更改记录中的字段

Question

I study reflection and try to change field's value in Record.

public record Account(Integer id, String login, Boolean blocked) {}

public class Main {
    public static void main(String[] args) {
        Account account = new Account(null, null, null);
        setFieldValue(account, "id", 1);
        setFieldValue(account, "login", "admin");
        setFieldValue(account, "blocked", false);
        System.out.println(account);
    }
    public static void setFieldValue(Object instance,
                                     String fieldName,
                                     Object value) {
        try {
            Field field = instance.getClass().getDeclaredField(fieldName);
            field.setAccessible(true);
            field.set(instance, value);
        } catch (NoSuchFieldException | IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}

If I convert Record to Class everything works, but with Record I get Exception

java.lang.IllegalAccessException: Can not set final java.lang.Integer field Account.id to java.lang.Integer
    at java.base/jdk.internal.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:76)
    at java.base/jdk.internal.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:80)
    at java.base/jdk.internal.reflect.UnsafeQualifiedObjectFieldAccessorImpl.set(UnsafeQualifiedObjectFieldAccessorImpl.java:79)
    at java.base/java.lang.reflect.Field.set(Field.java:799)

What do I have to do to make the code work with records?

Answer 1

In general the commenters saying that this "can't be done" or is "impossible" aren't wrong... unless you are willing to slightly bend the rules of the JVM:) For example, by using unsafe reflection to change the relevant value directly at the memory location in the record like this:

public static void setFieldValue(Object instance, String fieldName, Object value) {
    try {
        Field f = instance.getClass().getDeclaredField(fieldName);

        Field unsafeField = Unsafe.class.getDeclaredField("theUnsafe");
        unsafeField.setAccessible(true);
        Unsafe unsafe = (Unsafe) unsafeField.get(null);

        Field theInternalUnsafeField = Unsafe.class.getDeclaredField("theInternalUnsafe");
        theInternalUnsafeField.setAccessible(true);
        Object theInternalUnsafe = theInternalUnsafeField.get(null);

        Method offset = Class.forName("jdk.internal.misc.Unsafe").getMethod("objectFieldOffset", Field.class);
        unsafe.putBoolean(offset, 12, true);

        unsafe.putObject(instance, (long) offset.invoke(theInternalUnsafe, f), value);
    } catch (IllegalAccessException | NoSuchFieldException | ClassNotFoundException | NoSuchMethodException | InvocationTargetException e) {
        e.printStackTrace();
    }
}