本文源代碼基于 Android 7.0。
Init 程序孵化出 Zygote 程序，Zygote 程序是 Android 系統的第一個 Java 程序 (即虛拟機程序)，Zygote 是所有 Java 程序的父程序。
目錄：
- 概述
- Zygote 的啟動過程
- Java 層分析
- 總結
1. 概述
Zygote 是由 Init 程序通過解析 init.zygote.rc 檔案而建立的，zygote 所對應的可執行程式 app_process，所對應的源檔案是App_main.cpp，程序名為 zygote。
看看它的 rc 配置：
service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
class main
socket zygote stream 660 root system
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
onrestart restart media
onrestart restart netd
Zygote 重新開機的場景有幾個：
- servicemanager 程序被殺 (onresart)。
- surfaceflinger 程序被殺 (onresart)。
- Zygote 程序自身被殺 (oneshot=false)。
- system_server 程序被殺 (waitpid)。
Zygote 對應的入口代碼 app_main.cpp 函數調用流程大緻如下：
2. Zygote 的啟動過程
/base/cmds/app_process/app_main.cpp
- 2.1 app_main.main()
// Zygote啟動入口，app_main.cpp main函數
int main(int argc, char* const argv[])
{
//...
// 傳到的參數argv為“-Xzygote /system/bin --zygote --start-system-server”
AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
argc--;
argv++;
// --zygote : Start in zygote mode
// --start-system-server : Start the system server.
// --application : Start in application (stand alone, non zygote) mode.
// --nice-name : The nice name for this process.
int i;
for (i = 0; i < argc; i++) {
if (argv[i][0] != '-') {
break;
}
if (argv[i][1] == '-' && argv[i][2] == 0) {
++i; // Skip --.
break;
}
runtime.addOption(strdup(argv[i]));
}
// 參數解析
bool zygote = false;
bool startSystemServer = false;
bool application = false;
String8 niceName;
String8 className;
++i; // Skip unused "parent dir" argument.
while (i < argc) {
const char* arg = argv[i++];
if (strcmp(arg, "--zygote") == 0) {
zygote = true;
// 對于64位系統nice_name為zygote64; 32位系統為zygote
niceName = ZYGOTE_NICE_NAME;
} else if (strcmp(arg, "--start-system-server") == 0) {
startSystemServer = true;
} else if (strcmp(arg, "--application") == 0) {
application = true;
} else if (strncmp(arg, "--nice-name=", 12) == 0) {
niceName.setTo(arg + 12);
} else if (strncmp(arg, "--", 2) != 0) {
className.setTo(arg);
break;
} else {
--i;
break;
}
}
Vector<String8> args;
if (!className.isEmpty()) {
// 運作application或tool程式
args.add(application ? String8("application") : String8("tool"));
runtime.setClassNameAndArgs(className, argc - i, argv + i);
} else {
// 進入zygote模式，建立 /data/dalvik-cache路徑
maybeCreateDalvikCache();
if (startSystemServer) {
args.add(String8("start-system-server"));
}
char prop[PROP_VALUE_MAX];
if (property_get(ABI_LIST_PROPERTY, prop, NULL) == 0) {
LOG_ALWAYS_FATAL("app_process: Unable to determine ABI list from property %s.",
ABI_LIST_PROPERTY);
return 11;
}
String8 abiFlag("--abi-list=");
abiFlag.append(prop);
args.add(abiFlag);
// In zygote mode, pass all remaining arguments to the zygote
// main() method.
for (; i < argc; ++i) {
args.add(String8(argv[i]));
}
}
// 設定程序名
if (!niceName.isEmpty()) {
runtime.setArgv0(niceName.string());
set_process_name(niceName.string());
}
if (zygote) {
// 啟動AppRuntime
runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
} else if (className) {
runtime.start("com.android.internal.os.RuntimeInit", args, zygote);
} else {
// 沒有指定類名或zygote，參數錯誤
fprintf(stderr, "Error: no class name or --zygote supplied.\n");
app_usage();
LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
return 10;
}
}
我們以繼續往下看，runtime 為 AppRuntime，繼承于 AndroidRuntime：
if (zygote) {
// 啟動AppRuntime
runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
} else if (className) {
runtime.start("com.android.internal.os.RuntimeInit", args, zygote);
}
- 2.2 AndroidRuntime.start()
/base/core/jni/AndroidRuntime.cpp
// app_main.cpp -> main() -> AndroidRuntime.cpp -> start()
void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
static const String8 startSystemServer("start-system-server");
for (size_t i = 0; i < options.size(); ++i) {
if (options[i] == startSystemServer) {
const int LOG_BOOT_PROGRESS_START = 3000;
LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START, ns2ms(systemTime(SYSTEM_TIME_MONOTONIC)));
}
}
const char* rootDir = getenv("ANDROID_ROOT");
if (rootDir == NULL) {
rootDir = "/system";
if (!hasDir("/system")) {
LOG_FATAL("No root directory specified, and /android does not exist.");
return;
}
setenv("ANDROID_ROOT", rootDir, 1);
}
/* start the virtual machine */
JniInvocation jni_invocation;
jni_invocation.Init(NULL);
JNIEnv* env;
// 啟動虛拟機
if (startVm(&mJavaVM, &env, zygote) != 0) {
return;
}
onVmCreated(env);
// 注冊 jni 函數
if (startReg(env) < 0) {
ALOGE("Unable to register all android natives\n");
return;
}
jclass stringClass;
jobjectArray strArray;
jstring classNameStr;
stringClass = env->FindClass("java/lang/String");
assert(stringClass != NULL);
strArray = env->NewObjectArray(options.size() + 1, stringClass, NULL);
assert(strArray != NULL);
classNameStr = env->NewStringUTF(className);
assert(classNameStr != NULL);
env->SetObjectArrayElement(strArray, 0, classNameStr);
// 其中xxx為系統響應的cpu架構類型，比如arm64-v8a.
for (size_t i = 0; i < options.size(); ++i) {
jstring optionsStr = env->NewStringUTF(options.itemAt(i).string());
assert(optionsStr != NULL);
env->SetObjectArrayElement(strArray, i + 1, optionsStr);
}
// 将"com.android.internal.os.ZygoteInit"轉換為"com/android/internal/os/ZygoteInit"
// 反射調用ZygoteInit.java
char* slashClassName = toSlashClassName(className);
jclass startClass = env->FindClass(slashClassName);
if (startClass == NULL) {
ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
/* keep going */
} else {
jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
"([Ljava/lang/String;)V");
if (startMeth == NULL) {
ALOGE("JavaVM unable to find main() in '%s'\n", className);
/* keep going */
} else {
env->CallStaticVoidMethod(startClass, startMeth, strArray);
#if 0
if (env->ExceptionCheck())
threadExitUncaughtException(env);
#endif
}
}
// 釋放相應對象的記憶體空間
free(slashClassName);
ALOGD("Shutting down VM\n");
if (mJavaVM->DetachCurrentThread() != JNI_OK)
ALOGW("Warning: unable to detach main thread\n");
if (mJavaVM->DestroyJavaVM() != 0)
ALOGW("Warning: VM did not shut down cleanly\n");
}
主要工作包括：
- 啟動虛拟機。
- 注冊 JNI 函數。
- 反射調用 ZygoteInit.java。
- 2.3 AndroidRuntime.startVm()
/base/core/jni/AndroidRuntime.cpp
// 啟動虛拟機，建立Java虛拟機方法的主要篇幅是關于虛拟機參數的設定，下面隻列舉部分在調試優化過程中常用參數。
int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv, bool zygote)
{
JavaVMInitArgs initArgs;
char propBuf[PROPERTY_VALUE_MAX];
char stackTraceFileBuf[sizeof("-Xstacktracefile:")-1 + PROPERTY_VALUE_MAX];
char jniOptsBuf[sizeof("-Xjniopts:")-1 + PROPERTY_VALUE_MAX];
// ...
// JNI檢測功能，用于native層調用jni函數時進行正常檢測，比較弱字元串格式是否符合要求，資源是否正确釋放。
// 該功能一般用于早期系統調試或手機Eng版，對于User版往往不會開啟，引用該功能比較消耗系統CPU資源，降低系統性能。
bool checkJni = false;
property_get("dalvik.vm.checkjni", propBuf, "");
// ...
/*
* The default starting and maximum size of the heap. Larger
* values should be specified in a product property override.
