Reflection for replace static final String

Question

I am using reflection exactly according to:

But everytime I get this error and I don't know why.

12-02 17:53:58.650: W/System.err(10646): java.lang.NoSuchFieldException: modifiers
12-02 17:53:58.650: W/System.err(10646):    at java.lang.Class.getDeclaredField(Class.java:631)
12-02 17:53:58.650: W/System.err(10646):    at com.example.ref.MainActivity.setFinalStatic(MainActivity.java:56)
12-02 17:53:58.650: W/System.err(10646):    at com.example.ref.MainActivity.onCreate(MainActivity.java:26)
12-02 17:53:58.650: W/System.err(10646):    at android.app.Activity.performCreate(Activity.java:5206)
12-02 17:53:58.650: W/System.err(10646):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1083)
12-02 17:53:58.650: W/System.err(10646):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2064)
12-02 17:53:58.650: W/System.err(10646):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2125)
12-02 17:53:58.650: W/System.err(10646):    at android.app.ActivityThread.access$600(ActivityThread.java:140)

My code below:

    @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Log.i("aaa", "Before = " + securedField);
    try {
        Field stringField = MainActivity.class.getDeclaredField("securedField");
        stringField.setAccessible(true);
        setFinalStatic(stringField, new String("Screwed Data!"));
    } catch (Exception e) {
        e.printStackTrace();
    }
    Log.i("aaa", "After = " + securedField);
}

private void setFinalStatic(Field field, Object newValue) throws Exception {
    field.setAccessible(true);
    Field modifiersField = Field.class.getDeclaredField("modifiers");
    modifiersField.setAccessible(true);
    modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);
    field.set(null, newValue);
}

Does somebody know where the problem is? Thanks!

`NoSuchFieldException: modifiers` s modifiers valid field name? — Simon Dorociak, Dec 02 '13 at 17:17
What are you trying to achieve? To change a final field on Field.class? — emd, Dec 02 '13 at 17:17
Are you sure that the `Field` class actually has that field on it? It *is* `private`, after all, and it's possible that the implementation you're running it on (Android?) doesn't implement `getModifiers()` with a backing field. — chrylis -cautiouslyoptimistic-, Dec 02 '13 at 17:21
I have the same issue. If I run code in IDE, it works normally, but if I run mvn clean install, the same issue occurred — Dexter, Jun 30 '23 at 04:14

René Link · Answer 1 · 2015-10-13T12:41:26.957

Take a look at the source code of the android version of java.lang.reflect.Field.

It does not have a private field modifiers. Instead it looks like this

public int getModifiers() {
    return accessFlags & 0xffff;  // mask out bits not used by Java
}

You tried to adapt an implementation detail of the "normal" jvm to the android jvm. Since it is an implementation detail of another jvm it will not work with android.

But what you try to do will not work anyway. Keep in mind that the compiler inlines constants.

This will not work

class Greeting {

    public static final String SALUTATION = "Hello";

    public void greet(String name) {
        System.out.println(SALUTATION + " " + name);
    }
}

public class Main {

    public static void main(String[] args) throws Exception {
        Greeting greeting = new Greeting();
        greeting.greet("John");

        Field declaredField = Greeting.class.getDeclaredField("SALUTATION");
        setFinalStatic(declaredField, "Hey");

        greeting.greet("John");
    }

    private static void setFinalStatic(Field field, Object newValue)
            throws Exception {
        field.setAccessible(true);
        Field modifiersField = Field.class.getDeclaredField("modifiers");
        modifiersField.setAccessible(true);
        modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);
        field.set(null, newValue);
    }

}

The output will be

Hello John
Hello John

Also take a look at JLS, 13.4.9. final Fields and Constants for details.

Reflection for replace static final String

1 Answers1