Getting a live view of environment variables (Gradle and JDK 9)
david.holmes at oracle.com
Thu May 18 09:30:27 UTC 2017
On 18/05/2017 6:19 PM, Cédric Champeau wrote:
>> 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).
It's an OpenJDK flag, yes. But I would have expected there were already
differences in how you launch an IBM JVM versus OpenJDK or Oracle JVM - no?
>> 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 , and I have discussed this at Devoxx France
>>> 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  which is a
>>> long running process, benefiting from the JIT, aimed at effectively
>>> 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  as being sneaky
>>> 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
>>> true for long running process.
>>> - native programs can mutate the environment variables. Even if it's
>>> 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
>>> 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
>>> - 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
>>> 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
>>> 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
>>> you could provide us with an API to set environment variables directly
>>> Java. This would avoid JNI calls to do this. However, it’s not
>>> because the live view of environment variables would just work in this
>>> - Last, but we would really, really avoid to do this, spawn a new
>>> if we detect that the environment variables have changed (diff between
>>> 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
>>> 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
>>> 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).
>>>  https://github.com/adammurdoch/native-platform/issues/16
>>>  https://docs.gradle.org/current/userguide/gradle_daemon.html
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