On Thu, 3 Nov 2022 16:48:36 GMT, Mandy Chung <mch...@openjdk.org> wrote:
>> This change adds an option **EnableWaitForParallelLoad** to enable the >> legacy behavior where the VM will manage synchronization for multiple >> threads loading the same class using a non-parallel capable class loader >> that have released the class loader lock. The VM will break the class >> loader lock for parallel threads trying to load the class, and wait for the >> first thread that initiated loading the class to complete. The subsequent >> threads will use the result of the first thread, rather than get a >> LinkageError: duplicate class definition for loading the class without >> synchronization. >> Releasing the class loader lock was a common workaround for class loaders >> that used a non-hierarchical delegation scheme to avoid deadlock, before >> parallel capable class loading was added. >> https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/ClassLoader.html#registerAsParallelCapable() >> >> Tested with tier1-6 and internal applications. > > Adding core-libs-dev so that more developers know this behavioral change. > > *Background:* > > Prior to JDK 7, custom class loaders using non-hierarchical class delegation > model were prone to deadlock (see [1] for details). This workaround was > added in the VM in JDK 6 to allow parallel class loading for > non-parallel-capable class loaders to avoid deadlocks. The VM also supports > a common workaround at the time that some customers explicitly issue a wait() > on the class loader lock. > > Such workaround was kept to allow those customers to migrate to using > multi-thread-safe parallel-capable class loaders. It has been 11 years since > Java 7 and it's expected that those deadlock-prone custom class loaders > should have migrated to the parallel-capable class loaders. > > If customers have class loaders depending on this legacy behavior, they can > set `-XX:+EnableWaitForParallelLoad` flag to enable the legacy behavior and > migrate to parallel-capable class loaders as soon as possible. The > `EnableWaitForParallelLoad` VM flag will be removed in a future release. > > Refers to CSR [JDK-8295848](https://bugs.openjdk.org/browse/JDK-8295848) for > details. Thank you for the discussions, CSR, help and code reviews, @mlchung and @dholmes-ora . ------------- PR: https://git.openjdk.org/jdk/pull/10832
