android也是基于linux的系统，因此所有的进程都是从init进程开始的（直接或间接的从init进程fock出来的）。Zygote是受精卵进程，也是系统启动过程中由init进程创建的，具体的看下启动脚本/system/core/rootdir/init.zygote64.rc：

service zygote /system/bin/app_process64 -Xzygote /system/bin --zygote --start-system-server
class main
priority -20
user root
group root readproc
socket zygote stream 660 root system
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
onrestart restart audioserver
onrestart restart cameraserver
onrestart restart media
onrestart restart netd
onrestart restart wificond
writepid /dev/cpuset/foreground/tasks

可以看出，要执行的进程是/system/bin/app_process64。代码在/frameworks/base/cmds/app_process/app_main.cpp。入口函数是main:

......
// 创建AppRuntime
AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
......
while (i < argc) {const char* arg = argv[i++];if (strcmp(arg, "--zygote") == 0) {// 确认是zygote进程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;}
}
......
if (zygote) {// 执行zygote进程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.");
}
......

如果忽略掉参数这些细节，剩下的就是AppRuntime的建立和调用AppRuntime的start方法了，启动的是com.android.internal.os.ZygoteInit。
看下AppRuntime：

class AppRuntime : public AndroidRuntime
{
public:AppRuntime(char* argBlockStart, const size_t argBlockLength): AndroidRuntime(argBlockStart, argBlockLength), mClass(NULL){}......
};

构造函数中调用了基类的构造方法，基类在/frameworks/base/core/jni/AndroidRuntime.cpp：

AndroidRuntime::AndroidRuntime(char* argBlockStart, const size_t argBlockLength) :mExitWithoutCleanup(false),mArgBlockStart(argBlockStart),mArgBlockLength(argBlockLength)
{SkGraphics::Init();// There is also a global font cache, but its budget is specified by// SK_DEFAULT_FONT_CACHE_COUNT_LIMIT and SK_DEFAULT_FONT_CACHE_LIMIT.// Pre-allocate enough space to hold a fair number of options.mOptions.setCapacity(20);assert(gCurRuntime == NULL);        // one per processgCurRuntime = this;
}

保留了自己作为全局gCurRuntime。
直接看start方法：

void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{......JniInvocation jni_invocation;jni_invocation.Init(NULL);JNIEnv* env;// 启动虚拟机if (startVm(&mJavaVM, &env, zygote) != 0) {return;}// 回调虚拟机的创建onVmCreated(env);/** Register android functions.*/// 注册函数if (startReg(env) < 0) {ALOGE("Unable to register all android natives\n");return;}......jclass stringClass;jobjectArray strArray;jstring classNameStr;// 获得一个string的对象的引用stringClass = env->FindClass("java/lang/String");assert(stringClass != NULL);// 创建一个String数组对象strArray = env->NewObjectArray(options.size() + 1, stringClass, NULL);assert(strArray != NULL);classNameStr = env->NewStringUTF(className);assert(classNameStr != NULL);// 设置第一个string数组的第一个元素是classNameStr，在这里就是ZygoteInit的全名env->SetObjectArrayElement(strArray, 0, classNameStr);// 设置其他参数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);}/** Start VM.  This thread becomes the main thread of the VM, and will* not return until the VM exits.*/// 转换类中间的.为/，这里是转换格式char* slashClassName = toSlashClassName(className);// 从jni环境中找到这个类jclass startClass = env->FindClass(slashClassName);if (startClass == NULL) {ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);/* keep going */} else {// 调用找到的类的main方法，这里就是调用ZygoteInit的main方法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);......
}

关键部分已经给出了注释。总结一下：
1.启动虚拟机startVM；
2.通过startReg注册jni方法；
3.调用ZygoteInit类的main方法；
startVm基本上就是为这个进程建立一个Dalvik虚拟机环境，为当前线程初始化一个jni环境。startReg基本上是注册一大堆的jni方法，以供后面调用。不是本文重点，因此这里不再累述。

下面我们要关注的是ZygoteInit类的main方法了。
/frameworks/base/core/java/com/android/internal/os/ZygoteInit.java：

public static void main(String argv[]) {ZygoteServer zygoteServer = new ZygoteServer();......zygoteServer.registerServerSocket(socketName);......preload();......if (startSystemServer) {startSystemServer(abiList, socketName, zygoteServer);}......zygoteServer.runSelectLoop(abiList);......
}

1.创建ZygoteServer（可以看出是个cs架构的东西）;
2.注册socket（使用socket进行通讯方式）；
3.预加载；
4.启动SystemServer；
5.运行select循环体；
里面涉及到ZygoteHooks的运转，为了不影响整体，暂时做个标记，后面再阅读。
这里可以看到Zygote基本上是个cs架构的情况，并且通过socket进行这种架构的通讯。先来看看预加载过程：

