Documentation

Unity Engine

User Manual

Script Reference

Unity Engine

Calling C# code from Java and Kotlin plug-in code

Call C# script code from your Java and Kotlin plug-in code.
Read time 4 minutesLast updated 9 days ago
Java and Kotlin plug-in code can call your C# scripts, for example, to notify Unity of an Android event or to return a result from your plug-in code. There are two ways to do this:
  • Send a message to a GameObject with 
    UnitySendMessage
    .
  • Call a C# method through a callback interface with AndroidJavaProxy.
For the equivalent C# to Java and Kotlin implementation, refer to Calling Java and Kotlin plug-in code from C# scripts.
Note
To debug either approach, set AndroidJNIHelper.debug to 
true
to log the JNI activity that Unity performs for each call.

Send a message to a GameObject

Use 
UnitySendMessage
to send a string from Java or Kotlin plug-in code to a method on a named GameObject. This is a one-way call where the C# method receives the string but doesn't return a value to the plug-in. Use it for simple notifications.
The plug-in calls 
UnitySendMessage
with three arguments: the name of the target GameObject, the name of the method to call, and the string message.
The following Java example implements 
UnitySendMessage
:
package com.example.androidplugin;import com.unity3d.player.UnityPlayer;public class MessageSender {    public static void sendToUnity(String gameObjectName) {        UnityPlayer.UnitySendMessage(gameObjectName, "OnMessageReceived", "Hello from Java");    }}
The following Kotlin example implements 
UnitySendMessage
:
package com.example.androidpluginimport com.unity3d.player.UnityPlayerobject MessageSender {    @JvmStatic    fun sendToUnity(gameObjectName: String) {        UnityPlayer.UnitySendMessage(gameObjectName, "OnMessageReceived", "Hello from Kotlin")    }}
In your C# script, add the method that 
UnitySendMessage
calls. Ensure the script is attached to a GameObject whose name matches the first argument.
using UnityEngine;public class MessageReceiver : MonoBehaviour{    void Start()    {        gameObject.name = "MessageReceiver";        using var sender = new AndroidJavaClass("com.example.androidplugin.MessageSender");        sender.CallStatic("sendToUnity", gameObject.name);    }    void OnMessageReceived(string message)    {        Debug.Log("Message from plug-in: " + message);    }}
This example:
  1. Sets the GameObject's name to match the target name the plug-in sends.
  2. Calls the plug-in, which uses 
    UnitySendMessage
    to send a string back to the 
    OnMessageReceived
    method on the named GameObject.
  3. Logs the message that the plug-in sends.
Note
In this example, C# calls the plug-in directly to trigger the message, so you can run the example without an Android event. In a real project, the plug-in calls 
UnitySendMessage
in response to an Android event, such as a button tap or a lifecycle callback.
UnitySendMessage
has the following limitations:
  • You can only call C# methods with the signature 
    void MethodName(string message)
    .
  • Calls are asynchronous and have a delay of one frame.
  • Multiple GameObjects with the same name can cause conflicts.
To send data other than a string, serialize it to a JSON string. For more information, refer to Supported data types for Java/Kotlin and C# code.

Call a C# method through a callback interface

Use AndroidJavaProxy to implement a Java or Kotlin interface in C#. When the plug-in calls a method on the interface, Unity runs your C# implementation. Use this approach for typed callbacks, or to pass structured data or return values.
Define an interface in the plug-in and a class that calls it. The following plug-in code calls the interface method to pass a message back to C#.

Java plug-in code

package com.example.androidplugin;public class Greeter {    public interface GreetingCallback {        void onGreeting(String message);    }    public void requestGreeting(GreetingCallback callback) {        callback.onGreeting("Hello from Java");    }}
### Kotlin plug-in code
package com.example.androidpluginclass Greeter {    interface GreetingCallback {        fun onGreeting(message: String)    }    fun requestGreeting(callback: GreetingCallback) {        callback.onGreeting("Hello from Kotlin")    }}
In your C# script, create a class that derives from 
AndroidJavaProxy
and implements the interface. Pass an instance of this class to the plug-in.
using UnityEngine;public class GreetingExample : MonoBehaviour{    class GreetingCallback : AndroidJavaProxy    {        public GreetingCallback() : base("com.example.androidplugin.Greeter$GreetingCallback") { }        void onGreeting(string message)        {            Debug.Log("Greeting from plug-in: " + message);        }    }    void Start()    {        using var greeter = new AndroidJavaObject("com.example.androidplugin.Greeter");        greeter.Call("requestGreeting", new GreetingCallback());    }}
This example:
  1. Creates a C# 
    GreetingCallback
    class that derives from 
    AndroidJavaProxy
    and passes the interface name to the base constructor. Because 
    GreetingCallback
    is nested in the 
    Greeter
    class, the name uses the 
    $
    separator: 
    com.example.androidplugin.Greeter$GreetingCallback
    .
  2. Implements 
    onGreeting
    with a signature that matches the interface method. Unity automatically routes calls on the interface to this C# implementation.
  3. Creates an 
    AndroidJavaObject
    for the 
    Greeter
    plug-in class and calls 
    requestGreeting
    , passing the proxy. The plug-in then calls 
    onGreeting
    , which runs the C# implementation.
Note
In this example, the plug-in calls the interface method on Unity's main thread. If your plug-in calls the method from a background thread, the C# implementation also runs on the background thread. In this case, use AndroidApplication.InvokeOnUnityMainThread to run any Unity API calls on the main thread.
For more information on writing the plug-in code, refer to Java and Kotlin source plug-ins.

Additional resources