android启动过程分析--启动systemServer

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System Server是Android系统的核心，他在Dalvik虚拟机启动后立即开始初始化和运行。其它的系统服务在System Server进程的环境中运行.

systemServer是通过zygote启动的时候fork启动的，我们先看下它的启动大概流程：



创建systemServer进程

在zygote启动过程中，通过调用startSystemServer启动systemServer：

private static boolean startSystemServer()

throws MethodAndArgsCaller,RuntimeException {

/* Hardcoded command line to start thesystem server */

String args[] = {

"--setuid=1000",

"--setgid=1000",

"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,3001,3002,3003",

"--capabilities=130104352,130104352",

"--runtime-init",

"--nice-name=system_server",

"com.android.server.SystemServer",

};

ZygoteConnection.Arguments parsedArgs =null;

int pid;

try {

parsedArgs = newZygoteConnection.Arguments(args);

/*

* Enable debugging of the systemprocess if *either* the command line flags

* indicate it should be debuggableor the ro.debuggable system property

* is set to "1"

*/

int debugFlags = parsedArgs.debugFlags;

if("1".equals(SystemProperties.get("ro.debuggable")))

debugFlags |=Zygote.DEBUG_ENABLE_DEBUGGER;

/* Request to fork the systemserver process */

pid = Zygote.forkSystemServer(

parsedArgs.uid, parsedArgs.gid,

parsedArgs.gids,debugFlags, null,

parsedArgs.permittedCapabilities,

parsedArgs.effectiveCapabilities);

} catch (IllegalArgumentException ex) {

throw newRuntimeException(ex);

}

/* For child process */

if (pid == 0) {

handleSystemServerProcess(parsedArgs);

}

return true;

}

首先通过Zygote.forkSystemServer fork出一个子进程来，并传入了一些参数来设置新进程的一些信息，如uid、gid等，函数返回后，如果是子进程pid=0，进入handleSystemServerProcess：

private static voidhandleSystemServerProcess(

ZygoteConnection.ArgumentsparsedArgs)

throwsZygoteInit.MethodAndArgsCaller {

closeServerSocket();

/*

* Pass the remaining arguments toSystemServer.

* "--nice-name=system_servercom.android.server.SystemServer"

*/

RuntimeInit.zygoteInit(parsedArgs.remainingArgs);

/* should never reach here */

}

这里先关闭socket，然后调用zygoteInit

public static final voidzygoteInit(String[] argv)

throwsZygoteInit.MethodAndArgsCaller {

commonInit();

zygoteInitNative();

...

invokeStaticMain(startClass,startArgs);

}

调用commonInit初始化时区，zygoteInitNative是jni函数，最终会调用app_main.cpp中的onZygoteInit

virtual void onZygoteInit()

{

sp<ProcessState> proc =ProcessState::self();

if (proc->supportsProcesses()) {

LOGV("App process: startingthread pool.\n");

proc->startThreadPool();

}

}

这里启动线程池处理Binder事件 。

然后调用invokeStaticMain启动SystemServer的main函数中

public static void main(String[] args) {

if (System.currentTimeMillis() <EARLIEST_SUPPORTED_TIME) {

// If a device's clock is before1970 (before 0), a lot of

// APIs crash dealing with negativenumbers, notably

// java.io.File#setLastModified, soinstead we fake it and

// hope that time from cell towersor NTP fixes it

// shortly.

Slog.w(TAG, "System clock isbefore 1970; setting to 1970.");

SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);

}

if(SamplingProfilerIntegration.isEnabled()) {

SamplingProfilerIntegration.start();

timer = new Timer();

timer.schedule(new TimerTask() {

@Override

public void run() {

SamplingProfilerIntegration.writeSnapshot("system_server");

}

}, SNAPSHOT_INTERVAL,SNAPSHOT_INTERVAL);

}

// The system server has to run all ofthe time, so it needs to be

// as efficient as possible with itsmemory usage.

VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);

System.loadLibrary("android_servers");

init1(args);

}

首先检查系统时间设置和SamplingProfiler。然后加载一个叫android_servers的本地库，然后调用init1启动本地服务，因为后面要启动的java服务依赖这些服务。init1()也是个jni函数，最终调用system_init.cpp中的system_init()函数

extern "C" status_tsystem_init()

{

LOGI("Entered system_init()");

sp<ProcessState>proc(ProcessState::self());

sp<IServiceManager> sm =defaultServiceManager();

LOGI("ServiceManager: %p\n", sm.get());

sp<GrimReaper> grim = newGrimReaper();

sm->asBinder()->linkToDeath(grim,grim.get(), 0);

char propBuf[PROPERTY_VALUE_MAX];

property_get("system_init.startsurfaceflinger", propBuf,"1");

if (strcmp(propBuf, "1") == 0) {

// Start the SurfaceFlinger

SurfaceFlinger::instantiate();

}

// Start the sensor service

SensorService::instantiate();

// On the simulator, audioflinger et aldon't get started the

// same way as on the device, and we needto start them here

......

// the init function.

LOGI("System server: starting Androidruntime.\n");

AndroidRuntime* runtime =AndroidRuntime::getRuntime();

LOGI("System server: starting Androidservices.\n");

runtime->callStatic("com/android/server/SystemServer","init2");

if (proc->supportsProcesses()) {

LOGI("System server: enteringthread pool.\n");

ProcessState::self()->startThreadPool();

IPCThreadState::self()->joinThreadPool();

LOGI("System server: exitingthread pool.\n");

}

return NO_ERROR;

}

在这里根据设置是否启动SurfaceFlinger,启动SensorService,然后调用java函数init2启动java服务，启动线程池，由于前面已经调用过startThreadPool,故此次调用不做任何事情，最后把主要线程加入线程池。

public static final void init2() {

Slog.i(TAG, "Entered the Androidsystem server!");

Thread thr = new ServerThread();

thr.setName("android.server.ServerThread");

thr.start();

}

在这里,new一个ServerThread，并启动这个线程。进入它的run方法

public void run() {

...

try {

Slog.i(TAG, "EntropyService");

ServiceManager.addService("entropy", new EntropyService());

}

...

Slog.i(TAG, "ActivityManager");

context = ActivityManagerService.main(factoryTest);

Slog.i(TAG, "Package Manager");

pm = PackageManagerService.main(context,

factoryTest !=SystemServer.FACTORY_TEST_OFF);

ActivityManagerService.setSystemProcess();

...

Slog.i(TAG, "Window Manager");

wm = WindowManagerService.main(context,power,

factoryTest !=SystemServer.FACTORY_TEST_LOW_LEVEL);

((ActivityManagerService)ActivityManagerNative.getDefault())

.systemReady(new Runnable() {

...

}

...

Looper.loop();

Slog.d(TAG, "System ServerThread isexiting!");

}

在这里实现了Java Service的注册及初始化操作，PackageManagerService、ActivityManagerService等主要服务都是在这里启动的，ActivityManagerService.systemReady()函数来启动Home应用程序。

最后调用loop进入SystemServer事件处理循环。

到这里，SystemServer就已经启动起来了，各种服务也已经加载 了。