Know the Lock

What does Lock Mean in Javet?

V8 runtime runs in an isolated and single-threaded environment so that there is no race condition issue. How about playing V8 runtime in JVM among multiple threads? Yes, that is possible in 2 modes.

1. Implicit Mode

Javet automatically handles the lock() and unlock() regardless of thread context switch. So, in most of the cases, applications just call regular API without calling a single line of locking API.

2. Explicit Mode

In the explicit mode, applications just need to surround the code block with a V8Locker protected by try-with-resource. Internally, Javet allocates a long-live V8 locker instead of creating ad-hoc V8 locker per API call to achieve better performance.

try (V8Locker v8Locker = v8Runtime.getV8Locker()) {
    // Do whatever you want to do.


V8Locker cannot be nested, otherwise a checked exception will be thrown reporting lock conflict. Also, if the JS runtime type is Node.js, calling resetContext() or resetIsolate() may trigger core dump. Please refer to the source code for details.



The implicit mode comes with a slight performance overhead in V8 because every native call is protected by a new V8 locker.

The explicit mode is designed for performance sensitive work. In extreme performance test cases, the performance improvement may be up to 50% compared to the implicit mode.


The implicit mode is thread-safe because its locking granularity is at API call level. Multiple threads are free to share the same V8 runtime in the concurrent scenarios. But, please be careful that JavaScript variables may be changed by other threads. It's better not to touch the same JavaScript variable among multiple threads, otherwise, Javet may crash. Yes, crash, because Javet doesn't perform this kind of check.

The explicit mode is NOT thread-safe because it's designed for maximizing the performance in the single-threaded scenarios. Sharing V8 locker protected V8 runtime among multiple threads will result in Javet crash immediately.

Coroutines or Virtual Threads

Kotlin Coroutines and Java 21 Virtual Threads are incompatible with the V8 threading model. The V8 locking mechanism cannot handle the chaos from coroutines or virtual threads, because V8 sees the calls coming from the same OS thread and doesn't actually keep the calls in sequence. Calling Javet API in coroutines or virtual threads will result in the JVM crash, if an additional synchronization is absent.