Android系统开机过程分析

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当我们开机时，首先是启动Linux内核，在Linux内核中首先启动的是init进程，这个进程会去读取配置文件system\core\rootdir\init.rc配置文件，这个文件中配置了Android系统中第一个进程Zygote进程。Android中其他所有进程都是通过fork Zygote进程得来的。

在这里我只是先简单的分析代码中Android系统启动的大体过程：）

Step1 – init.rc

...
service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
socket zygote stream 666
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
onrestart restart media

...

Step2 – init.c

void service_start(struct service *svc, const char *dynamic_args)
{
pid_t pid;

...

pid = fork();//创建子进程

if (pid == 0) {//进入这个分支说明函数是在子进程中返回的，新的子进程就是zygote
struct socketinfo *si;

...

//创建通信使用的socket
for (si = svc->sockets; si; si = si->next) {
int s = create_socket(si->name,
!strcmp(si->type, "dgram") ?
SOCK_DGRAM : SOCK_STREAM,
si->perm, si->uid, si->gid);
if (s >= 0) {
publish_socket(si->name, s);//发布到系统中
}
}

...

//检查启动参数，如果没有就加载zygote应用程序文件
if (!dynamic_args) {
if (execve(svc->args[0], (char**) svc->args, (char**) ENV) < 0) {
ERROR("cannot execve('%s'): %s\n", svc->args[0], strerror(errno));
}
} else {
//否则就将参数合并到arg_ptrs数组中，在加载zygote应用程序文件
char *arg_ptrs[SVC_MAXARGS+1];
int arg_idx = svc->nargs;
char *tmp = strdup(dynamic_args);
char *next = tmp;
char *bword;

/* Copy the static arguments */
memcpy(arg_ptrs, svc->args, (svc->nargs * sizeof(char *)));

while((bword = strsep(&next, " "))) {
arg_ptrs[arg_idx++] = bword;
if (arg_idx == SVC_MAXARGS)
break;
}
arg_ptrs[arg_idx] = '\0';
//Step3
execve(svc->args[0], (char**) arg_ptrs, (char**) ENV);
}
_exit(127);
}
}

Step3 – App_main.cpp

int main(int argc, char* const argv[])
{
...

//创建AppRuntime，android运行时环境
AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));

...
int i;
...
++i;  // Skip unused "parent dir" argument.

//检查启动参数
while (i < argc) {
const char* arg = argv[i++];
if (strcmp(arg, "--zygote") == 0) {
zygote = true;

//zygote进程名字
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 (!niceName.isEmpty()) {
runtime.setArgv0(niceName.string());
//设置zygote进程名
set_process_name(niceName.string());
}

//zygote进程进入第一个分支
if (zygote) {
InitializeNativeLoader();
//Step3
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;
}
}

Step3 – AndroidRuntime.start

AppRuntime.start继承自AndroidRuntime.start

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;
}
onVmCreated(env);

//在虚拟机中注册JNI方法
if (startReg(env) < 0) {
ALOGE("Unable to register all android natives\n");
return;
}

...
jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
"([Ljava/lang/String;)V");
if (startMeth == NULL) {
ALOGE("JavaVM unable to find main() in '%s'\n", className);
} else {
//进入ZygoteInit.main
//Step4
env->
4000
;CallStaticVoidMethod(startClass, startMeth, strArray);
}

}

Step4 – ZygoteInit.main

//com.android.internal.os.ZygoteInit.java
public static void main(String argv[]) {
try {
...
//创建Server端socket，用来等待AMS请求
//Step4.1
registerZygoteSocket(socketName);

...
//这里startSystemServer==true
if (startSystemServer) {
//Step4.2
startSystemServer(abiList, socketName);
}
...
//等待AMS请求
//Step4.3
runSelectLoop(abiList);
...
}
}

Step4.1 – ZygoteInit.registerZygoteSocket

private static final String ANDROID_SOCKET_PREFIX = "ANDROID_SOCKET_";

private static void registerZygoteSocket(String socketName) {
if (sServerSocket == null) {
int fileDesc;
//获取socket描述字符串
final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName;
try {
//获取socket文件描述符
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);
//根据文件描述符创建server端socket
sServerSocket = new LocalServerSocket(fd);
} catch (IOException ex) {
throw new RuntimeException(
"Error binding to local socket '" + fileDesc + "'", ex);
}
}
}

STep4.2 – ZygoteInit.startSystemServer

private static boolean startSystemServer(String abiList, String socketName)
throws MethodAndArgsCaller, RuntimeException {
...
/* Hardcoded command line to start the system server */
//服务启动参数
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 {
...
/* Request to fork the system server process */
//通过fork创建服务
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);
}
//Step5
handleSystemServerProcess(parsedArgs);
}
return true;
}

