了解SystemServer之前，我们知道
- Android系统是基于Linux内核的，而在Linux系统中，所有的进程都是init进程的子孙进程，也就是说，所有的进程都是直接或者间接地由init进程fork出来的。Zygote进程也不例外，它是在系统启动的过程，由init进程创建的。在系统启动脚本system/core/rootdir/init.rc文件中。
- 系统启动时init进程会创建Zygote进程，Zygote进程负责后续Android应用程序框架层的其它进程的创建和启动工作。
- Zygote进程会首先创建一个SystemServer进程，SystemServer进程负责启动系统的关键服务，如AMS,PMS等。
- 下面查看一波源码，源码版本是2.3.7。
本文分析流程图
1>system/core/rootdir/init.rc
去打开一个叫做app_process的文件。
2>frameworks/base/cmds/app_process.rc
这里说下，从这一步到下一步之间这段过程都是在准备java vm以及android app运行环境，这期间的过程涉及比较多的细节，暂时TODO，我先理解从这一步到下一步则抵达了ZygoteInit.java。
3>frameworks\base\core\java\com\android\internal\os\ZygoteInit.java
我们看zygoteInit的main方法。精简之后就是这样：
public static void main(String argv[]) {
try {
......
if (argv[1].equals("true")) {
startSystemServer();
} else if (!argv[1].equals("false")) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
......
} catch (MethodAndArgsCaller caller) {
......
} catch (RuntimeException ex) {
......
}
}
startSystemServer()方法：
1)Prepare the arguments and fork for the system server process.，准备fork系统服务进程的参数。
2)Zygote.forkSystemServer(...)调用forkSystemServer方法发起一个fork请求。进入forkSystemServer方法。
/**
* Prepare the arguments and fork for the system server process.
*/
private static boolean startSystemServer()
throws MethodAndArgsCaller, RuntimeException {
/* Hardcoded command line to start the system server */
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003",
"--capabilities=130104352,130104352",
"--runtime-init",
"--nice-name=system_server",
"com.android.server.SystemServer",
};
ZygoteConnection.Arguments parsedArgs = null;
int pid;
try {
parsedArgs = new ZygoteConnection.Arguments(args);
......
if ("1".equals(SystemProperties.get("ro.debuggable")))
debugFlags |= Zygote.DEBUG_ENABLE_DEBUGGER;
/* Request to fork the system server process */
pid = Zygote.forkSystemServer(
parsedArgs.uid, parsedArgs.gid,
parsedArgs.gids, debugFlags, null,
parsedArgs.permittedCapabilities,
parsedArgs.effectiveCapabilities);
} catch (IllegalArgumentException ex) {
throw new RuntimeException(ex);
}
/* For child process */
if (pid == 0) {
handleSystemServerProcess(parsedArgs);
}
return true;
}
forkSystemServer调用到了Zygote.java的native public static int forkAndSpecialize()方法：
/**
* ......
*
* @return 0 if this is the child, pid of the child
* if this is the parent, or -1 on error.
*/
native public static int forkSystemServer(int uid, int gid,
int[] gids, int debugFlags, int[][] rlimits,
long permittedCapabilities, long effectiveCapabilities);我们去看看这个forkSystemServer方法的c++实现：
位置：dalvik\vm\native\dalvik_system_Zygote.c
static void Dalvik_dalvik_system_Zygote_forkSystemServer(
const u4* args, JValue* pResult)
{
pid_t pid;
pid = forkAndSpecializeCommon(args, true);
/* The zygote process checks whether the child process has died or not. */
if (pid > 0) {
int status;
LOGI("System server process %d has been created", pid);
gDvm.systemServerPid = 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.
*/
if (waitpid(pid, &status, WNOHANG) == pid) {
LOGE("System server process %d has died. Restarting Zygote!", pid);
kill(getpid(), SIGKILL);
}
}
RETURN_INT(pid);
}可以看到这个方法forkAndSpecializeCommon(args, true);传递了之前定义的args，forkAndSpecializeCommon这个方法代码我就不贴了，我们只需要知道这个方法内部又调用到了fork方法：
int fork(void)
{
int ret;
/* Posix mandates that the timers of a fork child process be
* disarmed, but not destroyed. To avoid a race condition, we're
* going to stop all timers now, and only re-start them in case
* of error, or in the parent process
*/
__timer_table_start_stop(1);
ret = __fork();
if (ret != 0) { /* not a child process */
__timer_table_start_stop(0);
} else {
/*
* Newly created process must update cpu accounting.
* Call cpuacct_add passing in our uid, which will take
* the current task id and add it to the uid group passed
* as a parameter.
*/
cpuacct_add(getuid());
}
return ret;
}再回顾一下我们之前的args参数：
private static boolean startSystemServer()
throws MethodAndArgsCaller, RuntimeException {
/* Hardcoded command line to start the system server */
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003",
"--capabilities=130104352,130104352",
"--runtime-init",
"--nice-name=system_server",
"com.android.server.SystemServer",
};
......
return true;
}com.android.server.SystemServer这也是我们判定当前fork的就是SystemServer进程的依据。
至此,SystemServer已经起来了。它会启动各种各样的服务，比如著名的AMS,PMS,WMS等，但是本文先不讨论。等下篇讲解Binder的时候再来看这部分代码。