Android provides a default Bluetooth stack that is divided into two layers: The Bluetooth Embedded System (BTE), which implements the core Bluetooth functionality, and the Bluetooth Application Layer (BTA), which communicates with Android framework applications.

To fully leverage the Bluetooth Low Energy APIs added in Android 5.0, you should implement the Android 6.0 Bluetooth HCI Requirements. That document initially was provided as the Android 5.0 Bluetooth HCI Requirements.

Architecture

A Bluetooth system service communicates with the Bluetooth stack through JNI and with applications through Binder IPC. The system service provides developers with access to various Bluetooth profiles. The following diagram shows the general structure of the Bluetooth stack:

Figure 1. Bluetooth architecture

Application framework

At the application framework level is application code, which utilizes the android.bluetooth APIs to interact with the Bluetooth hardware. Internally, this code calls the Bluetooth process through the Binder IPC mechanism.

Bluetooth system service

The Bluetooth system service, located in packages/apps/Bluetooth, is packaged as an Android app and implements the Bluetooth service and profiles at the Android framework layer. This app calls into the HAL layer via JNI.

JNI

The JNI code associated with android.bluetooth is located in packages/apps/Bluetooth/jni. The JNI code calls into the HAL layer and receives callbacks from the HAL when certain Bluetooth operations occur, such as when devices are discovered.

HAL

The hardware abstraction layer defines the standard interface that the android.bluetooth APIs and Bluetooth process call into and that you must implement to have your Bluetooth hardware function correctly. The header file for the Bluetooth HAL is hardware/libhardware/include/hardware/bluetooth.h. Additionally, please review all of the hardware/libhardware/include/hardware/bt_*.h files.

Bluetooth stack

The default Bluetooth stack is provided for you and is located in system/bt. The stack implements the generic Bluetooth HAL and customizes it with extensions and configuration changes.

Vendor extensions

To add custom extensions and an HCI layer for tracing, you can create a libbt-vendor module and specify these components.

Implementing the HAL

The Bluetooth HAL is located in /hardware/libhardware/include/hardware/bluetooth.h. Thus, the bluetooth.h file contains the basic interface for the Bluetooth stack, and you must implement its functions.

Profile-specific files are located in the same directory. For details, see the HAL File Reference.

The following is a partial list of the profile-related files. For the complete set, see the /hardware/libhardware/include/hardware/ directory:

• bt_av.h: Includes the interface definition for the A2DP profile.
• bt_gatt.h, bt_gatt_client.h, and bt_gatt_server.h: These include the interface definition for the GATT profile.
• bt_hf.h: Includes the interface definition for the HFP profile.
• bt_hh.h: Includes the interface definition for the HID host profile.
• bt_hl.h: Includes the interface definition for the HDP profile.
• bt_mce.h: Includes the interface definition for the MAP profile.
• bt_pan.h: Includes the interface definition for the PAN profile.
• bt_rc.h: Includes the interface definition for the AVRCP profile.
• bt_sock.h: Includes the interface definition for RFCOMM sockets.

Keep in mind that your Bluetooth implementation is not constrained to the features and profiles exposed in the HAL. You can find the default implementation located in the Bluetooth stack in the system/bt directory, which implements the default HAL and also extra features and customizations.

Customizing the Native Bluetooth Stack

If you are using the default Bluetooth stack, but want to make a few customizations, you can do the following:

• Custom Bluetooth profiles - If you want to add Bluetooth profiles that do not have HAL interfaces provided by Android, you must supply an SDK add-on download to make the profile available to app developers, make the APIs available in the Bluetooth system process app (packages/apps/Bluetooth), and add them to the default stack (system/bt).
• Custom vendor extensions and configuration changes - You can add things such as extra AT commands or device-specific configuration changes by creating a libbt-vendor module. See the /hardware/broadcom/libbt directory for an example.
• Host Controller Interface (HCI) - You can provide your own HCI by creating a libbt-hci module, which is mainly used for debug tracing. See the external/bluetooth/hci directory for an example.