How to create class with final static fields in any byte code library?

Question

I'm trying to create a simple class that contains static final object fields, using any byte code library. I have tried BCEL and Byte Buddy but had no success. The class I want to construct looks like this. Thanks.

public class ConstructedClass{

   public static final MyClass a = new MyClass();

   public static final MyClass b = new MyClass(); 
}

My attempt with BCEL:

ClassGen classGen=new ClassGen("org.test.lib.core", "java.lang.Object","core.java", Const.ACC_PUBLIC, null);
classGen.addEmptyConstructor(Const.ACC_PUBLIC); 

ConstantPoolGen constantPoolGen=classGen.getConstantPool();
int access_flags = Const.ACC_PUBLIC | Const.ACC_STATIC | Const.ACC_FINAL; 
final FieldGen FieldGen=new FieldGen( access_flags,Type.getType(Property.class), "test", constantPoolGen);
//FieldGen.setInitValue(new MyClass());

My second attempt also with BCEL:

private static final Type[] arg =  {Type.getType(MyClass.class)};
InstructionList init = new InstructionList();
InstructionFactory factory=new InstructionFactory(classGen);
//init.append(new PUSH(constantPoolGen, new MyClass())); 
init.append(factory.createInvoke(MyClass.class.getName(), "valueOf", 
             Type.getType(MyClass.class), arg, Const.INVOKESTATIC)); 
init.append(factory.createPutStatic("org.test.lib.core", "test", Type.getType(Property.class)));

The commented lines is where pushing my object didn't work.

I added the attempts, but with bytebuddy I only searched documentations and didn't write actual code. — gondor89
– gondor89, Commented Dec 19, 2017 at 13:10
I think the main problem is how to cast MyClass to ObjectType. Because The commented methods take only ObjectType, so where to insert MyClass object? — gondor89
– gondor89, Commented Dec 19, 2017 at 13:23
You can’t push MyClass objects. How should that work? Storing a MyClass instance inside a class file? You can generate code that will create a new MyClass instance and assign it to the field. That may be the equivalent of new MyClass() or an invocation of a factory method like MyClass.valueOf(), but when your factory method requires an existing instance as argument, you have a chicken-and-egg problem. — Holger
– Holger, Commented Dec 19, 2017 at 18:47

kaos · Accepted Answer · 2018-01-16 14:48:44Z

2

With ByteBuddy You can generate a static initialization block using ByteCodeAppender. This will result in a little different class then You wanted but i think close enough:

public class ConstructedClass {

   public static final MyClass a;

   static {
        a = new MyClass();
   } 
}

Generation code:

public class ByteBuddyTest {

    public static void main(String[] args) throws Exception {
        DynamicType.Loaded loaded =
            new ByteBuddy()
            .subclass(Object.class)
                .initializer( new ByteCodeAppender() {
                    @Override public Size apply(MethodVisitor methodVisitor, Implementation.Context implementationContext,
                                                MethodDescription instrumentedMethod) {
                        StackManipulation.Size size = new StackManipulation.Compound(
                            TypeCreation.of(new TypeDescription.ForLoadedType(MyClass.class)),
                            Duplication.SINGLE,
                            MethodInvocation.invoke(new TypeDescription.ForLoadedType(MyClass.class).getDeclaredMethods().filter(ElementMatchers.isDefaultConstructor()).getOnly()),
                            FieldAccess.forField(implementationContext.getInstrumentedType().getDeclaredFields().filter(ElementMatchers.named("a")).getOnly()).write()
                        ).apply(methodVisitor, implementationContext);

                        return new Size(size.getMaximalSize(), instrumentedMethod.getStackSize());
                    }
                })
            .name("org.test.lib.core.ConstructedClass")
            .modifiers(Opcodes.ACC_PUBLIC)
            .defineField("a", MyClass.class, Opcodes.ACC_PUBLIC | Opcodes.ACC_STATIC | Opcodes.ACC_FINAL)
            .make()
            .load(ByteBuddyTest.class.getClassLoader(), ClassLoadingStrategy.Default.INJECTION);

        Object obj = loaded.getLoaded().getConstructor().newInstance();
        System.out.println(obj.getClass().getField("a").get(obj));
    }

    public static class MyClass {
        public MyClass(String arg) {}

        public static MyClass createMyClass(String arg) {
            return new MyClass(arg);
        }
    }
}

Update for comment

To call a static factory method instead of constructor You just need to replace constructor call:

StackManipulation.Size size = new StackManipulation.Compound(
    new TextConstant("test"),
    MethodInvocation.invoke(new TypeDescription.ForLoadedType(MyClass.class).getDeclaredMethods().filter(ElementMatchers.named("createMyClass")).getOnly()),
         FieldAccess.forField(implementationContext.getInstrumentedType().getDeclaredFields().filter(ElementMatchers.named("a")).getOnly()).write()
).apply(methodVisitor, implementationContext);

Multiple fields

    DynamicType.Builder builder = new ByteBuddy().subclass(Object.class);
    List<String> fields = Lists.newArrayList("a", "b", "c");

    for (String str : fields) {
        builder = builder.defineField(str, MyClass.class, Opcodes.ACC_PUBLIC | Opcodes.ACC_STATIC | Opcodes.ACC_FINAL);
    }

    DynamicType.Loaded loaded = builder.make().load(ByteBuddyTest.class.getClassLoader(), ClassLoadingStrategy.Default.INJECTION);

    Object obj = loaded.getLoaded().getConstructor().newInstance();
    System.out.println(obj.getClass().getField("a"));
    System.out.println(obj.getClass().getField("c"));

edited Jan 16, 2018 at 14:48

answered Dec 27, 2017 at 22:55

kaos

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6 Comments

gondor89 Over a year ago

Thanks a lot. Can I initialize a with sth like this: a=B.createBClass("arg");

kaos Over a year ago

Yes You can, is quite simple if the argument can be a compile time constant. I updated the answer.

gondor89 Over a year ago

Many thanks you made my day, I've been searching for a proper docs for the past two days.

kaos Over a year ago

I found ByteBuddy unit tests a good place to search for examples not covered by docs.

kaos Over a year ago

I updated the answer, basically You don't have to chain all builder methods in one go.

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