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();
}
}

 

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),
};

 

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。

 

原文链接： http://blog.csdn.net/chief1985/archive/2009/09/14/4551255.aspx