Android源码学习之接着浅析SystemServer

    通过Android源码学习之浅析SystemServer脉络知道了SystemServer是怎么通过利用JNI，但相继的问题出现了：SystemServer是干嘛用的？本人从《深入理解Android 卷2》截取摘录这一问题的回答：

    SystemServer是什么？它是Android Java的两大支柱之一。另外一个支柱是专门负责孵化Java进程的Zygote。这两大支柱倒了一个，都会导致Android Java的崩溃（所有由Zygote孵化的Java进程都会被销毁，而SystemServer就是由Zygote孵化而来）。若Android Java真的崩溃了，则Linux系统中的进程init会重新启动“两大支柱”以重建Android Java。

    SystemServer和系统服务有着重要关系。Android系统中几乎所有的核心服务都在这个进程中，如ActivityManagerService、PowerManagerService和WindowManagerService等。那么，作为这些服务的大本营，SystemServer会是什么样的呢？

    其中“SystemServer会是什么样的呢？“知道了一些，但不知道SystemServer怎么就是服务的大本营了？在回去看看SystemServer.java。打开Source Insight项目，发现代码如下：

    public static final void init2() {
        Slog.i(TAG, "Entered the Android system server!");
        Thread thr = new ServerThread();
        thr.setName("android.server.ServerThread");
        thr.start();
    }

又看见ini2函数了，这个函数主要的功能是创建新的线程ServerThread，所以当它执行start时，我们应该找到这个类的override的run()函数，在同样的SystemServer.java中找到了ServerThread类的run函数，这个函数长的有点令人发指，但再仔细看发现其中有很多”重复的相似的代码“，各种***Service、null、ServiceManager.addService("***",new ***)和try{}catch(){}、以及Slog.i()等等，不愧是大本营，几乎所有的服务都在这里汇集ServiceManager.addService("***",new ***)，有人这些服务进行归类，一共六大类。我自己从这长长的run函数中截取皮毛代码，如下所示：

 LightsService lights = null;
        PowerManagerService power = null;
        BatteryService battery = null;
        AlarmManagerService alarm = null;
        NetworkManagementService networkManagement = null;
        NetworkStatsService networkStats = null;
        NetworkPolicyManagerService networkPolicy = null;
        ConnectivityService connectivity = null;
        WifiP2pService wifiP2p = null;
        WifiService wifi = null;
        IPackageManager pm = null;
        Context context = null;
        WindowManagerService wm = null;
        BluetoothService bluetooth = null;
        BluetoothA2dpService bluetoothA2dp = null;
        DockObserver dock = null;
        UsbService usb = null;
        UiModeManagerService uiMode = null;
        RecognitionManagerService recognition = null;
        ThrottleService throttle = null;
        NetworkTimeUpdateService networkTimeUpdater = null;

        // Critical services...
        try {
            Slog.i(TAG, "Entropy Service");
            ServiceManager.addService("entropy", new EntropyService());

            Slog.i(TAG, "Power Manager");
            power = new PowerManagerService();
            ServiceManager.addService(Context.POWER_SERVICE, power);

            Slog.i(TAG, "Activity Manager");
            context = ActivityManagerService.main(factoryTest);

            Slog.i(TAG, "Telephony Registry");
            ServiceManager.addService("telephony.registry", new TelephonyRegistry(context));

            AttributeCache.init(context);

            Slog.i(TAG, "Package Manager");
            // Only run "core" apps if we're encrypting the device.
            String cryptState = SystemProperties.get("vold.decrypt");
            boolean onlyCore = false;
            if (ENCRYPTING_STATE.equals(cryptState)) {
                Slog.w(TAG, "Detected encryption in progress - only parsing core apps");
                onlyCore = true;
            } else if (ENCRYPTED_STATE.equals(cryptState)) {
                Slog.w(TAG, "Device encrypted - only parsing core apps");

所以这里的最重要的一行代码就是ServiceManager.addService("***",new ***)，但自己初次分析源代码，还不知道这函数具体是怎么将各种服务添加到系统中的，所以这个ServiceManager类的分析，待到自己有能力了在做总结。高深的自己不懂，只能拿软柿子来捏一捏了，这么多**service，我选择了最简单的一个EntropyService分析（要是你读过了《深入理解Android》别拍砖啊，但求指导~~~）。
    找到该文件的137（貌似）代码---->ServiceManager.addService("entropy", new EntropyService());
    所以接着找到这个类EntropyService，类代码如下：

public EntropyService() {
        this(getSystemDir() + "/entropy.dat", "/dev/urandom");
    }

    /** Test only interface, not for public use */
    public EntropyService(String entropyFile, String randomDevice) {
        if (randomDevice == null) { throw new NullPointerException("randomDevice"); }
        if (entropyFile == null) { throw new NullPointerException("entropyFile"); }

        this.randomDevice = randomDevice;
        this.entropyFile = entropyFile;
        loadInitialEntropy();
        addDeviceSpecificEntropy();
        writeEntropy();
        scheduleEntropyWriter();
    }

     首先是调用自己的函数getSystemDir()，创建文件夹，然后返回路径名称，接着就是想在创建entropy.dat文件保存信息，最后调用另一个带两个参数的构造函数（有点废话），紧接着保存两个string参数、调用四个函数。字面的意思是初始化、添加、写入、按时间写。看第一个函数：

    private void loadInitialEntropy() {
        try {
            RandomBlock.fromFile(entropyFile).toFile(randomDevice, false);
        } catch (IOException e) {
            Slog.w(TAG, "unable to load initial entropy (first boot?)", e);
        }
    }

