Unity 2021.3.21f1 wants to access mUnityPlayer field

[Luluno01]

After upgrading to editor version 2021.3.21f1, Android build is no longer working:

E Unity   : AndroidJavaException: java.lang.NoSuchFieldError: no "Ljava/lang/Object;" field "mUnityPlayer" in class "Lcom/example/MainActivity;" or its superclasses
E Unity   : java.lang.NoSuchFieldError: no "Ljava/lang/Object;" field "mUnityPlayer" in class "Lcom/example/MainActivity;" or its superclasses
E Unity   :      at com.unity3d.player.UnityPlayer.nativeRender(Native Method)
E Unity   :      at com.unity3d.player.UnityPlayer.access$500(Unknown Source:0)
E Unity   :      at com.unity3d.player.UnityPlayer$e$1.handleMessage(Unknown Source:126)
E Unity   :      at android.os.Handler.dispatchMessage(Handler.java:102)
E Unity   :      at android.os.Looper.loop(Looper.java:233)
E Unity   :      at com.unity3d.player.UnityPlayer$e.run(Unknown Source:20)
E Unity   :   at UnityEngine.AndroidJNISafe.CheckException () [0x00000] in <00000000000000000000000000000000>:0
E Unity   :   at UnityEngine.AndroidJNISafe.GetFieldID (System.IntPtr clazz, System.String name, System.String sig) [0x00000] in <00000000000000000000000000000000>:0
E Unity   :   at UnityEngine._AndroidJNIHelper.GetFieldID (System.IntPtr jclass, System.String fieldName, System.Str

Somehow the Unity native code wants to access mUnityPlayer field of current activity class, which is never defined but it worked in previous versions. After looking into the built-in UnityPlayerActivity I found protected UnityPlayer mUnityPlayer; // don't change the name of this variable; referenced from native code, which has been there before it breaks. So it looks like there is change in the native code, but I can't find the source code of it.

[Luluno01]

Found a similar report https://stackoverflow.com/questions/75728722/flutter-unity-integration-errorandroidjavaexception-java-lang-nosuchfielderror

It seems Unity has changed the way it interacts with the hosting activity. Just don't know why the native code will need the mUnityPlayer field and what the native code is expecting from the hosting activity.

[anothercog]

I know this is an old issue, but it was stumping me for a while. I've found a solution.

The problem with many 3rd party libraries that require access to Unity is that they're grabbing the UnityPlayer from the mUnityPlayer variable in the MainActivity, which is fine if you're not using Unity as a library, but otherwise, if you're able to put your own mUnityPlayer variable in MainActivity populated with the UnityPlayer instance, these libraries will work fine.

My solution was to add the following to the bottom of the UPlayer constructor in \react-native-unity\android\src\main\java\com\azesmwayreactnativeunity\UPlayer.java

        try {
            activity.getClass().getMethod("setUnityPlayer", java.lang.Object.class).invoke(activity, unityPlayer);
        } catch (Exception e) {
            e.printStackTrace();
        }

Then your MainActivity.java class you need:

  public java.lang.Object mUnityPlayer = null;
  public void setUnityPlayer(java.lang.Object newUnityPlayer) {
    mUnityPlayer = newUnityPlayer;
  } 

The MainActivity.java being located in \android\app\src\main\java\com\myprojectname\MainActivity.java

Here's my full MainActivity.java:

package com.myprojectname;

import com.facebook.react.ReactActivity;
import com.facebook.react.ReactActivityDelegate;
import com.facebook.react.defaults.DefaultNewArchitectureEntryPoint;
import com.facebook.react.defaults.DefaultReactActivityDelegate;

public class MainActivity extends ReactActivity {

  public java.lang.Object mUnityPlayer = null;
  /**
   * Returns the name of the main component registered from JavaScript. This is used to schedule
   * rendering of the component.
   */
  @Override
  protected String getMainComponentName() {
    return "myprojectname";
  }

  /**
   * Returns the instance of the {@link ReactActivityDelegate}. Here we use a util class {@link
   * DefaultReactActivityDelegate} which allows you to easily enable Fabric and Concurrent React
   * (aka React 18) with two boolean flags.
   */
  @Override
  protected ReactActivityDelegate createReactActivityDelegate() {
    return new DefaultReactActivityDelegate(
        this,
        getMainComponentName(),
        // If you opted-in for the New Architecture, we enable the Fabric Renderer.
        DefaultNewArchitectureEntryPoint.getFabricEnabled());
  }

  public void setUnityPlayer(java.lang.Object newUnityPlayer) {
    mUnityPlayer = newUnityPlayer;
  } 
}

And the full UPlayer.java constructor

    public UPlayer(final Activity activity, final ReactNativeUnity.UnityPlayerCallback callback) throws ClassNotFoundException, InvocationTargetException, IllegalAccessException, InstantiationException {
        super();
        Class<?> _player = null;

        try {
            _player = Class.forName("com.unity3d.player.UnityPlayerForActivityOrService");
        } catch (ClassNotFoundException e) {
            _player = Class.forName("com.unity3d.player.UnityPlayer");
        }

        Constructor<?> constructor = _player.getConstructors()[1];
        unityPlayer = (UnityPlayer) constructor.newInstance(activity, new IUnityPlayerLifecycleEvents() {
            @Override
            public void onUnityPlayerUnloaded() {
                callback.onUnload();
            }

            @Override
            public void onUnityPlayerQuitted() {
                callback.onQuit();
            }
        });

        try {
            activity.getClass().getMethod("setUnityPlayer", java.lang.Object.class).invoke(activity, unityPlayer);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }