基于Android 6.0的源碼剖析， 分析Android啟動過程的Zygote進程

/frameworks/base/cmds/app_process/App_main.cpp （內含AppRuntime類） /frameworks/base/core/jni/AndroidRuntime.cpp /frameworks/base/core/java/com/android/internal/os/ZygoteInit.java /frameworks/base/core/java/com/android/internal/os/Zygote.java /frameworks/base/core/java/android/net/LocalServerSocket.java

一、啟動調用棧

Zygote啟動函數調用類的棧關系：

App_main.mainAndroidRuntime.startstartVmstartRegZygoteInit.mainregisterZygoteSocketpreloadstartSystemServerrunSelectLoop

二、App_main

Zygote是由init進程通過解析init.zygote.rc文件而創建的，zygote所對應的可執行程序app_process，所對應的源文件是App_main.cpp，進程名為zygote。

傳到main()的參數為?-Xzygote /system/bin --zygote --start-system-server

[–>App_main.cpp]

int main(int argc, char* const argv[]) {AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));//忽略第一個參數argc--;argv++;int i;for (i = 0; i < argc; i++) {if (argv[i][0] != '-') {break;}if (argv[i][1] == '-' && argv[i][2] == 0) {++i;break;}runtime.addOption(strdup(argv[i]));}//參數解析bool zygote = false;bool startSystemServer = false;bool application = false;String8 niceName;String8 className;++i;while (i < argc) {const char* arg = argv[i++];if (strcmp(arg, "--zygote") == 0) {zygote = true;niceName = ZYGOTE_NICE_NAME; //對于64位系統nice_name為zygote64,32位系統為zygote} 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()) {// 運行application或tool程序args.add(application ? String8("application") : String8("tool"));runtime.setClassNameAndArgs(className, argc - i, argv + i);} else {//進入zygote模式，創建 /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) {return 11;}String8 abiFlag("--abi-list=");abiFlag.append(prop);args.add(abiFlag);for (; i < argc; ++i) {args.add(String8(argv[i]));}}//設置進程名if (!niceName.isEmpty()) {runtime.setArgv0(niceName.string());set_process_name(niceName.string());}if (zygote) {// 啟動AppRuntime 【見小節3】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return 10;} }

采用cmd命令，是通過fork進程來執行相應的類：

app_process [可選參數] 命令所在路徑 啟動的類名 [可選參數]

三、AndroidRuntime

AndroidRuntime.cpp

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;}}const char* rootDir = getenv("ANDROID_ROOT");if (rootDir == NULL) {rootDir = "/system";if (!hasDir("/system")) {return;}setenv("ANDROID_ROOT", rootDir, 1);}JniInvocation jni_invocation;jni_invocation.Init(NULL);JNIEnv* env;// 虛擬機創建【見小節3.1】if (startVm(&mJavaVM, &env, zygote) != 0) {return;}onVmCreated(env);// JNI方法注冊【見小節3.2】if (startReg(env) < 0) {ALOGE("Unable to register all android natives\n");return;}jclass stringClass;jobjectArray strArray;jstring classNameStr;//等價 strArray= new String[options.size() + 1];stringClass = env->FindClass("java/lang/String");strArray = env->NewObjectArray(options.size() + 1, stringClass, NULL);//等價 strArray[0] = "com.android.internal.os.ZygoteInit"classNameStr = env->NewStringUTF(className);env->SetObjectArrayElement(strArray, 0, classNameStr);//等價 strArray[1] = "start-system-server"；// strArray[2] = "--abi-list=xxx"；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());env->SetObjectArrayElement(strArray, i + 1, optionsStr);}//將"com.android.internal.os.ZygoteInit"轉換為"com/android/internal/os/ZygoteInit"char* slashClassName = toSlashClassName(className);jclass startClass = env->FindClass(slashClassName);if (startClass == NULL) {ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);} else {jmethodID startMeth = env->GetStaticMethodID(startClass, "main","([Ljava/lang/String;)V");if (startMeth == NULL) {ALOGE("JavaVM unable to find main() in '%s'\n", className);} else {// 調用ZygoteInit.main()方法【見小節4.0】env->CallStaticVoidMethod(startClass, startMeth, strArray);}}free(slashClassName); //釋放相應對象的內存空間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"); }

