Returning byte array from Java/Android

Questions & Answers
1

I have several functions that make use of AndroidJavaObject.Call() to run Java code in my Unity project. 

AndroidJavaObject androidUtility = new AndroidJavaObject("com.my.java.library");

androidUtility.Call("myFunctionA", data1, data2);

androidUtility.Call<bool>("myFunctionB", data1, data2);

androidUtility.Call<byte[]>("myFunctionC", data1, data2);

The functions all get called. The call to myFunctionB returns a boolean. But the call to myFunctionC returns an empty array no matter what I put in the Java code.

Does AndroidJavaObject.Call() not work when returning arrays? Is there some special processing required?

Thanks for your help.

3

This seems to be an issue with little knowledge, so I thought I would reply even though this thread is quite old.

The problem is that an array is not a native type, so you can’t do what you are trying in the example. Arrays, as far as I can tell, must always be transferred in the form of an Object. The Unity JNIHelper class offers a method (ConvertFromJNIArray()) to change an AndroidJavaObject into a .NET array. The array has to be an array of native types from what I can tell (except string, which isn’t native, but it still works). There is also a ConvertToJNIArray that you can use to send an array to the Java side. Hopefully the code below will shed some light on it.

C# code:
    AndroidJavaClass cls = new AndroidJavaClass("com.mycompany.myproduct.myclass");
    AndroidJavaObject obj = cls.CallStatic<AndroidJavaObject>("getStringArray");
    if (obj.GetRawObject().ToInt32() != 0)
    {
      // String[] returned with some data!
      String[] result = AndroidJNIHelper.ConvertFromJNIArray<String[]>
                            (obj.GetRawObject());
      foreach (String str in result)
      {
        // Do something with the strings
      }
    }
    else
    {
      // null String[] returned
    }
    obj.Dispose();
    cls.Dispose();

java code:
public static String[] getStringArray()
{
  String[] strs = { "Test1", "Test2" };
  return strs;   // Or alternatively, return null;
}

Pay special attention to checking for null by using the obj.GetRawObject().ToInt32() method and testing it for 0. obj itself will not be null as it is a valid object, but if you call ConvertFromJNIArray(obj) with the obj’s Raw Object value of 0, it will crash.

2

Also here is an example from the (docs):

// Create a java.lang.String object holding the string "some string",
// and retrieve it's hash code.
function Start() {
    var jo = new AndroidJavaObject("java.lang.String", "some string");
    var hash = jo.Call.<int>("hashCode");
}

Modified version, to return an array:

function Start() {
    var jc = new AndroidJavaClass ("com.test.test");
    var doubleArary = jc.Call.<double[]>("getDoubleArray");
}

From logcat:

03-07 11:43:35.143: I/Unity(23708): getMethodID("com.test.test", "getDoubleArray", "()[D", non-static)
03-07 11:43:35.143: I/Unity(23708): FOUND double[] test.getDoubleArray();
03-07 11:43:35.153: W/dalvikvm(23708): JNI WARNING: received null array (Check_GetArrayLength)

The called java method:

public double[] getDoubleArray() {
	return new double[] { 1, 2, 3 };
}
4

Using Supernat’s answer

  1. you can feed bytes to POJO
public class ReadData {
    public byte[] Buffer;
    public ReadData(byte[] buffer) {
        Buffer=buffer;
    }
  }
  1. Call your method in Android (Java) code
callback.DidReceiveDataCallback(new ReadData(data));
  1. Render the bytes in Unity
public void DidReceiveDataCallback(AndroidJavaObject bytesObj)
        {
            Debug.Log("Unity callbacks: Receive Data ");
            AndroidJavaObject bufferObject = bytesObj.Get<AndroidJavaObject>("Buffer");
            byte[] buffer = AndroidJNIHelper.ConvertFromJNIArray<byte[]>(bufferObject.GetRawObject());
           // Do something with bytes
        }