Source Line Numbers from class files

Hi,
  I am working on getting the source code line numbers
from class files using BCEL. 

ConstantPool constants  =
java_class.getConstantPool();
ConstantPoolGen cp = new ConstantPoolGen(constants);

MethodGen methodGen = new MethodGen(method,
java_class.getClassName(), cp);
Instruction [] instructions =
methodGen.getInstructionList().getInstructions();
LineNumberGen[] aLineNumberGen =
methodGen.getLineNumbers();

I intend to use getSourceLine() API of LineNumberGen
class. But the array aLineNumberGen is returned of 0
length. Is there any setting of the compiler that will
save the line numbers in the class files, because of
which it is not working right now.

Thanks,
   Nikhil

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