[ammarahm-ed]

A Java call builds several JEnv objects on its way through the bridge, and each one called JavaVM::GetEnv. JEnv now reads the thread's JNIEnv from a thread_local and only asks the VM when none is set. Both the Node-API and the direct runtime get the change; it is the design from NativeScript/android#2092.

A JNIEnv is only valid while its thread stays attached, so the cache is set only where that lifetime is known:

• Runtime thread. Runtime::Init pins the env until DestroyRuntime. A worker thread also unpins it right before DetachCurrentThread.
• JNI entry points. initNativeScript, runModule, runScript, callJSMethodNative and createJSInstanceNative hold the env they were given in an EntryScope for the length of the call and then restore the previous value. A pinned env is left alone.
• Lazily attached threads. The env GetEnvSlow attaches is never cached, because nothing would tell us when it goes stale.

Results

V8 release build on a Sony SO-01M (Android 11, arm64), AOT-compiled with cmd package compile -m speed. One warm-up launch is discarded, then 5 launches; values are the median ns per call.

Case	Node-API before	after	Δ	Direct before	after	Δ
Void Method on instance	717	694	-3%	680	654	-4%
Void Static Method	362	355	-2%	377	330	-13%
multiply(a, b)	776	749	-3%	878	824	-6%
Instance add(a, b)	1,135	1,097	-3%	1,193	1,105	-7%
Return an Int	367	354	-3%	376	351	-7%
Pass an Int	626	607	-3%	693	636	-8%
Field on instance	910	819	-10%	882	821	-7%
Invoke int callback	10,851	10,264	-5%	11,006	10,623	-3%

The gain is a few percent per call, close to this device's run-to-run noise on single rows (about ±5% on primitive calls), so read the pattern across rows rather than any one cell.

All cases

Case	Node-API before	after	Δ	Direct before	after	Δ
Void Method on instance	717	694	-3%	680	654	-4%
Field on instance	910	819	-10%	882	821	-7%
Int Field on instance	425	381	-10%	387	369	-5%
Get Static Field	898	866	-4%	928	816	-12%
Void Static Method	362	355	-2%	377	330	-13%
multiply(a, b)	776	749	-3%	878	824	-6%
Pass and return string	1,979	1,850	-7%	1,975	2,059	+4%
Return a String	936	873	-7%	922	890	-3%
Return an Int	367	354	-3%	376	351	-7%
Pass a String	1,094	1,073	-2%	1,186	1,168	-2%
Return a Boolean	363	342	-6%	377	333	-12%
Return a Double	464	462	-0%	458	440	-4%
Pass an Int	626	607	-3%	693	636	-8%
Pass a Double	624	593	-5%	702	621	-12%
Pass a Boolean	701	724	+3%	670	633	-6%
Return an Int Array	26,453	27,509	+4%	17,506	17,994	+3%
Pass an Int Array	3,302	3,273	-1%	4,162	4,036	-3%
Return a String Array	27,122	27,111	-0%	19,109	19,576	+2%
Pass a String Array	5,691	5,420	-5%	7,092	6,872	-3%
Return a Double Array	26,921	27,135	+1%	20,357	20,574	+1%
Pass a Double Array	3,144	3,094	-2%	3,987	3,896	-2%
Return a Boolean Array	24,781	25,343	+2%	18,246	18,770	+3%
Pass a Boolean Array	3,014	2,926	-3%	3,842	3,757	-2%
Int Array[0] read	379	366	-3%	248	239	-4%
String Array[0] read	849	783	-8%	698	685	-2%
Double Array[0] read	492	446	-9%	348	343	-1%
Boolean Array[0] read	369	341	-7%	247	236	-4%
Int Array[0] write	546	458	-16%	389	375	-4%
String Array[0] write	1,289	1,128	-13%	1,141	1,060	-7%
Double Array[0] write	551	467	-15%	381	383	+1%
Boolean Array[0] write	552	458	-17%	394	388	-2%
Return a Date Object	29,666	31,058	+5%	23,960	24,197	+1%
Pass a Date Object	3,774	3,871	+3%	3,727	3,512	-6%
Invoke void callback	7,637	7,670	+0%	6,848	6,940	+1%
Instance add(a, b)	1,135	1,097	-3%	1,193	1,105	-7%
Pass and return non-ASCII string	2,007	1,870	-7%	1,959	2,083	+6%
Pass and return 1KB string	5,816	5,698	-2%	5,991	6,066	+1%
Return a Char	409	396	-3%	414	383	-8%
Return this	1,607	1,646	+2%	1,622	1,549	-5%
Return a new object	29,868	30,520	+2%	23,988	24,424	+2%
Call a method on a new object	38,563	38,321	-1%	33,581	33,041	-2%
Pass a 16 Int Array	4,512	4,350	-4%	5,325	5,313	-0%
Pass a 16 Double Array	5,351	4,882	-9%	6,043	5,733	-5%
Invoke int callback	10,851	10,264	-5%	11,006	10,623	-3%
Invoke string callback	8,868	8,652	-2%	8,511	8,281	-3%
Native heap after run (MB)	256 MB	270 MB	+5%	178 MB	204 MB	+15%

Only V8 was built and run.

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