本文轉載自Gityuan的博客，原文地址為：http://gityuan.com/2016/02/13/android-zygote/ 轉載請注明作者及原文鏈接。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 /frameworks/base/core/jni/AndroidRuntime.cpp/frameworks/base/core/java/com/android/internal/os/- ZygoteInit.java- Zygote.java- ZygoteConnection.java/frameworks/base/core/java/android/net/LocalServerSocket.java /system/core/libutils/Threads.cpp

一. 概述

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。

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-serverclass mainsocket zygote stream 660 root systemonrestart write /sys/android_power/request_state wakeonrestart write /sys/power/state ononrestart restart mediaonrestart restart netd

Zygote進程能夠重啟的地方:

• servicemanager進程被殺; (onresart)
• surfaceflinger進程被殺; (onresart)
• Zygote進程自己被殺; (oneshot=false)
• system_server進程被殺; (waitpid)

從App_main()開始，Zygote啟動過程的函數調用類大致流程如下：

二、Zygote啟動過程

2.1 App_main.main

[-> App_main.cpp]

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++; //忽略第一個參數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;//對于64位系統nice_name為zygote64; 32位系統為zygoteniceName = 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2.2】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;} }

2.2 start

[-> 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2.3】if (startVm(&mJavaVM, &env, zygote) != 0) {return;}onVmCreated(env);// JNI方法注冊【見小節2.4】if (startReg(env) < 0) {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) {...} else {jmethodID startMeth = env->GetStaticMethodID(startClass, "main","([Ljava/lang/String;)V");// 調用ZygoteInit.main()方法【見小節3.1】env->CallStaticVoidMethod(startClass, startMeth, strArray);}//釋放相應對象的內存空間free(slashClassName);mJavaVM->DetachCurrentThread();mJavaVM->DestroyJavaVM(); }

2.3 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;}//初始化虛擬機if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {ALOGE("JNI_CreateJavaVM failedn");return -1;} }

2.4 startReg

[–> AndroidRuntime.cpp]

int AndroidRuntime::startReg(JNIEnv* env) {//設置線程創建方法為javaCreateThreadEtc 【見小節2.4.1】androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);env->PushLocalFrame(200);//進程NI方法的注冊【見小節2.4.2】if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {env->PopLocalFrame(NULL);return -1;}env->PopLocalFrame(NULL);return 0; }

2.4.1 androidSetCreateThreadFunc

[-> Threads.cpp]

void androidSetCreateThreadFunc(android_create_thread_fn func) {gCreateThreadFn = func; }

虛擬機啟動后startReg()過程，會設置線程創建函數指針gCreateThreadFn指向javaCreateThreadEtc.

2.4.2 register_jni_procs

static int register_jni_procs(const RegJNIRec array[], size_t count, JNIEnv* env) {for (size_t i = 0; i < count; i++) {//【見小節2.4.3】if (array[i].mProc(env) < 0) {return -1;}}return 0; }

2.4.3 gRegJNI.mProc

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

array[i]是指gRegJNI數組, 該數組有100多個成員。其中每一項成員都是通過REG_JNI宏定義的：

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

可見，調用mProc，就等價于調用其參數名所指向的函數。 例如REG_JNI(register_com_android_internal_os_RuntimeInit).mProc也就是指進入register_com_android_internal_os_RuntimeInit方法，接下來就繼續以此為例來說明：

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 }, };

三. 進入Java層

前面[小節2.2]AndroidRuntime.start()執行到最后通過反射調用到ZygoteInit.main(),見下文:

3.1 ZygoteInit.main

[–>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]);}}...registerZygoteSocket(socketName); //為Zygote注冊socket【見小節3.2】preload(); // 預加載類和資源【見小節3.3】SamplingProfilerIntegration.writeZygoteSnapshot();gcAndFinalize(); //GC操作if (startSystemServer) {startSystemServer(abiList, socketName);//啟動system_server【見小節3.4】}runSelectLoop(abiList); //進入循環模式【見小節3.5】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。

3.2 registerZygoteSocket

[–>ZygoteInit.java]

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) {...}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) {...}} }

3.3 preload

[–>ZygoteInit.java]

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(); }

執行Zygote進程的初始化,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，在應用程序中以http://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：

3.4 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。

3.5 runSelectLoop

[–>ZygoteInit.java]

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。保存到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 {//處理輪詢狀態，當pollFds有事件到來則往下執行，否則阻塞在這里Os.poll(pollFds, -1);} catch (ErrnoException ex) {...}for (int i = pollFds.length - 1; i >= 0; --i) {//采用I/O多路復用機制，當接收到客戶端發出連接請求 或者數據處理請求到來，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，則意味著有客戶端連接請求；// 則創建ZygoteConnection對象,并添加到fds。ZygoteConnection newPeer = acceptCommandPeer(abiList);peers.add(newPeer);fds.add(newPeer.getFileDesciptor()); //添加到fds.} else {//i>0，則代表通過socket接收來自對端的數據，并執行相應操作【見小節3.6】boolean done = peers.get(i).runOnce();if (done) {peers.remove(i);fds.remove(i); //處理完則從fds中移除該文件描述符}}}} }

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。

3.6 runOnce

[-> ZygoteConnection.java]

boolean runOnce() throws ZygoteInit.MethodAndArgsCaller {String args[];Arguments parsedArgs = null;FileDescriptor[] descriptors;try {//讀取socket客戶端發送過來的參數列表args = readArgumentList();descriptors = mSocket.getAncillaryFileDescriptors();} catch (IOException ex) {...return true;}...try {//將binder客戶端傳遞過來的參數，解析成Arguments對象格式parsedArgs = new Arguments(args);...//【見小節7】pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid, parsedArgs.gids,parsedArgs.debugFlags, rlimits, parsedArgs.mountExternal, parsedArgs.seInfo,parsedArgs.niceName, fdsToClose, parsedArgs.instructionSet,parsedArgs.appDataDir);} catch (Exception e) {...}try {if (pid == 0) {//子進程執行IoUtils.closeQuietly(serverPipeFd);serverPipeFd = null;//進入子進程流程handleChildProc(parsedArgs, descriptors, childPipeFd, newStderr);return true;} else {//父進程執行IoUtils.closeQuietly(childPipeFd);childPipeFd = null;return handleParentProc(pid, descriptors, serverPipeFd, parsedArgs);}} finally {IoUtils.closeQuietly(childPipeFd);IoUtils.closeQuietly(serverPipeFd);} }

更多內容，見理解Android進程創建流程

四、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。

最后，介紹給通過cmd命令，來fork新進程來執行類中main方法的方式：(啟動后進入RuntimeInit.main)

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

總結

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