Alternatives for Unsafe field access

Uwe Schindler uschindler at
Thu Dec 8 21:13:35 UTC 2016


Of course, private field access is allowed to your own class if you use a private lookup object. The setAccessible hack is only needed when reflection would also be needed for frameworks or other "superusers".


Am 8. Dezember 2016 22:10:32 MEZ schrieb Uwe Schindler <uschindler at>:
>You can first do standard reflection, then use setAccessible and
>finally use the Lookup object to convert the now-accessible Field
>instance to a MethodHandle or VarHandle. You just have to know that
>way, existing since Java 7.
>Am 8. Dezember 2016 21:49:04 MEZ schrieb Nathan Mittler
><nathanmittler at>:
>>Hi Roger,
>>If I read that correctly, lookups still have to go through access
>>which would seem to imply that they can't be used for private field
>>Or am I misunderstanding?
>>On Thu, Dec 8, 2016 at 11:51 AM, Roger Riggs <Roger.Riggs at>
>>> Hi Nathan,
>>> Have you looked at VarHandles?  [1]
>>> It is possible to use MethodsHandles.Lookup to get a VarHandle to an
>>> unreflected field.
>>> Roger
>>> [1]
>>> /MethodHandles.Lookup.html
>>> On 12/8/2016 1:03 PM, Nathan Mittler wrote:
>>>> Hi everyone,
>>>> Apologies in advance if this isn't the correct forum for this
>>question. My
>>>> team is working on an experimental runtime for Google's protocol
>>>> which is currently relying on sun.misc.Unsafe to perform various
>>>> efficiently. I'm aware that the plan is to eventually remove Unsafe
>>>> altogether, and I want to make sure that we still have a way to
>>>> forward with future versions of Java.
>>>> The code is up on a github branch (
>>>> For an example of the sorts of things our runtime needs to do, you
>>>> look
>>>> at the serialization code (
>>>> ).
>>>> The idea is that the runtime dynamically determines information
>>>> message fields (e.g. field types, offsets) and stores it into a
>>>> buffer.  Our main need is fast read/write access to the private
>>fields of
>>>> our generated message classes. Java reflection would be too slow
>>>> require data representation as a list of objects, rather than a
>>>> buffer (much less compact and cache-friendly). Security
>>>> be a concern as well. At first glance at Java 9, I don't see any
>>>> facilities
>>>> that would help here.
>>>> This would seem to be a fairly common use case a low-level
>>>> framework, such as protobuf. Are there any thoughts on how such a
>>use case
>>>> could be supported post-Unsafe?
>>>> Thanks,
>>>> Nathan
>Uwe Schindler
>Achterdiek 19, 28357 Bremen

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