*/
// 對于不同的軟硬體環境，這些參數往往需要調整、優化，進而使系統達到最佳性能
parseRuntimeOption("dalvik.vm.heapstartsize", heapstartsizeOptsBuf, "-Xms", "4m");
parseRuntimeOption("dalvik.vm.heapsize", heapsizeOptsBuf, "-Xmx", "16m");
parseRuntimeOption("dalvik.vm.heapgrowthlimit", heapgrowthlimitOptsBuf, "-XX:HeapGrowthLimit=");
parseRuntimeOption("dalvik.vm.heapminfree", heapminfreeOptsBuf, "-XX:HeapMinFree=");
parseRuntimeOption("dalvik.vm.heapmaxfree", heapmaxfreeOptsBuf, "-XX:HeapMaxFree=");
parseRuntimeOption("dalvik.vm.heaptargetutilization",
heaptargetutilizationOptsBuf,
"-XX:HeapTargetUtilization=");
/*
* JIT related options.
*/
parseRuntimeOption("dalvik.vm.usejit", usejitOptsBuf, "-Xusejit:");
parseRuntimeOption("dalvik.vm.jitmaxsize", jitmaxsizeOptsBuf, "-Xjitmaxsize:");
parseRuntimeOption("dalvik.vm.jitinitialsize", jitinitialsizeOptsBuf, "-Xjitinitialsize:");
parseRuntimeOption("dalvik.vm.jitthreshold", jitthresholdOptsBuf, "-Xjitthreshold:");
property_get("dalvik.vm.usejitprofiles", useJitProfilesOptsBuf, "");
if (strcmp(useJitProfilesOptsBuf, "true") == 0) {
addOption("-Xjitsaveprofilinginfo");
}
parseRuntimeOption("dalvik.vm.jitprithreadweight",
jitprithreadweightOptBuf,
"-Xjitprithreadweight:");
parseRuntimeOption("dalvik.vm.jittransitionweight",
jittransitionweightOptBuf,
"-Xjittransitionweight:");
property_get("ro.config.low_ram", propBuf, "");
if (strcmp(propBuf, "true") == 0) {
addOption("-XX:LowMemoryMode");
}
parseRuntimeOption("dalvik.vm.gctype", gctypeOptsBuf, "-Xgc:");
parseRuntimeOption("dalvik.vm.backgroundgctype", backgroundgcOptsBuf, "-XX:BackgroundGC=");
// preloaded-classes檔案内容是由WritePreloadedClassFile.java生成的，
// 在ZygoteInit類中會預加載工作将其中的classes提前加載到記憶體，以提高系統性能
if (!hasFile("/system/etc/preloaded-classes")) {
ALOGE("Missing preloaded-classes file, /system/etc/preloaded-classes not found: %s\n",
strerror(errno));
return -1;
}
addOption("-Ximage-compiler-option");
addOption("--image-classes=/system/etc/preloaded-classes");
// If there is a compiled-classes file, push it.
if (hasFile("/system/etc/compiled-classes")) {
addOption("-Ximage-compiler-option");
addOption("--compiled-classes=/system/etc/compiled-classes");
}
property_get("dalvik.vm.image-dex2oat-flags", dex2oatImageFlagsBuf, "");
parseExtraOpts(dex2oatImageFlagsBuf, "-Ximage-compiler-option");
// Extra options for DexClassLoader.
parseCompilerRuntimeOption("dalvik.vm.dex2oat-Xms", dex2oatXmsFlagsBuf,
"-Xms", "-Xcompiler-option");
parseCompilerRuntimeOption("dalvik.vm.dex2oat-Xmx", dex2oatXmxFlagsBuf,
"-Xmx", "-Xcompiler-option");
if (skip_compilation) {
addOption("-Xcompiler-option");
addOption("--compiler-filter=verify-none");
// We skip compilation when a minimal runtime is brought up for decryption. In that case
// /data is temporarily backed by a tmpfs, which is usually small.
// If the system image contains prebuilts, they will be relocated into the tmpfs. In this
// specific situation it is acceptable to *not* relocate and run out of the prebuilts
// directly instead.
addOption("--runtime-arg");
addOption("-Xnorelocate");
} else {
parseCompilerOption("dalvik.vm.dex2oat-filter", dex2oatCompilerFilterBuf,
"--compiler-filter=", "-Xcompiler-option");
}
parseCompilerOption("dalvik.vm.dex2oat-threads", dex2oatThreadsBuf, "-j", "-Xcompiler-option");
parseCompilerOption("dalvik.vm.image-dex2oat-threads", dex2oatThreadsImageBuf, "-j",
"-Ximage-compiler-option");
// ...
