SystemServer是Android系统的核心，APK应用中可以直接交互的大部分系统服务都在该进程中执行，常见的比方WindowManagerServer（Wms）、ActivityManagerSystemService（AmS）、 PackageManagerServer（PmS）等，这些系统服务都是以一个线程的方式存在于SystemServer进程中。
startSystemServersystemServer是通过zygote启动的时候fork启动的，我们先看回到ZygoteInit.java类中看一下SystemServer的启动。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 the system server */
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003,3006,3007",
"--capabilities=130104352,130104352",
"--runtime-init",
"--nice-name=system_server",//终于的进程名
"com.android.server.SystemServer",
};
ZygoteConnection.Arguments parsedArgs = null;
int pid;
try {
parsedArgs = new ZygoteConnection.Arguments(args);
ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
/* 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);
}
/* For child process */
if (pid == 0) {
handleSystemServerProcess(parsedArgs);
}
return true;
}
首先通过Zygote.forkSystemServer fork出一个子进程来，并传入了一些參数来设置新进程的一些信息，如uid、gid等，函数返回后，假设是子进程pid=0，进入handleSystemServerProcess函数，例如以下：
private static void handleSystemServerProcess(
ZygoteConnection.Arguments parsedArgs)
throws ZygoteInit.MethodAndArgsCaller {
closeServerSocket();
// set umask to 0077 so new files and directories will default to owner-only permissions.
Libcore.os.umask(S_IRWXG | S_IRWXO);
//ps能够看到system 261 98 391508 49008 ffffffff 4005c880 S system_server，此处system_server就是那个parsedArgs.niceName
if (parsedArgs.niceName != null) {
Process.setArgV0(parsedArgs.niceName);
}
if (parsedArgs.invokeWith != null) {
WrapperInit.execApplication(parsedArgs.invokeWith,
parsedArgs.niceName, parsedArgs.targetSdkVersion,
null, parsedArgs.remainingArgs);
} else {
/*
* Pass the remaining arguments to SystemServer.
*/
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs);
}
/* should never reach here 注意不应该执行到这里*/
}
在此函数中首先调用 closeServerSocket()关闭从父进程zygote复制来的socket服务，由于仅仅要zygote去监听处理socket请求就够了。然后继续执行，RuntimeInit.zygoteInit（）会通过JNI调用app_main.cpp中的onZygoteInit,启动线程池处理Binder事件 。最后通过invokeStaticMain()函数抛出异常MethodAndArgsCaller。
public static class MethodAndArgsCaller extends Exception implements Runnable {
/** method to call */
private final Method mMethod;
/** argument array */
private final String[] mArgs;
public MethodAndArgsCaller(Method method, String[] args) {
mMethod = method;
mArgs = args;
}
public void run() {
try {
mMethod.invoke(null, new Object[] { mArgs });
} catch (IllegalAccessException ex) {
throw new RuntimeException(ex);
} catch (InvocationTargetException ex) {
Throwable cause = ex.getCause();
if (cause instanceof RuntimeException) {
throw (RuntimeException) cause;
} else if (cause instanceof Error) {
throw (Error) cause;
}
throw new RuntimeException(ex);
}
}
}
这个类继承自Exception，实现了Runnable。该异常被ZygoteInit.java类的main函数catch，并运行run()函数，通过反射启动SystemServer的main函数。
以下是ZygoteInit.java类的main函数捕获异常的代码：
} catch (MethodAndArgsCaller caller) {
caller.run();
}这样我们就启动了SystemServer.java的main函数了，源代码在 (base\services\java\com\android\server)。
SystemServer.java让我们来看看这个类中的部分代码：
/**
* This method is called from Zygote to initialize the system. This will cause the native
* services (SurfaceFlinger, AudioFlinger, etc..) to be started. After that it will call back
* up into init2() to start the Android services.
*/
native public static void init1(String[] args);
public static void main(String[] args) {
//首先检查系统时间设置和SamplingProfiler
if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
// If a device's clock is before 1970 (before 0), a lot of
// APIs crash dealing with negative numbers, notably
// java.io.File#setLastModified, so instead we fake it and
// hope that time from cell towers or NTP fixes it
// shortly.
Slog.w(TAG, "System clock is before 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", null);
}
}, SNAPSHOT_INTERVAL, SNAPSHOT_INTERVAL);
}
// Mmmmmm... more memory!
dalvik.system.VMRuntime.getRuntime().clearGrowthLimit();
// The system server has to run all of the time, so it needs to be
// as efficient as possible with its memory usage.
VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);
//载入动态库libandroid_servers.so
System.loadLibrary("android_servers");
//init1()函数是一个native函数，通过JNI终于调用system_init.cpp中的system_init()函数，内部会进行一些与Dalvik虚拟机相关的初始化工作。该函数运行完毕后，其内部会调用Java端的init2()函数，这就是为什么Java源代码中没有引用init2()的地方.
init1(args);
}
//基本的系统服 务都是在init2()函数中完毕的。
public static final void init2() {
Slog.i(TAG, "Entered the Android system server!");
//该函数首先创建了一个ServerThread对象，该对象是一个线程，然后直接运行该线程,从ServerThread的run()方法内部開始真正启动各种服务线程。
Thread thr = new ServerThread();
thr.setName("android.server.ServerThread");
thr.start();
}
main()函数首先会运行init1()函数,init1()函数是一个native函数，通过JNI终于调用system_init.cpp中的system_init()函数：
extern "C" status_t system_init()
{
ALOGI("Entered system_init()");
sp<ProcessState> proc(ProcessState::self());
sp<IServiceManager> sm = defaultServiceManager();
ALOGI("ServiceManager: %p\n", sm.get());
sp<GrimReaper> grim = new GrimReaper();
sm->asBinder()->linkToDeath(grim, grim.get(), 0);
char propBuf[PROPERTY_VALUE_MAX];
property_get("system_init.startsurfaceflinger", propBuf, "1");
if (strcmp(propBuf, "1") == 0) {
//启动SurfaceFlinger
SurfaceFlinger::instantiate();
}
property_get("system_init.startsensorservice", propBuf, "1");
if (strcmp(propBuf, "1") == 0) {
//启动sensor service
SensorService::instantiate();
}
// And now start the Android runtime. We have to do this bit
// of nastiness because the Android runtime initialization requires
// some of the core system services to already be started.
// All other servers should just start the Android runtime at
// the beginning of their processes's main(), before calling
// the init function.
ALOGI("System server: starting Android runtime.\n");
AndroidRuntime* runtime = AndroidRuntime::getRuntime();
ALOGI("System server: starting Android services.\n");
JNIEnv* env = runtime->getJNIEnv();
if (env == NULL) {
return UNKNOWN_ERROR;
}
//通过“com/android/server/SystemServer”找到SystemServer.java的类
jclass clazz = env->FindClass("com/android/server/SystemServer");
if (clazz == NULL) {
return UNKNOWN_ERROR;
}
//找到SystemServer.java的类的init2方法
jmethodID methodId = env->GetStaticMethodID(clazz, "init2", "()V");
if (methodId == NULL) {
return UNKNOWN_ERROR;
}
//执行init2()函数
env->CallStaticVoidMethod(clazz, methodId);
ALOGI("System server: entering thread pool.\n");
ProcessState::self()->startThreadPool();
IPCThreadState::self()->joinThreadPool();
ALOGI("System server: exiting thread pool.\n");
return NO_ERROR;
}
system_init()函数回调init2()函数，init2()函数启动ServerThread线程来加入各种服务以及其它的一些初始化工作，ServerThread是一个内部类，大体代码例如以下；
class ServerThread extends Thread {
...
@Override
public void run() {
EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN,
SystemClock.uptimeMillis());
Looper.prepareMainLooper();
...
String factoryTestStr = SystemProperties.get("ro.factorytest");
int factoryTest = "".equals(factoryTestStr) ? SystemServer.FACTORY_TEST_OFF
: Integer.parseInt(factoryTestStr);
final boolean headless = "1".equals(SystemProperties.get("ro.config.headless", "0"));
//定义变量,初始化服务，创建各种服务实例，如：电源、网络、Wifi、蓝牙，USB等
...
Context context = null;
...
// Create a shared handler thread for UI within the system server.
// This thread is used by at least the following components:
// - WindowManagerPolicy
// - KeyguardViewManager
// - DisplayManagerService
HandlerThread uiHandlerThread = new HandlerThread("UI");
uiHandlerThread.start();
Handler uiHandler = new Handler(uiHandlerThread.getLooper());
uiHandler.post(new Runnable() {
@Override
public void run() {
//Looper.myLooper().setMessageLogging(new LogPrinter(
// Log.VERBOSE, "WindowManagerPolicy", Log.LOG_ID_SYSTEM));
android.os.Process.setThreadPriority(
android.os.Process.THREAD_PRIORITY_FOREGROUND);
android.os.Process.setCanSelfBackground(false);
// For debug builds, log event loop stalls to dropbox for analysis.
if (StrictMode.conditionallyEnableDebugLogging()) {
Slog.i(TAG, "Enabled StrictMode logging for UI Looper");
}
}
});
// Create a handler thread just for the window manager to enjoy.
