Android 接口定义语言 (AIDL)

Android 接口定义语言 (AIDL) 与其他 IDL 类似:您可以使用它定义客户端和服务都同意的编程接口,以便使用进程间通信 (IPC) 进行相互通信。

在 Android 上,一个进程通常无法访问另一个进程的内存。为了进行通信,它们需要将自己的对象分解成操作系统可以理解的原语,并为您将这些对象编组到该边界上。用于执行该序列化的代码很难编写,因此 Android 会使用 AIDL 为您处理此事宜。

注意:只有在您允许来自不同应用的客户端访问您的服务以进行 IPC,并且您希望在服务中处理多线程时,才需要使用 AIDL。如果您不需要跨不同应用执行并发 IPC,请通过实现 Binder 来创建接口。如果您想执行 IPC,但不需要处理多线程,请使用 Messenger 实现接口。无论如何,请务必先了解绑定服务,然后再实现 AIDL。

在开始设计 AIDL 接口之前,请注意,对 AIDL 接口的调用是直接函数调用。不要对调用发生的线程做出假设。具体情况会有所不同,具体取决于调用是来自本地进程中的线程还是来自远程进程:

  • 从本地进程发出的调用会在发出调用的同一线程中执行。如果这是您的主界面线程,该线程会继续在 AIDL 接口中执行。如果是其他线程,则该线程会在服务中执行您的代码。因此,如果只有本地线程访问该服务,您可以完全控制哪些线程在其中执行。但如果是这种情况,请完全不要使用 AIDL;而是通过实现 Binder 来创建接口。
  • 来自远程进程的调用会从平台在您自己的进程中维护的线程池中调度。做好准备,以便处理来自未知线程的传入调用,同时发生多个调用。换句话说,AIDL 接口的实现必须完全线程安全。在同一远程对象上通过一个线程进行的调用会按顺序到达接收器端。
  • oneway 关键字用于修改远程调用的行为。使用此方法时,远程调用不会阻塞。它会发送交易数据,然后立即返回。接口实现最终会将此调用作为来自 Binder 线程池的常规调用(即常规远程调用)接收。如果将 oneway 与本地调用搭配使用,则不会产生任何影响,调用仍是同步的。

定义 AIDL 接口

使用 Java 编程语言语法在 .aidl 文件中定义 AIDL 接口,然后将其保存在托管服务的应用和绑定到该服务的任何其他应用的源代码的 src/ 目录中。

当您构建包含 .aidl 文件的每个应用时,Android SDK 工具都会根据 .aidl 文件生成 IBinder 接口,并将其保存在项目的 gen/ 目录中。该服务必须相应地实现 IBinder 接口。然后,客户端应用可以绑定到服务并调用 IBinder 中的方法来执行 IPC。

如需使用 AIDL 创建边界服务,请按以下步骤操作,后续部分将对这些步骤进行介绍:

  1. 创建 .aidl 文件

    此文件定义带有方法签名的编程接口。

  2. 实现接口

    Android SDK 工具会根据您的 .aidl 文件,使用 Java 编程语言生成接口。此接口有一个名为 Stub 的内部抽象类,该类会扩展 Binder 并实现 AIDL 接口中的方法。您必须扩展 Stub 类并实现相应方法。

  3. 向客户公开接口

    实现 Service 并替换 onBind(),以返回 Stub 类的实现。

注意:在首次发布后,您对 AIDL 接口所做的任何更改都必须保持向后兼容,以免破坏使用您服务的其他应用。也就是说,由于您的 .aidl 文件必须复制到其他应用,以便它们访问您的服务接口,因此您必须继续支持原始接口。

创建 .aidl 文件

AIDL 使用简单的语法,让您可以声明一个接口,其中包含一个或多个可接受参数和返回值的方法。参数和返回值可以是任何类型,甚至是其他 AIDL 生成的接口。

您必须使用 Java 编程语言构建 .aidl 文件。每个 .aidl 文件都必须定义一个接口,并且只需要接口声明和方法签名。

默认情况下,AIDL 支持下列数据类型:

