Android NDK (C++) Binder Examples

This is an Android Studio project that demonstrates 2 different scenarios.

How to implement an Android Service in C++ using NDK Binder APIs.
How to call an Android Service directly from the C++ JNI layer.

Example 1 : C++ Android Service and a client written in Java that consumes it

This example demonstrates how to ...

implement an Android Service binder in C++ using NDK Binder APIs (NDKBinderService).
implement a Java client that binds the NDK Binder Service and call the service APIs (JavaBinderClient).

To see this example in action, take the following steps.

Build and install NdkBinderService APK. It contains an Android Service, whose binder implementation is done in C++ JNI layer using NDK Binder APIs.

# You can issue following bash commands to build and install NdkBinderService APK on your device.
# Or if you don't want to use bash (terminal), you can use Android Studio as well to build the NdkBinderService APK.

$ ./gradlew NdkBinderService:assembleDebug

$ adb install -f NdkBinderService/build/outputs/apk/debug/NdkBinderService-debug.apk

Build and install JavaBinderClient APK. It contains an Android Activity, who binds the Service from NdkBinderService and talks to Service using Java Binder APIs.

# You can issue following bash commands to build and install JavaBinderClient APK on your device.
# Or if you don't want to use bash (terminal), you can use Android Studio as well to build the JavaBinderClient APK.

$ ./gradlew JavaBinderClient:assembleDebug

$ adb install -f JavaBinderClient/build/outputs/apk/debug/JavaBinderClient-debug.apk

Run JavaBinderClient's main Activity from Android app launcher.

Example 2 : C++ client that consumes a plain old AIDL service

This example demonstrates how to implement a client that binds an AIDL service and call the service APIs from C++ JNI layer.

Build and install JavaBinderService APK. It contains an Android Service implemented in Java.

# You can issue following bash commands to build and install JavaBinderService APK on your device.
# Or if you don't want to use bash (terminal), you can use Android Studio as well to build the JavaBinderService APK.

$ ./gradlew JavaBinderService:assembleDebug

$ adb install -f JavaBinderService/build/outputs/apk/debug/JavaBinderService-debug.apk

Build and install NdkBinderClient APK. It contains an Android Activity, who binds the Service from JavaBinderService and passes the IBinder object to C++ JNI layer to talk to the Service using NDK Binder APIs.

# You can issue following bash commands to build and install NdkBinderClient APK on your device.
# Or if you don't want to use bash (terminal), you can use Android Studio as well to build the NdkBinderClient APK.

$ ./gradlew NdkBinderClient:assembleDebug

$ adb install -f NdkBinderClient/build/outputs/apk/debug/NdkBinderClient-debug.apk

Run NdkBinderClient's main Activity from Android app launcher.

NDK Binder service implementation details

NdkBinderService : Android app (APK) module, containing a Java Service (MyService.java) that implements an AIDL (IMyService.aidl). Implementation (Binder native) is done in C++ JNI layer (MyService.cpp).

AIDL

Common/src/main/aidl/com/example/IMyService.aidl

package com.example;

import com.example.ComplexType;

interface IMyService
{
    void basicTypes(int anInt, long aLong, boolean aBoolean,
                    float aFloat, double aDouble, String aString);
    
    String complexType(in ComplexType aComplexObject);

    ComplexType returnComplexType(int anInt, long aLong,
                    boolean aBoolean, float aFloat,
                    double aDouble, String aString);
}

Common/src/main/aidl/com/example/ComplexType.aidl

package com.example;

parcelable ComplexType cpp_header "ComplexType.h";

There is a Gradle task (compileAidlNdk) to auto-generate NDK C++ binder source files.