HandlerThread wmHandlerThread = new HandlerThread("WindowManager");
wmHandlerThread.start();
Handler wmHandler = new Handler(wmHandlerThread.getLooper());
wmHandler.post(new Runnable() {
@Override
public void run() {
//Looper.myLooper().setMessageLogging(new LogPrinter(
// android.util.Log.DEBUG, TAG, android.util.Log.LOG_ID_SYSTEM));
android.os.Process.setThreadPriority(
android.os.Process.THREAD_PRIORITY_DISPLAY);
android.os.Process.setCanSelfBackground(false);
// For debug builds, log event loop stalls to dropbox for analysis.
if (StrictMode.conditionallyEnableDebugLogging()) {
Slog.i(TAG, "Enabled StrictMode logging for WM Looper");
}
}
});
// Critical services...
boolean onlyCore = false;
try {
// Wait for installd to finished starting up so that it has a chance to
// create critical directories such as /data/user with the appropriate
// permissions. We need this to complete before we initialize other services.
Slog.i(TAG, "Waiting for installd to be ready.");
installer = new Installer();
installer.ping();
//原来的EntropyService，熵混淆，添加随机数的不可预測性
Slog.i(TAG, "Entropy Mixer");
ServiceManager.addService("entropy", new EntropyMixer());//用Context.getSystemService (String name) 才获取到对应的服务
//向ServiceManager加入电源管理服务
Slog.i(TAG, "Power Manager");
power = new PowerManagerService();
ServiceManager.addService(Context.POWER_SERVICE, power);
//启动ActivityManagerService，注意返回了一个context
Slog.i(TAG, "Activity Manager");
context = ActivityManagerService.main(factoryTest);
//使用uiHandler、wmHandler
Slog.i(TAG, "Display Manager");
display = new DisplayManagerService(context, wmHandler, uiHandler);
ServiceManager.addService(Context.DISPLAY_SERVICE, display, true);
...
//使用wmHandler
Slog.i(TAG, "Input Manager");
inputManager = new InputManagerService(context, wmHandler);
//使用uiHandler、wmHandler
Slog.i(TAG, "Window Manager");
wm = WindowManagerService.main(context, power, display, inputManager,
uiHandler, wmHandler,
factoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL,
!firstBoot, onlyCore);
ServiceManager.addService(Context.WINDOW_SERVICE, wm);
ServiceManager.addService(Context.INPUT_SERVICE, inputManager);
ActivityManagerService.setSystemProcess();
...
Slog.i(TAG, "System Content Providers");
ActivityManagerService.installSystemProviders();
...
// only initialize the power service after we have started the lights service, content providers and the battery service.
power.init(context, lights, ActivityManagerService.self(), battery,
BatteryStatsService.getService(), display);
...
Slog.i(TAG, "Init Watchdog");
Watchdog.getInstance().init(context, battery, power, alarm,
ActivityManagerService.self());
...
ActivityManagerService.self().setWindowManager(wm);
...
//使用wmHandler
if (context.getResources().getBoolean(
com.android.internal.R.bool.config_dreamsSupported)) {
try {
Slog.i(TAG, "Dreams Service");
// Dreams (interactive idle-time views, a/k/a screen savers)
dreamy = new DreamManagerService(context, wmHandler);
ServiceManager.addService(DreamService.DREAM_SERVICE, dreamy);
} catch (Throwable e) {
reportWtf("starting DreamManagerService", e);
}
}
} catch (RuntimeException e) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting core service", e);
}
...
try {
Slog.i(TAG, "NetworkPolicy Service");
networkPolicy = new NetworkPolicyManagerService(
context, ActivityManagerService.self(), power,
networkStats, networkManagement);
ServiceManager.addService(Context.NETWORK_POLICY_SERVICE, networkPolicy);
} catch (Throwable e) {
reportWtf("starting NetworkPolicy Service", e);
}
...
}
...
if (safeMode) {
ActivityManagerService.self().showSafeModeOverlay();
}
...
//系统服务初始化准备就绪，通知各个模块
ActivityManagerService.self().systemReady(new Runnable() {
public void run() {
Slog.i(TAG, "Making services ready");
if (!headless) startSystemUi(contextF);
try {
if (mountServiceF != null) mountServiceF.systemReady();
} catch (Throwable e) {
reportWtf("making Mount Service ready", e);
}
...
try {
if (telephonyRegistryF != null) telephonyRegistryF.systemReady();
} catch (Throwable e) {
reportWtf("making TelephonyRegistry ready", e);
}
}
});
// For debug builds, log event loop stalls to dropbox for analysis.
if (StrictMode.conditionallyEnableDebugLogging()) {
Slog.i(TAG, "Enabled StrictMode for system server main thread.");
}
//開始处理消息的循环
Looper.loop();
Slog.d(TAG, "System ServerThread is exiting!");
}
static final void startSystemUi(Context context) {
Intent intent = new Intent();
intent.setComponent(new ComponentName("com.android.systemui",
"com.android.systemui.SystemUIService"));
//Slog.d(TAG, "Starting service: " + intent);
context.startServiceAsUser(intent, UserHandle.OWNER);
}
}到这里系统ApplicationFramework层的XxxServiceManager准备就绪
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