  • Java 编程语言中的所有基元类型(short 除外,例如 intlongcharboolean 等)
  • 任何类型的数组,例如 int[]MyParcelable[]
  • String
  • CharSequence
  • List

    List 中的所有元素都必须是此列表中支持的数据类型之一,或者您声明的其他 AIDL 生成的接口或 Parcelable 之一。List 可以选择用作参数化类型类,例如 List<String>。虽然方法是生成以使用 List 接口,但另一端收到的实际具体类始终是 ArrayList

  • Map

    Map 中的所有元素都必须是此列表中支持的数据类型之一,或者您声明的其他 AIDL 生成的接口或 Parcelable 之一。不支持参数化类型映射,例如采用 Map<String,Integer> 形式的映射。虽然生成的方法是使用 Map 接口,但另一端收到的实际具体类始终是 HashMap。考虑使用 Bundle 代替 Map

您必须为之前未列出的每个其他类型添加 import 语句,即使它们与接口定义在同一软件包中也是如此。

定义服务接口时,请注意:

  • 方法可以接受零个或多个参数,并且可以返回值或 void。
  • 所有非基元参数都需要一个方向标记,用于指明数据的传输方向:inoutinout(请参阅下面的示例)。

    基元、StringIBinder 和 AIDL 生成的接口默认为 in,无法采用其他值。

    注意:请将方向限制为真正需要的方向,因为封装参数的开销很高。

  • 除了 import 和 package 语句之前的注释外,.aidl 文件中包含的所有代码注释都包含在生成的 IBinder 接口中。
  • 可以在 AIDL 接口中定义字符串和整数常量,例如 const int VERSION = 1;
  • 方法调用由 transact() 代码调度,该代码通常基于接口中的方法索引。由于这会导致版本控制变得困难,因此您可以手动将事务代码分配给方法:void method() = 10;
  • 必须使用 @nullable 为可为 null 的参数和返回类型添加注解。

下面是一个示例 .aidl 文件:

// IRemoteService.aidl
package com.example.android;

// Declare any non-default types here with import statements.

/** Example service interface */
interface IRemoteService {
    /** Request the process ID of this service. */
    int getPid();

    /** Demonstrates some basic types that you can use as parameters
     * and return values in AIDL.
     */
    void basicTypes(int anInt, long aLong, boolean aBoolean, float aFloat,
            double aDouble, String aString);
}

.aidl 文件保存在项目的 src/ 目录中。在构建应用时,SDK 工具会在项目的 gen/ 目录中生成 IBinder 接口文件。生成的文件的名称与 .aidl 文件的名称一致,但扩展名为 .java。例如,IRemoteService.aidl 会导致 IRemoteService.java

如果您使用 Android Studio,增量 build 几乎会立即生成 binder 类。如果您不使用 Android Studio,Gradle 工具会在您下次构建应用时生成 binder 类。编写完 .aidl 文件后,立即使用 gradle assembleDebuggradle assembleRelease 构建项目,以便您的代码可以与生成的类相关联。

实现接口

构建应用时,Android SDK 工具会生成一个 .java 接口文件,该文件的名称以 .aidl 文件的名称命名。生成的接口包含一个名为 Stub 的子类,该子类是其父接口(例如 YourInterface.Stub)的抽象实现,并声明 .aidl 文件中的所有方法。

注意Stub 还定义了一些辅助方法,其中最值得注意的是 asInterface(),它接受一个 IBinder(通常是传递给客户端的 onServiceConnected() 回调方法的 IBinder),并返回桩接口的实例。如需详细了解如何进行此转换,请参阅调用 IPC 方法部分。

如需实现从 .aidl 生成的接口,请扩展生成的 Binder 接口(例如 YourInterface.Stub),并实现从 .aidl 文件继承的方法。

下面是使用匿名实例实现名为 IRemoteService 的接口(由前面的 IRemoteService.aidl 示例定义)的示例:

