（1）SystemServer启动
SystemServer的进程名实际上叫做“system_server”，通常简称为SS。
是系统中的服务驻留在其中，常见的比如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。
//frameworks/base/services/java/com/android/server/SystemServer.java
/**
* The main entry point from zygote.
*/
public static void main(String[] args) {
new SystemServer().run();
}
public SystemServer() {
// Check for factory test mode.
mFactoryTestMode = FactoryTest.getMode();
// Remember if it's runtime restart(when sys.boot_completed is already set) or reboot
mRuntimeRestart = "1".equals(SystemProperties.get("sys.boot_completed"));
mRuntimeStartElapsedTime = SystemClock.elapsedRealtime();
mRuntimeStartUptime = SystemClock.uptimeMillis();
}
SystemServer是一个Java类，从main函数入口分析，主要调用run方法
//frameworks/base/services/java/com/android/server/SystemServer.java
private void run() {
try {
traceBeginAndSlog("InitBeforeStartServices");
if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
Slog.w(TAG, "System clock is before 1970; setting to 1970.");
SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
}
String timezoneProperty = SystemProperties.get("persist.sys.timezone");
if (timezoneProperty == null || timezoneProperty.isEmpty()) {
Slog.w(TAG, "Timezone not set; setting to GMT.");
SystemProperties.set("persist.sys.timezone", "GMT");
}
if (!SystemProperties.get("persist.sys.language").isEmpty()) {
final String languageTag = Locale.getDefault().toLanguageTag();
SystemProperties.set("persist.sys.locale", languageTag);
SystemProperties.set("persist.sys.language", "");
SystemProperties.set("persist.sys.country", "");
SystemProperties.set("persist.sys.localevar", "");
}
// The system server should never make non-oneway calls
Binder.setWarnOnBlocking(true);
// Here we go!
Slog.i(TAG, "Entered the Android system server!");
int uptimeMillis = (int) SystemClock.elapsedRealtime();
EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, uptimeMillis);
if (!mRuntimeRestart) {
MetricsLogger.histogram(null, "boot_system_server_init", uptimeMillis);
}
SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());
// Mmmmmm... more memory!
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);
// Some devices rely on runtime fingerprint generation, so make sure
// we've defined it before booting further.
Build.ensureFingerprintProperty();
// Within the system server, it is an error to access Environment paths without
// explicitly specifying a user.
Environment.setUserRequired(true);
// Within the system server, any incoming Bundles should be defused
// to avoid throwing BadParcelableException.
BaseBundle.setShouldDefuse(true);
// Ensure binder calls into the system always run at foreground priority.
BinderInternal.disableBackgroundScheduling(true);
// Increase the number of binder threads in system_server
BinderInternal.setMaxThreads(sMaxBinderThreads);
// Prepare the main looper thread (this thread).
android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_FOREGROUND);
android.os.Process.setCanSelfBackground(false);
Looper.prepareMainLooper();
// Initialize native services.
System.loadLibrary("android_servers");
// Check whether we failed to shut down last time we tried.
// This call may not return.
performPendingShutdown();
// Initialize the system context.
createSystemContext();
// Create the system service manager.
mSystemServiceManager = new SystemServiceManager(mSystemContext);
mSystemServiceManager.setRuntimeRestarted(mRuntimeRestart);
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
// Prepare the thread pool for init tasks that can be parallelized
SystemServerInitThreadPool.get();
} finally {
traceEnd(); // InitBeforeStartServices
}
// Start services.
try {
traceBeginAndSlog("StartServices");
startBootstrapServices();
startCoreServices();
startOtherServices();
SystemServerInitThreadPool.shutdown();
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
traceEnd();
}
// For debug builds, log event loop stalls to dropbox for analysis.
if (StrictMode.conditionallyEnableDebugLogging()) {
Slog.i(TAG, "Enabled StrictMode for system server main thread.");
}
if (!mRuntimeRestart && !isFirstBootOrUpgrade()) {
int uptimeMillis = (int) SystemClock.elapsedRealtime();
MetricsLogger.histogram(null, "boot_system_server_ready", uptimeMillis);
final int MAX_UPTIME_MILLIS = 60 * 1000;
if (uptimeMillis > MAX_UPTIME_MILLIS) {
Slog.wtf(SYSTEM_SERVER_TIMING_TAG,
"SystemServer init took too long. uptimeMillis=" + uptimeMillis);
}
}
// Loop forever.
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
以上run方法的主要工作如下：
- 调整时间，如果系统时间比1970还要早，调整到1970年
- 设置语言
- 调整虚拟机堆内存大小和内存利用率
- 初始化Looper为mainLooper
- 装载库libandroid_server.so
- 初始化系统Context
- 创建SystemServiceManager负责系统Service启动
- 启动各种服务
- 调用Looper.loop()，进入处理消息的循环
这里简单介绍几个功能：
（1）初始化系统Context，可用于后续创建SystemServiceManager等
private void createSystemContext() {
ActivityThread activityThread = ActivityThread.systemMain();
mSystemContext = activityThread.getSystemContext();
mSystemContext.setTheme(DEFAULT_SYSTEM_THEME);
final Context systemUiContext = activityThread.getSystemUiContext();
systemUiContext.setTheme(DEFAULT_SYSTEM_THEME);
}
（2）创建SystemServiceManager实例
// Create the system service manager.
