[移动开发]Android SystemServer 进程

SystemServer 是系统的核心之一，大部分android提供的服务都运行在这个进程中，SystemServer 中运行的服务大概有八十多种。为了防止应用进程对系统造成破坏，Android应用进程没有权限直接访问设备的底层资源，只能通过 SystemServer 中的服务代理访问。通过 Binder 用户进程使用 SystemServer 中的服务并没有太多不便之处。

创建 SystemServer 进程

• 之前讲过，Init.rc 中引用了 import /init.${ro.zygote}.rc ，实际上对应zygote 服务启动参数里面包含如下内容：

service zygote /system/bin/app_process64 -Xzygote /system/bin --zygote --start-system-server

因此，在 ZygoteInit 的 main 方法中调用了如下代码创建 SystemServer

if (startSystemServer) {
                // 准备 SystemServer 参数并且 fork 出 SystemServer 进程
                Runnable r = forkSystemServer(abiList, socketName, zygoteServer);

                // {@code r == null} in the parent (zygote) process, and {@code r != null} in the
                // child (system_server) process.
                if (r != null) {
                    r.run();
                    return;
                }
            }

• forkSystemServer() 代码如下：

private static Runnable forkSystemServer(String abiList, String socketName,
            ZygoteServer zygoteServer) {
        // 代码略.....
        /* Hardcoded command line to start the system server */
        // 准备参数 ，可以看到 进程 ID 和 组ID 都为 1000
        String args[] = {
            "--setuid=1000",
            "--setgid=1000",
            "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1023,1024,1032,1065,3001,3002,3003,3006,3007,3009,3010",
            "--capabilities=" + capabilities + "," + capabilities,
            "--nice-name=system_server",
            "--runtime-args",
            "--target-sdk-version=" + VMRuntime.SDK_VERSION_CUR_DEVELOPMENT,
            "com.android.server.SystemServer",
        };
        ZygoteConnection.Arguments parsedArgs = null;

        int pid;

        try {
            parsedArgs = new ZygoteConnection.Arguments(args);
            ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
            ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);

            boolean profileSystemServer = SystemProperties.getBoolean(
                    "dalvik.vm.profilesystemserver", false);
            if (profileSystemServer) {
                parsedArgs.runtimeFlags |= Zygote.PROFILE_SYSTEM_SERVER;
            }

            /* Request to fork the system server process */
            pid = Zygote.forkSystemServer(
                    parsedArgs.uid, parsedArgs.gid,
                    parsedArgs.gids,
                    parsedArgs.runtimeFlags,
                    null,
                    parsedArgs.permittedCapabilities,
                    parsedArgs.effectiveCapabilities);
        } catch (IllegalArgumentException ex) {
            throw new RuntimeException(ex);
        }

        /* For child process */
        if (pid == 0) {
            if (hasSecondZygote(abiList)) {
                waitForSecondaryZygote(socketName);
            }

            zygoteServer.closeServerSocket();
            return handleSystemServerProcess(parsedArgs);
        }

        return null;
    }

forkSystemServer 主要做了以下几件事

• 第一件事 准备启动参数，可以看到 uid 和 gid 都为 1000；它的可执行文件为 package com.android.server.SystemServer

• 第二件事 调用 Zygote.forkSystemServer fork 出 SystemServer 进程。下面贴出了 Zygote.java 中 forkSystemServer 方法，其中也是调用了 nativeForkSystemServer() 方法。其对应的 native 函数在 com_android_internal_os_Zygote.cpp 文件中。

• forkSystemServer

    public static int forkSystemServer(int uid, int gid, int[] gids, int runtimeFlags,
            int[][] rlimits, long permittedCapabilities, long effectiveCapabilities) {
        VM_HOOKS.preFork();
        // Resets nice priority for zygote process.
        resetNicePriority();
        int pid = nativeForkSystemServer(
                uid, gid, gids, runtimeFlags, rlimits, permittedCapabilities, effectiveCapabilities);
        // Enable tracing as soon as we enter the system_server.
        if (pid == 0) {
            Trace.setTracingEnabled(true, runtimeFlags);
        }
        VM_HOOKS.postForkCommon();
        return pid;
    }

