上图描述的是蓝牙协议栈，通过该图，查看A2dp的代码在协议栈的调用流程。其分层架构如下： 1.蓝牙的系统服务service通过JNI与bluedroid协议栈进行通信。协议栈分为两层，Bluetooth Embedded System（BTE）和Bluetooth Application Layer（BTA）。这两层和framework层应用进行通信。 2.蓝牙服务通过Binder IPC通信与应用程序交互。 3.系统服务给开发者提供了获取各种profile的接口。
该层使用android.bluetooth APIs和Bluetooth hardware进行通信。通过Binder IPC机制和Bluetooth进程进行通信。该层代码位于frameworks/base/core/java/android/bluetooth/目录下。 如A2DP连接frameworks/base/core/java/android/bluetooth/BluetoothA2dp.java
public boolean connect(BluetoothDevice device) { if (DBG) log("connect(" + device + ")"); if (mService != null && isEnabled() && isValidDevice(device)) { try { return mService.connect(device); } catch (RemoteException e) { Log.e(TAG, "Stack:" + Log.getStackTraceString(new Throwable())); return false; } } if (mService == null) Log.w(TAG, "Proxy not attached to service"); return false; } 123456789101112131415该方法通过Binder IPC通信机制，调用packages/apps/Bluetooth/src/com/android/bluetooth/a2dp/A2dpService.java下的内部私有类。通过aidl机制实现。
<packages/apps/Bluetooth/src/com/android/bluetooth/a2dp/A2dpService.java> private static class BluetoothA2dpBinder extends IBluetoothA2dp.Stub { public boolean connect(BluetoothDevice device) { A2dpService service = getService(); if (service == null) return false; return service.connect(device); } } 12345678然后调用到A2dpService的connect的方法。
<packages/apps/Bluetooth/src/com/android/bluetooth/a2dp/A2dpService.java> public class A2dpService extends ProfileService { ... public boolean connect(BluetoothDevice device) { enforceCallingOrSelfPermission(BLUETOOTH_ADMIN_PERM, "Need BLUETOOTH ADMIN permission"); if (getPriority(device) == BluetoothProfile.PRIORITY_OFF) { return false; } ParcelUuid[] featureUuids = device.getUuids(); if ((BluetoothUuid.containsAnyUuid(featureUuids, A2DP_SOURCE_UUID)) && !(BluetoothUuid.containsAllUuids(featureUuids ,A2DP_SOURCE_SINK_UUIDS))) { Log.e(TAG,"Remote does not have A2dp Sink UUID"); return false; } int connectionState = mStateMachine.getConnectionState(device); if (connectionState == BluetoothProfile.STATE_CONNECTED || connectionState == BluetoothProfile.STATE_CONNECTING) { return false; } mStateMachine.sendMessage(A2dpStateMachine.CONNECT, device); return true; } .... private static class BluetoothA2dpBinder extends IBluetoothA2dp.Stub { public boolean connect(BluetoothDevice device) { A2dpService service = getService(); if (service == null) return false; return service.connect(device); } } } 1234567891011121314151617181920212223242526272829303132上述过程就是Bluetooth application framework与Bluetooth process的调用过程。
这部分代码位于packages/apps/Bluetooth下，在framework层实现蓝牙服务和各种profile，以apps形式存在。该层通过JNI调用HAL层代码。 A2dpService的connect方法会发送状态机改变消息， mStateMachine.sendMessage(A2dpStateMachine.CONNECT, device); 这个消息会被A2dpStateMachine对象的processMessage(Message)方法接收：
switch(message.what) { case CONNECT: BluetoothDevice device = (BluetoothDevice) message.obj; broadcastConnectionState(device, BluetoothProfile.STATE_CONNECTING, BluetoothProfile.STATE_DISCONNECTED); if (!connectA2dpNative(getByteAddress(device)) ) { broadcastConnectionState(device, BluetoothProfile.STATE_DISCONNECTED, BluetoothProfile.STATE_CONNECTING); break; } synchronized (A2dpStateMachine.this) { mTargetDevice = device; transitionTo(mPending); } // TODO(BT) remove CONNECT_TIMEOUT when the stack // sends back events consistently sendMessageDelayed(CONNECT_TIMEOUT, 30000); break; 1234567891011121314151617181920上述的connectA2dpNative（getByteAddress(device)）会通过JNI调用Native方法。