/*
* Retrieve the build fingerprint and provide it to the runtime. That way, ANR dumps will
* contain the fingerprint and can be parsed.
*/
parseRuntimeOption("ro.build.fingerprint", fingerprintBuf, "-Xfingerprint:");
initArgs.version = JNI_VERSION_1_4;
initArgs.options = mOptions.editArray();
initArgs.nOptions = mOptions.size();
initArgs.ignoreUnrecognized = JNI_FALSE;
// 初始化虛拟機
if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {
ALOGE("JNI_CreateJavaVM failed\n");
return -1;
}
return 0;
}
删減了一些代碼，主要是虛拟機參數的設定，然後建立。
- 2.4 AndroidRuntime.startReg()
/base/core/jni/AndroidRuntime.cpp
// 注冊所有的android natvie 方法
int AndroidRuntime::startReg(JNIEnv* env)
{
ATRACE_NAME("RegisterAndroidNatives");
androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);
ALOGV("--- registering native functions ---\n");
env->PushLocalFrame(200);
// 注冊JNI方法，其中gRegJNI是一個數組，記錄所有需要注冊的jni方法，其中有一項便是REG_JNI(register_android_os_Binder)，
if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
env->PopLocalFrame(NULL);
return -1;
}
env->PopLocalFrame(NULL);
//createJavaThread("fubar", quickTest, (void*) "hello");
return 0;
}
注冊所有的 android natvie 方法。
static const RegJNIRec gRegJNI[] = {
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_MemoryIntArray),
REG_JNI(register_android_util_PathParser),
REG_JNI(register_android_app_admin_SecurityLog),
REG_JNI(register_android_content_AssetManager),
REG_JNI(register_android_content_StringBlock),
REG_JNI(register_android_content_XmlBlock),
REG_JNI(register_android_text_AndroidCharacter),
REG_JNI(register_android_text_StaticLayout),
REG_JNI(register_android_text_AndroidBidi),
REG_JNI(register_android_view_InputDevice),
REG_JNI(register_android_view_KeyCharacterMap),
REG_JNI(register_android_os_Process),
REG_JNI(register_android_os_SystemProperties),
REG_JNI(register_android_os_Binder),
REG_JNI(register_android_os_Parcel),
REG_JNI(register_android_nio_utils),
REG_JNI(register_android_graphics_Canvas),
REG_JNI(register_android_graphics_Graphics),
REG_JNI(register_android_view_DisplayEventReceiver),
REG_JNI(register_android_view_RenderNode),
REG_JNI(register_android_view_RenderNodeAnimator),
REG_JNI(register_android_view_GraphicBuffer),
REG_JNI(register_android_view_DisplayListCanvas),
REG_JNI(register_android_view_HardwareLayer),
REG_JNI(register_android_view_ThreadedRenderer),
REG_JNI(register_android_view_Surface),
REG_JNI(register_android_view_SurfaceControl),
REG_JNI(register_android_view_SurfaceSession),
REG_JNI(register_android_view_TextureView),
REG_JNI(register_com_android_internal_view_animation_NativeInterpolatorFactoryHelper),
REG_JNI(register_com_google_android_gles_jni_EGLImpl),
REG_JNI(register_com_google_android_gles_jni_GLImpl),
REG_JNI(register_android_opengl_jni_EGL14),
REG_JNI(register_android_opengl_jni_EGLExt),
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_opengl_jni_GLES30),
REG_JNI(register_android_opengl_jni_GLES31),
REG_JNI(register_android_opengl_jni_GLES31Ext),
REG_JNI(register_android_opengl_jni_GLES32),
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_CreateJavaOutputStreamAdaptor),
REG_JNI(register_android_graphics_CanvasProperty),
REG_JNI(register_android_graphics_ColorFilter),
REG_JNI(register_android_graphics_DrawFilter),
REG_JNI(register_android_graphics_FontFamily),
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_SurfaceTexture),
REG_JNI(register_android_graphics_Typeface),
REG_JNI(register_android_graphics_Xfermode),
REG_JNI(register_android_graphics_YuvImage),
REG_JNI(register_android_graphics_drawable_AnimatedVectorDrawable),
REG_JNI(register_android_graphics_drawable_VectorDrawable),
REG_JNI(register_android_graphics_pdf_PdfDocument),
REG_JNI(register_android_graphics_pdf_PdfEditor),
REG_JNI(register_android_graphics_pdf_PdfRenderer),
REG_JNI(register_android_database_CursorWindow),
REG_JNI(register_android_database_SQLiteConnection),
REG_JNI(register_android_database_SQLiteGlobal),
REG_JNI(register_android_database_SQLiteDebug),
REG_JNI(register_android_os_Debug),