Common/build.gradle

plugins {
    id 'com.android.library'
}

android {
    compileSdkVersion 30
    buildToolsVersion '29.0.3'
    
    defaultConfig {
        minSdkVersion 29
        
        externalNativeBuild {
            cmake {
                cppFlags "-std=c++17"
            }
        }
    }
    
    externalNativeBuild {
        cmake {
            path "src/main/cpp/CMakeLists.txt"
            version "3.10.2"
        }
    }
    
    ...
}

task compileAidlNdk() {
    doLast {
        def aidlCpp = [android.sdkDirectory,
                       'build-tools',
                       android.buildToolsVersion,
                       'aidl'].join(File.separator)

        def outDir = [projectDir.absolutePath,
                      'src', 'main', 'cpp', 'aidl'].join(File.separator)

        def headerOutDir = [projectDir.absolutePath,
                           'src', 'main', 'cpp', 'includes'].join(File.separator)

        def searchPathForImports = [projectDir.absolutePath, 'src', 'main', 'aidl'].join(File.separator)

        def aidlFile = [projectDir.absolutePath,
                       'src', 'main', 'aidl',
                       'com', 'example', 'IMyService.aidl'].join(File.separator)

        exec {
            executable(aidlCpp)
            args('--lang=ndk',
                 '-o', outDir,
                 '-h', headerOutDir,
                 '-I', searchPathForImports,
                 aidlFile)
        }
    }
}

afterEvaluate {
    preBuild.dependsOn(compileAidlNdk)
}

Service implementation.

NdkBinderService/src/main/java/com/example/ndkbinderservice/MyService.java

package com.example.ndkbinderservice;

public class MyService extends Service
{
    static
    {
        System.loadLibrary("native-lib");
    }

    private IBinder mBinder;

    @Override
    public void onCreate()
    {
        super.onCreate();

        mBinder = createServiceBinder();
    }

    @Override
    public IBinder onBind(Intent intent)
    {
        return mBinder;
    }

    public native IBinder createServiceBinder();
}

Service's JNI library and NDK Binder implementations.

NdkBinderService/src/main/cpp/native-lib.cpp

#include <jni.h>
#include "MyService.h"

extern "C" JNIEXPORT jobject JNICALL
Java_com_example_ndkbinderservice_MyService_createServiceBinder(
        JNIEnv* env,
        jobject /* this */)
{
    static MyService myService;
    return env->NewGlobalRef(AIBinder_toJavaBinder(env, myService.asBinder().get()));
}

NdkBinderService/src/main/cpp/MyService.cpp

#include <aidl/com/example/BnMyService.h>

class MyService : public BnMyService
{
public:
    ScopedAStatus basicTypes(int32_t in_anInt, int64_t in_aLong,
                             bool in_aBoolean, float in_aFloat,
                             double in_aDouble,
                             const std::string& in_aString) override
    {
        return ScopedAStatus::ok();
    }

    ScopedAStatus complexType(const ComplexType& in_aComplexObject,
                              std::string* _aidl_return) override
    {
        char strBuf[1024];

        snprintf(strBuf, 1024,
                 "int=%d, long=%ld, bool=%d, float=%f, double=%lf, string=%s",
                 in_aComplexObject.i_Int,
                 in_aComplexObject.l_Long,
                 in_aComplexObject.b_Boolean,
                 in_aComplexObject.f_Float,
                 in_aComplexObject.d_Double,
                 in_aComplexObject.s_String.c_str());

        *_aidl_return = std::string(strBuf);

        return ScopedAStatus::ok();
    }

    ...
};

Common/src/main/cpp/includes/ComplexType.h

#include <android/binder_status.h>

class ComplexType
{
public:
    int i_Int;
    long l_Long;
    bool b_Boolean;
    float f_Float;
    double d_Double;
    std::string s_String;

public:
    binder_status_t readFromParcel(const AParcel* pParcel)
    {
        int32_t iNotNull;
        AParcel_readInt32(pParcel, &iNotNull);

        AParcel_readInt32(pParcel, &i_Int);

        int64_t aLong;
        AParcel_readInt64(pParcel, &aLong);
        l_Long = aLong;

        AParcel_readBool(pParcel, &b_Boolean);

        AParcel_readFloat(pParcel, &f_Float);

        AParcel_readDouble(pParcel, &d_Double);

        ndk::AParcel_readString(pParcel, &s_String);

        return STATUS_OK;
    }

    binder_status_t writeToParcel(AParcel* pParcel) const
    {
        int32_t iNotNull = 1;
        AParcel_writeInt32(pParcel, iNotNull);

        AParcel_writeInt32(pParcel, i_Int);

        AParcel_writeInt64(pParcel, l_Long);

        AParcel_writeBool(pParcel, b_Boolean);

        AParcel_writeFloat(pParcel, f_Float);

        AParcel_writeDouble(pParcel, d_Double);

        ndk::AParcel_writeString(pParcel, s_String);

        return STATUS_OK;
    }
};

NDK Binder client implementation details

NdkBinderClient : Android app (APK) module, containing a Java Activity (MainActivity.java) that binds an Android Service. IBinder object received onServiceConnection is passed to a C++ JNI layer (native-lib.cpp), and communication with the service happens in JNI layer.

