Android之初始化流程总结

Android设备上电至launcher出现的开机流程如上图所示，即主要分为三个阶段：Boot Loader，Linux Kernel和Android系统服务。默认情况下每个阶段都有各自的启动画面，启动画面的修改，是实际开发中经常遇到的需求。Boot Loader和Linux Kernel的启动，请参考Linux 启动流程图。本文主要集中在Android系统关键服务的启动解析。

1，init

init作为android中第一个被启动的进程（pid=0），它通过解析init.rc来启动android中大多数系统服务。具体语法格式请参考另一篇文章，Android启动之配置文件分析。

2，ServiceManager

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

由 Android启动之配置文件分析可知，servicemanager由init进程启动。由上图可知，ServiceManager所属class是core，其它同类的系统进程包括ueventd，console(system/bin/sh)，adbd等，这些进程会被同时启动或停止。另外，请留意其中的critical选项，说明它是关键进程。

3，Zygote

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

有上述可知：
ServiceName：zygote
Path: /system/bin/app_process
Arguments: -Xzygote /system/bin --zygote --start-system-server
Zygote所属class为main，而不是core，与其同class的系统进程有netd，debuggerd，rild等。

3.1, 我们来看下app_process的main函数

// frameworks/base/cmds/app_process/App_main.cpp

int main(int argc, char* const argv[])
{
    ......

    AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
    ......
    // --zygote : Start in zygote mode
    // --start-system-server : Start the system server.
    // --application : Start in application (stand alone, non zygote) mode.
    // --nice-name : The nice name for this process.
    ......
    bool zygote = false;
    bool startSystemServer = false;
    bool application = false;
    String8 niceName;
    String8 className;

    ++i;  // Skip unused "parent dir" argument.
    while (i < argc) {
        const char* arg = argv[i++];
        if (strcmp(arg, "--zygote") == 0) {
            zygote = true;
            niceName = ZYGOTE_NICE_NAME;
        } 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.setTo(arg + 12);
        } else if (strncmp(arg, "--", 2) != 0) {
            className.setTo(arg);
            break;
        } else {
            --i;
            break;
        }
    }
    ......

    if (startSystemServer) {
        args.add(String8("start-system-server"));
    }
    ......
    
    if (zygote) {
        runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
    } 
}

这个main函数很简单，但重要功能却是由AppRuntime完成。

3.2，AppRuntime::start()

// frameworks/base/core/jni/AndroidRuntime.cpp

/*
 * Start the Android runtime.  This involves starting the virtual machine
 * and calling the "static void main(String[] args)" method in the class
 * named by "className".
 *
 * Passes the main function two arguments, the class name and the specified
 * options string.
 */
void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
    ALOGD(">>>>>> START %s uid %d <<<<<<\n",
            className != NULL ? className : "(unknown)", getuid());

    static const String8 startSystemServer("start-system-server");

    /*
     * 'startSystemServer == true' means runtime is obsolete and not run from
     * init.rc anymore, so we print out the boot start event here.
     */
    for (size_t i = 0; i < options.size(); ++i) {
        if (options[i] == startSystemServer) {
           /* track our progress through the boot sequence */
           const int LOG_BOOT_PROGRESS_START = 3000;
           LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START,  ns2ms(systemTime(SYSTEM_TIME_MONOTONIC)));
        }
    }

    const char* rootDir = getenv("ANDROID_ROOT");
    if (rootDir == NULL) {
        rootDir = "/system";
        if (!hasDir("/system")) {
            LOG_FATAL("No root directory specified, and /android does not exist.");
            return;
        }
        setenv("ANDROID_ROOT", rootDir, 1);
    }

    //const char* kernelHack = getenv("LD_ASSUME_KERNEL");
    //ALOGD("Found LD_ASSUME_KERNEL='%s'\n", kernelHack);

    /* start the virtual machine */
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    if (startVm(&mJavaVM, &env, zygote) != 0) {//启动虚拟机
        return;
    }
    onVmCreated(env);//虚拟机启动后的回调

    /*
     * Register android functions.
     */
    if (startReg(env) < 0) {//注册JNI函数
        ALOGE("Unable to register all android natives\n");
        return;
    }

    /*
     * We want to call main() with a String array with arguments in it.
     * At present we have two arguments, the class name and an option string.
     * Create an array to hold them.
     */
    jclass stringClass;
    jobjectArray strArray;
    jstring classNameStr;

    stringClass = env->FindClass("java/lang/String");
    assert(stringClass != NULL);
    strArray = env->NewObjectArray(options.size() + 1, stringClass, NULL);
    assert(strArray != NULL);
    classNameStr = env->NewStringUTF(className);
    assert(classNameStr != NULL);
    env->SetObjectArrayElement(strArray, 0, classNameStr);

    for (size_t i = 0; i < options.size(); ++i) {
        jstring optionsStr = env->NewStringUTF(options.itemAt(i).string());
        assert(optionsStr != NULL);
        env->SetObjectArrayElement(strArray, i + 1, optionsStr);
    }

    /*
     * Start VM.  This thread becomes the main thread of the VM, and will
     * not return until the VM exits.
     */
    char* slashClassName = toSlashClassName(className);
    jclass startClass = env->FindClass(slashClassName);
    if (startClass == NULL) {
        ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
        /* keep going */
    } else {
        // 找到ZygoteInit类的static main函数
        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 {
            /*调用com.android.internal.os.ZygoteInit的main函数，进入Java 世界!*/
            env->CallStaticVoidMethod(startClass, startMeth, strArray);

#if 0
            if (env->ExceptionCheck())
                threadExitUncaughtException(env);
#endif
        }
    }
    free(slashClassName);

    ALOGD("Shutting down VM\n");
    if (mJavaVM->DetachCurrentThread() != JNI_OK)
        ALOGW("Warning: unable to detach main thread\n");
    if (mJavaVM->DestroyJavaVM() != 0)
        ALOGW("Warning: VM did not shut down cleanly\n");
}

3.3，ZygoteInit.java

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ZygoteInit::main() ->ZygoteInit::startSystemServer()->Zygote.forkSystemServer()->Zygote.nativeForkSystemServer()，最终调用fork()函数来创建SystemServer进程。Zygote.java位于frameworks/base/core/java/com/android/internal/os/Zygote.java

4，SystemServer

SystemServer::main()->SystemServer::run()

private void run() {
        ......
        // Initialize native services.
        System.loadLibrary("android_servers");
        ......
        // Create the system service manager.
        mSystemServiceManager = new SystemServiceManager(mSystemContext);
        LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

        // Start services.
        try {
            startBootstrapServices();
            startCoreServices();
            startOtherServices();
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        }
        ......
    }

至此，一些关键的本地native services 和 java services都已经被启动。其中，ActivityManagerService是导致Launcher被启动的关键。更多细节，请参考源码！

参考：

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