REG_JNI(register_android_os_FileObserver),
REG_JNI(register_android_os_MessageQueue),
REG_JNI(register_android_os_SELinux),
REG_JNI(register_android_os_Trace),
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_os_MemoryFile),
REG_JNI(register_com_android_internal_os_PathClassLoaderFactory),
REG_JNI(register_com_android_internal_os_Zygote),
REG_JNI(register_com_android_internal_util_VirtualRefBasePtr),
REG_JNI(register_android_hardware_Camera),
REG_JNI(register_android_hardware_camera2_CameraMetadata),
REG_JNI(register_android_hardware_camera2_legacy_LegacyCameraDevice),
REG_JNI(register_android_hardware_camera2_legacy_PerfMeasurement),
REG_JNI(register_android_hardware_camera2_DngCreator),
REG_JNI(register_android_hardware_Radio),
REG_JNI(register_android_hardware_SensorManager),
REG_JNI(register_android_hardware_SerialPort),
REG_JNI(register_android_hardware_SoundTrigger),
REG_JNI(register_android_hardware_UsbDevice),
REG_JNI(register_android_hardware_UsbDeviceConnection),
REG_JNI(register_android_hardware_UsbRequest),
REG_JNI(register_android_hardware_location_ActivityRecognitionHardware),
REG_JNI(register_android_hardware_location_ContextHubService),
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_RemoteDisplay),
REG_JNI(register_android_media_ToneGenerator),
REG_JNI(register_android_opengl_classes),
REG_JNI(register_android_server_NetworkManagementSocketTagger),
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_backup_FullBackup),
REG_JNI(register_android_app_Activity),
REG_JNI(register_android_app_ActivityThread),
REG_JNI(register_android_app_ApplicationLoaders),
REG_JNI(register_android_app_NativeActivity),
REG_JNI(register_android_util_jar_StrictJarFile),
REG_JNI(register_android_view_InputChannel),
REG_JNI(register_android_view_InputEventReceiver),
REG_JNI(register_android_view_InputEventSender),
REG_JNI(register_android_view_InputQueue),
REG_JNI(register_android_view_KeyEvent),
REG_JNI(register_android_view_MotionEvent),
REG_JNI(register_android_view_PointerIcon),
REG_JNI(register_android_view_VelocityTracker),
REG_JNI(register_android_content_res_ObbScanner),
REG_JNI(register_android_content_res_Configuration),
REG_JNI(register_android_animation_PropertyValuesHolder),
REG_JNI(register_com_android_internal_content_NativeLibraryHelper),
REG_JNI(register_com_android_internal_net_NetworkStatsFactory),
};
3. Java 層分析
app_main.main() 中最後會反射調用 ZygoteInit.java，接下來就開始進入 Java 層了。
/base/core/java/com/android/internal/os/ZygoteInit.java
- 3.1 ZygoteInit.main()
/**
* 執行Zygote程序的初始化, 對于類加載，采用反射機制 Class.forName() 方法來加載。
* 對于資源加載，主要是 com.android.internal.R.array.preloaded_drawables和com.android.internal.R.array.preloaded_color_state_lists，
* 在應用程式中以com.android.internal.R.xxx開頭的資源，便是此時由Zygote加載到記憶體的。
*
* zygote程序内加載了preload()方法中的所有資源，當需要fork新程序時，采用copy on write技術。
*
* @param argv
*/
public static void main(String argv[]) {
ZygoteHooks.startZygoteNoThreadCreation();
try {
Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "ZygoteInit");
// 開啟DDMS功能
RuntimeInit.enableDdms();
SamplingProfilerIntegration.start();
boolean startSystemServer = false;
String socketName = "zygote";
String abiList = null;
for (int i = 1; i < argv.length; i++) {
if ("start-system-server".equals(argv[i])) {
startSystemServer = true;
} else if (argv[i].startsWith(ABI_LIST_ARG)) {
abiList = argv[i].substring(ABI_LIST_ARG.length());
} else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
socketName = argv[i].substring(SOCKET_NAME_ARG.length());
} else {
throw new RuntimeException("Unknown command line argument: " + argv[i]);
}
}
if (abiList == null) {
throw new RuntimeException("No ABI list supplied.");
}
// 為Zygote注冊socket
registerZygoteSocket(socketName);
Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "ZygotePreload");
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
// 預加載類和資源
preload();
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
SystemClock.uptimeMillis());
Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
// Finish profiling the zygote initialization.
