android开机启动流程简单分析

android启动

当引导程序启动Linux内核后，会加载各种驱动和数据结构，当有了驱动以后，开始启动Android系统同时会加载用户级别的第一个进程init（system\core\init\init.cpp）代码如下：

int main(int argc, char** argv) {

.....
//创建文件夹，挂载
// Get the basic filesystem setup we need put together in the initramdisk
// on / and then we'll let the rc file figure out the rest.
if (is_first_stage) {
mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "mode=0755");
mkdir("/dev/pts", 0755);
mkdir("/dev/socket", 0755);
mount("devpts", "/dev/pts", "devpts", 0, NULL);
#define MAKE_STR(x) __STRING(x)
mount("proc", "/proc", "proc", 0, "hidepid=2,gid=" MAKE_STR(AID_READPROC));
mount("sysfs", "/sys", "sysfs", 0, NULL);
}
.....

//打印日志，设置log的级别
klog_init();
klog_set_level(KLOG_NOTICE_LEVEL);
.....

Parser& parser = Parser::GetInstance();
parser.AddSectionParser("service",std::make_unique<ServiceParser>());
parser.AddSectionParser("on", std::make_unique<ActionParser>());
parser.AddSectionParser("import", std::make_unique<ImportParser>());
// 加载init.rc配置文件
parser.ParseConfig("/init.rc");
}

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加载init.rc文件，会启动一个Zygote进程，此进程是Android系统的一个母进程，用来启动Android的其他服务进程，代码：

从Android L开始，在 /system/core/rootdir 目录中有 4 个 zygote 相关的启动脚本如下图：



在init.rc文件中，有如下代码：

import /init.environ.rc
import /init.usb.rc
import /init.${ro.hardware}.rc
import /init.usb.configfs.rc
import /init.${ro.zygote}.rc

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注意到上面的代码import /init.${ro.zygote}.rc，这里会读取ro.zygote这个属性，导入相应的init.rc文件。 

ro.zygote的属性可为：zygote32、zygote64、zygote32_64、zygote64_32。

对于这个属性的解释如下。

init.zygote32.rc：zygote 进程对应的执行程序是 app_process (纯 32bit 模式)
init.zygote64.rc：zygote 进程对应的执行程序是 app_process64 (纯 64bit 模式)
init.zygote32_64.rc：启动两个 zygote 进程 (名为 zygote 和 zygote_secondary)，对应的执行程序分别是 app_process32 (主模式)、app_process64。
init.zygote64_32.rc：启动两个 zygote 进程 (名为 zygote 和 zygote_secondary)，对应的执行程序分别是 app_process64 (主模式)、app_process32

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主流的机型属性都为zygote64_32，我们来看看init.zygote64_32.rc文件：

service zygote /system/bin/app_process64 -Xzygote /system/bin --zygote --start-system-server --socket-name=zygote
class main
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
writepid /dev/cpuset/foreground/tasks

service zygote_secondary /system/bin/app_process32 -Xzygote /system/bin --zygote --socket-name=zygote_secondary
class main
socket zygote_secondary stream 660 root system
onrestart restart zygote
writepid /dev/cpuset/foreground/tasks

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在 Linux 系统中，service 通常是一种被称为守护进程 (daemon) 的程序。它通常在系统启动时启动，并一直运行于后台，直到系统关闭时终止。

这里会以service方式来启动zygote进程，app_process的代码位于/frameworks/base/cmds/app_process/路径下，该路径下有一个文件app_main.cpp,入口是main函数。

接下来看app_main.cpp函数，这里实现从c++代码调到java代码：

int main(int argc, char* const argv[])
{
.....
//android运行时环境
AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
// Process command line arguments
// ignore argv[0]
argc--;
argv++;

.....
if (zygote) {
//启动java代码
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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ZygoteInit.java 代码：

/frameworks/base/core/Java/com/android/internal/os/ZygoteInit.java

public static void main(String argv[]) {
.....
try {
.....
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]);
}
}

.....
//预加载android依赖的文件
preload();
.....

if (startSystemServer) {
//启动系统服务
startSystemServer(abiList, socketName);
}

.....
} catch (MethodAndArgsCaller caller) {
caller.run();
} catch (Throwable ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}

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看这个类的main方法，最上面根据传入的参数判断是否将startSystemServer这个标记设为true，接着预加载android依赖的文件，最后根据上面设置的标记来判断是否能启动系统服务。

接着看ZygoteInit.java中main方法在最后调用startSystemServer方法具体内容

private static boolean startSystemServer(String abiList, String socketName)
throws MethodAndArgsCaller, RuntimeException {
.....
/* 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",
};
.....
try {
parsedArgs = new ZygoteConnection.Arguments(args);
ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);

//Zygote进程开始fork子进程，启动SystemServer
/* 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);
}
.....
}

