Question

I’ve been playing around with reflection in Java… and I’m a little bit baffled.

I was hoping that the program below would allow me to change the value of a public member variable within a class. However, I receive an IllegalArgumentException. Any ideas?

public class ColinTest {

    public String msg = "fail";

    public ColinTest() { }

    public static void main(String args[]) throws Exception {
        ColinTest test = new ColinTest();
        Class c = test.getClass();
        Field[] decfields = c.getDeclaredFields();
        decfields[0].set("msg", "success");

        System.out.println(ColinTest.msg)
    }
}

I receive this message –

Exception in thread "main" java.lang.IllegalArgumentException
    at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:37)
    at sun.reflect.UnsafeObjectFieldAccessorImpl.set(UnsafeObjectFieldAccessorImpl.java:57)
    at java.lang.reflect.Field.set(Field.java:656)
    at ColinTest.main(ColinTest.java:44)

Thanks.

Answer 1

The first argument of the Field.set method should be the object which you are reflecting on.

decfields[0].set("msg", "success");

Should read:

decfields[0].set(test, "success");

Furthermore, the final System.out.println call should refer to the test object rather than the class ColinTest, as I presume the intention is to output the contents of the test.msg field.

Update

As pointed out by toolkit and Chris, the Class.getDeclaredField method can be used to specify the name of the field in order to retrieve it:

Field msgField = test.getClass().getDeclaredField("msg");

// or alternatively:

Field msgField = ColinTest.class.getDeclaredField("msg");

Then, the set method of the msgField can be invoked as:

msgField.set(test, "success");

This way has its benefit, as already pointed out by toolkit, if there are more fields added to the object, the order of the fields that are returned by Class.getDeclaredFields may not necessarily return the field msg as the first element of the array. Depending on the order of the returned array to be a certain way may cause problems when changes are made to the class.

Therefore, it would probably be a better idea to use getDeclaredField and declare the name of the desired field.