    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");//预加载位于framework/base/preload-classes文件中的类preloadClasses();Trace.traceEnd(Trace.TRACE_TAG_DALVIK);Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PreloadResources");// 预加载资源preloadResources();Trace.traceEnd(Trace.TRACE_TAG_DALVIK);Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PreloadOpenGL");// 预加载资源preloadOpenGL();Trace.traceEnd(Trace.TRACE_TAG_DALVIK);//通过System.loadLibrary()方法，预加载"android","compiler_rt","jnigraphics"这3个共享库preloadSharedLibraries();//预加载文本连接符资源preloadTextResources();// webview的初始化// Ask the WebViewFactory to do any initialization that must run in the zygote process,// for memory sharing purposes.WebViewFactory.prepareWebViewInZygote();endIcuCachePinning();warmUpJcaProviders();Log.d(TAG, "end preload");}

看到了吧，都是android本身的一些资源的初始化过程，就是在这里完成的。
下面再看下startSystemServer：

private static boolean startSystemServer(String abiList, String socketName, ZygoteServer zygoteServer)throws Zygote.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,OsConstants.CAP_WAKE_ALARM);/* 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);}/* Hardcoded command line to start the system server */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 {parsedArgs = new ZygoteConnection.Arguments(args);ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);/* Request to fork the system server process */pid = Zygote.forkSystemServer(parsedArgs.uid, parsedArgs.gid,parsedArgs.gids,parsedArgs.debugFlags,null,parsedArgs.permittedCapabilities,parsedArgs.effectiveCapabilities);} catch (IllegalArgumentException ex) {throw new RuntimeException(ex);}/* For child process */if (pid == 0) {if (hasSecondZygote(abiList)) {waitForSecondaryZygote(socketName);}zygoteServer.closeServerSocket();handleSystemServerProcess(parsedArgs);}return true;}

其实主要的就是Zygote.forkSystemServer这句话，前面的都是参数的配置。再向下看一层/frameworks/base/core/java/com/android/internal/os/Zygote.java：

public static int forkSystemServer(int uid, int gid, int[] gids, int debugFlags,int[][] rlimits, long permittedCapabilities, long effectiveCapabilities) {VM_HOOKS.preFork();int pid = nativeForkSystemServer(uid, gid, gids, debugFlags, rlimits, permittedCapabilities, effectiveCapabilities);// Enable tracing as soon as we enter the system_server.if (pid == 0) {Trace.setTracingEnabled(true);}VM_HOOKS.postForkCommon();return pid;}

根据传递进来的uid,gid等调用函数nativeForkSystemServer，最终会在/frameworks/base/core/jni/com_android_internal_os_Zygote.cpp下的ForkAndSpecializeCommon中调用fork函数，那么实际上就可以知道，就是在c层fork分裂出一个进程来作为SystemServer。

现在我们回来看java层的ZygoteInit.java，继续看看与socket相关的部分，首先是registerServerSocket:

void registerServerSocket(String socketName) {if (mServerSocket == 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);mServerSocket = new LocalServerSocket(fd);} catch (IOException ex) {throw new RuntimeException("Error binding to local socket '" + fileDesc + "'", ex);}}}

这里设置了文件描述符，然后创建了LocalServerSocket赋值给了mServerSocket。/frameworks/base/core/java/android/net/LocalServerSocket.java：

public LocalServerSocket(FileDescriptor fd) throws IOException{impl = new LocalSocketImpl(fd);impl.listen(LISTEN_BACKLOG);localAddress = impl.getSockAddress();}

new出LocalSocketImpl后，直接就开始listen了。下面暂时不用特别看了吧，就是一个走的正常的网络socket了。这里应该就可以证明是以socket的方法进行的通讯。然后再来看看runSelectLoop：

void runSelectLoop(String abiList) throws Zygote.MethodAndArgsCaller {ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();fds.add(mServerSocket.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(this);if (done) {peers.remove(i);fds.remove(i);}}}}}

进入一个死循环，每次都将所有要观察的fd建立成数组，然后调用Os.poll(pollFds, -1)阻塞等待fd的变化。后面一个for循环是当fd有变化（即有客户端连接，也就是说有其他进程想要与ZygoteServer通讯），此时调用ZygoteConnection的runOnce方法。这个方法如果简单看下的话，最终是要调用Zygote.forkAndSpecialize分裂出进程来的，也就是说这个方法是一旦有连接建立后就表示有app启动了，此时就要fork分裂出新的进程来，代码暂时就不贴了。

至此为止，Zygote的过程基本分析完毕。总结一下：
1.系统启动，通过init进程会启动Zygote进程。确切的将是通过runtime调用了ZygoteInit，这个初始化过程；
2.Zygote是cs架构的，基于socket通讯机制的，在ZygoteInit过程中会启动ZygoteServer，为了等待接收socket的通讯来进行启动app进程的处理；
3.分裂出SystemServer进程，负责启动系统的一些关键服务。包括3类（广播类、核心类、其他类）；

最后附图一张便于理解：