3.1 虛擬機創建startVm

[–>AndroidRuntime.cpp]

創建Java虛擬機方法的主要篇幅是關于虛擬機參數的設置，下面只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) {// JNI檢測功能，用于native層調用jni函數時進行常規檢測，比較弱字符串格式是否符合要求，資源是否正確釋放。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, "");if (strcmp(propBuf, "true") == 0) {checkJni = true;} else if (strcmp(propBuf, "false") != 0) {property_get("ro.kernel.android.checkjni", propBuf, "");if (propBuf[0] == '1') {checkJni = true;}}if (checkJni) {addOption("-Xcheck:jni");}//虛擬機產生的trace文件，主要用于分析系統問題，路徑默認為/data/anr/traces.txtparseRuntimeOption("dalvik.vm.stack-trace-file", stackTraceFileBuf, "-Xstacktracefile:");//對于不同的軟硬件環境，這些參數往往需要調整、優化，從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=");//preloaded-classes文件內容是由WritePreloadedClassFile.java生成的，在ZygoteInit類中會預加載工作將其中的classes提前加載到內存，以提高系統性能if (!hasFile("/system/etc/preloaded-classes")) {return -1;}//創建虛擬機if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {ALOGE("JNI_CreateJavaVM failed\n");return -1;} }

3.2 JNI函數注冊startReg

[–>AndroidRuntime.cpp]

int AndroidRuntime::startReg(JNIEnv* env) {//設置線程創建方法為javaCreateThreadEtcandroidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);env->PushLocalFrame(200);//進程NI方法的注冊if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {env->PopLocalFrame(NULL);return -1;}env->PopLocalFrame(NULL);return 0; }

register_jni_procs

static int register_jni_procs(const RegJNIRec array[], size_t count, JNIEnv* env) {for (size_t i = 0; i < count; i++) {if (array[i].mProc(env) < 0) { 【見下文】return -1;}}return 0; }

gRegJNI

gRegJNI是一個數組,有100多個成員

static const RegJNIRec gRegJNI[] = {REG_JNI(register_com_android_internal_os_RuntimeInit),REG_JNI(register_android_os_Binder)，... }；

REG_JNI

#define REG_JNI(name) { name } struct RegJNIRec {int (*mProc)(JNIEnv*); };

調用mProc，那么gRegJNI數組的其中之一REG_JNI(register_com_android_internal_os_RuntimeInit)，等價于調用下面方法：

int register_com_android_internal_os_RuntimeInit(JNIEnv* env) {return jniRegisterNativeMethods(env, "com/android/internal/os/RuntimeInit",gMethods, NELEM(gMethods)); }//gMethods：java層方法名與jni層的方法的一一映射關系 static JNINativeMethod gMethods[] = {{ "nativeFinishInit", "()V",(void*) com_android_internal_os_RuntimeInit_nativeFinishInit },{ "nativeZygoteInit", "()V",(void*) com_android_internal_os_RuntimeInit_nativeZygoteInit },{ "nativeSetExitWithoutCleanup", "(Z)V",(void*) com_android_internal_os_RuntimeInit_nativeSetExitWithoutCleanup }, };

四 ZygoteInit

[–>ZygoteInit.java]

public static void main(String argv[]) {try {RuntimeInit.enableDdms(); //開啟DDMS功能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.");}registerZygoteSocket(socketName); //為Zygote注冊socket【見小節4.1】preload(); // 預加載類和資源【見小節4.2】SamplingProfilerIntegration.writeZygoteSnapshot();gcAndFinalize(); //GC操作if (startSystemServer) {startSystemServer(abiList, socketName);//啟動system_server【見小節4.3】}runSelectLoop(abiList); //進入循環模式【見小節4.4】closeServerSocket();} catch (MethodAndArgsCaller caller) {caller.run(); //啟動system_server中會講到。} catch (RuntimeException ex) {closeServerSocket();throw ex;} }