Kotlin

private val binder = object : IRemoteService.Stub() {

    override fun getPid(): Int =
            Process.myPid()

    override fun basicTypes(
            anInt: Int,
            aLong: Long,
            aBoolean: Boolean,
            aFloat: Float,
            aDouble: Double,
            aString: String
    ) {
        // Does nothing.
    }
}

Java

private final IRemoteService.Stub binder = new IRemoteService.Stub() {
    public int getPid(){
        return Process.myPid();
    }
    public void basicTypes(int anInt, long aLong, boolean aBoolean,
        float aFloat, double aDouble, String aString) {
        // Does nothing.
    }
};

现在,binderStub 类(一个 Binder)的实例,该类定义了服务的 IPC 接口。在下一步中,此实例会公开给客户端,以便客户端与服务交互。

实现 AIDL 接口时,请注意以下几条规则:

  • 无法保证传入调用会在主线程上执行,因此您需要从一开始就考虑多线程处理,并正确构建服务以实现线程安全。
  • 默认情况下,IPC 调用是同步的。如果您知道服务需要几毫秒以上的时间才能完成请求,请勿从 activity 的主线程调用该服务。这可能会导致应用挂起,进而导致 Android 显示“应用无响应”对话框。从客户端的单独线程中调用它。
  • 只有 Parcel.writeException() 参考文档下列出的异常类型会发送回调用方。

向客户端公开该接口

为服务实现接口后,您需要将其公开给客户端,以便客户端可以绑定到该接口。如需公开服务的接口,请扩展 Service 并实现 onBind(),以返回实现生成的 Stub 的类的实例,如前一部分所述。以下是一个向客户端公开 IRemoteService 示例接口的服务示例。

Kotlin

class RemoteService : Service() {

    override fun onCreate() {
        super.onCreate()
    }

    override fun onBind(intent: Intent): IBinder {
        // Return the interface.
        return binder
    }


    private val binder = object : IRemoteService.Stub() {
        override fun getPid(): Int {
            return Process.myPid()
        }

        override fun basicTypes(
                anInt: Int,
                aLong: Long,
                aBoolean: Boolean,
                aFloat: Float,
                aDouble: Double,
                aString: String
        ) {
            // Does nothing.
        }
    }
}

Java

public class RemoteService extends Service {
    @Override
    public void onCreate() {
        super.onCreate();
    }

    @Override
    public IBinder onBind(Intent intent) {
        // Return the interface.
        return binder;
    }

    private final IRemoteService.Stub binder = new IRemoteService.Stub() {
        public int getPid(){
            return Process.myPid();
        }
        public void basicTypes(int anInt, long aLong, boolean aBoolean,
            float aFloat, double aDouble, String aString) {
            // Does nothing.
        }
    };
}

现在,当客户端(例如 activity)调用 bindService() 以连接到此服务时,客户端的 onServiceConnected() 回调会收到服务的 onBind() 方法返回的 binder 实例。

客户端还必须有权访问接口类。因此,如果客户端和服务位于单独的应用中,则客户端的应用必须在其 src/ 目录中包含 .aidl 文件的副本,该副本会生成 android.os.Binder 接口,以便客户端访问 AIDL 方法。

当客户端在 onServiceConnected() 回调中收到 IBinder 时,必须调用 YourServiceInterface.Stub.asInterface(service) 将返回的参数转换为 YourServiceInterface 类型:

Kotlin

var iRemoteService: IRemoteService? = null

val mConnection = object : ServiceConnection {

    // Called when the connection with the service is established.
    override fun onServiceConnected(className: ComponentName, service: IBinder) {
        // Following the preceding example for an AIDL interface,
        // this gets an instance of the IRemoteInterface, which we can use to call on the service.
        iRemoteService = IRemoteService.Stub.asInterface(service)
    }

    // Called when the connection with the service disconnects unexpectedly.
    override fun onServiceDisconnected(className: ComponentName) {
        Log.e(TAG, "Service has unexpectedly disconnected")
        iRemoteService = null
    }
}

