文中的源代码版本为api23

Zygote进程启动流程分析

先说结论，zygote进程启动过程中主要做了下面这些事情：

1. 启动DVM虚拟机
2. 预加载部分资源，如一些通用类、通用资源、共享库等
3. 启动system_server进程
4. 创建一个ServerSocket，并进入死循环监听Socket消息。

我们知道在Android操作系统中，zygote进程占据着非常重要的地位，所有应用进程都是由zygote进程孵化而来。zygote进程启动的起点在app_main.cpp的main函数中(对这一点有疑问的童鞋可以参考这篇博客Android系统启动流程（一）解析init进程启动过程)，接下来，我们就以该函数为入口点，一步步分析zygote的启动流程。

app_main

main函数的入参读取自init.rc文件： -Xzygote /system/bin --zygote --start-system-server main函数的主要工作是解析部分参数，最终调用AppRuntime.start方法

int main(int argc, char* const argv[])
{//...AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));// Process command line arguments// ignore argv[0]argc--;argv++;//...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]));}//...++i;  // Skip unused "parent dir" argument.while (i < argc) {const char* arg = argv[i++];if (strcmp(arg, "--zygote") == 0) {zygote = true;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()) {//...} else {// We're in zygote mode.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]));}}if (!niceName.isEmpty()) {runtime.setArgv0(niceName.string());set_process_name(niceName.string());}if (zygote) {runtime.start("com.android.internal.os.ZygoteInit", args, zygote);} else if (className) {runtime.start("com.android.internal.os.RuntimeInit", args, zygote);} else {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;}
}
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AndroidRuntime.start

该函数的主要工作：

1. 调用startVm函数启动DVM虚拟机
2. 调用java层的ZygoteInit.main函数

void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{ALOGD(">>>>>> START %s uid %d <<<<<<\n",className != NULL ? className : "(unknown)", getuid());static const String8 startSystemServer("start-system-server");//...//启动虚拟机JniInvocation jni_invocation;jni_invocation.Init(NULL);JNIEnv* env;if (startVm(&mJavaVM, &env, zygote) != 0) {return;}onVmCreated(env);//...s/** Start VM.  This thread becomes the main thread of the VM, and will* not return until the VM exits.*/char* slashClassName = toSlashClassName(className);jclass startClass = env->FindClass(slashClassName);if (startClass == NULL) {ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);/* keep going */} else {//调用ZygoteInit.main函数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 0if (env->ExceptionCheck())threadExitUncaughtException(env);
#endif}}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");
}复制代码

ZygoteInit

main

main函数主要的任务是

1. 通过registerZygoteSocket创建一个ServerSocket由于后续进程间通信
2. 调用preload函数，预加载一些通用类(这些类定义在./frameworks/base/preloaded-classes文件中)、通用资源(如Drawaelb等，这些资源定义在./frameworks/base/core/res/res/values/arrays.xml中)、共享库等等。
3. 调用startSystemServer启动SystemServer进程
4. 调用runSelectLoop函数进入死循环，不停地检测是否有Socket通信

public static void main(String argv[]) {try {//...boolean startSystemServer = false;String socketName = "zygote";String abiList = null;//...//创建一个ServerSocketregisterZygoteSocket(socketName);//...//预先加载class、资源、共享库等等preload();//...//启动SystemServerif (startSystemServer) {startSystemServer(abiList, socketName);}runSelectLoop(abiList);closeServerSocket();} catch (MethodAndArgsCaller caller) {caller.run();} catch (RuntimeException ex) {Log.e(TAG, "Zygote died with exception", ex);closeServerSocket();throw ex;}
}static void preload() {Log.d(TAG, "begin preload");preloadClasses();preloadResources();preloadOpenGL();preloadSharedLibraries();preloadTextResources();// Ask the WebViewFactory to do any initialization that must run in the zygote process,// for memory sharing purposes.WebViewFactory.prepareWebViewInZygote();Log.d(TAG, "end preload");
}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);sServerSocket = new LocalServerSocket(fd);} catch (IOException ex) {throw new RuntimeException("Error binding to local socket '" + fileDesc + "'", ex);}}
}private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();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) {if ((pollFds[i].revents & POLLIN) == 0) {continue;}if (i == 0) {ZygoteConnection newPeer = acceptCommandPeer(abiList);peers.add(newPeer);fds.add(newPeer.getFileDesciptor());} else {boolean done = peers.get(i).runOnce();if (done) {peers.remove(i);fds.remove(i);}}}}
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总结

经过上面对于zygote进程启动流程的分析，我们可以总结出zygote进程启动过程中究竟做了什么事情

1. 启动DVM虚拟机
2. 预加载部分资源，如一些通用类、通用资源、共享库等
3. 启动system_server进程
4. 创建一个ServerSocket，并进入死循环监听Socket消息。