浅谈Android系统启动过程
转载请标明出处：http://blog.csdn.net/EdisonChang/article/details/50217225
本篇文章主要介绍Android启动过程，结合系统源码并参考了《深入理解Android内核设计思想》相关章节，在此对图书作者致敬。
Android是基于Linux内核的简易操作系统，init是Android中第一个启动的进程，init通过init.rc配置的命令陆续启动其他系统服务进程。与我们关系最为密切的，莫过于ServiceManager，Zygote和SystemServer。
ServiceManager 服务管理器
service servicemanager /system/bin/servicemanager
class core
user system
group system
critical
onrestart restart zygote
onrestart restart media
onrestart restart surfaceflinger
onrestart restart drm
ServiceManager是Linux程序，负责管理Android中各项服务，ServiceManager所属class是core，拥有core属性的进程会被同时启动和停止。cirtical选项说明它是系统的关键进程，如果进行在四分钟内异常退出超过四次，设备会重启并进入还原模式。
int ServiceManager::start(const char *name) int ServiceManager::stop(const char *name) bool ServiceManager::isRunning(const char *name) Zygote孵化进程
service zygote /system/bin/app_process -Xzygote /system/bin –zygote
–start-system-server
class main
socket zygote stream 666
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
onrestart restart media
onrestart restart netd
与ServiceManager类似，Zygote也由init解析脚本启动，Zygote所属的class为mian ,并不是core,它所在程序为app_processs。而且init.rc中在启动时指定了–zygote 和 –start–system-server 命令行参数，
int main(int argc, const char* const argv[]){ //************ while (i < argc) { const char* arg = argv[i++]; if (!parentDir) { parentDir = arg; } else if (strcmp(arg, "--zygote") == 0) { zygote = true; niceName = "zygote"; } else if (strcmp(arg, "--start-system-server") == 0) { startSystemServer = true; } else if (strcmp(arg, "--application") == 0) { application = true; } else if (strncmp(arg, "--nice-name=", 12) == 0) { niceName = arg + 12; } else { className = arg; break; } } 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) { // Remainder of args get passed to startup class main() runtime.mClassName = className; runtime.mArgC = argc - i; runtime.mArgV = argv + i; runtime.start("com.android.internal.os.RuntimeInit", application ? "application" : "tool"); } else { fprintf(stderr, "Error: no class name or --zygote supplied.\n"); app_usage(); LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied."); return 10; }}通过frameworks/base/cmds/app_process/app_main.cpp中app_processs的main方法可以看出这个时候就会调用runtime.start方法启动ZygoteInit预装载各种系统类和SystemServer。
Android 的系统服务——SystemServer
SystemServer提供了众多由java编写的各种系统服务，是Android进入Launcher前的最后准备。如果在init.rc中指定启动参数-start-system-server,那么ZygoteInit就会调用startSystemServer方法，ZygoteInit通过forkSystemServer来生成一个新的进程，用于承载各项系统服务。
/** * 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=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,3001,3002,3003,3006,3007", "--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); 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; }根据fork的特性，子进程和父进程将获得同样的代码环境，其中子进程的pid为0，如上系统源码，这时候就会调用handleSystemServerProcess方法处理各项系统服务 。
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 */ }WrapperInit.execApplication处理方法如下，通过–application 命令行启动app_process，调用com.android.internal.os.RuntimeInit 进行相应的初始化操作，
然后调用nativeFinishInit方法回调RuntimeInit 初始化结束状态。
public static void execApplication(String invokeWith, String niceName, int targetSdkVersion, FileDescriptor pipeFd, String[] args) { StringBuilder command = new StringBuilder(invokeWith); command.append(" /system/bin/app_process /system/bin --application"); if (niceName != null) { command.append(" '--nice-name=").append(niceName).append("'"); } command.append(" com.android.internal.os.WrapperInit "); command.append(pipeFd != null ? pipeFd.getInt$() : 0); command.append(' '); command.append(targetSdkVersion); Zygote.appendQuotedShellArgs(command, args); Zygote.execShell(command.toString()); } public static final void main(String[] argv) { if (argv.length == 2 && argv[1].equals("application")) { if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application"); redirectLogStreams(); } else { if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting tool"); } commonInit(); /* * Now that we're running in interpreted code, call back into native code * to run the system. */ nativeFinishInit(); if (DEBUG) Slog.d(TAG, "Leaving RuntimeInit!"); }欢迎补充和指正，多谢！
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