Java

IRemoteService iRemoteService;
private ServiceConnection mConnection = new ServiceConnection() {
    // Called when the connection with the service is established.
    public void onServiceConnected(ComponentName className, IBinder service) {
        // Following the preceding example for an AIDL interface,
        // this gets an instance of the IRemoteInterface, which we can use to call on the service.
        iRemoteService = IRemoteService.Stub.asInterface(service);
    }

    // Called when the connection with the service disconnects unexpectedly.
    public void onServiceDisconnected(ComponentName className) {
        Log.e(TAG, "Service has unexpectedly disconnected");
        iRemoteService = null;
    }
};

如需查看更多示例代码,请参阅 ApiDemos 中的 RemoteService.java 类。

通过 IPC 传递对象

在 Android 10(API 级别 29 或更高版本)中,您可以直接在 AIDL 中定义 Parcelable 对象。此处还支持作为 AIDL 接口参数和其他 Parcelable 的类型。这样可以避免手动编写序列化代码和自定义类的额外工作。不过,这也会创建一个裸结构体。如果需要自定义访问器或其他功能,请改为实现 Parcelable

package android.graphics;

// Declare Rect so AIDL can find it and knows that it implements
// the parcelable protocol.
parcelable Rect {
    int left;
    int top;
    int right;
    int bottom;
}

上述代码示例会自动生成一个包含整数字段 lefttoprightbottom 的 Java 类。系统会自动实现所有相关的封装容器代码,并且无需添加任何实现即可直接使用该对象。

您还可以通过 IPC 接口将自定义类从一个进程发送到另一个进程。不过,请确保您的类的代码可供 IPC 通道的另一端使用,并且您的类必须支持 Parcelable 接口。支持 Parcelable 非常重要,因为它可让 Android 系统将对象分解为可跨进程封装的基本元素。

如需创建支持 Parcelable 的自定义类,请执行以下操作:

  1. 让您的类实现 Parcelable 接口。
  2. 实现 writeToParcel,该方法会获取对象的当前状态并将其写入 Parcel
  3. 向您的类添加一个名为 CREATOR 的静态字段,该字段是一个实现 Parcelable.Creator 接口的对象。
  4. 最后,创建一个用于声明 Parcelable 类的 .aidl 文件,如以下 Rect.aidl 文件所示。

    如果您使用的是自定义构建流程,请.aidl 文件添加到 build 中。与 C 语言中的头文件类似,此 .aidl 文件不会被编译。

AIDL 会在其生成的代码中使用这些方法和字段来编组和解组您的对象。

例如,以下 Rect.aidl 文件用于创建可分割的 Rect 类:

package android.graphics;

// Declare Rect so AIDL can find it and knows that it implements
// the parcelable protocol.
parcelable Rect;

下面的示例展示了 Rect 类如何实现 Parcelable 协议。

Kotlin

import android.os.Parcel
import android.os.Parcelable

class Rect() : Parcelable {
    var left: Int = 0
    var top: Int = 0
    var right: Int = 0
    var bottom: Int = 0

    companion object CREATOR : Parcelable.Creator<Rect> {
        override fun createFromParcel(parcel: Parcel): Rect {
            return Rect(parcel)
        }

        override fun newArray(size: Int): Array<Rect?> {
            return Array(size) { null }
        }
    }

    private constructor(inParcel: Parcel) : this() {
        readFromParcel(inParcel)
    }

    override fun writeToParcel(outParcel: Parcel, flags: Int) {
        outParcel.writeInt(left)
        outParcel.writeInt(top)
        outParcel.writeInt(right)
        outParcel.writeInt(bottom)
    }

    private fun readFromParcel(inParcel: Parcel) {
        left = inParcel.readInt()
        top = inParcel.readInt()
        right = inParcel.readInt()
        bottom = inParcel.readInt()
    }

    override fun describeContents(): Int {
        return 0
    }
}

Java

import android.os.Parcel;
import android.os.Parcelable;

public final class Rect implements Parcelable {
    public int left;
    public int top;
    public int right;
    public int bottom;

