Why It No Longer Works

The reason this no longer works in Java 12 is due to JDK-8210522. This CSR says:

Summary

Core reflection has a filtering mechanism to hide security and integrity sensitive fields and methods from Class getXXXField(s) and getXXXMethod(s). The filtering mechanism has been used for several releases to hide security sensitive fields such as System.security and Class.classLoader.

This CSR proposes to extend the filters to hide fields from a number of highly security sensitive classes in java.lang.reflect and java.lang.invoke.

Problem

Many of classes in java.lang.reflect and java.lang.invoke packages have private fields that, if accessed directly, will compromise the runtime or crash the VM. Ideally all non-public/non-protected fields of classes in java.base would be filtered by core reflection and not be readable/writable via the Unsafe API but we are no where near this at this time. In the mean-time the filtering mechanism is used as a band aid.

Solution

Extend the filter to all fields in the following classes:

java.lang.ClassLoader
java.lang.reflect.AccessibleObject
java.lang.reflect.Constructor
java.lang.reflect.Field
java.lang.reflect.Method

and the private fields in java.lang.invoke.MethodHandles.Lookup that are used for the lookup class and access mode.

Specification

There are no specification changes, this is filtering of non-public/non-protected fields that nothing outside of java.base should rely on. None of the classes are serializable.

Basically, they filter out the fields of java.lang.reflect.Field so you can’t abuse them—as you’re currently trying to do. You should find another way to do what you need; the answer by Eugene appears to provide at least one option.

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Proper Fix

The proper way to drop a final modifier is to instrument the running program, and have your agent redefine the class. If you do this when the class is first loaded, it’s no different than having modified the class file before the JVM was even started. In other words, it’s like the final modifier was never present.

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Workaround

Obligatory Warning: The developers of Java obviously don’t want you to be able to change a final field into a non-final field without actually changing the class file (e.g., by recompiling the source code, instrumentation, etc.). Use any hack at your own risk; it may have unintended side-effects, work only some times, and/or stop working in a future release.

Use java.lang.invoke

The following uses the java.lang.invoke package. For whatever reason, the same restrictions applied to the Reflection API are not applied to the Invoke API (at least up to and including Java 17; continue reading for more information).

The example modifies the EMPTY_ELEMENTDATA final field of the ArrayList class. This field normally contains an empty array that’s shared between all ArrayList instances when initialized with a capacity of 0. The below sets the field to {"Hello", "World!"}, and as you can see by running the program, this results in the list instance containing elements that were never added to it.

Java 12 – 17

I tested this on Java 16.0.2 and Java 17.0.3, both downloaded from https://adoptium.net/.

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.ArrayList;

public class Main {

  private static final VarHandle MODIFIERS;

  static {
    try {
      var lookup = MethodHandles.privateLookupIn(Field.class, MethodHandles.lookup());
      MODIFIERS = lookup.findVarHandle(Field.class, "modifiers", int.class);
    } catch (IllegalAccessException | NoSuchFieldException ex) {
      throw new RuntimeException(ex);
    }
  }

  public static void main(String[] args) throws Exception {
    var emptyElementDataField = ArrayList.class.getDeclaredField("EMPTY_ELEMENTDATA");
    // make field non-final
    MODIFIERS.set(emptyElementDataField, emptyElementDataField.getModifiers() & ~Modifier.FINAL);
    
    // set field to new value
    emptyElementDataField.setAccessible(true);
    emptyElementDataField.set(null, new Object[] {"Hello", "World!"});

    var list = new ArrayList<>(0);

    // println uses toString(), and ArrayList.toString() indirectly relies on 'size'
    var sizeField = ArrayList.class.getDeclaredField("size");
    sizeField.setAccessible(true);
    sizeField.set(list, 2); // the new "empty element data" has a length of 2

    System.out.println(list);
  }
}

Run the code with:

javac Main.java
java --add-opens=java.base/java.lang.reflect=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED Main

^{Note: I tried to use the “single source file” feature, but that resulted in a ConcurrentModificationException. As pointed out in the comments, this is likely due to some JIT optimization (e.g., the static final field has been inlined, because the JVM does not expect such a field to be able to change).}

Output:

[Hello, World!]

Java 18+

Unfortunately, the above results in the following exception on Java 18.0.1 (downloaded from https://adoptium.net/):

Exception in thread "main" java.lang.UnsupportedOperationException
        at java.base/java.lang.invoke.VarForm.getMemberName(VarForm.java:114)
        at Main.main(Main.java:23)

Where line 23 is:

MODIFIERS.set(emptyElementDataField, emptyElementDataField.getModifiers() & ~Modifier.FINAL);