android java代码的启动：app_process
在android上，如果你直接用dalivik去加载framework.jar，你会发现里面的大部分native方法无法使用。同时，在 /init.rc里面，你也找不到dalvikvm。在ps列表，也没有dalvikvm。那android是怎么启动java程序的呢？
在android上，java程序是通过app_process启动的。在/init.rc里面，有如下一段代码：
service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
socket zygote stream 666
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
app_process的命令行参数如下：
app_process [java-options] cmd-dir start-class-name [options]
因此，init.rc里面的各个参数的对应关系为：
-Xzygote： java-options,这些参数会传给dalvik，这些参数必须以-开头，一旦遇到不是以-开头的或者--，代表java-options结束。
/system/bin：cmd-dir，也就是当前目录，文件操作的父路径将为此路径。
start-class-name:空
--zygote --start-system-server:选项。
- if (i < argc) {
- arg = argv[i++];
- if (0 == strcmp( "--zygote" , arg)) {
- bool startSystemServer = (i < argc) ?
- strcmp(argv[i], "--start-system-server" ) == 0 : false ;
- setArgv0(argv0, "zygote" );
- set_process_name("zygote" );
- runtime.start("com.android.internal.os.ZygoteInit" ,
- startSystemServer);
- } else {
- set_process_name(argv0);
- runtime.mClassName = arg;
- // Remainder of args get passed to startup class main()
- runtime.mArgC = argc-i;
- runtime.mArgV = argv+i;
- LOGV("App process is starting with pid=%d, class=%s./n" ,
- getpid(), runtime.getClassName());
- runtime.start();
- }
- }
- if (i < argc) {
- arg = argv[i++];
- if (0 == strcmp("--zygote", arg)) {
- bool startSystemServer = (i < argc) ?
- strcmp(argv[i], "--start-system-server") == 0 : false;
- setArgv0(argv0, "zygote");
- set_process_name("zygote");
- runtime.start("com.android.internal.os.ZygoteInit",
- startSystemServer);
- } else {
- set_process_name(argv0);
- runtime.mClassName = arg;
- // Remainder of args get passed to startup class main()
- runtime.mArgC = argc-i;
- runtime.mArgV = argv+i;
- LOGV("App process is starting with pid=%d, class=%s./n",
- getpid(), runtime.getClassName());
- runtime.start();
- }
- }
if (i < argc) { arg = argv[i++]; if (0 == strcmp("--zygote", arg)) { bool startSystemServer = (i < argc) ? strcmp(argv[i], "--start-system-server") == 0 : false; setArgv0(argv0, "zygote"); set_process_name("zygote"); runtime.start("com.android.internal.os.ZygoteInit", startSystemServer); } else { set_process_name(argv0); runtime.mClassName = arg; // Remainder of args get passed to startup class main() runtime.mArgC = argc-i; runtime.mArgV = argv+i; LOGV("App process is starting with pid=%d, class=%s./n", getpid(), runtime.getClassName()); runtime.start(); } }
从app_process的main函数（在app_main.cpp里面）可以看出，app_process有两种启动方式：一种是 init.rc里面的这种方式，这种方式将会以zygote模式启动com.android.internal.os.ZygoteInit，并将进程名称改为zygote；另外一种是以非zygote模拟启动com.android.internal.os.RuntimeInit，并调用它的main 方法，main的最后会执行finishInit，finishInit是一个native方法，这个方法会调用app_process的 onStarted方法，在onStarted里面将会调用真正要执行的class。
无论app_process，它都将会调用frameworks/base/core/jni/AndroidRuntime.cpp里面的start
方法（496行）。这个start方法会根据android属性系统设置dalvik的参数，并初始化java代码的native方法，最终启动dalvik。需要注意的是，java代码的大部分native方法（差不多80%）都是在这里面初始化的,这些初始化代码会检查java代码的正确性（例如是否有对应的 native方法，是否有需要的属性等），在初始化过程中，任何一步的错误都将导致进程退出。整个native方法初始化过程如下：start（AndroidRuntime.cpp，766行）->startReg(AndroidRuntime.cpp，1136 行)-> register_jni_procs(AndroidRuntime.cpp，1011行)。初始化的java class有（AndroidRuntime.cpp，1018行）：
- 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_KeyCharacterMap),
- REG_JNI(register_android_os_Process),
- REG_JNI(register_android_os_Binder),
- REG_JNI(register_android_os_Hardware),
- 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_graphics_Bitmap),
- REG_JNI(register_android_graphics_BitmapFactory),
- 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_com_android_internal_graphics_NativeUtils),
- REG_JNI(register_android_database_CursorWindow),
