SystemServer类解析
安卓系统有一个system_server进程，该进程由zygote进程创建，负责运行所有的系统服务，该进程的入口类是frameworks/base/services/java/com/android/server/SystemServer.java。下面来解析一下该类的主要功能。
该类有一个main方法，说起main方法学过java的都熟悉，main方法是一段java程序的入口，也可以说是一个java进程的入口。我们来看一下该方法源码（源码版本是android 7.1.1）：
/**
* The main entry point from zygote.
*/
public static void main(String[] args) {
new SystemServer().run();
}从该方法的源码可知，该main方法是zygote创建system_server进程的入口，方法里面只是新创建了SystemServer对象，然后调用其run方法。
我们来看一下SystemServer.run()都干了什么：
private void run() {
//此处省略n行代码
// Start services.
try {
Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartServices");
startBootstrapServices();
startCoreServices();
startOtherServices();
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
//此处省略n行代码
}run方法我们只看与服务调度有关的逻辑，其他逻辑屏蔽掉。从上面代码看，run方法会调用三个startService方法来分别启动不同性质的服务：startBootstrapSerivces()、startCoreServices()、startOtherServices()。
1.startBootstrapSerivces()
/**
* Starts the small tangle of critical services that are needed to get
* the system off the ground. These services have complex mutual dependencies
* which is why we initialize them all in one place here. Unless your service
* is also entwined in these dependencies, it should be initialized in one of
* the other functions.
*/
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.
Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "InitPowerManagement");
mActivityManagerService.initPowerManagement();
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
// 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.
traceBeginAndSlog("StartPackageManagerService");
mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
mFirstBoot = mPackageManagerService.isFirstBoot();
mPackageManager = mSystemContext.getPackageManager();
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
// Manages A/B OTA dexopting. This is a bootstrap service as we need it to rename
// A/B artifacts after boot, before anything else might touch/need them.
// Note: this isn't needed during decryption (we don't have /data anyways).
if (!mOnlyCore) {
boolean disableOtaDexopt = SystemProperties.getBoolean("config.disable_otadexopt",
false);
if (!disableOtaDexopt) {
traceBeginAndSlog("StartOtaDexOptService");
try {
OtaDexoptService.main(mSystemContext, mPackageManagerService);
} catch (Throwable e) {
reportWtf("starting OtaDexOptService", e);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
}
}
traceBeginAndSlog("StartUserManagerService");
mSystemServiceManager.startService(UserManagerService.LifeCycle.class);
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
// 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();
}先看一下爱该方法的注释，可知该方法启动一些极重要的services，这些服务相互依赖，所以统一在这里进行初始化。再看一下方法里面的代码，创建了一个Installer、ActivityManagerService、PowerManagerService、DisplayManagerService、PackageManagerService、UserManagerService等，ActivityManagerService和Installer、PowerManagerService是有关联的：
mActivityManagerService.setInstaller(installer);
mActivityManagerService.initPowerManagement();这些服务都是继承了一个叫SystemService的抽象类，该抽象类里面定义了一个抽象方法onStart，系统服务实现这个方法。SystemServiceManager.startService最终调用的就是这个onStart方法来启动系统服务，来看一下源码吧：
/**
* Starts a service by class name.
*
* @return The service instance.
*/
@SuppressWarnings("unchecked")
public SystemService startService(String className) {
final Class<SystemService> serviceClass;
try {
serviceClass = (Class<SystemService>)Class.forName(className);
} catch (ClassNotFoundException ex) {
Slog.i(TAG, "Starting " + className);
throw new RuntimeException("Failed to create service " + className
+ ": service class not found, usually indicates that the caller should "
+ "have called PackageManager.hasSystemFeature() to check whether the "
+ "feature is available on this device before trying to start the "
+ "services that implement it", ex);
}
return startService(serviceClass);
}
/**
* Creates and starts a system service. The class must be a subclass of
* {@link com.android.server.SystemService}.
*
* @param serviceClass A Java class that implements the SystemService interface.
* @return The service instance, never null.
* @throws RuntimeException if the service fails to start.
*/
@SuppressWarnings("unchecked")
public <T extends SystemService> T startService(Class<T> serviceClass) {
try {
final String name = serviceClass.getName();
Slog.i(TAG, "Starting " + name);
Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartService " + 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<T> 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;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
}我们以Installer服务来举例说明启动流程，看一下其onStart方法实现：
@Override
public void onStart() {
Slog.i(TAG, "Waiting for installd to be ready.");
mInstaller.waitForConnection();
}2 startCoreServices()
/**
* Starts some essential services that are not tangled up in the bootstrap process.
*/
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));
// Tracks whether the updatable WebView is in a ready state and watches for update installs.
mWebViewUpdateService = mSystemServiceManager.startService(WebViewUpdateService.class);
}发方法里面启动了BateerService、UsageStatsService、WebViewUpdateService。
我们来看一下BatteryService的onStart方法：
@Override
public void onStart() {
IBinder b = ServiceManager.getService("batteryproperties");
final IBatteryPropertiesRegistrar batteryPropertiesRegistrar =
IBatteryPropertiesRegistrar.Stub.asInterface(b);
try {
batteryPropertiesRegistrar.registerListener(new BatteryListener());
} catch (RemoteException e) {
// Should never happen.
}
mBinderService = new BinderService();
publishBinderService("battery", mBinderService);
publishLocalService(BatteryManagerInternal.class, new LocalService());
}我们注意一下publishBinderService方法，这个方法的定义SystemService抽象类中，我们看一下其定义：
/**
* Publish the service so it is accessible to other services and apps.
*/
protected final void publishBinderService(String name, IBinder service) {
publishBinderService(name, service, false);
}
/**
* Publish the service so it is accessible to other services and apps.
*/
protected final void publishBinderService(String name, IBinder service,
boolean allowIsolated) {
ServiceManager.addService(name, service, allowIsolated);
}我们再看一下ServiceManager的addService方法：
/**
* Place a new @a service called @a name into the service
* manager.
*
* @param name the name of the new service
* @param service the service object
* @param allowIsolated set to true to allow isolated sandboxed processes
* to access this service
*/
public static void addService(String name, IBinder service, boolean allowIsolated) {
try {
getIServiceManager().addService(name, service, allowIsolated);
} catch (RemoteException e) {
Log.e(TAG, "error in addService", e);
}
}该方法调用了ServiceManagerNative.addService方法：
public void addService(String name, IBinder service, boolean allowIsolated)
throws RemoteException {
Parcel data = Parcel.obtain();
Parcel reply = Parcel.obtain();
data.writeInterfaceToken(IServiceManager.descriptor);
data.writeString(name);
data.writeStrongBinder(service);
data.writeInt(allowIsolated ? 1 : 0);
mRemote.transact(ADD_SERVICE_TRANSACTION, data, reply, 0);
reply.recycle();
data.recycle();
}然后是跟Native层的服务交互了。此处把service添加到了ServiceManagerNative里面了，getSystemService方法获取系统服务的时候会用到此处逻辑，参考getSystemService追根溯源。
3 startOtherServices()
ServiceManager.addService("scheduling_policy", new SchedulingPolicyService());
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
mSystemServiceManager.startService(TelecomLoaderService.class);startOtherServices方法里面启动了很多服务，启动方式有两种，如上段代码所示，一种是直接new一个服务对象，然后调用ServiceManager.addService方法添加到ServiceManagerNative中，这种方式启动的服务一般是继承了某个接口的Stub类；另一种是通过SystemServiceManager.startService方法来启动，这种服务是继承了SystemService抽象类。
另外补充一下，在run方法里面创建了Looper，可见该进程通信也是使用Handler的。