android9启动进程,androidSource/9 SystemServer进程启动流程.md at master · hualw/androidSource · GitHub...

于 2021-05-28 10:22:34 发布
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文章标签： android9启动进程

转载请标明出处：一片枫叶的专栏

上面一文中我们讲过android系统中比较重要的几个进程：init进程，Zygote进程，SystemServer进程已经各种应用进程，其中Zygote进程是整个android系统的根进程，包含SystemServer进程已经各种应用进程在内的进程都是通过Zygote进程fork出来的，具体可参见：android源码解析之(八)-->Zygote进程启动流程

那么SystemServer进程是做什么用的呢？

其实SystemServer进程主要的作用是在这个进程中启动各种系统服务，比如ActivityManagerService，PackageManagerService，WindowManagerService服务，以及各种系统性的服务其实都是在SystemServer进程中启动的，而当我们的应用需要使用各种系统服务的时候其实也是通过与SystemServer进程通讯获取各种服务对象的句柄的。

由上一篇文章我们知道SystemServer进程其实也是有Zygote进程fork出来的，并且执行其main方法，那么这里我们以android23的源码为例，看一下SystemServer的main方法的执行逻辑：

/**

* The main entry point from zygote.

*/

public static void main(String[] args) {

new SystemServer().run();

}

这里比较简单，只是new出一个SystemServer对象并执行其run方法，查看SystemServer类的定义我们知道其实final类型的，所以我们一般不能重写或者继承。

然后我们查看run方法：

private void run() {

// 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.

if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {

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

SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);

}

// If the system has "persist.sys.language" and friends set, replace them with

// "persist.sys.locale". Note that the default locale at this point is calculated

// using the "-Duser.locale" command line flag. That flag is usually populated by

// AndroidRuntime using the same set of system properties, but only the system_server

// and system apps are allowed to set them.

//

// NOTE: Most changes made here will need an equivalent change to

// core/jni/AndroidRuntime.cpp

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", "");

}

// Here we go!

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

EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, SystemClock.uptimeMillis());

// In case the runtime switched since last boot (such as when

// the old runtime was removed in an OTA), set the system

// property so that it is in sync. We can't do this in

// libnativehelper's JniInvocation::Init code where we already

// had to fallback to a different runtime because it is

// running as root and we need to be the system user to set

// the property. http://b/11463182

SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());

// Enable the sampling profiler.

if (SamplingProfilerIntegration.isEnabled()) {

SamplingProfilerIntegration.start();

mProfilerSnapshotTimer = new Timer();

mProfilerSnapshotTimer.schedule(new TimerTask() {

@Override

public void run() {

SamplingProfilerIntegration.writeSnapshot("system_server", null);

}

}, SNAPSHOT_INTERVAL, SNAPSHOT_INTERVAL);

}

// 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);

// Ensure binder calls into the system always run at foreground priority.

BinderInternal.disableBackgroundScheduling(true);

// 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);

LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

// Start services.

try {

startBootstrapServices();

startCoreServices();

startOtherServices();

} catch (Throwable ex) {

Slog.e("System", "******************************************");

Slog.e("System", "************ Failure starting system services", ex);

throw ex;

}

// For debug builds, log event loop stalls to dropbox for analysis.

if (StrictMode.conditionallyEnableDebugLogging()) {

Slog.i(TAG, "Enabled StrictMode for system server main thread.");

}

// Loop forever.

Looper.loop();

throw new RuntimeException("Main thread loop unexpectedly exited");

}

好吧，代码比较多，慢慢看。。。

if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {

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

SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);

}

首先判断系统当前时间，若当前时间小于1970年1月1日，则一些初始化操作可能会处所，所以当系统的当前时间小于1970年1月1日的时候，设置系统当前时间为该时间点。

然后代码：

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", "");

}

主要是设置系统的语言环境等；

下面的主要是设置虚拟机运行内存，加载运行库，设置SystemServer的异步消息，具体的异步消息机制可参见：android源码解析之(二)-->异步消息机制

然后下面的代码是：

// Initialize the system context.

createSystemContext();

// Create the system service manager.

mSystemServiceManager = new SystemServiceManager(mSystemContext);

LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

// Start services.

try {

startBootstrapServices();

startCoreServices();

startOtherServices();

} catch (Throwable ex) {

Slog.e("System", "******************************************");

Slog.e("System", "************ Failure starting system services", ex);

throw ex;

}

首先调用createSystemContext()方法：

private void createSystemContext() {

ActivityThread activityThread = ActivityThread.systemMain();

mSystemContext = activityThread.getSystemContext();

mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar);

}

可以看到在SystemServer进程中也存在着Context对象，并且是通过ActivityThread.systemMain方法创建context的，这一部分的逻辑以后会通过介绍Activity的启动流程来介绍，这里就不在扩展，只知道在SystemServer进程中也需要创建Context对象。

