在进入SystemServer的main函数前，Zygote先会启动Binder线程池，用于与其他进程的通信。

public static final void zygoteInit(int targetSdkVersion, String[] argv,
            ClassLoader classLoader) throws Zygote.MethodAndArgsCaller {
        RuntimeInit.redirectLogStreams();
        RuntimeInit.commonInit();
     //调用native层代码，启动Binder线程池
        ZygoteInit.nativeZygoteInit();
        RuntimeInit.applicationInit(targetSdkVersion, argv, classLoader);
    }

RuntimeInit.applicationInit内部主要调用invokeStaticMain，该方法内部通过反射得到SystemerServer类，再反射调用其main方法。

private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader)
            throws Zygote.MethodAndArgsCaller {
        Class<?> cl;

        try {
            cl = Class.forName(className, true, classLoader);
        } catch (ClassNotFoundException ex) {
            throw new RuntimeException(
                    "Missing class when invoking static main " + className,
                    ex);
        }
        Method m;
        try {
            m = cl.getMethod("main", new Class[] { String[].class });
        } catch (NoSuchMethodException ex) {
            throw new RuntimeException(
                    "Missing static main on " + className, ex);
        } catch (SecurityException ex) {
            throw new RuntimeException(
                    "Problem getting static main on " + className, ex);
        }
        int modifiers = m.getModifiers();
        if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
            throw new RuntimeException(
                    "Main method is not public and static on " + className);
        }
     //若抛出异常，则会调用 Zygote.MethodAndArgsCaller，这个方法最终也是通 
  过反射调用
     //SystemServer的main(),这里就不贴代码了，自己翻一下看看
        throw new Zygote.MethodAndArgsCaller(m, argv);
    }

再看SystemServer的man函数，主要调用了run()方法。

public static void main(String[] args) {
        new SystemServer().run();
    }

private void run() {
        try {
   
            VMRuntime.getRuntime().clearGrowthLimit();
            BaseBundle.setShouldDefuse(true);
            BinderInternal.disableBackgroundScheduling(true);
            BinerInternal.setMaxThreads(sMaxBinderThreads);
            // Prepare the main looper thread (this thread).
            android.os.Process.setThreadPriority(
                android.os.Process.THREAD_PRIORITY_FOREGROUND);
            android.os.Process.setCanSelfBackground(false);
              //创建消息Looper
            Looper.prepareMainLooper();
            // 加载native动态库
            System.loadLibrary("android_servers");
            performPendingShutdown();
            // 创建系统的上线
            createSystemContext();
            // 创建SystemServiceManager，用于对服务进行创建、启动和生命周期的管理
            mSystemServiceManager = new SystemServiceManager(mSystemContext);
            LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
        }

        // Start services.
        try {
            Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartServices");
         //启动系统的引导服务
            startBootstrapServices();
         //启动系统的核心服务
            startCoreServices();
            //启动其他服务
            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);
        }
        Looper.loop();
        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

简单列举几个三类服务

小结

1.启动Binder线程池，用于进程间通讯
2.创建SystemServerManager用于对各类服务的管理
3.启动三类系统服务

系统服务也已经启动，下面来看看应用启动过程Android应用程序启动分析