Zygote，SystemServer启动分析
内核启动完成之后，Android的Init进程就会接管后续系统的初始化工作。
init进程启动之后，就会开始解析系统的*.rc 脚本，解析完成之后就开始启动各种service服务，然后修改权限，挂载相关目录，创建相关文件，目录。下面下简单介绍一下脚本中相关command和action的对应关系
struct {
const char *name;
int (*func)(int nargs, char **args);
unsigned char nargs;
unsigned char flags;
} keyword_info[KEYWORD_COUNT] = {
[ K_UNKNOWN ] = { "unknown", 0, 0, 0 },
#include "keywords.h"
};结构体keyword_info数组保存command，action的映射关系
int lookup_keyword(const char *s)
{
switch (*s++) {
case 'c':
if (!strcmp(s, "opy")) return K_copy;
if (!strcmp(s, "apability")) return K_capability;
if (!strcmp(s, "hdir")) return K_chdir;
if (!strcmp(s, "hroot")) return K_chroot;
if (!strcmp(s, "lass")) return K_class;
if (!strcmp(s, "lass_start")) return K_class_start;
if (!strcmp(s, "lass_stop")) return K_class_stop;
if (!strcmp(s, "lass_reset")) return K_class_reset;
if (!strcmp(s, "onsole")) return K_console;
if (!strcmp(s, "hown")) return K_chown;
if (!strcmp(s, "hmod")) return K_chmod;
if (!strcmp(s, "ritical")) return K_critical;
break;
case 'd':
if (!strcmp(s, "isabled")) return K_disabled;
if (!strcmp(s, "omainname")) return K_domainname;
break;
case 'e':
if (!strcmp(s, "xec")) return K_exec;
if (!strcmp(s, "xport")) return K_export;
break;
case 'g':
if (!strcmp(s, "roup")) return K_group;
break;
case 'h':
if (!strcmp(s, "ostname")) return K_hostname;
break;
case 'i':
if (!strcmp(s, "oprio")) return K_ioprio;
if (!strcmp(s, "fup")) return K_ifup;
if (!strcmp(s, "nsmod")) return K_insmod;
if (!strcmp(s, "mport")) return K_import;
break;
case 'k':
if (!strcmp(s, "eycodes")) return K_keycodes;
break;
case 'l':
if (!strcmp(s, "oglevel")) return K_loglevel;
if (!strcmp(s, "oad_persist_props")) return K_load_persist_props;
break;
case 'm':
if (!strcmp(s, "kdir")) return K_mkdir;
if (!strcmp(s, "ount_all")) return K_mount_all;
if (!strcmp(s, "ount")) return K_mount;
break;
case 'o':
if (!strcmp(s, "n")) return K_on;
if (!strcmp(s, "neshot")) return K_oneshot;
if (!strcmp(s, "nrestart")) return K_onrestart;
break;
case 'p':
if (!strcmp(s, "owerctl")) return K_powerctl;
case 'r':
if (!strcmp(s, "estart")) return K_restart;
if (!strcmp(s, "estorecon")) return K_restorecon;
if (!strcmp(s, "mdir")) return K_rmdir;
if (!strcmp(s, "m")) return K_rm;
break;
case 's':
if (!strcmp(s, "eclabel")) return K_seclabel;
if (!strcmp(s, "ervice")) return K_service;
if (!strcmp(s, "etcon")) return K_setcon;
if (!strcmp(s, "etenforce")) return K_setenforce;
if (!strcmp(s, "etenv")) return K_setenv;
if (!strcmp(s, "etkey")) return K_setkey;
if (!strcmp(s, "etprop")) return K_setprop;
if (!strcmp(s, "etrlimit")) return K_setrlimit;
if (!strcmp(s, "etsebool")) return K_setsebool;
if (!strcmp(s, "ocket")) return K_socket;
if (!strcmp(s, "tart")) return K_start;
if (!strcmp(s, "top")) return K_stop;
if (!strcmp(s, "wapon_all")) return K_swapon_all;
if (!strcmp(s, "ymlink")) return K_symlink;
if (!strcmp(s, "ysclktz")) return K_sysclktz;
break;
case 't':
if (!strcmp(s, "rigger")) return K_trigger;
break;
case 'u':
if (!strcmp(s, "ser")) return K_user;
break;
case 'w':
if (!strcmp(s, "rite")) return K_write;
if (!strcmp(s, "ait")) return K_wait;
break;
}
return K_UNKNOWN;
}lookup_keyword函数描述了操作名称，操作索引的关系，keywords.h则列举出了具体的操作方法
#define KEYWORD(symbol, flags, nargs, func) K_##symbol,
enum {
K_UNKNOWN,
#endif
KEYWORD(capability, OPTION, 0, 0)