<packages/apps/Bluetooth/jni/com_android_bluetooth_a2dp.cpp> static jboolean connectA2dpNative(JNIEnv *env, jobject object, jbyteArray address) { jbyte *addr; bt_bdaddr_t * btAddr; bt_status_t status; ALOGI("%s: sBluetoothA2dpInterface: %p", __FUNCTION__, sBluetoothA2dpInterface); if (!sBluetoothA2dpInterface) return JNI_FALSE; addr = env->GetByteArrayElements(address, NULL); btAddr = (bt_bdaddr_t *) addr; if (!addr) { jniThrowIOException(env, EINVAL); return JNI_FALSE; } if ((status = sBluetoothA2dpInterface->connect((bt_bdaddr_t *)addr)) != BT_STATUS_SUCCESS) { ALOGE("Failed HF connection, status: %d", status); } env->ReleaseByteArrayElements(address, addr, 0); return (status == BT_STATUS_SUCCESS) ? JNI_TRUE : JNI_FALSE; } 12345678910111213141516171819202122和JNI相关的代码在package/apps/Bluetooth/jni目录下，上一节的connectA2dpNative方法就是这个目录的代码。JNI层代码调用到HAL代码并且从HAL接收到回调。 A2dp连接中，会调用sBluetoothA2dpInterface->connect实现。
<packages/apps/Bluetooth/jni/com_android_bluetooth_a2dp.cpp> static void initNative(JNIEnv *env, jobject object) { const bt_interface_t* btInf; bt_status_t status; ... if ( (sBluetoothA2dpInterface = (btav_interface_t *) btInf->get_profile_interface(BT_PROFILE_ADVANCED_AUDIO_ID)) == NULL) { ALOGE("Failed to get Bluetooth A2DP Interface"); return; } if ( (status = sBluetoothA2dpInterface->init(&sBluetoothA2dpCallbacks)) != BT_STATUS_SUCCESS) { ALOGE("Failed to initialize Bluetooth A2DP, status: %d", status); sBluetoothA2dpInterface = NULL; return; } mCallbacksObj = env->NewGlobalRef(object); } 12345678910111213141516171819在initNative中获得sBluetoothA2dpInterface对象。
硬件抽象层定义了android.bluetooth APIs和Bluetooth process调用的接口。Bluetooth 的HAL层代码在： hardware/libhardware/include/hardware/bluetooth.h，HAL层的以下代码也要重视：
hardware/libhardware/include/hardware/bt_common_types.h //A2DP profile定义 hardware/libhardware/include/hardware/bt_av.h //GATT profile 定义 hardware/libhardware/include/hardware/bt_gatt_types.h hardware/libhardware/include/hardware/bt_gatt_client.h hardware/libhardware/include/hardware/bt_gatt_server.h hardware/libhardware/include/hardware/bt_gatt.h //HFP profile定义 hardware/libhardware/include/hardware/bt_hf.h //HDP profile 定义 hardware/libhardware/include/hardware/bt_hl.h //MAP profile hardware/libhardware/include/hardware/bt_mce.h //HFP client profile hardware/libhardware/include/hardware/bt_hf_client.h //pan profile hardware/libhardware/include/hardware/bt_pan.h //AVRCP profile hardware/libhardware/include/hardware/bt_rc.h hardware/libhardware/include/hardware/bt_hh.h //SDP profile hardware/libhardware/include/hardware/bt_sdp.h //RFCOMM sockets hardware/libhardware/include/hardware/bt_sock.h 12345678910111213141516171819202122232425JNI中的sBluetoothA2dpInterface是btav_interface_t结构体，位于hardware/libhardware/include/hardware/bt_av.h中，定义为：
typedef struct { /** set to sizeof(btav_interface_t) */ size_t size; /** * Register the BtAv callbacks */ bt_status_t (*init)( btav_callbacks_t* callbacks ); /** connect to headset */ bt_status_t (*connect)( bt_bdaddr_t *bd_addr ); /** dis-connect from headset */ bt_status_t (*disconnect)( bt_bdaddr_t *bd_addr ); /** Closes the interface. */ void (*cleanup)( void ); } btav_interface_t; 1234567891011121314151617协议栈在/system/bt下，实现了HAL层和扩展配置等。包括BTA和BTE量大组件，BTA实现了蓝牙设备管理、状态管理及一些应用规范。而BTE则通过HCI与厂商蓝牙芯片交互以实现了蓝牙协议栈的通用功能和相关协议。另外，BTE还包括一个统一内核接口（GKI），蓝牙芯片厂商可借助GKI快速轻松得移植蓝牙协议栈到其他操作系统或手机平台上。 A2dp的连接将调用
<system/bt/btif/src/btif_av.c> static bt_status_t connect_int(bt_bdaddr_t *bd_addr, uint16_t uuid) { btif_av_connect_req_t connect_req; connect_req.target_bda = bd_addr; connect_req.uuid = uuid; BTIF_TRACE_EVENT("%s", __FUNCTION__); btif_sm_dispatch(btif_av_cb.sm_handle, BTIF_AV_CONNECT_REQ_EVT, (char*)&connect_req); return BT_STATUS_SUCCESS; } 123456789101112增加自定义扩展或者HCI层追踪，可以创建libbt-vendor模块并指定这些组件。
