Getting a live view of environment variables (Gradle and JDK 9)

Cédric Champeau cedric.champeau at
Thu May 18 08:19:32 UTC 2017

> Can you elaborate as to why specifying the "big kill switch"
> --permit-illegal-access is not viable? Specifically if you use:
> -XX:+IgnoreUnrecognizedVMOptions --permit-illegal-access
> it should work on 9 and be ignored on earlier releases.
mmm that's interesting, I actually forgot this flag exists. However, it's
an OpenJDK/Oracle JDK specific flag, right? (Thinking of users running IBM
JDK typically).

> Thanks,
> David
> On 11/05/2017 7:37 AM, Cédric Champeau wrote:
>> Hi all,
>> I'm writing this on behalf of the Gradle team. This email is closely
>> related to the other thread just posted today, but just a timeline
>> coincidence (just like the email exchange I had today about this with Alan
>> Bateman ;)) and not exactly the same issue.
>> We are in the process of making sure Gradle runs properly on JDK 9, but
>> there's an issue which is unresolved so far, and probably requires a new
>> API. It's described at [1], and I have discussed this at Devoxx France
>> with
>> Rémi Forax who suggested to post something here.
>> In short, Gradle is a build tool which supports building a variety of
>> different things, from Java to C++. The JVM happens to be its runtime
>> environment, and Gradle has what we call the Gradle Daemon [2] which is a
>> long running process, benefiting from the JIT, aimed at effectively
>> running
>> builds. When the build starts, a client connects to the daemon and sends a
>> "build request". A daemon will run a single build at a time, and there are
>> several cases which would trigger a new daemon to spawn (the daemon JVM
>> arguments are one) but the environment variables are *not*. Since the
>> daemon is a long running process, it is possible that the environment
>> variables are mutated between the moment the daemon was spawned (in a
>> previous build) and the moment the build is executed.
>> What we do, now, is to send the environment variables of the client to the
>> daemon, which *mutates* the existing environment variables map provided by
>> System.getenv. This is exactly what is described in [1] as being sneaky
>> (it
>> is) and broken in JDK 9 (since the underlying map doesn't exist anymore).
>> However, there are valid use cases for this:
>>    - in practice, environment variables are not immutable. It is
>> especially
>>    true for long running process.
>>    - native programs can mutate the environment variables. Even if it's
>> not
>>    recommended, it is possible and legal.
>>    - Gradle runs in a "blackbox": we don't know what plugins are doing.
>>    Even if we provide an API which gives access to "environment
>> variables",
>>    and that those environment variables are not the ones returned by
>>    System.getenv, plugin authors would have to use this new API to get
>>    correct information. However, they may use libraries which access
>>    System.getenv directly, or use native APIs which would get out-of-sync
>>    information.
>>    - we need to propagate the environment to forked process (typically,
>>    forked compilers and worker daemons)
>> This means that today, we use JNI to effectively mutate the environment
>> variables of running process (that’s one of the purposes of the
>> native-platform project). Then, we mutate the backing map of the JDK to
>> reflect those changes, otherwise the mutation is not visible from Java
>> code.
>> What can we do now?
>>    - Have the JDK honor the fact that environment variables *can* be
>>    mutated, because it just happens. In short, don't create an immutable
>> copy
>>    of environment variables at startup, but provide a live view of the
>>    environment variables (using the existing APIs, System.getenv, would
>> be the
>>    best thing because it would be immediately visible to all consumers,
>>    including 3rd party code run in plugins). In addition (but not
>> mandatory),
>>    you could provide us with an API to set environment variables directly
>> from
>>    Java. This would avoid JNI calls to do this. However, it’s not
>> mandatory,
>>    because the live view of environment variables would just work in this
>> case.
>>    - Last, but we would really, really avoid to do this, spawn a new
>> daemon
>>    if we detect that the environment variables have changed (diff between
>> what
>>    the client has and the daemon sees). The major drawback of this
>> approach is
>>    that it kills performance, since a new daemon would have to be
>> spawned. And
>>    it is likely to do so each time something (through native code, for
>>    example), mutates environment variables. A very simple example is
>> the PWD environment
>>    variables on Linux which contains the working directory. Basically
>> changing
>>    the directory would be enough to spawn a new daemon. Another example
>> is the
>>    TERM_SESSION_ID one, which means that 2 different terminals would force
>>    us to spawn 2 different Gradle daemons. We could, of course, have a
>> list of
>>    “blessed” environments variables that we don’t trust, but it’s very
>> easily
>>    broken, and no good design. That’s why, even if it’s possible, we don’t
>>    consider this a solution.
>> Thanks for considering our request, which is currently a blocker for us
>> (understand, have Gradle running properly under JDK 9).
>> [1]
>> [2]

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