KEYWORD(chdir, COMMAND, 1, do_chdir)
KEYWORD(chroot, COMMAND, 1, do_chroot)
KEYWORD(class, OPTION, 0, 0)
KEYWORD(class_start, COMMAND, 1, do_class_start)
KEYWORD(class_stop, COMMAND, 1, do_class_stop)
KEYWORD(class_reset, COMMAND, 1, do_class_reset)
KEYWORD(console, OPTION, 0, 0)
KEYWORD(critical, OPTION, 0, 0)
KEYWORD(disabled, OPTION, 0, 0)
KEYWORD(domainname, COMMAND, 1, do_domainname)
KEYWORD(exec, COMMAND, 1, do_exec)
KEYWORD(export, COMMAND, 2, do_export)
KEYWORD(group, OPTION, 0, 0)
KEYWORD(hostname, COMMAND, 1, do_hostname)
KEYWORD(ifup, COMMAND, 1, do_ifup)
KEYWORD(insmod, COMMAND, 1, do_insmod)
KEYWORD(import, SECTION, 1, 0)
KEYWORD(keycodes, OPTION, 0, 0)
KEYWORD(mkdir, COMMAND, 1, do_mkdir)
KEYWORD(mount_all, COMMAND, 1, do_mount_all)
KEYWORD(mount, COMMAND, 3, do_mount)
KEYWORD(on, SECTION, 0, 0)
KEYWORD(oneshot, OPTION, 0, 0)
KEYWORD(onrestart, OPTION, 0, 0)
KEYWORD(powerctl, COMMAND, 1, do_powerctl)
KEYWORD(restart, COMMAND, 1, do_restart)
KEYWORD(restorecon, COMMAND, 1, do_restorecon)
KEYWORD(rm, COMMAND, 1, do_rm)
KEYWORD(rmdir, COMMAND, 1, do_rmdir)
KEYWORD(seclabel, OPTION, 0, 0)
KEYWORD(service, SECTION, 0, 0)
KEYWORD(setcon, COMMAND, 1, do_setcon)
KEYWORD(setenforce, COMMAND, 1, do_setenforce)
KEYWORD(setenv, OPTION, 2, 0)
KEYWORD(setkey, COMMAND, 0, do_setkey)
KEYWORD(setprop, COMMAND, 2, do_setprop)
KEYWORD(setrlimit, COMMAND, 3, do_setrlimit)
KEYWORD(setsebool, COMMAND, 2, do_setsebool)
KEYWORD(socket, OPTION, 0, 0)
KEYWORD(start, COMMAND, 1, do_start)
KEYWORD(stop, COMMAND, 1, do_stop)
KEYWORD(swapon_all, COMMAND, 1, do_swapon_all)
KEYWORD(trigger, COMMAND, 1, do_trigger)
KEYWORD(symlink, COMMAND, 1, do_symlink)
KEYWORD(sysclktz, COMMAND, 1, do_sysclktz)
KEYWORD(user, OPTION, 0, 0)
KEYWORD(wait, COMMAND, 1, do_wait)
KEYWORD(write, COMMAND, 2, do_write)
KEYWORD(copy, COMMAND, 2, do_copy)
KEYWORD(chown, COMMAND, 2, do_chown)
KEYWORD(chmod, COMMAND, 2, do_chmod)
KEYWORD(loglevel, COMMAND, 1, do_loglevel)
KEYWORD(load_persist_props, COMMAND, 0, do_load_persist_props)
KEYWORD(ioprio, OPTION, 0, 0)
#ifdef __MAKE_KEYWORD_ENUM__
KEYWORD_COUNT,
};所有的动作都在这里定义了，chmod，mkdir，chmon，mount，service，等等，这里我们主要看service，后面的zygote启动需要用到，在rc 脚本中存在指令service zygote /system/bin/app_process -Xzygote /system/bin –zygote –start-system-server，Init进程在解析完rc 脚本之后会启动app_precess。Init进程的service_start函数会去创建新的进程，并在子进程空间启动app_process进程，但是启动之后进程名称换成zygote了，通过ps指令查看zygote进程的父进程就是Init（PID=1）。
void service_start(struct service *svc, const char *dynamic_args)
{
......
......
pid = fork();
if (pid == 0) {
......
if (needs_console) {
setsid();
open_console();
} else {
zap_stdio();
}
setpgid(0, getpid());
......
if (!dynamic_args) {
if (execve(svc->args[0], (char**) svc->args, (char**) ENV) < 0) {
ERROR("cannot execve('%s'): %s\n", svc->args[0], strerror(errno));
}
} else {
......
execve(svc->args[0], (char**) arg_ptrs, (char**) ENV);
}
_exit(127);
}
......
if (properties_inited())
notify_service_state(svc->name, "running");
}上面fork创建子进程，新进程空间execve直接启动app_process，接着app_main.c 主函数main会被调用。
int main(int argc, char* const argv[])
{
......
if (niceName && *niceName) {
setArgv0(argv0, niceName);
set_process_name(niceName);
}
runtime.mParentDir = parentDir;
if (zygote) {
runtime.start("com.android.internal.os.ZygoteInit",
startSystemServer ? "start-system-server" : "");
} else if (className) {
......
} else {
......
