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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