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
作者： Leo_wl
出处： http://www.cnblogs.com/Leo_wl/
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