SamplingProfilerIntegration.writeZygoteSnapshot();
// Do an initial gc to clean up after startup
Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PostZygoteInitGC");
// GC操作
gcAndFinalize();
Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
// Disable tracing so that forked processes do not inherit stale tracing tags from
// Zygote.
Trace.setTracingEnabled(false);
// Zygote process unmounts root storage spaces.
Zygote.nativeUnmountStorageOnInit();
ZygoteHooks.stopZygoteNoThreadCreation();
if (startSystemServer) {
// 啟動system_server程序
startSystemServer(abiList, socketName);
}
Log.i(TAG, "Accepting command socket connections");
// 進入循環模式
runSelectLoop(abiList);
closeServerSocket();
} catch (MethodAndArgsCaller caller) {
caller.run(); // 啟動system_server中會講到
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}
主要包括以下幾個工作：
- 開啟 DDMS 功能。
- 為 Zygote 注冊 socket。
- 預加載類。
- 預加載資源。
- 啟動 system_server 程序。
- 進入循環模式。
- 3.2 ZygoteInit.registerZygoteSocket()
// 為zygote注冊socket
private static void registerZygoteSocket(String socketName) {
if (sServerSocket == null) {
int fileDesc;
final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName;
try {
String env = System.getenv(fullSocketName);
fileDesc = Integer.parseInt(env);
} catch (RuntimeException ex) {
throw new RuntimeException(fullSocketName + " unset or invalid", ex);
}
try {
FileDescriptor fd = new FileDescriptor();
// 設定檔案描述符
fd.setInt$(fileDesc);
// 建立Socket的本地服務端
sServerSocket = new LocalServerSocket(fd);
} catch (IOException ex) {
throw new RuntimeException(
"Error binding to local socket '" + fileDesc + "'", ex);
}
}
}
- 3.3 ZygoteInit.preload()
// 預加載
static void preload() {
Log.d(TAG, "begin preload");
Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "BeginIcuCachePinning");
beginIcuCachePinning();
Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PreloadClasses");
// 預加載位于/system/etc/preloaded-classes檔案中的類
preloadClasses();
Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PreloadResources");
// 預加載資源，包含drawable和color資源
preloadResources();
Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PreloadOpenGL");
// 預加載OpenGL
preloadOpenGL();
Trace.traceEnd(Trace.TRACE_TAG_DALVIK);
// 通過System.loadLibrary()方法，
// 預加載"android","compiler_rt","jnigraphics"這3個共享庫
preloadSharedLibraries();
// 預加載 文本連接配接符資源
preloadTextResources();
// Ask the WebViewFactory to do any initialization that must run in the zygote process,
// for memory sharing purposes.
// 僅用于zygote程序，用于記憶體共享的程序
WebViewFactory.prepareWebViewInZygote();
endIcuCachePinning();
warmUpJcaProviders();
Log.d(TAG, "end preload");
}
- 3.4 ZygoteInit.startSystemServer()
/**
* 啟動SystemServer程序
*
* 準備參數并fork新程序，從上面可以看出system server程序參數資訊為uid=1000,gid=1000,程序名為sytem_server，
* 從zygote程序fork新程序後，需要關閉zygote原有的socket。另外，對于有兩個zygote程序情況，需等待第2個zygote建立完成。
*
* @param abiList
* @param socketName
* @return
* @throws MethodAndArgsCaller
* @throws RuntimeException
*/
private static boolean startSystemServer(String abiList, String socketName)
throws MethodAndArgsCaller, RuntimeException {
long capabilities = posixCapabilitiesAsBits(
OsConstants.CAP_IPC_LOCK,
OsConstants.CAP_KILL,
OsConstants.CAP_NET_ADMIN,
OsConstants.CAP_NET_BIND_SERVICE,
