SystemServer会开启很多服务，这些服务的创建流程类似，以Installer为例子

1.startBootstrapServices 

//frameworks\base\services\java\com\android\server\SystemServer.javaprivate void startBootstrapServices() {Installer installer = mSystemServiceManager.startService(Installer.class);
}

2.startService

通过反射创建service

//android10\frameworks\base\services\core\java\com\android\server\SystemServiceManager.java@SuppressWarnings("unchecked")public <T extends SystemService> T startService(Class<T> serviceClass) {try {final String name = serviceClass.getName();//要创建的服务的类名final T service;try {//通过反射创建服务Constructor<T> constructor = serviceClass.getConstructor(Context.class);service = constructor.newInstance(mContext);} catch (InstantiationException ex) {//......} catch (IllegalAccessException ex) {//......} catch (NoSuchMethodException ex) {//......} catch (InvocationTargetException ex) {//......}startService(service);return service;} finally {Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);}}

3.startService

调用service的onStart方法

//android10\frameworks\base\services\core\java\com\android\server\SystemServiceManager.java// Services that should receive lifecycle events.private final ArrayList<SystemService> mServices = new ArrayList<SystemService>();//Installer服务继承自SystemServicepublic void startService(@NonNull final SystemService service) {// Register it.mServices.add(service); //添加到ArrayList// Start it.try {service.onStart();//调用service的onStart方法} catch (RuntimeException ex) {//......}}