• nativeForkSystemServer
  Zygote 会检查 SystemServer 是否启动成功，如果启动失败则会重启Zygote进程

static jint com_android_internal_os_Zygote_nativeForkSystemServer(
        JNIEnv* env, jclass, uid_t uid, gid_t gid, jintArray gids,
        jint runtime_flags, jobjectArray rlimits, jlong permittedCapabilities,
        jlong effectiveCapabilities) {
        // fork 
  pid_t pid = ForkAndSpecializeCommon(env, uid, gid, gids,
                                      runtime_flags, rlimits,
                                      permittedCapabilities, effectiveCapabilities,
                                      MOUNT_EXTERNAL_DEFAULT, NULL, NULL, true, NULL,
                                      NULL, false, NULL, NULL);
  if (pid > 0) {
      // The zygote process checks whether the child process has died or not.
      ALOGI("System server process %d has been created", pid);
      gSystemServerPid = pid;
      // There is a slight window that the system server process has crashed
      // but it went unnoticed because we haven't published its pid yet. So
      // we recheck here just to make sure that all is well.
      int status;
      if (waitpid(pid, &status, WNOHANG) == pid) {
          ALOGE("System server process %d has died. Restarting Zygote!", pid);
          RuntimeAbort(env, __LINE__, "System server process has died. Restarting Zygote!");
      }

      bool low_ram_device = GetBoolProperty("ro.config.low_ram", false);
      bool per_app_memcg = GetBoolProperty("ro.config.per_app_memcg", low_ram_device);
      if (per_app_memcg) {
          // Assign system_server to the correct memory cgroup.
          // Not all devices mount /dev/memcg so check for the file first
          // to avoid unnecessarily printing errors and denials in the logs.
          if (!access("/dev/memcg/system/tasks", F_OK) &&
                !WriteStringToFile(StringPrintf("%d", pid), "/dev/memcg/system/tasks")) {
              ALOGE("couldn't write %d to /dev/memcg/system/tasks", pid);
          }
      }
  }
  return pid;
}

上面代码调用了 ForkAndSpecializeCommon() 函数来fork 进程，在这个方法中调用了 SetSignalHandlers(); 函数。
还调用了 SigChldHandler 函数处理 SIGCHLD 信号，代码如下

static void SigChldHandler(int /*signal_number*/) {

	//...
	while ((pid = waitpid(-1, &status, WNOHANG)) > 0) {
		// ...
		if (pid == gSystemServerPid) {
		// 如果 SystemServer 死亡则退出 zygote 
     	ALOGE("Exit zygote because system server (%d) has terminated", pid);
     	kill(getpid(), SIGKILL);
    }
	}
	// ...
}

继续回到 Zygote.Init 的 forkSystemServer函数

第三件事：fork() 出子进程后，调用了 handleSystemServerProcess() 来初始化 SystemServer

        /* For child process */
        if (pid == 0) {
            if (hasSecondZygote(abiList)) {
                waitForSecondaryZygote(socketName);
            }

            zygoteServer.closeServerSocket();
            return handleSystemServerProcess(parsedArgs);
        }

• handleSystemServerProcess() 如下注释操作；最后会执行到 SystemServer 的 main() 方法

    /**
     * Finish remaining work for the newly forked system server process.
     */
    private static Runnable handleSystemServerProcess(ZygoteConnection.Arguments parsedArgs) {
        // set umask to 0077 so new files and directories will default to owner-only permissions.
        // 将 SystemServer 进程umask设置成 0077 这样只有SystemServer可以访问
        Os.umask(S_IRWXG | S_IRWXO);

        if (parsedArgs.niceName != null) {
            // 修改进程名为参数传入的 niceName （system_server）
            Process.setArgV0(parsedArgs.niceName);
        }

