Accessing private fields with synthetic methods in Java, see also https://blog.gypsyengineer.com/en/security/accessing-private-fields-with-synthetic-methods-in-java.html

AccessPrivateField.java

package com.gypsyengineer.innerclass.field;

public class AccessPrivateField {
    public static void test01() throws Exception {
        System.out.println("Test #1: try to modify a private field with the same classloader");
        System.out.println("         (no exception is expected, 'oops' should be printed out)");

        Outer outer = new Outer();
        go(outer);
        outer.check();
    }

    // Modify a private field by calling a synthetic method
    public static void go(Object t) throws Exception {
        Method m = t.getClass().getDeclaredMethod(
                "access$002", t.getClass(), int.class);
        m.invoke(null, t, -1);
    }
}