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。