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 test02(ClassLoader cl) throws Exception {
        System.out.println("Test #2: try to modify a private field with different classloader");
        System.out.println("         (an exception is expected)");

        // Load AccessPrivateField class with different classloader
        Class clazz = cl.loadClass(
            "com.gypsyengineer.innerclass.field.AccessPrivateField");

        // Make sure that we loaded a new class
        if (AccessPrivateField.class.equals(clazz)) {
            throw new RuntimeException("Couldn't load different AccessPrivateField with new class loader");
        }

        // Get "go" method from clazz, and run with an instance of Outer
        // Note that Outer class and clazz were loaded by different classloaders
        Outer outer = new Outer();
        Method m = clazz.getDeclaredMethod("go", Object.class);
        m.invoke(null, outer);
    }

    // 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);
    }
}