SystemServer进程

前言

在systemServer启动文章中我们讲了在SystemServer.java的main方法里面调用new SystemServer().run（）方法启动System_server进程。那么我们接着看一下SystemServer.java具体做了哪些事情？
##SystemServer的run方法介绍

frameworks\base\services\java\com\android\server/SystemServer.java
   public SystemServer() {
        // Check for factory test mode.
		//获取工厂测试模式
        mFactoryTestMode = FactoryTest.getMode();

        // Record process start information.
        // Note SYSPROP_START_COUNT will increment by *2* on a FDE device when it fully boots;
        // one for the password screen, second for the actual boot.
        //记录进程的开始信息
        mStartCount = SystemProperties.getInt(SYSPROP_START_COUNT, 0) + 1;
        mRuntimeStartElapsedTime = SystemClock.elapsedRealtime();
        mRuntimeStartUptime = SystemClock.uptimeMillis();
        Process.setStartTimes(mRuntimeStartElapsedTime, mRuntimeStartUptime);

        // Remember if it's runtime restart(when sys.boot_completed is already set) or reboot
        // We don't use "mStartCount > 1" here because it'll be wrong on a FDE device.
        // TODO: mRuntimeRestart will *not* be set to true if the proccess crashes before
        // sys.boot_completed is set. Fix it.
        mRuntimeRestart = "1".equals(SystemProperties.get("sys.boot_completed"));
    }

这里可以看到在调用run方法之前，先会调用SystemServer的构造函数创建SystemServer对象，在构造函数中只是获取了工厂测试模式，记录了进程开始的时间的操作，接着看run方法

    private void run() {
        TimingsTraceAndSlog t = new TimingsTraceAndSlog();
        try {
            t.traceBegin("InitBeforeStartServices");

            // Record the process start information in sys props.
            //在system prop文件中记录进程启动信息
            SystemProperties.set(SYSPROP_START_COUNT, String.valueOf(mStartCount));
            SystemProperties.set(SYSPROP_START_ELAPSED, String.valueOf(mRuntimeStartElapsedTime));
            SystemProperties.set(SYSPROP_START_UPTIME, String.valueOf(mRuntimeStartUptime));

            EventLog.writeEvent(EventLogTags.SYSTEM_SERVER_START,
                    mStartCount, mRuntimeStartUptime, mRuntimeStartElapsedTime);

            //
            // Default the timezone property to GMT if not set.
            //
            //如果时间不正确调整系统时间
            String timezoneProperty = SystemProperties.get("persist.sys.timezone");
            if (!isValidTimeZoneId(timezoneProperty)) {
                Slog.w(TAG, "persist.sys.timezone is not valid (" + timezoneProperty
                        + "); setting to GMT.");
                SystemProperties.set("persist.sys.timezone", "GMT");
            }

            // 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
            //设置系统语言
            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", "");
            }

            // The system server should never make non-oneway calls
            Binder.setWarnOnBlocking(true);
            // The system server should always load safe labels
            PackageItemInfo.forceSafeLabels();

            // Default to FULL within the system server.
            SQLiteGlobal.sDefaultSyncMode = SQLiteGlobal.SYNC_MODE_FULL;

            // Deactivate SQLiteCompatibilityWalFlags until settings provider is initialized
            SQLiteCompatibilityWalFlags.init(null);

            // Here we go!
            Slog.i(TAG, "Entered the Android system server!");
            final long uptimeMillis = SystemClock.elapsedRealtime();
            EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, uptimeMillis);
            if (!mRuntimeRestart) {
                FrameworkStatsLog.write(FrameworkStatsLog.BOOT_TIME_EVENT_ELAPSED_TIME_REPORTED,
                        FrameworkStatsLog
                                .BOOT_TIME_EVENT_ELAPSED_TIME__EVENT__SYSTEM_SERVER_INIT_START,
                        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
            //设置属性persist.sys.dalvik.vm.lib.2的值为当前虚拟机的运行库路径
            
            SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());

            // Mmmmmm... more memory!
            VMRuntime.getRuntime().clearGrowthLimit();

            // 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);

            // Within the system server, any incoming Bundles should be defused
            // to avoid throwing BadParcelableException.
            BaseBundle.setShouldDefuse(true);

            // Within the system server, when parceling exceptions, include the stack trace
            Parcel.setStackTraceParceling(true);

            // Ensure binder calls into the system always run at foreground priority.
            //确保对系统的绑定调用始终以前台优先级运行。
            BinderInternal.disableBackgroundScheduling(true);

            // Increase the number of binder threads in system_server
            //设置binder 的最大线程数量 是31个
            BinderInternal.setMaxThreads(sMaxBinderThreads);

            // Prepare the main looper thread (this thread).
            //准备主线程looper
            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);

            SystemServiceRegistry.sEnableServiceNotFoundWtf = true;