NdkBinderClient/src/main/java/com/example/ndkbinderclient/MainActivity.java

public class MainActivity extends AppCompatActivity implements ServiceConnection
{
    static
    {
        System.loadLibrary("native-lib");
    }

    private volatile boolean mIsServiceConnected = false;
    private final ConditionVariable mServiceConnectionWaitLock = new ConditionVariable();

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @Override
    protected void onResume()
    {
        super.onResume();

        Intent intent = new Intent();
        intent.setClassName("com.example.javabinderservice",
                "com.example.javabinderservice.MyService");

        bindService(intent, this, BIND_AUTO_CREATE);

        new Thread(new Runnable()
        {
            @Override
            public void run()
            {
                // Not connected to service yet?
                while(!mIsServiceConnected)
                {
                    mServiceConnectionWaitLock.block(); // waits for service connection
                }

                talkToService();
            }
        }).start();
    }

    @Override
    protected void onPause()
    {
        unbindService(this);

        mIsServiceConnected = false;

        onServiceDisconnected();

        super.onPause();
    }

    @Override
    public void onServiceConnected(ComponentName componentName, IBinder iBinder)
    {
        onServiceConnected(iBinder);

        mIsServiceConnected = true;

        mServiceConnectionWaitLock.open(); // breaks service connection waits
    }

    @Override
    public void onServiceDisconnected(ComponentName componentName)
    {
        mIsServiceConnected = false;

        onServiceDisconnected();
    }

    public native void onServiceConnected(IBinder binder);
    public native void onServiceDisconnected();
    public native String talkToService();
}

JNI library implementation.

NdkBinderClient/src/main/cpp/native-lib.cpp

#include <jni.h>
#include <aidl/com/example/IMyService.h>
#include <android/binder_ibinder_jni.h>

std::shared_ptr<IMyService> g_spMyService;

extern "C" JNIEXPORT void JNICALL
Java_com_example_ndkbinderclient_MainActivity_onServiceConnected(
        JNIEnv* env,
        jobject /* this */,
        jobject binder)
{
    AIBinder* pBinder = AIBinder_fromJavaBinder(env, binder);

    const ::ndk::SpAIBinder spBinder(pBinder);
    g_spMyService = IMyService::fromBinder(spBinder);
}

extern "C" JNIEXPORT void JNICALL
Java_com_example_ndkbinderclient_MainActivity_onServiceDisconnected(
        JNIEnv* env,
        jobject /* this */)
{
    g_spMyService = nullptr;
}

extern "C" JNIEXPORT jstring JNICALL
Java_com_example_ndkbinderclient_MainActivity_talkToService(
        JNIEnv* env,
        jobject /* this */)
{
    ScopedAStatus basicTypesResult = g_spMyService->basicTypes(2021, 65535000,
            true, 3.14f, 3.141592653589793238, "Hello, World!");

    if(basicTypesResult.isOk())
    {
        ...
    }
    else
    {
        ...
    }

    ComplexType ct(2021, 65535000, true, 3.14f,3.141592653589793238,
            "Hello, World!");

    std::string sReturnedString;

    ScopedAStatus complexTypeResult = g_spMyService->complexType(ct, &sReturnedString);

    if(complexTypeResult.isOk())
    {
        ...
    }
    else
    {
        ...
    }

    ComplexType returnedComplexObject;

    ScopedAStatus returnComplexTypeResult = g_spMyService->returnComplexType(2021,
            65535000, true, 3.14f, 3.141592653589793238,
            "Hello, World!", &returnedComplexObject);

    if(returnComplexTypeResult.isOk())
    {
        ...
    }
    else
    {
        ...
    }

    std::string sRet;
    returnedComplexObject.toString(&sRet);

    return env->NewStringUTF(sRet.c_str());
}

lakinduboteju / AndroidNdkBinderExamples