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1、概述
在Android系统中，所有的应用程序进程以及系统服务进程SystemServer都是由Zygote进程fork出来的，这也许就是为什么要把它称为Zygote（受精卵）的原因，而系统中比较重要的ActivityManagerService、PackageManagerService，WindowManagerService以及PowerManagerService等也是由其子进程SystemServer进程创建而来的。既然Zygote进程在Android系统中有着如此重要的地位，本文将详细分析它的启动过程。
这里先看一下手机中的进程图：
由图可以看出，Zygote进程的ID是2452，而system_server，systemui，settings等进程的父进程ID也都是2452，所以可以说这些子进程都是由Zygote进程创建而来的。
大家应该都知道，Android系统是基于Linux内核的，而在Linux系统中，所有的进程都是init进程的子进程，即所有的进程都是直接或者间接的由init进程fork出来的。Zygote进程也不例外，它是在系统启动的过程，由init进程创建的。由于本文是基于Android 5.1版本，所以我们这里可以看到system/core/rootdir/目录下存在init.zygote32.rc，init.zygote32_64.rc，init.zygote64.rc，init.zygote64_32.rc等文件，我们这里只看一个init.zygote32.rc脚本。
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前面的关键字service是要告诉init进程创建一个名为”zygote”的进程，这个zygote进程要执行的程序是/system/bin/app_process，后面是要传给app_process的参数。
了解到这里以后我们知道要创建Zygote进程需执行的程序便是system/bin/app_process了，它的源代码位于frameworks/base/cmds/app_process/app_main.cpp文件中，入口函数是main，我们进入该方法。
2、源码分析
（1）app_main.main()
源码：frameworks/base/cmds/app_process/app_main.cpp
int main(int argc, char* const argv[])
{
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) {
// Older kernels don't understand PR_SET_NO_NEW_PRIVS and return
// EINVAL. Don't die on such kernels.
if (errno != EINVAL) {
LOG_ALWAYS_FATAL("PR_SET_NO_NEW_PRIVS failed: %s", strerror(errno));
return 12;
}
}
AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
// Process command line arguments
// ignore argv[0]
argc--;
argv++;
// Everything up to '--' or first non '-' arg goes to the vm.
//
// The first argument after the VM args is the "parent dir", which
// is currently unused.
//
// After the parent dir, we expect one or more the following internal
// arguments :
//
// --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.
//
// For non zygote starts, these arguments will be followed by
// the main class name. All remaining arguments are passed to
// the main method of this class.
//
// For zygote starts, all remaining arguments are passed to the zygote.
// main function.
//
// Note that we must copy argument string values since we will rewrite the
// entire argument block when we apply the nice name to argv0.
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]));
}
// Parse runtime arguments. Stop at first unrecognized option.
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;
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()) {
// We're not in zygote mode, the only argument we need to pass
// to RuntimeInit is the application argument.
//
// The Remainder of args get passed to startup class main(). Make
// copies of them before we overwrite them with the process name.
args.add(application ? String8("application") : String8("tool"));
runtime.setClassNameAndArgs(className, argc - i, argv + i);
} 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;
}
}这里主要是最后调用了runtime对象的start方法，并将com.android.internal.os.ZygoteInit类作为参数传入该方法，这里的runtime对象就是AppRuntime，我们来看其源码（此类就在app_main.cpp文件中）。
class AppRuntime : public AndroidRuntime{
//......
}可以发现该类是继承自AndroidRuntime，而其start方法也是父类AndroidRuntime的，所以我们进入父类的start方法。
（2）AndroidRuntime.start()
源码：frameworks/base/core/jni/AndroidRuntime.cpp
/*
* Start the Android runtime. This involves starting the virtual machine
* and calling the "static void main(String[] args)" method in the class
* named by "className".
*
* Passes the main function two arguments, the class name and the specified
* options string.
*/
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");
/*
* 'startSystemServer == true' means runtime is obsolete and not run from
* init.rc anymore, so we print out the boot start event here.
*/
for (size_t i = 0; i < options.size(); ++i) {
if (options[i] == startSystemServer) {
/* track our progress through the boot sequence */
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);
}
//const char* kernelHack = getenv("LD_ASSUME_KERNEL");
//ALOGD("Found LD_ASSUME_KERNEL='%s'\n", kernelHack);
/* start the virtual machine */
JniInvocation jni_invocation;
jni_invocation.Init(NULL);
/*通过调用startVm来启动虚拟机*/
if (startVm(&mJavaVM, &env, zygote) != 0) {
return;
}
onVmCreated(env);
/*
* Register android functions.
*/
/*通过startReg()来注册JNI*/
if (startReg(env) < 0) {
ALOGE("Unable to register all android natives\n");
return;
}
/*
* We want to call main() with a String array with arguments in it.
* At present we have two arguments, the class name and an option string.
* Create an array to hold them.
*/
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);
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.