然后通过SystemServiceManager的构造方法创建了一个新的SystemServiceManager对象，我们知道SystemServer进程主要是用来构建系统各种service服务的，而SystemServiceManager就是这些服务的管理对象。

然后调用：

LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

是将SystemServiceManager对象保存SystemServer进程中的一个数据结构中。

最后开始执行：

// Start services.

try {

startBootstrapServices();

startCoreServices();

startOtherServices();

} catch (Throwable ex) {

Slog.e("System", "******************************************");

Slog.e("System", "************ Failure starting system services", ex);

throw ex;

}

里面主要涉及了是三个方法：

startBootstrapServices() 主要用于启动系统Boot级服务

startCoreServices() 主要用于启动系统核心的服务

startOtherServices() 主要用于启动一些非紧要或者是非需要及时启动的服务

下面我们重点介绍这三个启动服务的方法，包括启动那些系统服务已经如何启动系统服务等。

首先看一下startBootstrapServices方法：

private void startBootstrapServices() {

// Wait for installd to finish 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.

Installer installer = mSystemServiceManager.startService(Installer.class);

// Activity manager runs the show.

mActivityManagerService = mSystemServiceManager.startService(

ActivityManagerService.Lifecycle.class).getService();

mActivityManagerService.setSystemServiceManager(mSystemServiceManager);

mActivityManagerService.setInstaller(installer);

// Power manager needs to be started early because other services need it.

// Native daemons may be watching for it to be registered so it must be ready

// to handle incoming binder calls immediately (including being able to verify

// the permissions for those calls).

mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);

// Now that the power manager has been started, let the activity manager

// initialize power management features.

mActivityManagerService.initPowerManagement();

// 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);

// We need the default display before we can initialize the package manager.

mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);

// 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.

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

mPackageManagerService = PackageManagerService.main(mSystemContext, installer,

mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);

mFirstBoot = mPackageManagerService.isFirstBoot();

mPackageManager = mSystemContext.getPackageManager();

Slog.i(TAG, "User Service");

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();

// The sensor service needs access to package manager service, app ops

// service, and permissions service, therefore we start it after them.

startSensorService();

}

首先执行：

Installer installer = mSystemServiceManager.startService(Installer.class);

mSystemServiceManager是系统服务管理对象，在main方法中已经创建完成，这里我们看一下其startService方法的具体实现：

public T startService(Class serviceClass) {

final String name = serviceClass.getName();

Slog.i(TAG, "Starting " + name);

// Create the service.

if (!SystemService.class.isAssignableFrom(serviceClass)) {

throw new RuntimeException("Failed to create " + name

+ ": service must extend " + SystemService.class.getName());

}

final T service;

try {

Constructor constructor = serviceClass.getConstructor(Context.class);

service = constructor.newInstance(mContext);

} catch (InstantiationException ex) {

throw new RuntimeException("Failed to create service " + name

+ ": service could not be instantiated", ex);

} catch (IllegalAccessException ex) {

throw new RuntimeException("Failed to create service " + name

+ ": service must have a public constructor with a Context argument", ex);

} catch (NoSuchMethodException ex) {

throw new RuntimeException("Failed to create service " + name

+ ": service must have a public constructor with a Context argument", ex);

} catch (InvocationTargetException ex) {

throw new RuntimeException("Failed to create service " + name

+ ": service constructor threw an exception", ex);

}

// Register it.

mServices.add(service);

// Start it.

try {

service.onStart();

} catch (RuntimeException ex) {

throw new RuntimeException("Failed to start service " + name

+ ": onStart threw an exception", ex);

}

return service;

}

可以看到我们通过反射器构造方法创建出服务类，然后添加到SystemServiceManager的服务列表数据中，最后调用了service.onStart()方法，因为我们传递的是Installer.class，我们这里我们查看一下Installer的onStart方法：

@Override

public void onStart() {

Slog.i(TAG, "Waiting for installd to be ready.");

mInstaller.waitForConnection();

}

很简单就是执行了mInstaller的waitForConnection方法，这里简单介绍一下Installer类，该类是系统安装apk时的一个服务类，继承SystemService(系统服务的一个抽象接口)，我们需要在启动完成Installer服务之后才能启动其他的系统服务。

然后查看waitForConnection()方法：

public void waitForConnection() {

for (;;) {

if (execute("ping") >= 0) {

return;

}

Slog.w(TAG, "installd not ready");

SystemClock.sleep(1000);

}

}

通过追踪代码可以发现，其在不断的通过ping命令连接Zygote进程(SystemServer和Zygote进程通过socket方式通讯，其他进程通过Binder方式通讯)；

