1.SystemServer简介
Android系统中的好多功能能够运行起来，在代码层面好多都是以服务的方式实现的。而几乎所有的服务都是在SystemServer中创建的。SystemServer作为Android系统的一个核心进程，它是在zygote进程中孕育出来的。
那么zygote进程是怎么来的呢？再次我稍作解释。
我们知道，Android系统是以Linux为内核的，在Linux系统中，所有的进程无一例外的都是由init进程直接或者间接创建的，也就是说所有的进程都是init进程的后代（这个和我们现在所讲的，中国人都是炎黄子孙一个道理，炎黄相当于是init进程，我们这些人都是其子孙进程）。那么zygote进程也不例外，它就是在系统启动过程中由init进程创建的。在系统启动脚本system/core/rootdir/init.rc文件中，我们可以看到启动Zygote进程的脚本命令：
import /init.${ro.zygote}.rc
service servicemanager /system/bin/servicemanager
class core
user system
group system
critical
onrestart restart healthd
onrestart restart zygote
onrestart restart media
onrestart restart surfaceflinger
onrestart restart drm
service surfaceflinger /system/bin/surfaceflinger
class core
user system
group graphics drmrpc
onrestart restart zygote在rootdir中还有4个和zygote相关的脚本：init.zygote32.rc、init.zygote32_64.rc、init.zygote64.rc、init.zygote64_32.rc，这个是为了区分64位和32才这么做的。
我们可以看一下init.zygote32.rc中的内容：
service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
class main
socket zygote stream 660 root system
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
onrestart restart media
onrestart restart netd那么SystemServer又是怎样创建的呢？答案是：在ZygoteInit中创建的，看下边的代码。
public class ZygoteInit {
private static final String TAG = "Zygote";
public static void main(String argv[]) {
try {
// Start profiling the zygote initialization.
SamplingProfilerIntegration.start();
boolean startSystemServer = false;
String socketName = "zygote";
String abiList = null;
for (int i = 1; i < argv.length; i++) {
if ("start-system-server".equals(argv[i])) {
startSystemServer = true;
} else if (argv[i].startsWith(ABI_LIST_ARG)) {
abiList = argv[i].substring(ABI_LIST_ARG.length());
} else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
socketName = argv[i].substring(SOCKET_NAME_ARG.length());
} else {
throw new RuntimeException("Unknown command line argument: " + argv[i]);
}
}
if (abiList == null) {
throw new RuntimeException("No ABI list supplied.");
}
registerZygoteSocket(socketName);
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
preload();
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
SystemClock.uptimeMillis());
// Finish profiling the zygote initialization.
SamplingProfilerIntegration.writeZygoteSnapshot();
// Do an initial gc to clean up after startup
gc();
// Disable tracing so that forked processes do not inherit stale tracing tags from
// Zygote.
Trace.setTracingEnabled(false);
if (startSystemServer) {
startSystemServer(abiList, socketName);/*启动SystemServer的地方*/
}
Log.i(TAG, "Accepting command socket connections");
runSelectLoop(abiList);
closeServerSocket();
} catch (MethodAndArgsCaller caller) {
caller.run();
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}我们在来看看startSystemServer方法的代码实现：
/*
* Prepare the arguments and fork for the system server process.
*/
private static boolean startSystemServer(String abiList, String socketName)
int pid;
try {
parsedArgs = new ZygoteConnection.Arguments(args);
ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
/* Request to fork the system server process */
pid = Zygote.forkSystemServer(/*这里fork了SystemServer*/
parsedArgs.uid, parsedArgs.gid,
parsedArgs.gids,
parsedArgs.debugFlags,
null,
parsedArgs.permittedCapabilities,
parsedArgs.effectiveCapabilities);
} catch (IllegalArgumentException ex) {
throw new RuntimeException(ex);
}
/* For child process */
if (pid == 0) {
if (hasSecondZygote(abiList)) {
waitForSecondaryZygote(socketName);
}
handleSystemServerProcess(parsedArgs);
}
return true;
}2.以太网服务的启动
以太网在Android中能够运行，也是作为一个系统级服务来执行的，那么自然我们能联想到它是在SystemServer中启动的。代码路径如下：
frameworks/base/services/java/com/android/server/SystemServer.java
public final class SystemServer {
private static final String TAG = "SystemServer";
.......................
private static final String ETHERNET_SERVICE_CLASS =
"com.android.server.ethernet.EthernetService";
/**
* The main entry point from zygote.
*/
public static void main(String[] args) {
new SystemServer().run();
}
private void run() {
// Create the system service manager.
mSystemServiceManager = new SystemServiceManager(mSystemContext);
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
// Start services.
/* SystemServer中启动服务的三个方法 */
try {
startBootstrapServices();
startCoreServices();
startOtherServices(); /* 这个方法里启动了以太网服务进程 */
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
}
}
}
private void startOtherServices() {
if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_ETHERNET)) {
mSystemServiceManager.startService(ETHERNET_SERVICE_CLASS);
}
}至此，Android以太网服务进程就算启动起来了
以太网服务启动后，以太网卡对应的端口又是怎样up起来的呢？又是怎样分配到地址的呢？我的另一篇博文《Android5.1系统启动过程中启动有线网卡并为其分配静态IP地址》http://blog.csdn.net/moyu123456789/article/details/50002099中有比较明确的解释。