在異常捕獲后調用的方法caller.run()，會在后續的system_server文章會講到。

4.1 registerZygoteSocket

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();fd.setInt$(fileDesc); //設置文件描述符sServerSocket = new LocalServerSocket(fd); //創建Socket的本地服務端} catch (IOException ex) {throw new RuntimeException("Error binding to local socket '" + fileDesc + "'", ex);}} }

4.2 preload

執行Zygote進程的初始化

static void preload() {//預加載位于/system/etc/preloaded-classes文件中的類preloadClasses(); //預加載資源，包含drawable和color資源preloadResources();//預加載OpenGLpreloadOpenGL();//通過System.loadLibrary()方法，//預加載"android","compiler_rt","jnigraphics"這3個共享庫preloadSharedLibraries();//預加載 文本連接符資源preloadTextResources();//僅用于zygote進程，用于內存共享的進程WebViewFactory.prepareWebViewInZygote(); }

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，在應用程序中以com.android.internal.R.xxx開頭的資源，便是此時由Zygote加載到內存的。

zygote進程內加載了preload()方法中的所有資源，當需要fork新進程時，采用copy on write技術，如下：

4.3 startSystemServer

ZygoteInit.java

private static boolean startSystemServer(String abiList, String socketName)throws MethodAndArgsCaller, RuntimeException {long capabilities = posixCapabilitiesAsBits(OsConstants.CAP_BLOCK_SUSPEND,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);//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","--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，生成目標格式parsedArgs = new ZygoteConnection.Arguments(args);ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);// fork子進程，用于運行system_serverpid = Zygote.forkSystemServer(parsedArgs.uid, parsedArgs.gid,parsedArgs.gids,parsedArgs.debugFlags,null,parsedArgs.permittedCapabilities,parsedArgs.effectiveCapabilities);} catch (IllegalArgumentException ex) {throw new RuntimeException(ex);}//進入子進程system_serverif (pid == 0) {if (hasSecondZygote(abiList)) {waitForSecondaryZygote(socketName);}// 完成system_server進程剩余的工作handleSystemServerProcess(parsedArgs);}return true; }

準備參數并fork新進程，從上面可以看出system server進程參數信息為uid=1000,gid=1000,進程名為sytem_server，從zygote進程fork新進程后，需要關閉zygote原有的socket。另外，對于有兩個zygote進程情況，需等待第2個zygote創建完成。更多詳情見Android系統啟動-systemServer上篇。

4.4 runSelectLoop

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進程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 {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多路復用機制，當客戶端發出連接請求或者數據處理請求時，跳過continue，執行后面的代碼if ((pollFds[i].revents & POLLIN) == 0) {continue;}if (i == 0) {//創建客戶端連接ZygoteConnection newPeer = acceptCommandPeer(abiList);peers.add(newPeer);fds.add(newPeer.getFileDesciptor());} else {//處理客戶端數據事務boolean done = peers.get(i).runOnce();if (done) {peers.remove(i);fds.remove(i);}}}} }

Zygote采用高效的I/O多路復用機制，保證在沒有客戶端連接請求或數據處理時休眠，否則響應客戶端的請求。

五、總結

Zygote啟動過程的調用流程圖：

解析init.zygote.rc中的參數，創建AppRuntime并調用AppRuntime.start()方法；

調用AndroidRuntime的startVM()方法創建虛擬機，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，用于響應客戶端請求；

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ap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進程，也是上層framework的運行載體。

zygote功成身退，調用runSelectLoop()，隨時待命，當接收到請求創建新進程請求時立即喚醒并執行相應工作。

原文地址：http://gityuan.com/2016/02/05/android-init/

總結

以上是生活随笔為你收集整理的Android系统启动-zygote篇的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。