    public static final Parcelable.Creator<Rect> CREATOR = new Parcelable.Creator<Rect>() {
        public Rect createFromParcel(Parcel in) {
            return new Rect(in);
        }

        public Rect[] newArray(int size) {
            return new Rect[size];
        }
    };

    public Rect() {
    }

    private Rect(Parcel in) {
        readFromParcel(in);
    }

    public void writeToParcel(Parcel out, int flags) {
        out.writeInt(left);
        out.writeInt(top);
        out.writeInt(right);
        out.writeInt(bottom);
    }

    public void readFromParcel(Parcel in) {
        left = in.readInt();
        top = in.readInt();
        right = in.readInt();
        bottom = in.readInt();
    }

    public int describeContents() {
        return 0;
    }
}

Rect 类中的封装非常简单。查看 Parcel 的其他方法,了解您可以将哪些其他类型的值写入 Parcel

警告:请注意从其他进程接收数据的安全影响。在本例中,Rect 会从 Parcel 读取四个数字,但您必须确保这些数字在调用方尝试执行的任何操作的接受范围内。如需详细了解如何保护应用免受恶意软件的侵害,请参阅安全提示

带软件包参数(包含 Parcelable 类型)的方法

如果某个方法接受预计包含 Parcelable 的 Bundle 对象,请务必先通过调用 Bundle.setClassLoader(ClassLoader) 设置 Bundle 的类加载器,然后再尝试从 Bundle 读取数据。否则,即使您的应用中正确定义了 parcelable,您也会遇到 ClassNotFoundException

例如,请考虑以下 .aidl 文件示例:

// IRectInsideBundle.aidl
package com.example.android;

/** Example service interface */
interface IRectInsideBundle {
    /** Rect parcelable is stored in the bundle with key "rect". */
    void saveRect(in Bundle bundle);
}
如以下实现所示,在读取 Rect 之前,ClassLoader 会在 Bundle 中明确设置:

Kotlin

private val binder = object : IRectInsideBundle.Stub() {
    override fun saveRect(bundle: Bundle) {
      bundle.classLoader = classLoader
      val rect = bundle.getParcelable<Rect>("rect")
      process(rect) // Do more with the parcelable.
    }
}

Java

private final IRectInsideBundle.Stub binder = new IRectInsideBundle.Stub() {
    public void saveRect(Bundle bundle){
        bundle.setClassLoader(getClass().getClassLoader());
        Rect rect = bundle.getParcelable("rect");
        process(rect); // Do more with the parcelable.
    }
};

调用 IPC 方法

如需调用使用 AIDL 定义的远程接口,请在调用类中执行以下步骤:

  1. 在项目的 src/ 目录中添加 .aidl 文件。
  2. 声明 IBinder 接口的实例,该实例是根据 AIDL 生成的。
  3. 实现 ServiceConnection
  4. 调用 Context.bindService(),并传入 ServiceConnection 实现。
  5. onServiceConnected() 的实现中,您会收到一个名为 serviceIBinder 实例。调用 YourInterfaceName.Stub.asInterface((IBinder)service) 以将返回的参数转换为 YourInterface 类型。
  6. 调用您在接口中定义的方法。始终捕获 DeadObjectException 异常,系统会在连接中断时抛出此异常。此外,还要捕获 SecurityException 异常,当 IPC 方法调用中涉及的两个进程具有冲突的 AIDL 定义时,系统会抛出此类异常。
  7. 如需断开连接,请使用接口实例调用 Context.unbindService()

调用 IPC 服务时,请注意以下几点:

  • 对象是跨进程计数的引用。
  • 您可以将匿名对象作为方法参数发送。

如需详细了解如何绑定到服务,请参阅绑定服务概览

以下示例代码展示了如何调用 AIDL 创建的服务,该代码摘自 ApiDemos 项目中的 Remote Service 示例。

Kotlin

private const val BUMP_MSG = 1

class Binding : Activity() {

    /** The primary interface you call on the service.  */
    private var mService: IRemoteService? = null

