Bidirectional link between java application and an unreal game


I am thinking about making a bidirectional link between a java application which controls some parts of an unreal game. Setting and getting game values. Probably a dll can make it. I know how to use a dll in an unreal game and in a java application.

I don’t know how to call a game method by the java application. E.g. I want to read the window size of the game or set the player’s position. Would be callbacks, provided by the dll to the game, a solution?


Short answer; using a dll does not work! Each process runs its own copy of the dll in his process space. Sharing dll data is not possible. Inter process communication do this.

Did you managed to do it and could you explain to me how? :open_mouth: I’m looking for a similar fix

In case someone else needs this here is how it’s done. This link between c++ and java made possible with jni.
The easiest way to access java is to modify game activity class with upl using a tag . For example:


      public void testFunction()
      Log.debug("caled java code");

Then in c++ function you call:

static jmethodID javaMethodId;


	JNIEnv* env = FAndroidApplication::GetJavaEnv();
	javaMethodId = FJavaWrapper::FindMethod(env, 
        FJavaWrapper::GameActivityClassID, "testFunction", "()V", false);
	FJavaWrapper::CallVoidMethod(env, FJavaWrapper::GameActivityThis, javaMethodId);

You need to know the sinature of java function, you can find it here.
This is a common knowledge that could be found, I just summarised it. What I couldn’t find is how to call back from java to c++ and here is how you do it.
Again using simply game activity you add function declaration of c++ method in java as described previously:

      private static native void howToGetString(String code);

In c++(I used the same file from where I call java class) you do the following:

#include <jni.h>

static FCriticalSection ReceiversLock;

static FString OurString = "";

extern "C"
	JNIEXPORT void Java_com_epicgames_ue4_GameActivity_howToGetString(JNIEnv * jni, jclass clazz, jstring code)
		const char* charsId = jni->GetStringUTFChars(code, 0);
		OurString = FString(UTF8_TO_TCHAR(charsId));
		jni->ReleaseStringUTFChars(code, charsId);

Now you can call function howToGetString("a string ") and it will call you c++ class. I used it in OnActivityResult to get data from my intent.
Hope this will help someone.

1 Like

Lovely. Works just like described. Thanks!