本文对Android framework层的API函数与其对应本地方法的映射、注册原理进行了介绍，通过本文，应该会对我们频繁调用的Android API的调用过程及实现原理有更加深入的认识。
我们知道，Android平台是Java、C/C++等多种混合语言写成的，我们在使用Android SDK提供的framework层的API来进行应用开发时通常调用的是Java方法，而实际上，这些Java API很多时候只是一个入口，方法功能的真正实现是通过JNI调用到framework层的native本地方法来实现的。比如，我们在使用Canvas的drawBitmap方法来绘制图片时，drawBitmap的真正实现就是通过JNI调用C++的同名方法来实现的。
下面，我们通过开机启动后本地方法的注册流程来具体的介绍Android API与对应本地方法究竟是如何关联起来的。
Android在开机启动后，经过一系列的加载流程，将进入\frameworks\base\cmds\ app_process\ App_main.cpp的main()方法，在main()方法中，会调用\frameworks\base\core\jni\AndroidRuntime.cpp的start()方法。事实上，Android API与本地方法的注册关联主要就是在AndroidRuntime.cpp模块里完成的。
在AndroidRuntime.cpp的start()方法中，会经过如下的调用流程，即：
void AndroidRuntime::start() --->
void AndroidRuntime::start(const char* className, const bool startSystemServer) --->
int AndroidRuntime::startReg(JNIEnv* env)
其中，startReg函数主要就是用来进行函数注册的。我们深入到该函数内部，可以看到如下的代码片段：
if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) { env->PopLocalFrame(NULL); return -1; }
好了，流程介绍到这里，我们已经定位到了Android API与本地方法的注册关联的关键之处，实际上，API与本地方法的注册关键就是在此处完成的。标记为红色的register_jni_procs及其参数gRegJNI是注册关联的两大要素，下面我们对其分别作具体分析。
首先看gRegJNI的定义：
static const RegJNIRec gRegJNI[] = { REG_JNI(register_android_debug_JNITest), REG_JNI(register_com_android_internal_os_RuntimeInit), REG_JNI(register_android_os_SystemClock), REG_JNI(register_android_util_EventLog), REG_JNI(register_android_util_Log), REG_JNI(register_android_util_FloatMath), REG_JNI(register_android_text_format_Time), REG_JNI(register_android_pim_EventRecurrence), REG_JNI(register_android_content_AssetManager), REG_JNI(register_android_content_StringBlock), REG_JNI(register_android_content_XmlBlock), REG_JNI(register_android_emoji_EmojiFactory), REG_JNI(register_android_security_Md5MessageDigest), REG_JNI(register_android_text_AndroidCharacter), REG_JNI(register_android_text_AndroidBidi), REG_JNI(register_android_text_KeyCharacterMap), REG_JNI(register_android_os_Process), REG_JNI(register_android_os_Binder), REG_JNI(register_android_view_Display), REG_JNI(register_android_nio_utils), REG_JNI(register_android_graphics_PixelFormat), REG_JNI(register_android_graphics_Graphics), REG_JNI(register_android_view_Surface), REG_JNI(register_android_view_ViewRoot), REG_JNI(register_com_google_android_gles_jni_EGLImpl), REG_JNI(register_com_google_android_gles_jni_GLImpl), REG_JNI(register_android_opengl_jni_GLES10), REG_JNI(register_android_opengl_jni_GLES10Ext), REG_JNI(register_android_opengl_jni_GLES11), REG_JNI(register_android_opengl_jni_GLES11Ext), REG_JNI(register_android_opengl_jni_GLES20), REG_JNI(register_android_graphics_Bitmap), REG_JNI(register_android_graphics_BitmapFactory), REG_JNI(register_android_graphics_BitmapRegionDecoder), REG_JNI(register_android_graphics_Camera), REG_JNI(register_android_graphics_Canvas), REG_JNI(register_android_graphics_ColorFilter), REG_JNI(register_android_graphics_DrawFilter), REG_JNI(register_android_graphics_Interpolator), REG_JNI(register_android_graphics_LayerRasterizer), REG_JNI(register_android_graphics_MaskFilter), REG_JNI(register_android_graphics_Matrix), REG_JNI(register_android_graphics_Movie), REG_JNI(register_android