Re: Trying to change value of static final field

I believe the compiler is allowed to propagate static finals. Check that the
original bytecode actually accesses the field via GETSTATIC (rather than
directly using its value via LDC #....)

 - Godmar

On Tue, Feb 23, 2010 at 12:40 AM, <[email protected]> wrote:

> Hi,
>
> I'm trying to change the value of a static final field in a class using a
> java
> agent that uses ASM.
>
> In the code being changed, if I access the field using reflection then the
> value has changed, but if I access the field using the variable name then
> it
> still has the old value.
>
> An example of some output where I've changed the value of a static final
> String
> is as follows:
>
> Value of the static field is: Initial value for static field
> Accessed via reflection, field name is: str With Value: Modified value for
> static field
>
> Am I doing something wrong when I'm trying to change the field value? Here
> are
> some code samples to show what I'm doing.
>
> The class that is being modified is as follows:
>
> package test.javaagent;
>
> import java.lang.reflect.Field;
>
> import org.junit.Test;
>
> import junit.framework.TestCase;
>
> public class ClassWithStaticField extends TestCase {
>        private final static String str = "Initial value for static field";
>
>        @Test
>        public void test() throws IllegalArgumentException,
> IllegalAccessException {
>                ClassWithStaticField.printValue();
>        }
>
>        public static void printValue() throws IllegalArgumentException,
>                        IllegalAccessException {
>                System.out.println("Value of the static field is: " + str);
>
>                Field[] fields =
> ClassWithStaticField.class.getDeclaredFields();
>                for (Field f : fields) {
>                        System.out.println("Accessed via reflection, field
> name
> is: " + f.getName()
>                                        + " With Value: " +
> f.get(ClassWithStaticField.class));
>                }
>        }
> }
>
>
>
> The classes making up the Java Agent and Class Adapters etc:
>
> package test.javaagent;
>
> import java.lang.instrument.ClassFileTransformer;
> import java.lang.instrument.IllegalClassFormatException;
> import java.lang.instrument.Instrumentation;
> import java.security.ProtectionDomain;
>
> import org.objectweb.asm.ClassAdapter;
> import org.objectweb.asm.ClassReader;
> import org.objectweb.asm.ClassVisitor;
> import org.objectweb.asm.ClassWriter;
> import org.objectweb.asm.FieldVisitor;
>
> /*
>  * Java Agent entry
>  */
> public class ModifyFieldExample {
>        public static void premain(String agentArgs, Instrumentation inst) {
>                inst.addTransformer(new MyClassTransformer());
>        }
> }
>
> class MyClassTransformer implements ClassFileTransformer {
>
>        public byte[] transform(ClassLoader arg0, String arg1, Class<?>
> arg2,
>                        ProtectionDomain arg3, byte[] arg4)
>                        throws IllegalClassFormatException {
>                if (arg1.endsWith("ClassWithStaticField")) {
>                        byte[] transformClass = arg4.clone();
>                        ClassReader cr = new ClassReader(transformClass);
>                        ClassWriter cw = new ClassWriter(cr, 0);
>
>                        MyClassAdapter ca = new MyClassAdapter(cw);
>
>                        cr.accept(ca, 0);
>
>                        byte[] transformedBytes = cw.toByteArray();
>                        return transformedBytes;
>                }
>                return null;
>        }
>
> }
>
> class MyClassAdapter extends ClassAdapter {
>
>        public MyClassAdapter(ClassVisitor arg0) {
>                super(arg0);
>        }
>
>        @Override
>        public FieldVisitor visitField(int arg0, String arg1, String arg2,
>                        String arg3, Object arg4) {
>                if (arg4.equals("Initial value for static field")) {
>                        return cv.visitField(arg0, arg1, arg2, arg3,
>                                        "Modified value for static field");
>                }
>                return cv.visitField(arg0, arg1, arg2, arg3, arg4);
>        }
>
> }
>
> Thanks in advance.
>
> Cristy
>
>
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