Supported configurations

Use the patched JDK 25 or GraalVM 25.3.4.1 on macOS 26 arm64, Linux x86_64 or Windows x86_64. The Linux build has been exercised on Ubuntu 22.04 with glibc 2.35; the Windows build on Windows 11 with Visual Studio 2022. Select Jam with -XX:+UnlockExperimentalVMOptions -XX:+UseJamGC and set equal initial and maximum heap sizes. The build guide gives the toolchain and commands. Native executables use native-image --gc=jam from the patched GraalVM; see Native Image for its separate sizing and pinning rules.

HotSpot heap and compiler

Jam collects both generations. New objects normally enter young; minor collections retain young survivors or promote the whole live nursery when it fits. Major collections compact both generations independently. Java soft, weak, final and phantom references use the VM's reference processor alongside the generalized weak policy.

The current heap uses fixed capacities and stop-the-world collection. It needs ordinary object headers, eight-byte alignment, normal pages and nonzero-base, shift-three compressed oops. Each generation, including its guard and copy reserve, must fit within a 16 GiB domain. Class unloading and CDS heap loading are disabled.

Interpreter, C1, C2 and the patched Graal compiler use Jam's card barriers. The GraalVM build includes patched libgraal. The VM rejects a compiler that does not recognize Jam before it can install Java code. Other operating systems and instruction sets still need validation.

Guest API

Use jam.vm.Weak to register weak associations and install JVM runnables. Any caller can pump the shared queue. There is no automatic finalizer thread or wakeup notification; the caller supplies the pump schedule.

Tokens are local to a JVM or Native Image isolate and never reused. Dead registrations currently retain metadata, so native memory use grows with lifetime registrations. Allocation failure while growing that metadata terminates instead of throwing Java OutOfMemoryError. Keep this in mind for long-running, weak-heavy workloads.

Use the packaged runtime when moving an installation out of its build checkout.

Next steps

The thc owner can use the existing Java hooks to lower weak primitives and wrap guest finalizers in runnables. GHC C finalizers and weak-thread resurrection need additional runtime support.

Within the collector, the next work includes reclaiming weak metadata, indexed weak processing, selective promotion, adaptive capacities and parallel VM scanning. Jam's SIMD kernels and work scheduler remain the implementation base.