FAQ

What is CratonVM, in one sentence?

A Java Virtual Machine written entirely in Rust, with a custom x86-64 JIT compiler, that runs Java SE 8-25 bytecode with or without a JDK installed.

Do I need a JDK installed to run it?

No. CratonVM boots against a real JDK when it detects one, and otherwise runs against its own built-in synthetic standard library: no JAVA_HOME, no rt.jar. You do need a javac somewhere to compile .java to .class; CratonVM runs the resulting bytecode. See JDK Modes.

Is it production-ready?

No. CratonVM is experimental, research-grade software. It is not certified, has not had a formal security audit, and must not run untrusted code in security-sensitive environments. See the Security Overview and Known Limitations.

How fast is it?

Current performance depends heavily on JIT configuration. In an older snapshot taken before back-edge OSR became the default, with CRATONVM_JIT_OSR=1 CRATONVM_JIT_THRESHOLD=1, the Arithmetic, Sieve, and Matrix QuickBench kernels are roughly 1.3x-1.6x slower than HotSpot C2, but recursive Fibonacci is still 13.8x slower and Binary Trees remains much slower. Current dev enables OSR by default; use CRATONVM_JIT_OSR=0 only when you want the old OSR-off lane for diagnosis. See Benchmarks.

Why is my allocation-heavy program slow, or seemingly hung?

Two likely causes: (1) the heap is too small and the collector is thrashing; (2) GC throughput on short-lived allocation is a known weak spot. Raise -Xmx explicitly, especially inside containers where ergonomic defaults may not match the actual workload. See Memory & GC and Containers.

How do I run a JAR?

cratonvm --jar app.jar [args...]. The main class comes from the JAR manifest; --classpath is ignored in this mode. There is no main-class auto-detection from a bare classpath; name the class explicitly otherwise.

I get "native method not found" / NoSuchMethodError. Why?

Your program uses a standard-library method CratonVM has not implemented yet. Audit exactly what is missing with --XX:AuditMissingNatives or dump it to JSON with --dump-missing-natives. See Debugging & Diagnostics.

Does it do TLS / HTTPS?

Not as a full JSSE stack. There is no backed SSLContext/SSLEngine/ HttpsURLConnection. Terminate TLS in front of the VM (reverse proxy or sidecar). The cryptographic primitives (digests, AES-GCM, HMAC, RSA, DSA, ECDSA/Ed25519, PBKDF2, ML-KEM/ML-DSA) are available; see Cryptography.

What platforms are supported?

Linux and Windows are the primary targets; macOS is best-effort. The JIT is x86-64 only, and the interpreter runs everywhere. See the Platform Support Matrix.

Can I embed it in my own application?

Yes, from Rust via cratonvm-embed, or from any FFI language via the libcratonvm C ABI, which also exposes the JNI Invocation API. Drive a VM from the thread that created it; one VM per process is the tested configuration. See Embedding.

How do I turn the JIT off?

--nojit or CRATONVM_DISABLE_JIT=1. This is the fastest way to tell whether a wrong result or hang is an interpreter or JIT issue.

How do I report a bug?

File on the GitHub issue tracker with a minimal Java reproduction, the exact command line, expected-vs-actual output, and whether it reproduces under --nojit / --synthetic-jdk. Report security issues privately per the repository's SECURITY.md. See Troubleshooting.

What license is it under?

Apache License 2.0.