Changing private final fields via reflection

Question

class WithPrivateFinalField {
    private final String s = \"I’m totally safe\";
    public String toString() {
        return \"s = \" + s;
    }
}
WithPrivateFinal         


        

Answer 1

This answer is more than exhaustive on the topic.

JLS 17.5.3 Subsequent Modification of Final Fields

Even then, there are a number of complications. If a final field is initialized to a compile-time constant in the field declaration, changes to the final field may not be observed, since uses of that final field are replaced at compile time with the compile-time constant.

But, if you read the paragraph above very carefully, you may find a way around here (set the private final field in the constructor instead of in the field definition):

import java.lang.reflect.Field;


public class Test {

  public static void main(String[] args) throws Exception {
    WithPrivateFinalField pf = new WithPrivateFinalField();
    System.out.println(pf);
    Field f = pf.getClass().getDeclaredField("s");
    f.setAccessible(true);
    System.out.println("f.get(pf): " + f.get(pf));
    f.set(pf, "No, you’re not!");
    System.out.println(pf);
    System.out.println("f.get(pf): " + f.get(pf));
  }

  private class WithPrivateFinalField {
    private final String s;

    public WithPrivateFinalField() {
      this.s = "I’m totally safe";
    }
    public String toString() {
      return "s = " + s;
    }
  }

}

The output is then as follows:

s = I’m totally safe
f.get(pf): I’m totally safe
s = No, you’re not!
f.get(pf): No, you’re not!

Hope this helps a bit.