*/
char* slashClassName = toSlashClassName(className);
jclass startClass = env->FindClass(slashClassName);
if (startClass == NULL) {
ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
/* keep going */
} else {
/*调用传递过来的com.android.internal.os.ZygoteInit类的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 0
if (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");
}这段代码主要就做了三件事情：
1、调用startVm方法来启动虚拟机
2、调用startReg来注册JNI
3、调用CallStaticVoidMethod方法来调用ZygoteInit类的main方法
（3）AndroidRuntime.startReg()
源码：frameworks/base/core/jni/AndroidRuntime.cpp
/*
* Register android native functions with the VM.
*/
/*static*/
int AndroidRuntime::startReg(JNIEnv* env)
{
//......
if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
env->PopLocalFrame(NULL);
return -1;
}
env->PopLocalFrame(NULL);
//createJavaThread("fubar", quickTest, (void*) "hello");
return 0;
}这里通过register_jni_procs方法加载大量的gRegJNI数组文件，我们看一下他们的具体实现。
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_FloatMath),
REG_JNI(register_android_content_AssetManager),
REG_JNI(register_android_content_StringBlock),
REG_JNI(register_android_content_XmlBlock),
REG_JNI(register_android_emoji_EmojiFactory),
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_Graphics),
REG_JNI(register_android_view_DisplayEventReceiver),
REG_JNI(register_android_view_RenderNode),
REG_JNI(register_android_view_RenderNodeAnimator),
//......
}
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) {
return -1;
}
}
return 0;
}由此处代码可知，其通过for循环遍历JNI数组gRegJNI进行注册。
（4）ZygoteInit.main()
源码：frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
public static void main(String argv[]) {
try {
// Start profiling the zygote initialization.
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.");
}
//（1）创建socket接口用来和ActivityManagerService通信
registerZygoteSocket(socketName);
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
//（2）预加载应用程序框架中的类和资源
preload();
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
SystemClock.uptimeMillis());
// Finish profiling the zygote initialization.
SamplingProfilerIntegration.writeZygoteSnapshot();
// Do an initial gc to clean up after startup
gc();
// Disable tracing so that forked processes do not inherit stale tracing tags from
// Zygote.
Trace.setTracingEnabled(false);
//(3)调用startSystemServer方法来启动SystemServer服务
if (startSystemServer) {
startSystemServer(abiList, socketName);
}
Log.i(TAG, "Accepting command socket connections");
//runSelectLoop在之前创建的socket上进入无限循环等待AMS请求创建新的应用程序进程
runSelectLoop(abiList);
closeServerSocket();
} catch (MethodAndArgsCaller caller) {
caller.run();
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}这里用一张图来表示以上步骤（图片来源于网络）
接下来将对以上步骤进行详细分析。
（5）ZygoteInit.registerZygoteSocket()
源码：frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
/**
* Registers a server socket for zygote command connections
*
* @throws RuntimeException when open fails
*/
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 {
sServerSocket = new LocalServerSocket(
createFileDescriptor(fileDesc));
} catch (IOException ex) {
throw new RuntimeException(
"Error binding to local socket '" + fileDesc + "'", ex);
}
}
}这里传进来的参数socketName值为”zygote”，然后据此创建一个LocalServerSocket对象并将其赋值给sServerSocket静态变量，并在init.rc中有生成该socket的相关内容。
（6）ZygoteInit.preload()
源码：frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
static void preload() {
Log.d(TAG, "begin preload");
/* SPRD:modify preload class and preload resource in parallel mode @{ */
new Thread("preloadClsses"){
@Override
public void run() {
//加载Framework类
preloadClasses();
mConnectedSignal.countDown();
}
}.start();
new Thread("preloadResources"){
public void run() {
//加载资源文件
preloadResources();
mConnectedSignal.countDown();
}
}.start();
waitForLatch(mConnectedSignal);
/* @} */
//加载OpenGL和共享库
preloadOpenGL();
preloadSharedLibraries();
// 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");
}这里主要是预加载一些Framework类和资源文件以使应用程序在使用这些类和资源时，可以做到直接获取。
我们先来看一下preloadClasses这个方法。
/**
* Performs Zygote process initialization. Loads and initializes
* commonly used classes.
*
* Most classes only cause a few hundred bytes to be allocated, but
* a few will allocate a dozen Kbytes (in one case, 500+K).
*/
private static void preloadClasses() {
final VMRuntime runtime = VMRuntime.getRuntime();
/**
* The path of a file that contains classes to preload.
*/
//private static final String PRELOADED_CLASSES = "/system/etc/preloaded-classes";
InputStream is;
try {
//使用流来加载PRELOADED_CLASSES文件
is = new FileInputStream(PRELOADED_CLASSES);
} catch (FileNotFoundException e) {
return;
}
Log.i(TAG, "Preloading classes...");
long startTime = SystemClock.uptimeMillis();
//......
// Start with a clean slate.
System.gc();
runtime.runFinalizationSync();
Debug.startAllocCounting();
try {
BufferedReader br
= new BufferedReader(new InputStreamReader(is), 256);
int count = 0;
String line;
//逐行读取字符串
while ((line = br.readLine()) != null) {
// Skip comments and blank lines.
line = line.trim();
if (line.startsWith("#") || line.equals("")) {
continue;
}
try {
//加载读取的类
Class.forName(line);
//......