OsConstants.CAP_NET_BROADCAST,
OsConstants.CAP_NET_RAW,
OsConstants.CAP_SYS_MODULE,
OsConstants.CAP_SYS_NICE,
OsConstants.CAP_SYS_RESOURCE,
OsConstants.CAP_SYS_TIME,
OsConstants.CAP_SYS_TTY_CONFIG
);
/* Containers run without this capability, so avoid setting it in that case */
if (!SystemProperties.getBoolean(PROPERTY_RUNNING_IN_CONTAINER, false)) {
capabilities |= posixCapabilitiesAsBits(OsConstants.CAP_BLOCK_SUSPEND);
}
// 參數準備
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,3001,3002,3003,3006,3007,3009,3010",
"--capabilities=" + capabilities + "," + capabilities,
"--nice-name=system_server",
"--runtime-args",
"com.android.server.SystemServer",
};
ZygoteConnection.Arguments parsedArgs = null;
int pid;
try {
// 用于解析參數，生成目标格式，使用socket fork
parsedArgs = new ZygoteConnection.Arguments(args);
ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
// fork子程序，用于運作system_server
pid = Zygote.forkSystemServer(
parsedArgs.uid, parsedArgs.gid,
parsedArgs.gids,
parsedArgs.debugFlags,
null,
parsedArgs.permittedCapabilities,
parsedArgs.effectiveCapabilities);
} catch (IllegalArgumentException ex) {
throw new RuntimeException(ex);
}
//進入子程序system_server
if (pid == 0) {
if (hasSecondZygote(abiList)) {
waitForSecondaryZygote(socketName);
}
// 完成system_server程序剩餘的工作
handleSystemServerProcess(parsedArgs);
}
return true;
}
fork 子程序，運作 system_server。
- 3.5 ZygoteInit.runSelectLoop()
// Zygote采用高效的I/O多路複用機制，保證在沒有用戶端連接配接請求或資料處理時休眠，否則響應用戶端的請求。
private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {
ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();
// sServerSocket是socket通信中的服務端，即zygote程序。儲存到fds[0]
fds.add(sServerSocket.getFileDescriptor());
peers.add(null);
while (true) {
StructPollfd[] pollFds = new StructPollfd[fds.size()];
for (int i = 0; i < pollFds.length; ++i) {
pollFds[i] = new StructPollfd();
pollFds[i].fd = fds.get(i);
pollFds[i].events = (short) POLLIN;
}
try {
// 處理輪詢狀态，當pollFds有事件到來則往下執行，否則阻塞在這裡
Os.poll(pollFds, -1);
} catch (ErrnoException ex) {
throw new RuntimeException("poll failed", ex);
}
for (int i = pollFds.length - 1; i >= 0; --i) {
// 采用I/O多路複用機制，當接收到用戶端發出連接配接請求 或者資料處理請求到來，則往下執行；
// 否則進入continue，跳出本次循環。
if ((pollFds[i].revents & POLLIN) == 0) {
continue;
}
if (i == 0) {
// 即fds[0]，代表的是sServerSocket，則意味着有用戶端連接配接請求；
// 則建立ZygoteConnection對象,并添加到fds。
ZygoteConnection newPeer = acceptCommandPeer(abiList);
peers.add(newPeer);
fds.add(newPeer.getFileDesciptor());
} else {
// i>0，則代表通過socket接收來自對端的資料，并執行相應操作
boolean done = peers.get(i).runOnce();
if (done) {
peers.remove(i);
// 處理完則從fds中移除該檔案描述符
fds.remove(i);
}
}
}
}
}
Zygote 采用高效的 I/O 多路複用機制，保證在沒有用戶端連接配接請求或資料處理時休眠，否則響應用戶端的請求。
4. 總結
- 解析 init.zygote.rc 中的參數，建立 AppRuntime 并調用 AppRuntime.start() 方法。
- 調用 AndroidRuntime 的 startVM() 方法建立虛拟機，再調用 startReg() 注冊 JNI 函數。
- 通過 JNI 方式調用 ZygoteInit.main()，第一次進入Java世界。
- registerZygoteSocket() 建立 socket 通道，zygote 作為通信的服務端，用于響應用戶端請求。
- preload() 預加載通用類、drawable 和 color 資源、openGL 以及共享庫以及 WebView，用于提高 app 啟動效率。
- zygote 完畢大部分工作，接下來再通過 startSystemServer()，fork 得力幫手 system_server 程序，也是上層 framework 的運作載體。
- zygote 功成身退，調用 runSelectLoop()，随時待命，當接收到請求建立新程序請求時立即喚醒并執行相應工作。
- 最後，介紹給通過 cmd 指令，來 fork 新程序來執行類中 main 方法的方式：(啟動後進入 RuntimeInit.main)。