Step4.3 – ZygoteInit.runSelectLoop

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);

//循环等待请求
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 {
//收到请求创建进程
boolean done = peers.get(i).runOnce();
if (done) {
peers.remove(i);
fds.remove(i);
}
}
}
}
}

Step5 ZygoteInit.handleSystemServerProcess

private static void handleSystemServerProcess(ZygoteConnection.Arguments parsedArgs)
throws ZygoteInit.MethodAndArgsCaller {

//由于system进程复制了zygote进程的地址空间，因此它也会获得zygote进程在启动过程中创建的socket，
//由于system进程不需要这个socket，所以关闭
closeServerSocket();
...
//Step6
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
}

Step6 – RuntimeInit.zygoteInit

public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
//初始化时区，键盘布局等通用信息
commonInit();
//在system进程中启动一个binder线程池
nativeZygoteInit();
//Step7
applicationInit(targetSdkVersion, argv, classLoader);
}

Step7 – RuntimeInit.applicationInit

private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {

// We want to be fairly aggressive about heap utilization, to avoid
// holding on to a lot of memory that isn't needed.
VMRuntime.getRuntime().setTargetHeapUtilization(0.75f);
VMRuntime.getRuntime().setTargetSdkVersion(targetSdkVersion);
...
// Remaining arguments are passed to the start class's static main
//进入com.android.server.SystemServer.main
//Step8
invokeStaticMain(args.startClass, args.startArgs, classLoader);
}

Step8 – SystemServer.main

//com.android.server.SystemServer.java
public static void main(String[] args) {
//Step9
new SystemServer().run();
}

Step9 – SystemServer.run

private void run() {
try {
...

// Here we go!
Slog.i(TAG, "Entered the Android system server!");

...

// Initialize native services.
System.loadLibrary("android_servers");

...

// Initialize the system context.
createSystemContext();

// Create the system service manager.
mSystemServiceManager = new SystemServiceManager(mSystemContext);

LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}

// Start services.
try {
//Step9.1
startBootstrapServices();
//Step9.2
startCoreServices();
startOtherServices();
} catch (Throwable ex) {
...
throw ex;
} finally {
...
}
// Loop forever.
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}

Step9.1 – SystemServer.startBootstrapServices

private void startBootstrapServices() {
//启动Installer Server
Installer installer = mSystemServiceManager.startService(Installer.class);

// 启动AMS
mActivityManagerService = mSystemServiceManager.startService(
ActivityManagerService.Lifecycle.class).getService();
mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
mActivityManagerService.setInstaller(installer);

//启动PMS
mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);

...

// Manages LEDs and display backlight so we need it to bring up the display.
mSystemServiceManager.startService(LightsService.class);

// Display manager is needed to provide display metrics before package manager
// starts up.
mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);

// Only run "core" apps if we're encrypting the device.
String cryptState = SystemProperties.get("vold.decrypt");
if (ENCRYPTING_STATE.equals(cryptState)) {
Slog.w(TAG, "Detected encryption in progress - only parsing core apps");
mOnlyCore = true;
} else if (ENCRYPTED_STATE.equals(cryptState)) {
Slog.w(TAG, "Device encrypted - only parsing core apps");
mOnlyCore = true;
}

// Start the package manager.
traceBeginAndSlog("StartPackageManagerService");
mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
mFirstBoot = mPackageManagerService.isFirstBoot();
mPackageManager = mSystemContext.getPackageManager();

ServiceManager.addService(Context.USER_SERVICE, UserManagerService.getInstance());

// Initialize attribute cache used to cache resources from packages.
AttributeCache.init(mSystemContext);

// Set up the Application instance for the system process and get started.
mActivityManagerService.setSystemProcess();

//启动传感器服务
startSensorService();
}

Step9.2 – SystemServer.startCoreServices

private void startCoreServices() {
// Tracks the battery level.  Requires LightService.
mSystemServiceManager.startService(BatteryService.class);

// Tracks application usage stats.
mSystemServiceManager.startService(UsageStatsService.class);
mActivityManagerService.setUsageStatsManager(
LocalServices.getService(UsageStatsManagerInternal.class));
// Update after UsageStatsService is available, needed before performBootDexOpt.
mPackageManagerService.getUsageStatsIfNoPackageUsageInfo();

// Tracks whether the updatable WebView is in a ready state and watches for update installs.
mSystemServiceManager.startService(WebViewUpdateService.class);
}