- 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_Exec),
- REG_JNI(register_android_os_FileObserver),
- REG_JNI(register_android_os_FileUtils),
- 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_wifi_WifiManager),
- 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_Database),
- REG_JNI(register_android_bluetooth_HeadsetBase),
- REG_JNI(register_android_bluetooth_BluetoothAudioGateway),
- REG_JNI(register_android_bluetooth_RfcommSocket),
- REG_JNI(register_android_bluetooth_ScoSocket),
- REG_JNI(register_android_server_BluetoothDeviceService),
- REG_JNI(register_android_server_BluetoothEventLoop),
- REG_JNI(register_android_server_BluetoothA2dpService),
- REG_JNI(register_android_message_digest_sha1),
- REG_JNI(register_android_ddm_DdmHandleNativeHeap),
- REG_JNI(register_android_util_Base64),
- REG_JNI(register_android_location_GpsLocationProvider),
- };
- 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_KeyCharacterMap),
- REG_JNI(register_android_os_Process),
- REG_JNI(register_android_os_Binder),
- REG_JNI(register_android_os_Hardware),
- 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_graphics_Bitmap),
- REG_JNI(register_android_graphics_BitmapFactory),
- 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_com_android_internal_graphics_NativeUtils),
- REG_JNI(register_android_database_CursorWindow),
- 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_Exec),
- REG_JNI(register_android_os_FileObserver),
- REG_JNI(register_android_os_FileUtils),
- 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_wifi_WifiManager),
- 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_Database),
- REG_JNI(register_android_bluetooth_HeadsetBase),
- REG_JNI(register_android_bluetooth_BluetoothAudioGateway),
- REG_JNI(register_android_bluetooth_RfcommSocket),
- REG_JNI(register_android_bluetooth_ScoSocket),
- REG_JNI(register_android_server_BluetoothDeviceService),
- REG_JNI(register_android_server_BluetoothEventLoop),
- REG_JNI(register_android_server_BluetoothA2dpService),
- REG_JNI(register_android_message_digest_sha1),
- REG_JNI(register_android_ddm_DdmHandleNativeHeap),
- REG_JNI(register_android_util_Base64),
- REG_JNI(register_android_location_GpsLocationProvider),
- };
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_KeyCharacterMap), REG_JNI(register_android_os_Process), REG_JNI(register_android_os_Binder), REG_JNI(register_android_os_Hardware), 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_graphics_Bitmap), REG_JNI(register_android_graphics_BitmapFactory), 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_com_android_internal_graphics_NativeUtils), REG_JNI(register_android_database_CursorWindow), 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_Exec), REG_JNI(register_android_os_FileObserver), REG_JNI(register_android_os_FileUtils), 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_wifi_WifiManager), 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_Database), REG_JNI(register_android_bluetooth_HeadsetBase), REG_JNI(register_android_bluetooth_BluetoothAudioGateway), REG_JNI(register_android_bluetooth_RfcommSocket), REG_JNI(register_android_bluetooth_ScoSocket), REG_JNI(register_android_server_BluetoothDeviceService), REG_JNI(register_android_server_BluetoothEventLoop), REG_JNI(register_android_server_BluetoothA2dpService), REG_JNI(register_android_message_digest_sha1), REG_JNI(register_android_ddm_DdmHandleNativeHeap), REG_JNI(register_android_util_Base64), REG_JNI(register_android_location_GpsLocationProvider), };
这些初始化的native方法是java代码与底层服务打交道的接口，因此我们必须初始化这些方法。
以app_process启动java class的另外一个好处是它会初始化IBinder，这样就可以在java代码和jni代码里面接收IBinder消息。