创建对象时如何处理注释和设置默认值

Question

I wanted to write code, which looks like below.

MyClass a = new @MyAnnotation MyClass()

Assume MyClass method look like below

   public class MyClass {

    int a;

    public void setValue(int a) {
        this.a = a;
    }

}

So when some one creats object of MyClass with the annotation, it should set a default value to some x(say 20).

In checker framework, it is allowing us to annotate in the code as below.

MyClass a = new @MyAnnotation MyClass()

Does any one know how can we get split this into two parts at the time of compiling like below?

   MyClass a = new MyClass();
   a.setValue(20);

or if we can able to pass defauilt value like below is also fine.

MyClass a = new @MyAnnotation(value=20) MyClass()

Can this be achieved using annotation processor/ need to use bytecode processor like ASM to get it done? Updated with more meaningful way.

Answer 1

Are you writing a custom type-system via meta-annotations? That is, are you using the SubtypingChecker or are have you created your own subtype of BaseTypeChecker?

The short answer is, with meta-annotations you cannot specify this sort of behavior. The long answer is, if you have a custom checker, then there are a number of ways you can do this.

Keep in mind, however, that writing a custom checker is a fair bit more complicated then using meta-annotations. But, if you have a custom checker, you can do one of the following:

Option A: Polymorphic Constructors Steps:

1. Create an @PolyMyAnnotation polymorphic annotation, see Polymorphic Qualifiers
2. Create a PolyMorphic constructor for MyClass, eg @PolyMyAnnotation MyClass(@PolyMyAnnotation int a) {}
3. Create a TreeAnnotator that adds @MyAnnotation to the type's of int literals. See RegexTreeAnnotator.visitLiteral for an example.
4. Add your TreeAnnotator to the list of TreeAnnotators used by your AnnotatedTypeFactory. (See RegexAnnotatedTypeFactory

Option B: Use A TreeAnnotator Steps:

1. Create a class that extends TreeAnnotator
2. Override visitNewClass
3. Identify if the object being constructed is an instance of MyClass (see ElementUtils.getQualifiedClassName and ElementUtils.isObjectType to see how you might do this).
4. If the class being constructed is an instance of MyClass, if so, see if the argument is an Integer literal.
5. If the item is a literal, parse the literal and add it's value to the type being constructed. Note, if you would like this to handle non-literals, eg int a = 20; new MyClass(a)

Then you can use the ValueChecker. IF your type factory extends ValueAnnotatedTypeFactory it will have support for constant propagation. The data flow API also provides some support for constant propagation.

Option C: Customize dataflow's CFAbstractTransfer Function However, if you have a custom checker, you can override CFAbstractTransfer.visitMethodInvocation to detect when setValue is called and refine it accordingly. You can look at RegexTransfer for an example, or search for methods that extend CFAbstractTransfer or implement TransferFunction.

That said, dataflow is not a substitute for a type-state system (see a discussion of this at Typestate checkers ).