Question

I have a legacy (not modifiable :-() code, that have a static initializer block.
I would like to create a subclass – but without running the static block.

Is there any way doing this? For example: class.ForName method has a second argument which is responsibe for initializing the class – but sadly, it doesn’t work for me (probably means something else ..): http://docs.oracle.com/javase/6/docs/api/java/lang/Class.html#forName(java.lang.String, boolean, java.lang.ClassLoader)

UPDATE: the above mentioned class.forName does not trigget initialization – but asking for a newInstance does 🙁

Thanks,
krisy

Answer 1

You can always patch the old class with ASM. Generating a new class from the old bytecode by ignoring the clinit block should be easy.

import org.objectweb.asm.*;
import org.objectweb.asm.commons.EmptyVisitor;
import java.io.*;

public class ClinitKiller {
    public static void main (String[] args) {
        final InputStream input = ClinitKiller.class.getResourceAsStream(Test.class.getName() + ".class");
        try {
            final byte[] bytes = instrument(input);
            FileOutputStream out = new FileOutputStream("/tmp/Test.class");
            out.write(bytes);
            out.close();
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static byte[] instrument(InputStream is) throws IOException {
        ClassReader reader = new ClassReader(is);
        ClassWriter writer = new ClassWriter(0);
        ClinitKillerClassAdapter classAdapter = new ClinitKillerClassAdapter(writer);
        reader.accept(classAdapter, 0);
        return writer.toByteArray();
    }
}

class ClinitKillerClassAdapter extends ClassAdapter {
    public ClinitKillerClassAdapter(final ClassVisitor cv) {
        super(cv);
    }

    public MethodVisitor visitMethod(final int access, final String name, final String desc, final String signature, final String[] exceptions) {
        if (name.equals("<clinit>")) {
            return new EmptyVisitor();
        }
        return cv.visitMethod(access, name, desc, signature, exceptions);
    }
}

Here is the before and after for the following class:

public class Test {
    private static final String value;
    static {
        System.out.println("Test static");
        value = "test value";
    }
    public static void main(String[] args) {
        System.out.println(value);
    }
}

Before:

javap -c Test
Compiled from "Test.java"
public class Test extends java.lang.Object{
    public Test();
    Code:
    0:  aload_0
    1:  invokespecial   #1; //Method java/lang/Object."<init>":()V
    4:  return

    public static void main(java.lang.String[]);
    Code:
    0:  getstatic   #2; //Field java/lang/System.out:Ljava/io/PrintStream;
    3:  getstatic   #3; //Field value:Ljava/lang/String;
    6:  invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
    9:  return

    static {};
    Code:
    0:  getstatic   #2; //Field java/lang/System.out:Ljava/io/PrintStream;
    3:  ldc #5; //String Test static
    5:  invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
    8:  ldc #6; //String test value
    10: putstatic   #3; //Field value:Ljava/lang/String;
    13: return

}

Output:
Test static
test value

After:

javap -c Test
Compiled from "Test.java"
public class Test extends java.lang.Object{
    public Test();
    Code:
    0:  aload_0
    1:  invokespecial   #11; //Method java/lang/Object."<init>":()V
    4:  return

    public static void main(java.lang.String[]);
    Code:
    0:  getstatic   #21; //Field java/lang/System.out:Ljava/io/PrintStream;
    3:  getstatic   #23; //Field value:Ljava/lang/String;
    6:  invokevirtual   #29; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
    9:  return

}

Output:
null