Allow direct field access in Java Forms

[mkurz]

Fixes https://discuss.lightbend.com/t/databinder-field-access-instead-of-using-accessors/2468

Adds play.forms.binding.directFieldAccess config to switch on direct field access for all forms. To control specific forms only this pull request adds form.withDirectFieldAccess(...) to switch direct field access on or off - or if null falls back to config fallback.

[marcospereira]

Made some comments. I wonder if the default for play.forms.binding.directFieldAccess should be true. WDYT?

[mkurz]

@marcospereira I addressed all your comments plus added two more tests.

I would definitely not change play.forms.binding.directFieldAccess to true by default.

First of all you have to know it's either / or. Either you choose initDirectFieldAccess (which tries to access fields directly) or initBeanPropertyAccess (which tries to use getters and is the default). It's just not possible to mix these two.

These points come into my mind:

1. If we switch to initDirectFieldAccess=true then the getter methods would not be called anymore - but maybe in some cases such getters are not simple getters but execute logic before returning a value (even if it's only trimming a string before returning it). That logic wouldn't be executed anymore -> not backward compatible.

2. There may be security problems. Direct field access tries to make private fields accessible via reflection (this calls that). That could cause security exceptions. We don't know which JVMs and/or security managers/settings Play users run their Play apps with, which may don't allow setting (certain) form fields accessible. Again not backward compatible.

3. I am not 100% sure how that direct field access works in regards to forms. Would it maybe try to access other private fields as well, which are not meant to be form fields and do something with them? I don't know.

As you can see even though I think that our tests would pass (because it looks in our code it's not a problem to make the private fields accessible and we use simple getters without logic everywhere) I would not change the default because that possible would break backward compatibility in user apps.

[marcospereira]

Thanks for the explanation, @mkurz.

I agree that we should not change the default to true (I was under the impression that the binder was smart enough to handle both cases).

There may be security problems. Direct field access tries to make private fields accessible via reflection (this calls that). That could cause security exceptions. We don't know which JVMs and/or security managers/settings Play users run their Play apps with, which may don't allow setting (certain) form fields accessible. Again not backward compatible.

Hum... I can foresee a warning about illegal reflective access in the future then when using Java 8+. But still, this is a configuration that is off by default. Anyway, it would be good to have a note/warning on our docs about it. Could you add a section on JavaForms that explains the use of directFieldAccess and its consequences?

[mkurz]

@marcospereira I added docs that show the usage of direct field access and mentions the risk of warnings and exceptions.
Please have a look once more, thanks Marcos!

[mkurz]

@marcospereira Are the added docs ok for you?

[marcospereira]

LGTM.

Thank you, @mkurz.

[marcospereira]

Backport to 2.7.x: a262d90