上两篇聊了Launcher之后的流程，现在咱们再看一下之前的流程 --- 也就是Zygote进程启动之后：
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Zygote, 意为“受精卵”,安卓进程孵化器
上图可以看出，由linux kernel 的 init （一生万物）进程可以启动Zygote进程，Zygote又启动dalvik，初始化java环境，app需要的资源，系统服务... fork（孵化）桌面Launcher以及各个app进程
在系统启动脚本system/core/rootdir/init.rc文件中，我们可以看到启动Zygote进程的脚本命令：
service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
socket zygote stream 666 # zygote需要一个套接字
onrestart write /sys/android_power/request_state wake # zygote重启的话，需要执行这个操作
onrestart write /sys/power/state on
onrestart restart media
onrestart restart netd
app_process对应的源码在frameworks/base/cmds/app_process目录下，其入口函数main在文件app_main.cpp中：
/*
* 启动zygote的方式为/system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
* 所以 argc == 5
* argv里头存的就是这5个参数argv[0]=="/system/bin/app_process" ,argv[1] == "-Xzygote"....
*/
int main(int argc, char* const argv[])
{
......
// These are global variables in ProcessState.cpp
mArgC = argc;
mArgV = argv;
mArgLen = 0;
for (int i=0; imain函数主要就是创建了runtime实例，并且解析参数，然后调用runtime的start函数，接着我们分析AppRuntime的start函数：
/*
* 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.
*/
// 首先我们明确下传进来的参数 className == "com.android.internal.os.ZygoteInit" options == "start-system-server"
void AndroidRuntime::start(const char* className, const char* options)
{
ALOGD("\n>>>>>> AndroidRuntime START %s <<<<<<\n",
className != NULL ? className : "(unknown)");
/*
* 'startSystemServer == true' means runtime is obsolete and not run from
* init.rc anymore, so we print out the boot start event here.
*/
if (strcmp(options, "start-system-server") == 0) {
/* 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); //配置ANDROID_ROOT环境变量
}
//const char* kernelHack = getenv("LD_ASSUME_KERNEL");
//ALOGD("Found LD_ASSUME_KERNEL='%s'\n", kernelHack);
//1：调用startVm函数创建虚拟机；
/* start the virtual machine */
JniInvocation jni_invocation;
jni_invocation.Init(NULL);
JNIEnv* env;
if (startVm(&mJavaVM, &env) != 0) { // 创建虚拟机
return;
}
onVmCreated(env);
//2：调用startReg函数注册Android Natvie函数；
/*
* Register android functions.
*/
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;
jstring optionsStr;
stringClass = env->FindClass("java/lang/String");
assert(stringClass != NULL);
strArray = env->NewObjectArray(2, stringClass, NULL);
assert(strArray != NULL);
classNameStr = env->NewStringUTF(className);
assert(classNameStr != NULL);
env->SetObjectArrayElement(strArray, 0, classNameStr);
optionsStr = env->NewStringUTF(options);
env->SetObjectArrayElement(strArray, 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 {
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 {
//3：让虚拟机去执行com.android.internal.os.ZygoteInit的main函数。
/* 调用com.android.internal.os.ZygoteInit的main函数，strArray是参数，数组里面有两个元素,
className == "com.android.internal.os.ZygoteInit" options == "start-system-server" */
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");
}
start函数主要做了以下几件事情：
1：调用startVm函数创建虚拟机；
2：调用startReg函数注册Android Natvie函数；
3：让虚拟机去执行com.android.internal.os.ZygoteInit的main函数。
public static void main(String argv[]) {
try {
// Start profiling the zygote initialization.
