CODEWITHSUNDEEP
javareflectionannotations
FabianCook
FabianCook

Reputation: 20557

Using reflection, get the parent class/field of an object

Is it possible in Java to get the 'parent' class of an object using an instance of it and its class type?

Lets say we have these two class types and the annotation type:

public class Foo{

}

public class Bar{

    @TestAnnotation
    public Foo mFoo = new Foo();
}

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface TestAnnotation{

}

I would want to get the Field object of mFoo using an instance of Foo, otherwise null, with this I would get the annotation using TestAnnotation test = field.TestAnnotation(SQLiteID.class);

The end result I want is to know whether the instance of Foo is part of another type that contains a certain annotation

This seems rather far fetched so I don't expect it to be possible.

Upvotes: 1

Views: 3081

Answers (3)

chrylis -cautiouslyoptimistic-
chrylis -cautiouslyoptimistic-

Reputation: 77177

This isn't possible in Java. It would be possible by writing a JVM agent but would probably have a severe performance penalty because of all the bookkeeping.

Upvotes: 1

user2864740
user2864740

Reputation: 61875

This is not possible/feasible in general.

The same object instance can have many names - that is, the same object can be assigned to many different static fields, instance fields, parameters, local variables (etc) simultaneously.

All "solutions" involve some form of iteration over all such variables, which is rarely feasible and such general designs should be avoided. Even with a reduced domain (such as with annotations) a similar issue remains - i.e. how to find all such roots (objects instances with members having a certain annotation) of said domain?

Upvotes: 4

Sotirios Delimanolis
Sotirios Delimanolis

Reputation: 279960

I would want to get the Field object of mFoo using an instance of Foo.

This is doing things backwards and not possible.

You can get the Field of Bar by testing its type and then try to retrieve a specific annotation

Field[] fields = Bar.class.getDeclaredFields();
for (Field field : fields) {
    if (Foo.class.isAssignableFrom(field.getType())) {
        TestAnnotation testAnnotation = field.getAnnotation(TestAnnotation.class);
        System.out.println(testAnnotation); // might be null, if annotation doesn't exist
    }
}

Now if you have an instance of Foo you can also compare it to the field value of an instance of Bar.

Upvotes: 2

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