        final String systemServerClasspath = Os.getenv("SYSTEMSERVERCLASSPATH");
        if (systemServerClasspath != null) {
            performSystemServerDexOpt(systemServerClasspath);
            // Capturing profiles is only supported for debug or eng builds since selinux normally
            // prevents it.
            boolean profileSystemServer = SystemProperties.getBoolean(
                    "dalvik.vm.profilesystemserver", false);
            if (profileSystemServer && (Build.IS_USERDEBUG || Build.IS_ENG)) {
                try {
                    prepareSystemServerProfile(systemServerClasspath);
                } catch (Exception e) {
                    Log.wtf(TAG, "Failed to set up system server profile", e);
                }
            }
        }
        // invokeWith 通常为 null
        if (parsedArgs.invokeWith != null) {
            String[] args = parsedArgs.remainingArgs;
            // If we have a non-null system server class path, we'll have to duplicate the
            // existing arguments and append the classpath to it. ART will handle the classpath
            // correctly when we exec a new process.
            if (systemServerClasspath != null) {
                String[] amendedArgs = new String[args.length + 2];
                amendedArgs[0] = "-cp";
                amendedArgs[1] = systemServerClasspath;
                System.arraycopy(args, 0, amendedArgs, 2, args.length);
                args = amendedArgs;
            }

            WrapperInit.execApplication(parsedArgs.invokeWith,
                    parsedArgs.niceName, parsedArgs.targetSdkVersion,
                    VMRuntime.getCurrentInstructionSet(), null, args);

            throw new IllegalStateException("Unexpected return from WrapperInit.execApplication");
        } else {
            ClassLoader cl = null;
            if (systemServerClasspath != null) {
                cl = createPathClassLoader(systemServerClasspath, parsedArgs.targetSdkVersion);

                Thread.currentThread().setContextClassLoader(cl);
            }

            /*
             * Pass the remaining arguments to SystemServer. 在这里面会执行 SystemServer 的 main 方法。
             */
            return ZygoteInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
        }

        /* should never reach here */
    }

SystemServer 初始化

• SystemServer 的入口如下：new 了一个 SystemServer 并执行 run 方法

    /**
     * The main entry point from zygote.
     */
    public static void main(String[] args) {
        new SystemServer().run();
    }

• run
  主要任务：
  设置 JVM 运行库路径
  调整系统时间
  设置虚拟机堆利用率
  装载 libandroid_services.so 库
  获取 system context.设置 Context
  创建并运行所有 Java 服务
  Looper.loop() 开始消息循环

private void run() {
        try {
            traceBeginAndSlog("InitBeforeStartServices");
            // 如果时间不正确调整系统时间
            if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
                Slog.w(TAG, "System clock is before 1970; setting to 1970.");
                SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
            }

            //
            // Default the timezone property to GMT if not set.
            // 默认 GMT 的时区 
            //
            String timezoneProperty =  SystemProperties.get("persist.sys.timezone");
            if (timezoneProperty == null || timezoneProperty.isEmpty()) {
                Slog.w(TAG, "Timezone not set; setting to GMT.");
                SystemProperties.set("persist.sys.timezone", "GMT");
            }

            // 设置语言
            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.setForceSafeLabels(true);

            // 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!");
            int uptimeMillis = (int) SystemClock.elapsedRealtime();
            EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, uptimeMillis);
            if (!mRuntimeRestart) {
                MetricsLogger.histogram(null, "boot_system_server_init", 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 | xq oqi'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
            // 下面设置一些虚拟机参数 运行路径
            SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());

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

            // The system server has to run all of the time, so it needs to be
            // as efficient as possible with its memory usage.
            VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);

            // 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.
            BinderInternal.disableBackgroundScheduling(true);

            // Increase the number of binder threads in system_server
            BinderInternal.setMaxThreads(sMaxBinderThreads);

            // 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);

            // Initialize native services. 装载 libandroid_servers.so 库
            System.loadLibrary("android_servers");

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

            // Initialize the system context. 
            createSystemContext();

            // Create the system service manager. 创建 SystemServiceManager
            mSystemServiceManager = new SystemServiceManager(mSystemContext);
            mSystemServiceManager.setStartInfo(mRuntimeRestart,
                    mRuntimeStartElapsedTime, mRuntimeStartUptime);
            LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
            // Prepare the thread pool for init tasks that can be parallelized
            SystemServerInitThreadPool.get();
        } finally {
            traceEnd();  // InitBeforeStartServices
        }

        // Start services. 启动所有的 Java service 
        try {
            traceBeginAndSlog("StartServices");
            startBootstrapServices();
            startCoreServices();
            startOtherServices();
            SystemServerInitThreadPool.shutdown();
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        } finally {
            traceEnd();
        }

