
> Thanks
> Matt
> ______________________________________________________________
> Verschicken Sie romantische, coole und witzige Bilder per SMS!
> Jetzt bei WEB.DE FreeMail: http://f.web.de/?mc=021193
>
>
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