Call Java and Kotlin plug-in code from C# scripts

To call Java code from C# scripts, Unity provides C# APIs that communicate with the Android Java Native Interface (JNI) through C++. Unity provides both a low level and a high level API that you can use to interact with Java code using JNI.

Low-level API

The low-level AndroidJNI class wraps JNI calls and provides static methods that directly map to JNI methods. The AndroidJNIHelper API provides helper functionality that is primarily used by the high-level API, but they can be useful in certain situations.

High-level API

The high-level AndroidJavaObject, AndroidJavaClass, and AndroidJavaProxy APIs automate a lot of tasks required for JNI calls. They also use caching to make calls to Java faster. The combination of AndroidJavaObject and AndroidJavaClass is built on top of AndroidJNI and AndroidJNIHelper, but they also contain additional functionality such as static methods that you can use to access static members of Java classes.

Instances of AndroidJavaObject and AndroidJavaClass have a one-to-one mapping to an instance of java.lang.Object and java.lang.Class respectively. They provide three types of interactions with Java/Kotlin code:

• Call a method.
• Get the value of a field.
• Set the value of a field.

Each interaction also has a static version:

• CallStatic to call a static method.
• GetStatic to get the value of a static field.
• SetStatic to set the value of a static field.

When you get the value of a field or call a method that returns a value, you use generics to specify the return type. When you set the value of a field, you also use generics to specify the type of the field that you are setting. For methods that don’t return a value, there is a regular, non-generic, version of Call.

Important: You must access any non-primitive type as an AndroidJavaObject. The only exception is a string which you access directly, even though they don’t represent a primitive type in Java.

例

This section contains code samples that show how to use the high-level AndroidJavaObject and AndroidJavaClass APIs.

Get the hash code for a Java string

The following code sample creates an instance of java.lang.String initialized with a string, and retrieves the hash value for that string.

using UnityEngine;
public class JavaExamples
{
    public static int GetJavaStringHashCode(string text)
    {
        using (AndroidJavaObject jo = new AndroidJavaObject("java.lang.String", text))
        {
            int hash = jo.Call<int>("hashCode");
            return hash;
        }
    }
}

This example:

1. Creates an AndroidJavaObject that represents a java.lang.String. The AndroidJavaObject constructor takes at least one parameter, which is the name of the class to construct an instance of. Any parameters after the class name are for the constructor call on the object, in this case the text parameter from GetJavaStringHashCode.
2. Calls hashCode() to get the hash code of the string. This call uses the int generic type parameter for Call because hashCode() returns the hash code as an integer.

Note: You can’t use dotted notation to instantiate a nested Java class. You must use the $ separator to instantiate inner classes. For example, Use android.view.ViewGroup$LayoutParams or android/view/ViewGroup$LayoutParams, where the LayoutParams class is nested in the ViewGroup class.

Get the cache directory

The following code sample shows how to get the cache directory for the current application in C# without using plug-ins.

using UnityEngine;

public class JavaExamples
{
    public static string GetApplicationCacheDirectory()
    {
       using (AndroidJavaClass unityPlayer = new AndroidJavaClass("com.unity3d.player.UnityPlayer"))
        using (AndroidJavaObject currentActivity = unityPlayer.GetStatic<AndroidJavaObject>("currentActivity"))
        using (AndroidJavaObject javaFile = currentActivity.Call<AndroidJavaObject>("getCacheDir"))
        {
            string cacheDirectory = javaFile.Call<string>("getCanonicalPath");
            return cacheDirectory;
        }
    }
}

This example:

1. Creates an AndroidJavaClass to represent com.unity3d.player.UnityPlayer. It’s best practice to use AndroidJavaClass instead of AndroidJavaObject to access static members.
2. Creates an AndroidJavaObject to represent the current activity, which is a static member of com.unity3d.player.UnityPlayer.
3. Calls getCacheDir() on the Activity object, which returns a File object that represents the cache directory.
4. Calls getCanonicalPath() on the File object can to get the cache directory as a string.

Note: This example is for reference purposes. Instead, to access the application’s cache and file directory use the Application.temporaryCachePath and Application.persistentDataPath APIs.

Pass data from Java to Unity

The following code sample shows how to pass data from Java to Unity using UnitySendMessage.

using UnityEngine;

public class JavaExamples : MonoBehaviour
{

    private void Start()
    {
        AndroidJNIHelper.debug = true;
        using (AndroidJavaClass jc = new AndroidJavaClass("com.unity3d.player.UnityPlayer"))
        {
            jc.CallStatic("UnitySendMessage", "My GameObject", "JavaMessage", "NewMessage");
        }
    }

    private void JavaMessage(string message)
    {
        Debug.Log("message from java: " + message);
    }
}

This example:

1. Creates an AndroidJavaClass to represent com.unity3d.player.UnityPlayer.
2. Calls the staticUnitySendMessage method that is a member of com.unity3d.player.UnityPlayer.

Although you call UnitySendMessage from within Unity, it relays the message using Java, then Java calls back to the native/Unity code to deliver the message to the object, in this example it is the GameObject named “My GameObject” which has an attached script that contains a method called JavaMessage.

Best practice

This section describes best practices to be aware of when you call Java and Kotlin plug-in code from C# scripts.

Minimize JNI calls

Using the Java Native Interface (JNI), through either the high-level or low-level C# API is resource intensive and can be slow. To improve performance, and also code clarity, it’s best practice to keep the number of JNI calls low.

To avoid unnecessary JNI calls, the high-level C# API caches the ID of each Java method that you call. This means that subsequent calls to the same method are not as resource intensive as the first call. The calls don’t need to be during the same frame or even from the same AndroidJavaObject/AndroidJavaClass instance. If you use the low-level API and want this performance benefit, you must cache method ID yourself. Otherwise, it’s best practice to use the high-level API.

Note: Unity maintains the cache until the application closes. This includes while the application is in the background.

Garbage collection

You should wrap any instance of AndroidJavaObject or AndroidJavaClass with a using statement to ensure Unity destroys them as soon as possible. If you don’t use using, Unity’s garbage collector should still release all created instances, but you lose control of when this will be.

The following code sample shows how to use using statements to get the system language in an optimal way:

void Start () 
{ 
    using (AndroidJavaClass cls = new AndroidJavaClass("java.util.Locale")) 
    using(AndroidJavaObject locale = cls.CallStatic<AndroidJavaObject>("getDefault")) 
    { 
        Debug.Log("current lang = " + locale.Call<string>("getDisplayLanguage")); 
    } 
}

Note: To see a record of the garbage collector’s activity in Android Logcat, set AndroidJNIHelper.debug to true.