    /** Another interface you use on the service.  */
    internal var secondaryService: ISecondary? = null

    private lateinit var killButton: Button
    private lateinit var callbackText: TextView
    private lateinit var handler: InternalHandler

    private var isBound: Boolean = false

    /**
     * Class for interacting with the main interface of the service.
     */
    private val mConnection = object : ServiceConnection {

        override fun onServiceConnected(className: ComponentName, service: IBinder) {
            // This is called when the connection with the service is
            // established, giving us the service object we can use to
            // interact with the service.  We are communicating with our
            // service through an IDL interface, so get a client-side
            // representation of that from the raw service object.
            mService = IRemoteService.Stub.asInterface(service)
            killButton.isEnabled = true
            callbackText.text = "Attached."

            // We want to monitor the service for as long as we are
            // connected to it.
            try {
                mService?.registerCallback(mCallback)
            } catch (e: RemoteException) {
                // In this case, the service crashes before we can
                // do anything with it. We can count on soon being
                // disconnected (and then reconnected if it can be restarted)
                // so there is no need to do anything here.
            }

            // As part of the sample, tell the user what happened.
            Toast.makeText(
                    this@Binding,
                    R.string.remote_service_connected,
                    Toast.LENGTH_SHORT
            ).show()
        }

        override fun onServiceDisconnected(className: ComponentName) {
            // This is called when the connection with the service is
            // unexpectedly disconnected&mdash;that is, its process crashed.
            mService = null
            killButton.isEnabled = false
            callbackText.text = "Disconnected."

            // As part of the sample, tell the user what happened.
            Toast.makeText(
                    this@Binding,
                    R.string.remote_service_disconnected,
                    Toast.LENGTH_SHORT
            ).show()
        }
    }

    /**
     * Class for interacting with the secondary interface of the service.
     */
    private val secondaryConnection = object : ServiceConnection {

        override fun onServiceConnected(className: ComponentName, service: IBinder) {
            // Connecting to a secondary interface is the same as any
            // other interface.
            secondaryService = ISecondary.Stub.asInterface(service)
            killButton.isEnabled = true
        }

        override fun onServiceDisconnected(className: ComponentName) {
            secondaryService = null
            killButton.isEnabled = false
        }
    }

    private val mBindListener = View.OnClickListener {
        // Establish a couple connections with the service, binding
        // by interface names. This lets other applications be
        // installed that replace the remote service by implementing
        // the same interface.
        val intent = Intent(this@Binding, RemoteService::class.java)
        intent.action = IRemoteService::class.java.name
        bindService(intent, mConnection, Context.BIND_AUTO_CREATE)
        intent.action = ISecondary::class.java.name
        bindService(intent, secondaryConnection, Context.BIND_AUTO_CREATE)
        isBound = true
        callbackText.text = "Binding."
    }

    private val unbindListener = View.OnClickListener {
        if (isBound) {
            // If we have received the service, and hence registered with
            // it, then now is the time to unregister.
            try {
                mService?.unregisterCallback(mCallback)
            } catch (e: RemoteException) {
                // There is nothing special we need to do if the service
                // crashes.
            }

            // Detach our existing connection.
            unbindService(mConnection)
            unbindService(secondaryConnection)
            killButton.isEnabled = false
            isBound = false
            callbackText.text = "Unbinding."
        }
    }

    private val killListener = View.OnClickListener {
        // To kill the process hosting the service, we need to know its
        // PID.  Conveniently, the service has a call that returns
        // that information.
        try {
            secondaryService?.pid?.also { pid ->
                // Note that, though this API lets us request to
                // kill any process based on its PID, the kernel
                // still imposes standard restrictions on which PIDs you
                // can actually kill. Typically this means only
                // the process running your application and any additional
                // processes created by that app, as shown here. Packages
                // sharing a common UID are also able to kill each
                // other's processes.
                Process.killProcess(pid)
                callbackText.text = "Killed service process."
            }
        } catch (ex: RemoteException) {
            // Recover gracefully from the process hosting the
            // server dying.
            // For purposes of this sample, put up a notification.
            Toast.makeText(this@Binding, R.string.remote_call_failed, Toast.LENGTH_SHORT).show()
        }
    }