_graphics_NinePatch), REG_JNI(register_android_graphics_Paint), REG_JNI(register_android_graphics_Path), REG_JNI(register_android_graphics_PathMeasure), REG_JNI(register_android_graphics_PathEffect), REG_JNI(register_android_graphics_Picture), REG_JNI(register_android_graphics_PorterDuff), REG_JNI(register_android_graphics_Rasterizer), REG_JNI(register_android_graphics_Region), REG_JNI(register_android_graphics_Shader), REG_JNI(register_android_graphics_Typeface), REG_JNI(register_android_graphics_Xfermode), REG_JNI(register_android_graphics_YuvImage), REG_JNI(register_com_android_internal_graphics_NativeUtils), REG_JNI(register_android_database_CursorWindow), REG_JNI(register_android_database_SQLiteCompiledSql), REG_JNI(register_android_database_SQLiteDatabase), REG_JNI(register_android_database_SQLiteDebug), REG_JNI(register_android_database_SQLiteProgram), REG_JNI(register_android_database_SQLiteQuery), REG_JNI(register_android_database_SQLiteStatement), REG_JNI(register_android_os_Debug), REG_JNI(register_android_os_FileObserver), REG_JNI(register_android_os_FileUtils), REG_JNI(register_android_os_MessageQueue), REG_JNI(register_android_os_ParcelFileDescriptor), REG_JNI(register_android_os_Power), REG_JNI(register_android_os_StatFs), REG_JNI(register_android_os_SystemProperties), REG_JNI(register_android_os_UEventObserver), REG_JNI(register_android_net_LocalSocketImpl), REG_JNI(register_android_net_NetworkUtils), REG_JNI(register_android_net_TrafficStats), REG_JNI(register_android_net_wifi_WifiManager), REG_JNI(register_android_nfc_NdefMessage), REG_JNI(register_android_nfc_NdefRecord), REG_JNI(register_android_os_MemoryFile), REG_JNI(register_com_android_internal_os_ZygoteInit), REG_JNI(register_android_hardware_Camera), REG_JNI(register_android_hardware_SensorManager), REG_JNI(register_android_media_AudioRecord), REG_JNI(register_android_media_AudioSystem), REG_JNI(register_android_media_AudioTrack), REG_JNI(register_android_media_JetPlayer), REG_JNI(register_android_media_ToneGenerator), REG_JNI(register_android_opengl_classes), REG_JNI(register_android_bluetooth_HeadsetBase), REG_JNI(register_android_bluetooth_BluetoothAudioGateway), REG_JNI(register_android_bluetooth_BluetoothSocket), REG_JNI(register_android_bluetooth_ScoSocket), REG_JNI(register_android_server_BluetoothService), REG_JNI(register_android_server_BluetoothEventLoop), REG_JNI(register_android_server_BluetoothA2dpService), REG_JNI(register_android_server_Watchdog), REG_JNI(register_android_message_digest_sha1), REG_JNI(register_android_ddm_DdmHandleNativeHeap), REG_JNI(register_android_backup_BackupDataInput), REG_JNI(register_android_backup_BackupDataOutput), REG_JNI(register_android_backup_FileBackupHelperBase), REG_JNI(register_android_backup_BackupHelperDispatcher), REG_JNI(register_android_app_NativeActivity), REG_JNI(register_android_view_InputChannel), REG_JNI(register_android_view_InputQueue), REG_JNI(register_android_view_KeyEvent), REG_JNI(register_android_view_MotionEvent), REG_JNI(register_android_content_res_ObbScanner), REG_JNI(register_android_content_res_Configuration), };