}
count++;
}
//......
}
//......
}
}函数比较简单，就是以行读取preloaded-classes文件的内容，我们来看一下该文件的内容。
preloaded-classes这个文件主要由frameworks\base\tools\preload工具自动生成的，工具会判断每个类加载的时间是否大于1250ms，超过这个时间就会被写到preloaded-classes文件中，最后由zygote来预加载。它总共有几千多行的类，可见Zygote进程初始化的时候任务还是很繁重的，这也是导致Android系统启动慢的原因之一。
preloadResources和preloadOpenGL等也就不做解释了，大体处理都差不多，接下来我们进入启动SystemServer的方法中。
（7）ZygoteInit.startSystemServer()
源码：frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
/**
* Prepare the arguments and fork for the system server process.
*/
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
);
/* Hardcoded command line to start the system server */
//SystemServer启动参数
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1032,3001,3002,3003,3006,3007",
"--capabilities=" + capabilities + "," + capabilities,
"--runtime-init",
"--nice-name=system_server",// 进程名为system_server
"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 */
//Zygote通过forkSystemServer来启动SystemServer
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);
}
handleSystemServerProcess(parsedArgs);
}
return true;
}args[]数组中保存了启动SystemServer的参数，这些参数无非就是设置SystemServier名称，uid和gid，以及进程分组等，最后一个字符串参数com.android.server.SystemServer用于指定SystemServer类。
然后Zygote通过forkSystemServer方法创建新的进程来启动SystemServer组件，紧接着执行handleSystemServerProcess方法（此方法用于执行SystemServer进程的方法，我们后续分析）。
（8）ZygoteInit.runSelectLoop()
源码：frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
/**
* Runs the zygote process's select loop. Accepts new connections as
* they happen, and reads commands from connections one spawn-request's
* worth at a time.
*
* @throws MethodAndArgsCaller in a child process when a main() should
* be executed.
*/
private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {
ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();
FileDescriptor[] fdArray = new FileDescriptor[4];
// sServerSocket是之前在registerZygoteSocket创建的服务端socket
fds.add(sServerSocket.getFileDescriptor());
peers.add(null);
int loopCount = GC_LOOP_COUNT;
//这里通过无限循环来等待AMS连接socket
while (true) {
int index;
/*
* Call gc() before we block in select().
* It's work that has to be done anyway, and it's better
* to avoid making every child do it. It will also
* madvise() any free memory as a side-effect.
*
* Don't call it every time, because walking the entire
* heap is a lot of overhead to free a few hundred bytes.
*/
if (loopCount <= 0) {
gc();
loopCount = GC_LOOP_COUNT;
} else {
loopCount--;
}
try {
fdArray = fds.toArray(fdArray);
/*
selectReadable是一个native函数，内部调用select等待客户端
的连接，客户端连接上之后就会返回。
返回值：
<0: 内部发生错误
=0: 该客户端第一次连接到服务端
>0: 客户端与服务端已经建立连接，并开始发送数据
*/
index = selectReadable(fdArray);
} catch (IOException ex) {
throw new RuntimeException("Error in select()", ex);
}
if (index < 0) {
throw new RuntimeException("Error in select()");
/*返回0，表明该客户端第一次请求服务端，服务端调用accept与客户端建立连接,客户端在zygote中以ZygoteConnection对象表示
*/
} else if (index == 0) {
ZygoteConnection newPeer = acceptCommandPeer(abiList);
peers.add(newPeer);
fds.add(newPeer.getFileDescriptor());
} else {
boolean done;
/* 返回>0，表明发送数据的客户端的index，peers.get(index)取得发送数据客户端的ZygoteConnection对象，之后调用runOnce方法来创建应用程序
*/
done = peers.get(index).runOnce();
// 请求处理完成之后，移除与该客户端的连接
if (done) {
peers.remove(index);
fds.remove(index);
}
}
}
}runSelectLoop函数的逻辑比较简单，主要有两点：
1、处理客户端的连接和请求，其中客户端在zygote进程中使用ZygoteConnection对象表示。
2、客户的请求由ZygoteConnection的runOnce来处理。
好啦，到此Zygote进程就启动完成了，接下来我们总结一下其启动过程。
3、总结
Zygote是Android系统中创建java世界的盘古，它创建了第一个java虚拟机。同时，它又是女娲，它成功的繁殖了framework的核心system_server进程。主要步骤如下：
1、创建AppRuntime对象，并调用其start函数，之后zygote的核心初始化都由AppRuntime中。
2、调用startVm创建java虚拟机，然后调用startReg来注册JNI函数。
3、通过JNI调用com.android.internal.os.ZygoteInit类的main函数，从此进入了java世界。
4、调用registerZygoteSocket创建可以响应子孙后代请求的socket，同时zygote调用preload函数预加载了常用的类、资源等，为java世界添砖加瓦。
5、调用startSystemServer函数分裂了一个子进程system_server来为java世界服务。
6、Zygote完成了java世界的初创工作，便调用runSelectLoop来让自己沉沉的睡去，之后如果收到子孙后代的请求，它便会醒来为他们工作。