SamplingProfilerIntegration.start();
// 1、创建一个套接字，用于监听ams发过来的fork请求
registerZygoteSocket();
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
preload();
// 2、加载classes 和resources, 后面会详细分析
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();
// If requested, start system server directly from Zygote
if (argv.length != 2) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
if (argv[1].equals("start-system-server")) {
//3、 创建system server进程，ams wms pms等常见service都在该进程里面
startSystemServer();
} else if (!argv[1].equals("")) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
Log.i(TAG, "Accepting command socket connections");
if (ZYGOTE_FORK_MODE) {
runForkMode();
} else {
// 4、进入循环监听模式，监听外来请求
runSelectLoopMode();
}
closeServerSocket();
} catch (MethodAndArgsCaller caller) {
caller.run();
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}
com.android.internal.os.ZygoteInit的main函数主要做了四件事情：
1：调用registerZygoteSocket()创建一个套接字，用于监听ams发过来的fork请求，如下：
private static void registerZygoteSocket(String socketName) {
if (sServerSocket == null) {
int fileDesc;
//此处的socket name，就是zygote
final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName;
try {
//记得么？在init.zygote.rc被加载时，就会创建一个名为zygote的socket
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();
//获取zygote socket的文件描述符
fd.setInt$(fileDesc);
//将zygote socket包装成一个server socket
sServerSocket = new LocalServerSocket(fd);
} catch (IOException ex) {
throw new RuntimeException("Error binding to local socket '" + fileDesc + "'", ex);
}
}
}
2：调用preload()预加载classes 和resources，如下；
static void preload() {
Log.d(TAG, "begin preload");
//读取文件framework/base/preloaded-classes，然后通过反射加载对应的类
//需要加载数千个类，启动慢的原因之一
preloadClasses();
//负载加载一些常用的系统资源
preloadResources();
//图形相关的
preloadOpenGL();
//一些必要库
preloadSharedLibraries();
//好像是语言相关的字符信息
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");
}
3：调用startSystemServer()创建system server进程，ams wms pms等常见service都在该进程里面，如下；
private static boolean startSystemServer(String abiList, String socketName) {
//准备capabilities参数
........
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=.........",
"--capabilities=" + capabilities + "," + capabilities,
"--nice-name=system_server",
"--runtime-args",
"com.android.server.SystemServer",
};
ZygoteConnection.Arguments parsedArgs = null;
int pid;
try {
//将上面准备的参数，按照ZygoteConnection的风格进行封装
parsedArgs = new ZygoteConnection.Arguments(args);
...........
//通过fork"分裂"出system server
/* 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);
}
if (pid == 0) {
............
//pid = 0, 在进程system server中
//system server进程处理自己的工作
handleSystemServerProcess(parsedArgs);
}
return true;
}
4：调用runSelectLoopMode()进入循环监听模式，监听外来请求，如下。
private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {
ArrayList fds = new ArrayList();
ArrayList peers = new ArrayList();
//首先将server socket加入到fds
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);
//关注事件到来
pollFds[i].events = (short) POLLIN;
}
try {
//等待事件到来
Os.poll(pollFds, -1);
} catch (ErrnoException ex) {
throw new RuntimeException("poll failed", ex);
}
//注意这里是倒序的
for (int i = pollFds.length - 1; i >= 0; --i) {
if ((pollFds[i].revents & POLLIN) == 0) {
continue;
}
//server socket最先加入fds， 因此这里是server socket收到数据
if (i == 0) {
//收到新的建立通信的请求，建立通信连接
ZygoteConnection newPeer = acceptCommandPeer(abiList);
//加入到peers和fds
peers.add(newPeer);
fds.add(newPeer.getFileDesciptor());
} else {
//其它通信连接收到数据，runOnce执行对应命令
boolean done = peers.get(i).runOnce();
if (done) {
//对应通信连接不再需要执行其它命令，关闭并移除
peers.remove(i);
fds.remove(i);
}
}
}
}
}
从上面代码可知，初始时，fds中仅有server socket，因此当有数据到来时，将执行i ==0 的部分。
此时，显然是需要创建新的通信连接，因此acceptCommandPeer将被调用。
private static ZygoteConnection acceptCommandPeer(String abiList) {
try {
return new ZygoteConnection(sServerSocket.accept(), abiList);
} catch (IOException ex) {
throw new RuntimeException("IOException during accept()", ex);
}
}
总结
1：acceptCommandPeer封装了socket的accpet函数。于是我们知道，对应的新的连接，zygote将会创建出一个新的socket与其通信，并将该socket加入到fds中。因此，一旦通信连接建立后，fds中将会包含有多个socket。
2：当poll监听到这一组sockets上有数据到来时，就会从阻塞中恢复。于是，我们需要判断到底是哪个socket收到了数据。
3：在runSelectLoop中采用倒序的方式轮询，由于server socket第一个被加入到fds，因此最后轮询到的socket才需要处理新建连接的操作；其它socket收到数据时，仅需要调用zygoteConnection的runonce函数执行数据对应的操作。
4：若一个连接处理完所有对应消息后，该连接对应的socket和连接等将被移除。
那么从 system server 到 ActivityManager 到 Launcher 又是如何沟通的呢，也就是第一篇提到的
AMS.startProcessLocked()-->Process.start()-->zygoteSendArgsAndGetResult()-->ZygoteInit.invokeStaticMain(cloader, className, mainArgs)(ActivityThread.main)
system server，具体的过程在介绍system server时再分析