总结：

在开始执行启动服务之前总是会先尝试通过socket方式连接Zygote进程，在成功连接之后才会开始启动其他服务。

继续来看startBootstrapServices方法：

// Activity manager runs the show.

mActivityManagerService = mSystemServiceManager.startService(

ActivityManagerService.Lifecycle.class).getService();

mActivityManagerService.setSystemServiceManager(mSystemServiceManager);

mActivityManagerService.setInstaller(installer);

这段代码主要是用于启动ActivityManagerService服务，并为其设置SysServiceManager和Installer。ActivityManagerService是系统中一个非常重要的服务，Activity，service，Broadcast，contentProvider都需要通过其余系统交互。

首先看一下Lifecycle类的定义：

public static final class Lifecycle extends SystemService {

private final ActivityManagerService mService;

public Lifecycle(Context context) {

super(context);

mService = new ActivityManagerService(context);

}

@Override

public void onStart() {

mService.start();

}

public ActivityManagerService getService() {

return mService;

}

}

可以看到其实ActivityManagerService的一个静态内部类，在其构造方法中会创建一个ActivityManagerService，通过刚刚对Installer服务的分析我们知道，SystemServiceManager的startService方法会调用服务的onStart()方法，而在Lifecycle类的定义中我们看到其onStart()方法直接调用了mService.start()方法，mService是Lifecycle类中对ActivityManagerService的引用，所以我们可以看一下ActivityManagerService的start方法的实现：

private void start() {

Process.removeAllProcessGroups();

mProcessCpuThread.start();

mBatteryStatsService.publish(mContext);

mAppOpsService.publish(mContext);

Slog.d("AppOps", "AppOpsService published");

LocalServices.addService(ActivityManagerInternal.class, new LocalService());

}

由于ActivityManagerService的创建过程比较复杂这里不做过多的分析了，主要是在其构造方法中初始化了一些变量。

然后是启动PowerManagerService服务：

mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);

启动方式跟上面的ActivityManagerService服务相似都会调用其构造方法和onStart方法，PowerManagerService主要用于计算系统中和Power相关的计算，然后决策系统应该如何反应。同时协调Power如何与系统其它模块的交互，比如没有用户活动时，屏幕变暗等等。

然后是启动LightsService服务

mSystemServiceManager.startService(LightsService.class);

主要是手机中关于闪光灯，LED等相关的服务；也是会调用LightsService的构造方法和onStart方法；

然后是启动DisplayManagerService服务

mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);

主要是手机显示方面的服务；

然后是启动PackageManagerService，该服务也是android系统中一个比较重要的服务，包括多apk文件的安装，解析，删除，卸载等等操作。

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

mPackageManagerService = PackageManagerService.main(mSystemContext, installer,

mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);

mFirstBoot = mPackageManagerService.isFirstBoot();

mPackageManager = mSystemContext.getPackageManager();

可以看到PackageManagerService服务的启动方式与其他服务的启动方式有一些区别，直接调用了PackageManagerService的静态main方法，这里我们看一下其main方法的具体实现：

public static PackageManagerService main(Context context, Installer installer,

boolean factoryTest, boolean onlyCore) {

PackageManagerService m = new PackageManagerService(context, installer,

factoryTest, onlyCore);

ServiceManager.addService("package", m);

return m;

}

可以看到也是直接使用new的方式创建了一个PackageManagerService对象，并在其构造方法中初始化相关变量，最后调用了ServiceManager.addService方法，主要是通过Binder机制与JNI层交互，这里不再扩展。

然后启动UserManagerService和SensorService，至此startBootstrapServices方法执行完成。

然后查看startCoreServices方法：

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);

}

可以看到这里启动了BatteryService(电池相关服务)，UsageStatsService，WebViewUpdateService服务等。

最后看一下startOtherServices方法，主要用于启动系统中其他的服务，代码很多，这里就不贴代码了，启动的流程和ActivityManagerService的流程类似，会调用服务的构造方法与onStart方法初始化变量。

总结：

SystemServer进程是android中一个很重要的进程由Zygote进程启动；

SystemServer进程主要用于启动系统中的服务；

SystemServer进程启动服务的启动函数为main函数；

SystemServer在执行过程中首先会初始化一些系统变量，加载类库，创建Context对象，创建SystemServiceManager对象等之后才开始启动系统服务；

SystemServer进程将系统服务分为三类：boot服务，core服务和other服务，并逐步启动

SertemServer进程在尝试启动服务之前会首先尝试与Zygote建立socket通讯，只有通讯成功之后才会开始尝试启动服务；

创建的系统服务过程中主要通过SystemServiceManager对象来管理，通过调用服务对象的构造方法和onStart方法初始化服务的相关变量；

服务对象都有自己的异步消息对象，并运行在单独的线程中；

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