看似简单，它是调用了RandomBlock类的静态函数fromFile，然后再写入，意思就是从"entropy.dat"写入"/dev/urandom"中，具体是什么现在也不懂，看看RandomBlock类。

class RandomBlock {

    private static final String TAG = "RandomBlock";
    private static final boolean DEBUG = false;
    private static final int BLOCK_SIZE = 4096;
    private byte[] block = new byte[BLOCK_SIZE];

    private RandomBlock() { }

    static RandomBlock fromFile(String filename) throws IOException {
        if (DEBUG) Slog.v(TAG, "reading from file " + filename);
        InputStream stream = null;
        try {
            stream = new FileInputStream(filename);
            return fromStream(stream);
        } finally {
            close(stream);
        }
    }

    private static RandomBlock fromStream(InputStream in) throws IOException {
        RandomBlock retval = new RandomBlock();
        int total = 0;
        while(total < BLOCK_SIZE) {
            int result = in.read(retval.block, total, BLOCK_SIZE - total);
            if (result == -1) {
                throw new EOFException();
            }
            total += result;
        }
        return retval;
    }

    void toFile(String filename, boolean sync) throws IOException {
        if (DEBUG) Slog.v(TAG, "writing to file " + filename);
        RandomAccessFile out = null;
        try {
            out = new RandomAccessFile(filename, sync ? "rws" : "rw");
            toDataOut(out);
            truncateIfPossible(out);
        } finally {
            close(out);
        }
    }

    private static void truncateIfPossible(RandomAccessFile f) {
        try {
            f.setLength(BLOCK_SIZE);
        } catch (IOException e) {
            // ignore this exception.  Sometimes, the file we're trying to
            // write is a character device, such as /dev/urandom, and
            // these character devices do not support setting the length.
        }
    }

    private void toDataOut(DataOutput out) throws IOException {
        out.write(block);
    }

    private static void close(Closeable c) {
        try {
            if (c == null) {
                return;
            }
            c.close();
        } catch (IOException e) {
            Slog.w(TAG, "IOException thrown while closing Closeable", e);
        }
    }
}

这类够绝的，不是static就是private，连构造函数都private了，明白了，先是从文件entropy.dat读出数据流，保存到block字符数组中，然后写入到urandom中，这里有两个文件操作的类FileInputStream和RandomAccessFile，让我想到了《Head First Design Pattern》中有个（装饰模式？）介绍过怎么解读Java的文件操作类之间的关系，回头好好复习一下。
     第一关键函数读完了，接着第二个addDeviceSpecificEntropy函数，看代码：

 /**
     * Add additional information to the kernel entropy pool.  The
     * information isn't necessarily "random", but that's ok.  Even
     * sending non-random information to {@code /dev/urandom} is useful
     * because, while it doesn't increase the "quality" of the entropy pool,
     * it mixes more bits into the pool, which gives us a higher degree
     * of uncertainty in the generated randomness.  Like nature, writes to
     * the random device can only cause the quality of the entropy in the
     * kernel to stay the same or increase.
     *
     * <p>For maximum effect, we try to target information which varies
     * on a per-device basis, and is not easily observable to an
     * attacker.
     */
    private void addDeviceSpecificEntropy() {
        PrintWriter out = null;
        try {
            out = new PrintWriter(new FileOutputStream(randomDevice));
            out.println("Copyright (C) 2009 The Android Open Source Project");
            out.println("All Your Randomness Are Belong To Us");
            out.println(START_TIME);
            out.println(START_NANOTIME);
            out.println(SystemProperties.get("ro.serialno"));
            out.println(SystemProperties.get("ro.bootmode"));
            out.println(SystemProperties.get("ro.baseband"));
            out.println(SystemProperties.get("ro.carrier"));
            out.println(SystemProperties.get("ro.bootloader"));
            out.println(SystemProperties.get("ro.hardware"));
            out.println(SystemProperties.get("ro.revision"));
            out.println(new Object().hashCode());
            out.println(System.currentTimeMillis());
            out.println(System.nanoTime());
        } catch (IOException e) {
            Slog.w(TAG, "Unable to add device specific data to the entropy pool", e);
        } finally {
            if (out != null) {
                out.close();
            }
        }
    }

看着字面的理解就是首先将一些文本信息，如”Copyright (C) 2009 The Android Open Source Project“写入到这个urandom设备（姑且认为是urandom文件）中，接着将SystemProperties获取的东东写入，最后写入系统时间等，现在看看SystemProperties到底是什么东西了。看代码：

View Code

好嘛~~~这个又是和Native有关了，留给自己接着分析了（也给大家自己分析）~~~
    第三个函数了，writeEntropy()看代码：

    private void writeEntropy() {
        try {
            RandomBlock.fromFile(randomDevice).toFile(entropyFile, true);
        } catch (IOException e) {
            Slog.w(TAG, "unable to write entropy", e);
        }
    }

这不就是和之前的相似吗？直接将urando设备的内容读出写入到entropy.dat中。
    第四个函数了，scheduleEntropyWriter，看代码：

    private void scheduleEntropyWriter() {
        mHandler.removeMessages(ENTROPY_WHAT);
        mHandler.sendEmptyMessageDelayed(ENTROPY_WHAT, ENTROPY_WRITE_PERIOD);
    }

接着看看mHandler它是如何定义操作的：

    /**
     * Handler that periodically updates the entropy on disk.
     */
    private final Handler mHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            if (msg.what != ENTROPY_WHAT) {
                Slog.e(TAG, "Will not process invalid message");
                return;
            }
            writeEntropy();
            scheduleEntropyWriter();
        }
    };

具体意思就是向这个类每三个小时发送一个消息，当消息到达之后，该类会再次调用writeEntropy()。。。

    现在知道这个服务是怎么进展的，但具体启动这个服务干嘛用的，有知道的教教我~~~

 

 

 

 

 

 

 

标签:  android entropyService