Using JNI without having to add jvm.dll path inside Environment Variables (User or System)

sValentin 41

In order to use JNI, you have to add inside Environment Variables (User or System) the path to were jvm.dll is located. You have a few ways to do that, but it is possible. But, is there a way to not have to add the path inside Environment Variables and instead when you call the function to create a VM from C++ to tell it where it is located via JavaVMInitArgs, or maybe JavaVMOption, or in any other way, but like I said without having to add the path in Environment Variables?

Minxin Yu 10,031 Reputation points Microsoft Vendor

2023-01-26T07:56:04.7566667+00:00

Are you using C++ in visual studio?
sValentin 41 Reputation points

2023-01-26T08:03:37.24+00:00

Yes, Visual Studio Community 2019.
sValentin 41 Reputation points

2023-01-26T08:04:17.7666667+00:00

Yes, Visual Studio Community 2019.

//First comment wasn't showing up, that is why there are 2.
sValentin 41 Reputation points

2023-01-26T08:07:28.5566667+00:00

Unfortunately that isn't an option for me, as the project is of NMake type.

//Edit: Something might be wrong with the site. I tried to edit a comment and got error 404, and this comment should have been placed in the answer below not here.
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Minxin Yu 10,031 Reputation points Microsoft Vendor

2023-01-26T09:32:17.2466667+00:00

Is your problem with compiling the program or running it?
RLWA32 40,651 Reputation points

2023-01-26T10:37:14.17+00:00

There are a number of Windows API functions that enable a process to alter the DLL search path at runtime. For example, SetDllDirectoryW, SetDefaultDllDirectories, AddDllDirectory.

Depending on when jvm.dll need to be accessible it may also be necessary to use delay-loading. I'm not a JNI user so I can't give you detailed guidance or an example.
sValentin 41 Reputation points

2023-01-26T11:03:40.85+00:00

AddDllDirectory is looking like it would be the thing I need, but unfortunately I'm getting some errors when building the project with Unreal Engine when I use that or LoadLibraryW. I made a thread in their forum, maybe someone can help me there.
RLWA32 40,651 Reputation points

2023-01-26T11:18:42.53+00:00

Another possibility is to use use a loader process. The loader process would pass an environment block that you have customized to include the needed path to jvm.dll to the CreateProcessW function which starts your target application. That way, no modification would be needed to your actual User or System environment variables.
Viorel 112.5K Reputation points

2023-01-26T20:28:04.9833333+00:00

Instead of setting the environment variables manually, maybe you can use the _putenv and other functions to adjust the variables of the current process before starting VM?

RLWA32 40,651

Further to Viorel's suggestion following is some sample code that I had used when experimenting with delay loading and dll search paths --

BOOL AddToEnvironmentPath(LPCTSTR pszPath)
{
    BOOL fSuccess = FALSE;
    size_t count = 0;
    LPTSTR pszDupPath = NULL;
    _ASSERTE(pszPath != NULL);

    if (!_tdupenv_s(&pszDupPath, &count, _T("PATH")))
    {
        size_t len = _tcslen(pszPath) + count + 1; // bump for semicolon, count already includes null terminator
        LPTSTR pszNew = (LPTSTR)malloc(len * sizeof(TCHAR));
        if (pszNew)
        {
            if (!_tcscpy_s(pszNew, len, pszDupPath) &&
                !_tcscat_s(pszNew, len, _T(";")) &&
                !_tcscat_s(pszNew, len, pszPath) &&
                !_tputenv_s(_T("PATH"), pszNew))
            {
                fSuccess = TRUE;
            }

            free(pszNew);
        }

        free(pszDupPath);
    }

    return fSuccess;
}

sValentin 41 Reputation points

2023-01-27T07:20:26.48+00:00

I'm already using something to add automatically what I need into User Path (as this doesn't require admin as System Path does), and change that value in case the location is changed because of user action, but I wanted to try and skip that process. But it is fine, I'll stick to that, as it is working as I want. I just wanted to maybe find a version that doesn't require that, but it seems with JNI it is more different, I tried some functions that with other DLL were working but with JNI it doesn't.

Your suggestion using CreateProcessW can work, but I got lost at some point, as C++ isn't the language I normally use.

Like I said, I'll stick to what I have now, but thank you for all of your suggestions! It is still good to know about them, maybe I'll use them in the future for something else.

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sValentin 41

I'm going to stick with the way I have now which is to add automatically what I need into User Path (as this doesn't require admin as System Path does), and change that value in case the location is changed because of user action.

It would have been nice to skip that process of having to add a variable into PATH, but as I already have something that works, I'll stick with that and continue the project, and maybe when it is finished I'll take another look at this.

Thank you all for all of your suggestions!

As a note, to skip having to add a variable to the PATH, can be done using CreateProcessW. I didn't got to a point where the code would work due to my lack of C++ knowledge. I'll come back to this when the project is done.

RLWA32 40,651 Reputation points

2023-01-27T07:45:36.99+00:00

Just to be clear, if you used the code that I posted inside your application that uses JNI the path environment variable that would be changed is private to that process. The actual USER and SYSTEM environment variables would not be affected. The posted code does not require elevated privileges.

sValentin 41

#include <filesystem>
#include <string>
#include "pch.h"
#include "Your_JNI File.h"

void setVariable(std::string variable) {
	HKEY hKey;
	DWORD dwResult = 0;
	if (RegCreateKeyEx(HKEY_CURRENT_USER, TEXT("Environment"), 0, NULL, REG_OPTION_NON_VOLATILE, KEY_ALL_ACCESS, NULL, &hKey, &dwResult) == ERROR_SUCCESS) {
		RegSetValueExA(hKey, "Path", 0, REG_EXPAND_SZ, (const BYTE*)variable.c_str(), variable.size() + 1);
	}
	if (hKey) {
		RegCloseKey(hKey);
	}
	SendNotifyMessage(HWND_BROADCAST, WM_SETTINGCHANGE, 0, (LPARAM)TEXT("Environment"));
}

std::string getVariable() {
	char* buffer = new char[16383];
	DWORD size = 16383;
	HKEY hKey;
	if (RegOpenKeyExA(HKEY_CURRENT_USER, "Environment", 0, KEY_READ, &hKey) == ERROR_SUCCESS) {
		if (RegQueryValueExA(hKey, "Path", NULL, NULL, (LPBYTE) buffer, &size) != ERROR_SUCCESS) {
			strcpy_s(buffer, 13, "PathNotFound");
		} else {
			RegCloseKey(hKey);
		}
	} else {
		strcpy_s(buffer, 13, "PathNotFound");
	}
	std::string data = buffer;
	delete[] buffer;
	return data;
}

JNIEXPORT jstring JNICALL Java_Your_JNI_ Function_Get(JNIEnv* env, jclass) {
	return env->NewStringUTF(getVariable().c_str());
}

JNIEXPORT void JNICALL Java_Your_JNI_ Function_Set(JNIEnv* env, jclass, jstring jPath) {
	jboolean isCopy;
	const char* cPath = env->GetStringUTFChars(jPath, &isCopy);
	env->ReleaseStringUTFChars(jPath, cPath);
	setVariable(cPath);
}