            // Initialize native services.
            //装载android_servers.so库，初始化native层service
            System.loadLibrary("android_servers");

            // Allow heap / perf profiling.
            initZygoteChildHeapProfiling();

            // Debug builds - spawn a thread to monitor for fd leaks.
            if (Build.IS_DEBUGGABLE) {
                spawnFdLeakCheckThread();
            }

            // Check whether we failed to shut down last time we tried.
            // This call may not return.
            performPendingShutdown();

            // Initialize the system context.
            //初始化SystemContext
            createSystemContext();

            // Call per-process mainline module initialization.
            //按照流程调用主线程模块初始化
            ActivityThread.initializeMainlineModules();

            // Sets the dumper service
            ServiceManager.addService("system_server_dumper", mDumper);
            mDumper.addDumpable(this);

            // Create the system service manager.
            //创建SystemServiceManager对象
            mSystemServiceManager = new SystemServiceManager(mSystemContext);
            mSystemServiceManager.setStartInfo(mRuntimeRestart,
                    mRuntimeStartElapsedTime, mRuntimeStartUptime);
            mDumper.addDumpable(mSystemServiceManager);

            LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
            // Prepare the thread pool for init tasks that can be parallelized
            SystemServerInitThreadPool tp = SystemServerInitThreadPool.start();
            mDumper.addDumpable(tp);

            // Load preinstalled system fonts for system server, so that WindowManagerService, etc
            // can start using Typeface. Note that fonts are required not only for text rendering,
            // but also for some text operations (e.g. TextUtils.makeSafeForPresentation()).
            if (Typeface.ENABLE_LAZY_TYPEFACE_INITIALIZATION) {
                Typeface.loadPreinstalledSystemFontMap();
            }

            // Attach JVMTI agent if this is a debuggable build and the system property is set.
            if (Build.IS_DEBUGGABLE) {
                // Property is of the form "library_path=parameters".
                String jvmtiAgent = SystemProperties.get("persist.sys.dalvik.jvmtiagent");
                if (!jvmtiAgent.isEmpty()) {
                    int equalIndex = jvmtiAgent.indexOf('=');
                    String libraryPath = jvmtiAgent.substring(0, equalIndex);
                    String parameterList =
                            jvmtiAgent.substring(equalIndex + 1, jvmtiAgent.length());
                    // Attach the agent.
                    try {
                        Debug.attachJvmtiAgent(libraryPath, parameterList, null);
                    } catch (Exception e) {
                        Slog.e("System", "*************************************************");
                        Slog.e("System", "********** Failed to load jvmti plugin: " + jvmtiAgent);
                    }
                }
            }
        } finally {
            t.traceEnd();  // InitBeforeStartServices
        }

        // Setup the default WTF handler
        RuntimeInit.setDefaultApplicationWtfHandler(SystemServer::handleEarlySystemWtf);

        // Start services.
        //调用startBootstrapServices(),startCoreService(),startOtherService()启动java所有的服务
            t.traceBegin("StartServices");
            startBootstrapServices(t);
            startCoreServices(t);
            startOtherServices(t);
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        } finally {
            t.traceEnd(); // StartServices
        }

        StrictMode.initVmDefaults(null);

        if (!mRuntimeRestart && !isFirstBootOrUpgrade()) {
            final long uptimeMillis = SystemClock.elapsedRealtime();
            FrameworkStatsLog.write(FrameworkStatsLog.BOOT_TIME_EVENT_ELAPSED_TIME_REPORTED,
                    FrameworkStatsLog.BOOT_TIME_EVENT_ELAPSED_TIME__EVENT__SYSTEM_SERVER_READY,
                    uptimeMillis);
            final long maxUptimeMillis = 60 * 1000;
            if (uptimeMillis > maxUptimeMillis) {
                Slog.wtf(SYSTEM_SERVER_TIMING_TAG,
                        "SystemServer init took too long. uptimeMillis=" + uptimeMillis);
            }
        }

		//进入looper循环
        // Loop forever.
        Looper.loop();
        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

通过上面代码可知run方法的主要任务是：
1、通过SystemProperties设置各种属性
2、准备主Looper线程
3、加载native service so （System.loadLibrary(“android_servers”);
4、初始化mSystemContext
5、初始化mSystemServiceManager
6、startBootstrapServer()开启引导服务
7、startCoreService() 开启核心服务
8、startOtherService() 开启其他服务，在这个方法里面还开起了SystemUI
9,Looper.looper()开始消息循环

SystemServiceManager.startService和ServiceManager.addservice的区别

（1）由mSystemServiceManager.startService()启动的service必须是SystemService的子类，可以直接extends SystemService，也可以通过静态内部类Lifecycle extends SystemServicce。它的时序图如下图，最终也会走到ServiceManager的addService()方法

(2)由SystemManager.addService()添加的service只要是IBinder即可，不需要是SystemService的子类。