Calling a java method in android from Unity

Question

Our project uses it's custom video player script and makes a call to the android side within the Update function - to update audio angles as this is a 360 degree video player. There is a large amount of garbage collection going on per frame based on profiler sample results. We are passing two float arguments to the java function. Does anyone know why there is a lot of GC happening in this call? Pasting the code here:

 horizontal = eulerAngles [1] * Mathf.Deg2Rad;
 vertical = eulerAngles [0] * Mathf.Deg2Rad;
 _JO.Call(SET_AUDIO_ANGLE, -horizontal, vertical);

Answer 1

Two things: First the signature of the Call method is

 public void Call(string methodName, params object[] args)

That means each time you call it you will create a new "object[]" with your two parameters. The second thing is your two float parameters need to be boxed in order to be stored in the array.

Unfortunately there's nothing you can do against the boxing beside avoid calling the method when the angles haven't changed. You may use a minimum difference threshold. The object array can be cached. So create the array once and reuse it.

edit

Well there's actually a way around that boxing problem. The "Call" method is rather convenient to use but creates a lot of "internal garbage". The actual JNI interface doesn't take an "object[]" but rather a "jvalue[]". jvalue is a struct with overlapping fields for the various types.

So the solution would be to:

Use AndroidJNIHelper.GetMethodID to get back an IntPtr to the method id . You have to pass the native class reference, the method name, the object[] with the parameters and "false" (since it's an instance method, not a static method). You store that reference next to your actual "_JO".
Beside that you create a jvalue array of size 2. This array should be cached and reused.
Finally you can call the method with AndroidJNI.CallVoidMethod. You need to pass the native object pointer which you get from _JO.GetRawObject(), the method ID we stored and the jvalue array. Before you call that method of course you have to update the values in the array. The struct type has a field named "f" which is of type float. There you can assign your two values.

So it would look something like this:

 AndroidJavaObject _JO;
 System.IntPtr methodID;
 jvalue[] parameters = new jvalue[2];
 
 void Init()
 {
     // make sure your _JO object is already created at this point.
     //methodID = AndroidJNIHelper.GetMethodID(_JO.GetRawClass(), SET_AUDIO_ANGLE, new object[]{0f,0f}, false);
     // just noticed that there is also a GetMethodID version that takes a signature string. This avoids
     // the temp object array. The array is only used to create that signature string from the types
     methodID = AndroidJNIHelper.GetMethodID(_JO.GetRawClass(), SET_AUDIO_ANGLE, "(FF)V", false);
 }
 
 void UpdateAudioAngles(Vector3 aEulerAngles)
 {
     parameters[0].f = -aEulerAngles.y * Mathf.Deg2Rad;
     parameters[1].f = aEulerAngles.x * Mathf.Deg2Rad;
     AndroidJNI.CallVoidMethod(_JO.GetRawObject(), methodID, parameters);
 }

Note: Be aware that "CallVoidMethod" might throw an exception. Unity actually has an internal class (in all their wiseness they makes all useful classes internal) that wraps the call in a try catch block and performs some special Java exception handling in case of an exception. This is missing in this case. AS long as there is no error in your java method you should be fine. Don't ask me what happens when an exception is thrown. You might just try it ^^.

ps: i guessed "SET_AUDIO_ANGLE" is actually a string constant that holds the method name, right?

Calling a java method in android from Unity

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