    // ----------------------------------------------------------------------
    // Code showing how to deal with callbacks.
    // ----------------------------------------------------------------------

    /**
     * This implementation is used to receive callbacks from the remote
     * service.
     */
    private val mCallback = object : IRemoteServiceCallback.Stub() {
        /**
         * This is called by the remote service regularly to tell us about
         * new values.  Note that IPC calls are dispatched through a thread
         * pool running in each process, so the code executing here is
         * NOT running in our main thread like most other things. So,
         * to update the UI, we need to use a Handler to hop over there.
         */
        override fun valueChanged(value: Int) {
            handler.sendMessage(handler.obtainMessage(BUMP_MSG, value, 0))
        }
    }

    /**
     * Standard initialization of this activity.  Set up the UI, then wait
     * for the user to interact with it before doing anything.
     */
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        setContentView(R.layout.remote_service_binding)

        // Watch for button taps.
        var button: Button = findViewById(R.id.bind)
        button.setOnClickListener(mBindListener)
        button = findViewById(R.id.unbind)
        button.setOnClickListener(unbindListener)
        killButton = findViewById(R.id.kill)
        killButton.setOnClickListener(killListener)
        killButton.isEnabled = false

        callbackText = findViewById(R.id.callback)
        callbackText.text = "Not attached."
        handler = InternalHandler(callbackText)
    }

    private class InternalHandler(
            textView: TextView,
            private val weakTextView: WeakReference<TextView> = WeakReference(textView)
    ) : Handler() {
        override fun handleMessage(msg: Message) {
            when (msg.what) {
                BUMP_MSG -> weakTextView.get()?.text = "Received from service: ${msg.arg1}"
                else -> super.handleMessage(msg)
            }
        }
    }
}

Java

public static class Binding extends Activity {
    /** The primary interface we are calling on the service. */
    IRemoteService mService = null;
    /** Another interface we use on the service. */
    ISecondary secondaryService = null;

    Button killButton;
    TextView callbackText;

    private InternalHandler handler;
    private boolean isBound;

    /**
     * Standard initialization of this activity. Set up the UI, then wait
     * for the user to interact with it before doing anything.
     */
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.remote_service_binding);

        // Watch for button taps.
        Button button = (Button)findViewById(R.id.bind);
        button.setOnClickListener(mBindListener);
        button = (Button)findViewById(R.id.unbind);
        button.setOnClickListener(unbindListener);
        killButton = (Button)findViewById(R.id.kill);
        killButton.setOnClickListener(killListener);
        killButton.setEnabled(false);

        callbackText = (TextView)findViewById(R.id.callback);
        callbackText.setText("Not attached.");
        handler = new InternalHandler(callbackText);
    }

    /**
     * Class for interacting with the main interface of the service.
     */
    private ServiceConnection mConnection = new ServiceConnection() {
        public void onServiceConnected(ComponentName className,
                IBinder service) {
            // This is called when the connection with the service is
            // established, giving us the service object we can use to
            // interact with the service.  We are communicating with our
            // service through an IDL interface, so get a client-side
            // representation of that from the raw service object.
            mService = IRemoteService.Stub.asInterface(service);
            killButton.setEnabled(true);
            callbackText.setText("Attached.");

            // We want to monitor the service for as long as we are
            // connected to it.
            try {
                mService.registerCallback(mCallback);
            } catch (RemoteException e) {
                // In this case the service crashes before we can even
                // do anything with it. We can count on soon being
                // disconnected (and then reconnected if it can be restarted)
                // so there is no need to do anything here.
            }

            // As part of the sample, tell the user what happened.
            Toast.makeText(Binding.this, R.string.remote_service_connected,
                    Toast.LENGTH_SHORT).show();
        }

        public void onServiceDisconnected(ComponentName className) {
            // This is called when the connection with the service is
            // unexpectedly disconnected&mdash;that is, its process crashed.
            mService = null;
            killButton.setEnabled(false);
            callbackText.setText("Disconnected.");

