Best Practices

Thread, Engine and Pool

  • Always get 1 Javet engine from the pool in 1 thread.

  • If multiple context is required in 1 thread, there are 2 options.

    • Call resetContext() between context switch.

    • Obtain multiple V8Runtime instances.

  • Do not pass Javet engine to other threads.

  • Always return Javet engine to pool in the end via try-with-resource or calling close() explicitly.

  • Subclass Javet engine pool and Javet engine to complete your customization. Indeed, they are open to full customization.

Resource Management

  • Dangling V8 objects will be forced to be recycled by Javet under the following scenarios and corresponding log will reflect that. Keeping an eye on the log helps address memory leak issues in the early stage.

    • Engine is closed.

    • Pool is closed.

    • Context is reset.

    • Isolate is reset.

  • Always apply try-with-resource to Javet objects regardless of primitive or reference if they are not returned to Javet.

  • Always prohibit calling close() of Javet objects if they will be returned to Javet.

  • If the lifecycle of V8 objects is uncertain, calling setWeak() is the only way so that calling close() is no longer required. Be careful, calling close() after calling setWeak() may lead to V8 core dump immediately.

  • In performance sensitive scenarios, please explicitly acquire V8Locker.

  • Use V8Scope for preventing memory leak when exception is thrown.


  • Malicious scripts are recommended to be executed in V8 mode or in vm module in Node.js mode.

  • eval can be disabled in Javet.

  • V8 0-day vulnerable issues most likely impact Node.js because the embedded V8 in Node.js is very old. It's recommended to use the V8 mode to minimize the risk.


  • Modularize the code as much as possible so that performance is maximized.

  • Always register unhandled rejection event.

  • In non-module mode (similar to V8 in web browser), always put launch script in a dedicated folder whose parent folder contains node_modules and avoid require modules in the same folder.

  • In module mode (similar to V8 in Node.js), be aware that the execution result is a promise and the behavior is different from native Node.js runtime behavior unless await() is called.