        StrictMode.initVmDefaults(null);

        if (!mRuntimeRestart && !isFirstBootOrUpgrade()) {
            int uptimeMillis = (int) SystemClock.elapsedRealtime();
            MetricsLogger.histogram(null, "boot_system_server_ready", uptimeMillis);
            final int MAX_UPTIME_MILLIS = 60 * 1000;
            if (uptimeMillis > MAX_UPTIME_MILLIS) {
                Slog.wtf(SYSTEM_SERVER_TIMING_TAG,
                        "SystemServer init took too long. uptimeMillis=" + uptimeMillis);
            }
        }

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

• createSystemContext() ; 调用了 ActivityThread的静态方法 systemMain();得到了 ActivityThread对象，然后通过activityThread.getSystemContext 获取 Context 对象。并设置主题

    private void createSystemContext() {
        ActivityThread activityThread = ActivityThread.systemMain();
        mSystemContext = activityThread.getSystemContext();
        mSystemContext.setTheme(DEFAULT_SYSTEM_THEME);

        final Context systemUiContext = activityThread.getSystemUiContext();
        systemUiContext.setTheme(DEFAULT_SYSTEM_THEME);
    }

• ActivityThread.systemMain();
  从下面代码中可以看出，在 systemMain中创建了 ActivityThread 对象。然后使用 true 调用了 thread.attach(true, 0);

    public static ActivityThread systemMain() {
        // The system process on low-memory devices do not get to use hardware
        // accelerated drawing, since this can add too much overhead to the
        // process.
        if (!ActivityManager.isHighEndGfx()) {
            ThreadedRenderer.disable(true);
        } else {
            ThreadedRenderer.enableForegroundTrimming();
        }
        ActivityThread thread = new ActivityThread();
        thread.attach(true, 0);
        return thread;
    }

• 为什么 SystemServer 需要创建 ActivityThread 呢？

Zygote启动应用后将执行 main 方法，这里实际上 SystemServer 不仅是一个后台进程，它也是运行着组件 Service 的进程。很多系统对话框都是从 SystemServer 弹出的。因此 SystemServer 也需要上下文环境，attach方法传入 true 代表 SystemServer 中调用的。主要代码代码如下：

// 如果传入 true 走下面代码 这里就像创建普通应用一样 创建了ContextImpl 还执行了 application 的 onCreate 方法
android.ddm.DdmHandleAppName.setAppName("system_process",
                    UserHandle.myUserId());
            try {                mInstrumentation = new Instrumentation();
                mInstrumentation.basicInit(this);
                ContextImpl context = ContextImpl.createAppContext(
                        this, getSystemContext().mPackageInfo);
                mInitialApplication = context.mPackageInfo.makeApplication(true, null);
                mInitialApplication.onCreate();
            } catch (Exception e) {
                throw new RuntimeException(
                        "Unable to instantiate Application():" + e.toString(), e);
            }

那么 SystemServer 对应 的 apk 是哪个呢？

ContextImpl context = ContextImpl.createAppContext(
                        this, getSystemContext().mPackageInfo);

上面这行代码 getSystemContext() 在 mContetxt为null的情况 调用了 ContextImpl.createSystemContext

    public ContextImpl getSystemContext() {
        synchronized (this) {
            if (mSystemContext == null) {
                mSystemContext = ContextImpl.createSystemContext(this);
            }
            return mSystemContext;
        }
    }

• ContextImpl.createSystemContext

    static ContextImpl createSystemContext(ActivityThread mainThread) {
        LoadedApk packageInfo = new LoadedApk(mainThread);
        ContextImpl context = new ContextImpl(null, mainThread, packageInfo, null, null, null, 0,
                null);
        context.setResources(packageInfo.getResources());
        context.mResources.updateConfiguration(context.mResourcesManager.getConfiguration(),
                context.mResourcesManager.getDisplayMetrics());
        return context;
    }

LoadedApk 用来保存一个 apk 信息，这个构造方法传入为使用 “android” 为包名而 framework_res.apk 的包名为 android 。所以启动的为 framework_res.apk。再回述前面 attach 创建的 ContextImpl 实际上是复制 mSystemContext 对象，创建的 application 为 framework_res.apk。所以 systemMain 创建了 framework_res.apk 的上下文环境。