android启动过程分析--启动systemServer
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。