mSystemServiceManager = new SystemServiceManager(mSystemContext);
mSystemServiceManager.setStartInfo(mRuntimeRestart,
mRuntimeStartElapsedTime, mRuntimeStartUptime);
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
可用于后面通过SystemServiceManager.start方法启动各种服务。
（3）创建和启动服务有两种方式
//（1）通过SystemServiceManager.start
mSystemServiceManager.startService(AlarmManagerService.class);
mSystemServiceManager.startService(DisplayManagerService.class);
mSystemServiceManager.startService(
ActivityManagerService.Lifecycle.class).getService();
//（2）通过ServiceManager.addService
ServiceManager.addService("telephony.registry", telephonyRegistry);
ServiceManager.addService(Context.SYSTEM_UPDATE_SERVICE,
new SystemUpdateManagerService(context));
ServiceManager.addService(Context.MEDIA_ROUTER_SERVICE, mediaRouter);
（4）启动系统各种核心服务
traceBeginAndSlog("StartBatteryService");
// Tracks the battery level. Requires LightService.
mSystemServiceManager.startService(BatteryService.class);
traceEnd();
traceBeginAndSlog("StartActivityManager");
mActivityManagerService = mSystemServiceManager.startService(
ActivityManagerService.Lifecycle.class).getService();
mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
mActivityManagerService.setInstaller(installer);
traceEnd();
traceBeginAndSlog("StartPackageManagerService");
mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
mFirstBoot = mPackageManagerService.isFirstBoot();
mPackageManager = mSystemContext.getPackageManager();
traceEnd();
traceBeginAndSlog("StartDisplayManager");
mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);
traceEnd();
（2）AMS.systemReady
systemReady是在SystemServer.startOtherServices的最后被调用，主要的动作是标记和等待各个服务启动完成如等待PMS启动结束，紧接着启动SystemUI和启动Launcher。
//frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
public void systemReady(final Runnable goingCallback, TimingsTraceLog traceLog) {
//startSystemUi
traceBeginAndSlog("StartSystemUI");
try {
startSystemUi(context, windowManagerF);
} catch (Throwable e) {
reportWtf("starting System UI", e);
}
traceEnd();
//startHomeActivity
traceLog.traceBegin("PhaseActivityManagerReady");
startHomeActivityLocked(currentUserId, "systemReady");
...
}
startSystemUi（SystemUI启动）
SystemUI是在AMS.systemReady时启动，主要是通过Intent启动包名为com.android.systemui组件名为SystemUIService的服务，然后通过调用WindowManager.onSystemUiStarted方法调用KeyguardService启动锁屏服务。SystemUI启动成功后表示系统的通知栏和导航栏已经初始化成功，接下来就是启动Launcher。
static final void startSystemUi(Context context, WindowManagerService windowManager) {
Intent intent = new Intent();
intent.setComponent(new ComponentName("com.android.systemui",
"com.android.systemui.SystemUIService"));
intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);
//Slog.d(TAG, "Starting service: " + intent);
context.startServiceAsUser(intent, UserHandle.SYSTEM);
windowManager.onSystemUiStarted();
}
startHomeActivity（Launcher启动）
Launcher的启动是在AMS的最后，通过startHomeActivityLocked来启动Launcher的Activity并将其置于Activity栈顶，然后通过resumeFocusedStackTopActivityLocked将栈顶的Activity显示到界面上，launcher的启动就已经完成了。
Intent getHomeIntent() {
Intent intent = new Intent(mTopAction, mTopData != null ? Uri.parse(mTopData) : null);
intent.setComponent(mTopComponent);
intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);
if (mFactoryTest != FactoryTest.FACTORY_TEST_LOW_LEVEL) {
intent.addCategory(Intent.CATEGORY_HOME);
}
return intent;
}
boolean startHomeActivityLocked(int userId, String reason) {
if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL
&& mTopAction == null) {
// We are running in factory test mode, but unable to find
// the factory test app, so just sit around displaying the
// error message and don't try to start anything.
return false;
}
//获得HomeActivity的Intent
Intent intent = getHomeIntent();
ActivityInfo aInfo = resolveActivityInfo(intent, STOCK_PM_FLAGS, userId);
if (aInfo != null) {
intent.setComponent(new ComponentName(aInfo.applicationInfo.packageName, aInfo.name));
// Don't do this if the home app is currently being
// instrumented.
aInfo = new ActivityInfo(aInfo);
aInfo.applicationInfo = getAppInfoForUser(aInfo.applicationInfo, userId);
ProcessRecord app = getProcessRecordLocked(aInfo.processName,
aInfo.applicationInfo.uid, true);
if (app == null || app.instr == null) {
intent.setFlags(intent.getFlags() | FLAG_ACTIVITY_NEW_TASK);
final int resolvedUserId = UserHandle.getUserId(aInfo.applicationInfo.uid);
// For ANR debugging to verify if the user activity is the one that actually
// launched.
final String myReason = reason + ":" + userId + ":" + resolvedUserId;
//用于启动HomeActivity
mActivityStartController.startHomeActivity(intent, aInfo, myReason);
}
} else {
Slog.wtf(TAG, "No home screen found for " + intent, new Throwable());
}
return true;
}
接下来我们画一个简单的UML图来表示Android开机全过程。
具体后续的开机动画（bootanimation）如何启动和退出的，我们将在后面的博文中在进行详细分析。