}
}
app_process 通过AppRuntime.start，AndroidRuntime.start 一步步启动虚拟机
void AndroidRuntime::start(const char* className, const char* options)
{
......
JNIEnv* env;
if (startVm(&mJavaVM, &env) != 0) {
return;
}
onVmCreated(env);
/* * Register android functions. */
if (startReg(env) < 0) {
ALOGE("Unable to register all android natives\n");
return;
}
......
/* * Start VM. This thread becomes the main thread of the VM, and will * not return until the VM exits. */
......
if (startClass == NULL) {
......
} else {
jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
"([Ljava/lang/String;)V");
if (startMeth == NULL) {
ALOGE("JavaVM unable to find main() in '%s'\n", className);
/* keep going */
} else {
env->CallStaticVoidMethod(startClass, startMeth, strArray);
}
}
......
}通过反射机制构造ZygoteInit类，获取静态方法main，并调用。
public static void main(String argv[]) {
try {
// Start profiling the zygote initialization.
SamplingProfilerIntegration.start();
registerZygoteSocket();
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 requested, start system server directly from Zygote
if (argv.length != 2) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
if (argv[1].equals("start-system-server")) {
startSystemServer();
} else if (!argv[1].equals("")) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
Log.i(TAG, "Accepting command socket connections");
runSelectLoop();
closeServerSocket();
} catch (MethodAndArgsCaller caller) {
caller.run();
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}ZygoteInit.main 注册Zygote通信通道，启动SystemServer，使用select监听“/dev/socket/zygote”设备随时准备（runSelectLoop）孵化其它应用进程，下面继续看SystemServer的启动
private static boolean startSystemServer()
throws MethodAndArgsCaller, RuntimeException {
......
/* Hardcoded command line to start the system server */
String args[] = {
"--setuid=1000",
"--setgid=1000",
"--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1032,3001,3002,3003,3006,3007",
"--capabilities=" + capabilities + "," + capabilities,
"--runtime-init",
"--nice-name=system_server",
"com.android.server.SystemServer",
};
ZygoteConnection.Arguments parsedArgs = null;
int pid;
try {
parsedArgs = new ZygoteConnection.Arguments(args);
ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
/* Request to fork the system server process */
pid = Zygote.forkSystemServer(
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) {
handleSystemServerProcess(parsedArgs);
}
return true;
}接着Zygote通过forkSystemServer创建SystemServer需要的进程空间，handleSystemServerProcess则在子进程空间构建SystemServer类，并call静态方法main
private static void handleSystemServerProcess(
ZygoteConnection.Arguments parsedArgs)
throws ZygoteInit.MethodAndArgsCaller {
closeServerSocket();
// set umask to 0077 so new files and directories will default to owner-only permissions.
Libcore.os.umask(S_IRWXG | S_IRWXO);
if (parsedArgs.niceName != null) {
Process.setArgV0(parsedArgs.niceName);
}
if (parsedArgs.invokeWith != null) {
WrapperInit.execApplication(parsedArgs.invokeWith,
parsedArgs.niceName, parsedArgs.targetSdkVersion,
null, parsedArgs.remainingArgs);
} else {
/* * Pass the remaining arguments to SystemServer. */
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs);
}
/* should never reach here */
}RuntimeInit继续向下走
public static final void zygoteInit(int targetSdkVersion, String[] argv)
throws ZygoteInit.MethodAndArgsCaller {
if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote");
redirectLogStreams();
commonInit();
nativeZygoteInit();
applicationInit(targetSdkVersion, argv);
}nativeZygoteInit 方法会进入AppRuntime::onZygoteInit启动线程池，而applicationInit进入invokeStaticMain完成最后的动作。
private static void invokeStaticMain(String className, String[] argv)
throws ZygoteInit.MethodAndArgsCaller {
Class<?> cl;
try {
cl = Class.forName(className);
} catch (ClassNotFoundException ex) {
throw new RuntimeException(
"Missing class when invoking static main " + className,
ex);
}
Method m;
try {
m = cl.getMethod("main", new Class[] { String[].class });
} catch (NoSuchMethodException ex) {
throw new RuntimeException(
"Missing static main on " + className, ex);
} catch (SecurityException ex) {
throw new RuntimeException(
"Problem getting static main on " + className, ex);
}
int modifiers = m.getModifiers();
if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
throw new RuntimeException(
"Main method is not public and static on " + className);
}
/* * This throw gets caught in ZygoteInit.main(), which responds * by invoking the exception's run() method. This arrangement * clears up all the stack frames that were required in setting * up the process. */
throw new ZygoteInit.MethodAndArgsCaller(m, argv);
}invokeStaticMain可以看到先将传进入来的“com.android.server.SystemServer”构建出来，cl.getMethod(“main”, new Class[] { String[].class })完成main的调用，至此SystemServer启动，后续将在进入Android framework层AMS，IMS，PMS，WMS等服务的初始化。
上述有很多进程的创建，关于进程组的设置暂未研究。