system_server进程启动原理

https://zhuanlan.zhihu.com/p/343233982
https://blog.csdn.net/xingyu19911016/article/details/128619982
https://blog.csdn.net/qq_21154101/article/details/89455884

一、初始化（InitBeforeStartServices）

• 准备MainLooper线程

  // Prepare the main looper thread (this thread).
  android.os.Process.setThreadPriority(
      android.os.Process.THREAD_PRIORITY_FOREGROUND);
  android.os.Process.setCanSelfBackground(false);
  Looper.prepareMainLooper();
  Looper.getMainLooper().setSlowLogThresholdMs(
      SLOW_DISPATCH_THRESHOLD_MS, SLOW_DELIVERY_THRESHOLD_MS);

• 加载native库

  // Initialize native services.
  System.loadLibrary("android_servers");

• createSystemContext

  613     private void createSystemContext() {
  614         ActivityThread activityThread = ActivityThread.systemMain();
  615         mSystemContext = activityThread.getSystemContext();
  616         mSystemContext.setTheme(DEFAULT_SYSTEM_THEME);
  617 
  618         final Context systemUiContext = activityThread.getSystemUiContext();
  619         systemUiContext.setTheme(DEFAULT_SYSTEM_THEME);
  620     }

• 创建SystemServiceManager实例

  501             // Create the system service manager.
  502             mSystemServiceManager = new SystemServiceManager(mSystemContext);
  503             mSystemServiceManager.setStartInfo(mRuntimeRestart,
  504                     mRuntimeStartElapsedTime, mRuntimeStartUptime);
  505             LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
  506             // Prepare the thread pool for init tasks that can be parallelized

• 创建线程池
  507 SystemServerInitThreadPool.get();

二、启动服务

 512         // Start services.
 513         try {
 514             traceBeginAndSlog("StartServices");
 515             startBootstrapServices();
 516             startCoreServices();
 517             startOtherServices();
 518             SystemServerInitThreadPool.shutdown();
 519         } catch (Throwable ex) {
 520             Slog.e("System", "******************************************");
 521             Slog.e("System", "************ Failure starting system services", ex);
 522             throw ex;
 523         } finally {
 524             traceEnd();
 525         }

1.服务的多种创建启动方式

方式一：通过SystemServiceManager.java的startService方法

大多数的系统服务都是通过此方法启动的

SystemServiceManager.java源码位置：frameworks/base/services/core/java/com/android/server/SystemServiceManager.java

startService有3个重载方法，最终调用方法三，所有的服务都是SystemService的子类。

//方法一
public SystemService startService(String className)
//方法二
public <T extends SystemService> T startService(Class<T> serviceClass)

//方法三
124     public void startService(@NonNull final SystemService service) {
125         // Register it.
            //mServices是一个ArrayList
126         mServices.add(service);
127         // Start it.
128         long time = SystemClock.elapsedRealtime();
129         try {
130             service.onStart();
131         } catch (RuntimeException ex) {
132             throw new RuntimeException("Failed to start service " + service.getClass().getName()
133                     + ": onStart threw an exception", ex);
134         }
135         warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onStart");
136     }

方式二：常规new服务类

• 静态main方法
  PackageManagerService、OtaDexoptService、WindowManagerService
• 静态create方法
  NetworkManagementService、IpSecService、NsdService、NetworkStatsService
• 直接New

2.缓存服务

• LocalServices.addService
  将服务缓存起来，供system_server进程中的其它服务调用。

SystemServer会调用此方法，其它的服务的构造方法一般都会调用此方法。

• ServiceManager.addService
  ServiceManager.java类见http://xinyiworld.top/wordpress_it/?p=14112

将服务的Binder缓存起来，供其它进程（一般是应用进程）跨进程调用。
绝大多数的服务都是在SystemServer类中通过ServiceManager.addService注册，也有部分部分是在本类中调用ServiceManager.addService注册的，如ActivityManagerService与PackageManagerService

3.开机启动执行各个阶段的回调

138     /**
139      * Starts the specified boot phase for all system services that have been started up to
140      * this point.
141      *
142      * @param phase The boot phase to start.
143      */
144     public void startBootPhase(final int phase) {
145         if (phase <= mCurrentPhase) {
146             throw new IllegalArgumentException("Next phase must be larger than previous");
147         }
148         mCurrentPhase = phase;
149 
150         Slog.i(TAG, "Starting phase " + mCurrentPhase);
151         try {
152             Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "OnBootPhase " + phase);
153             final int serviceLen = mServices.size();
154             for (int i = 0; i < serviceLen; i++) {
155                 final SystemService service = mServices.get(i);
156                 long time = SystemClock.elapsedRealtime();
157                 Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, service.getClass().getName());
158                 try {
159                     service.onBootPhase(mCurrentPhase);
160                 } catch (Exception ex) {
161                     throw new RuntimeException("Failed to boot service "
162                             + service.getClass().getName()
163                             + ": onBootPhase threw an exception during phase "
164                             + mCurrentPhase, ex);
165                 }
166                 warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onBootPhase");
167                 Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
168             }
169         } finally {
170             Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
171         }
172     }

在SystemServiceManager.java的startService方法中mServices会将各个SystemService缓存起来，此处再遍历执行各个SystemService的onBootPhase方法。

3.具体源码分析

3-1)startBootstrapServices

• 启动WatchDog子线程，监听系统服务。

• Reading configuration
  SystemServerInitThreadPool.get().submit(SystemConfig::getInstance, TAG_SYSTEM_CONFIG);
  这个代码按道理说会报错，SystemConfig都没有实现runnable接口，怎么能加入线程池执行？