            // As part of the sample, tell the user what happened.
            Toast.makeText(Binding.this, R.string.remote_service_disconnected,
                    Toast.LENGTH_SHORT).show();
        }
    };

    /**
     * Class for interacting with the secondary interface of the service.
     */
    private ServiceConnection secondaryConnection = new ServiceConnection() {
        public void onServiceConnected(ComponentName className,
                IBinder service) {
            // Connecting to a secondary interface is the same as any
            // other interface.
            secondaryService = ISecondary.Stub.asInterface(service);
            killButton.setEnabled(true);
        }

        public void onServiceDisconnected(ComponentName className) {
            secondaryService = null;
            killButton.setEnabled(false);
        }
    };

    private OnClickListener mBindListener = new OnClickListener() {
        public void onClick(View v) {
            // Establish a couple connections with the service, binding
            // by interface names. This lets other applications be
            // installed that replace the remote service by implementing
            // the same interface.
            Intent intent = new Intent(Binding.this, RemoteService.class);
            intent.setAction(IRemoteService.class.getName());
            bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
            intent.setAction(ISecondary.class.getName());
            bindService(intent, secondaryConnection, Context.BIND_AUTO_CREATE);
            isBound = true;
            callbackText.setText("Binding.");
        }
    };

    private OnClickListener unbindListener = new OnClickListener() {
        public void onClick(View v) {
            if (isBound) {
                // If we have received the service, and hence registered with
                // it, then now is the time to unregister.
                if (mService != null) {
                    try {
                        mService.unregisterCallback(mCallback);
                    } catch (RemoteException e) {
                        // There is nothing special we need to do if the service
                        // crashes.
                    }
                }

                // Detach our existing connection.
                unbindService(mConnection);
                unbindService(secondaryConnection);
                killButton.setEnabled(false);
                isBound = false;
                callbackText.setText("Unbinding.");
            }
        }
    };

    private OnClickListener killListener = new OnClickListener() {
        public void onClick(View v) {
            // To kill the process hosting our service, we need to know its
            // PID.  Conveniently, our service has a call that returns
            // that information.
            if (secondaryService != null) {
                try {
                    int pid = secondaryService.getPid();
                    // Note that, though this API lets us request to
                    // kill any process based on its PID, the kernel
                    // still imposes standard restrictions on which PIDs you
                    // can actually kill.  Typically this means only
                    // the process running your application and any additional
                    // processes created by that app as shown here. Packages
                    // sharing a common UID are also able to kill each
                    // other's processes.
                    Process.killProcess(pid);
                    callbackText.setText("Killed service process.");
                } catch (RemoteException ex) {
                    // Recover gracefully from the process hosting the
                    // server dying.
                    // For purposes of this sample, put up a notification.
                    Toast.makeText(Binding.this,
                            R.string.remote_call_failed,
                            Toast.LENGTH_SHORT).show();
                }
            }
        }
    };

    // ----------------------------------------------------------------------
    // Code showing how to deal with callbacks.
    // ----------------------------------------------------------------------

    /**
     * This implementation is used to receive callbacks from the remote
     * service.
     */
    private IRemoteServiceCallback mCallback = new IRemoteServiceCallback.Stub() {
        /**
         * This is called by the remote service regularly to tell us about
         * new values.  Note that IPC calls are dispatched through a thread
         * pool running in each process, so the code executing here is
         * NOT running in our main thread like most other things. So,
         * to update the UI, we need to use a Handler to hop over there.
         */
        public void valueChanged(int value) {
            handler.sendMessage(handler.obtainMessage(BUMP_MSG, value, 0));
        }
    };

    private static final int BUMP_MSG = 1;

    private static class InternalHandler extends Handler {
        private final WeakReference<TextView> weakTextView;

        InternalHandler(TextView textView) {
            weakTextView = new WeakReference<>(textView);
        }

        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case BUMP_MSG:
                    TextView textView = weakTextView.get();
                    if (textView != null) {
                        textView.setText("Received from service: " + msg.arg1);
                    }
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    }
}