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SystemServer.java 代码

/frameworks/base/services/java/com/android/server/SystemServer.java

/**
* The main entry point from zygote.
*/
public static void main(String[] args) {
new SystemServer().run();
}

private void run() {
......
// Start services.
try {
Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartServices");
startBootstrapServices();
//启动核心Service
startCoreServices();
//启动其他Service
startOtherServices();
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}

// Loop forever.
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}

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启动SystemServer.java的startOtherServices方法

/**
* Starts a miscellaneous grab bag of stuff that has yet to be refactored
* and organized.
*/
private void startOtherServices() {
.....
// We now tell the activity manager it is okay to run third party
// code.  It will call back into us once it has gotten to the state
// where third party code can really run (but before it has actually
// started launching the initial applications), for us to complete our
// initialization.
mActivityManagerService.systemReady(new Runnable() {
@Override
public void run() {
Slog.i(TAG, "Making services ready");
.....
}
}
.....
}

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ActivityManagerService.java中的mStackSupervisor.resumeFocusedStackTopActivityLocked()

public void systemReady(final Runnable goingCallback) {
.....
Slog.i(TAG, "System now ready");
EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_AMS_READY,
SystemClock.uptimeMillis());
.....

synchronized (this) {
// Only start up encryption-aware persistent apps; once user is
// unlocked we'll come back around and start unaware apps
startPersistentApps(PackageManager.MATCH_DIRECT_BOOT_AWARE);
.....
//调用ActivityStackSupervisor中的resumeFocusedStackTopActivityLocked方法
mStackSupervisor.resumeFocusedStackTopActivityLocked();
mUserController.sendUserSwitchBroadcastsLocked(-1, currentUserId);
}
}

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ActivityStackSupervisor.java中的resumeFocusedStackTopActivityLocked方法

boolean resumeFocusedStackTopActivityLocked() {
return resumeFocusedStackTopActivityLocked(null, null, null);
}

boolean resumeFocusedStackTopActivityLocked(
ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
if (targetStack != null && isFocusedStack(targetStack)) {
return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
}
final ActivityRecord r = mFocusedStack.topRunningActivityLocked();
if (r == null || r.state != RESUMED) {
//调用ActivityStack中的resumeTopActivityUncheckedLocked来保证栈中的顶层Activity被重新启动
mFocusedStack.resumeTopActivityUncheckedLocked(null, null);
}
return false;
}

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ActivityStack.java中的resumeTopActivityUncheckedLocked方法

boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {

....

boolean result = false;
try {
// Protect against recursion.
mStackSupervisor.inResumeTopActivity = true;
if (mService.mLockScreenShown == ActivityManagerService.LOCK_SCREEN_LEAVING) {
mService.mLockScreenShown = ActivityManagerService.LOCK_SCREEN_HIDDEN;
mService.updateSleepIfNeededLocked();
}
result = resumeTopActivityInnerLocked(prev, options);
} finally {
mStackSupervisor.inResumeTopActivity = false;
}
return result;
}

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private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
if (DEBUG_LOCKSCREEN) mService.logLockScreen("");
if (!mService.mBooting && !mService.mBooted) {
// Not ready yet!
return false;
}
.....
if (next == null) {
// There are no more activities!
final String reason = "noMoreActivities";
final int returnTaskType = prevTask == null || !prevTask.isOverHomeStack()
? HOME_ACTIVITY_TYPE : prevTask.getTaskToReturnTo();
if (!mFullscreen && adjustFocusToNextFocusableStackLocked(returnTaskType, reason)) {
// Try to move focus to the next visible stack with a running activity if this
// stack is not covering the entire screen.
return mStackSupervisor.resumeFocusedStackTopActivityLocked(
mStackSupervisor.getFocusedStack(), prev, null);
}

// Let's just start up the Launcher...
ActivityOptions.abort(options);
if (DEBUG_STATES) Slog.d(TAG_STATES,
"resumeTopActivityLocked: No more activities go home");
if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
// Only resume home if on home display
//调用ActivityStackSupervisor中的方法来恢复 Launcher所在的栈
return isOnHomeDisplay() &&
mStackSupervisor.resumeHomeStackTask(returnTaskType, prev, reason);
}
.....
}

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ActivityStackSupervisor.java中resumeHomeStackTask方法

boolean resumeHomeStackTask(int homeStackTaskType, ActivityRecord prev, String reason) {
if (!mService.mBooting && !mService.mBooted) {
// Not ready yet!
return false;
}

.....

mHomeStack.moveHomeStackTaskToTop(homeStackTaskType);
ActivityRecord r = getHomeActivity();
final String myReason = reason + " resumeHomeStackTask";

// Only resume home activity if isn't finishing.
if (r != null && !r.finishing) {
mService.setFocusedActivityLocked(r, myReason);
return resumeFocusedStackTopActivityLocked(mHomeStack, prev, null);
}
启动launcher应用的锁屏界面
return mService.startHomeActivityLocked(mCurrentUser, myReason);
}

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Android系统启动完成，打开了Launcher应用的Home界面。