Android——启动过程详解

                                                                                          Android从Kernel启动有4个步骤（以android4.2为例）

(1) init进程启动

(2) Native服务启动

(3) System Server，Android服务启动

(4) Home启动

总体启动框架图如：

 

第一步：initial进程(system/core/init)

    init进程，它是一个由内核启动的用户级进程。内核自行启动（已经被载入内存，开始运行，并已初始化所有的设备驱动程序和数据结构等）之后，就通过启动一个用户级程序init的方式，完成引导进程。init始终是第一个进程.

Init进程一起来就根据init.rc脚本文件建立了几个基本的服务：

•  servicemanamger
•  zygote

最后Init并不退出，而是担当起property service的功能。

1.1进程启动

/system/core/Init中的init.c入口：

int main(int argc, char **argv)
{
...
...
/* clear the umask */
umask(0);

/* Get the basic filesystem setup we need put
 * together in the initramdisk on / and then we'll
 * let the rc file figure out the rest.
 */
        /*创建基本的文件系统*/
mkdir("/dev", 0755);
mkdir("/proc", 0755);
mkdir("/sys", 0755);
...
INFO("reading config file\n");
/*解析init.rc文件*/
init_parse_config_file("/init.rc");
...
/* execute all the boot actions to get us started */
/*触发需要执行的action*/
action_for_each_trigger("init", action_add_queue_tail);
...

for(;;)
{
int nr, i, timeout = -1;
                        /*执行当前action的一个command*/
execute_one_command();
...
/*loop 处理来自property, signal的event*/
for (i = 0; i < fd_count; i++)
{
if (ufds[i].revents == POLLIN)
{
if (ufds[i].fd == get_property_set_fd())
handle_property_set_fd();
else if (ufds[i].fd == get_keychord_fd())
handle_keychord();
else if (ufds[i].fd == get_signal_fd())
handle_signal();
}
}
}

}

Init.rc是Android自己规定的初始化脚本(Android Init Language, system/core/init/readme.txt)

该脚本包含四个类型的声明：

• Actions
• Commands
• Services
• Options.

1.2 解析init.rc中的service

system/core/init/下的init_parser.c中的 init_parse_config_file("/init.rc")——> parse_config(fn, data)——>parse_new_section(&state, kw, nargs, args)——>parse_service(state, nargs, args)——>  list_add_tail(&service_list, &svc->slist);

添加service到service_list

init_parser.c解析：

static list_declare(service_list);
static list_declare(action_list);
static list_declare(action_queue);

1.3 启动native service

execute_one_command()：从action_queue链表上移除头结点(action)

class_start default对应的入口函数，主要用于启动native service

system/core/init/ builtins.c中的：

int do_class_start(int nargs, char **args)
{
        /* Starting a class does not start services
         * which are explicitly disabled.  They must
         * be started individually.
         */
    service_for_each_class(args[1], service_start_if_not_disabled);
    return 0;
}

init_parser.c中的 service_for_each_class(...)：遍历service_list链表上的所有结点

static void service_start_if_not_disabled(struct service *svc)
{
    if (!(svc->flags & SVC_DISABLED)) {
        service_start(svc, NULL);
    }
}

如果不是disabled就启动service。

init.c中的service_start(...)调用fork()创建进程，调用execve(...)调用执行新的service。

system/core/init/property_service.c中的handle_property_set_fd处理系统属性服务请求，如：service, wlan和dhcp.

property_service服务可以参考Android——SystemProperties的应用

system/core/init/keycords.c中的handle_keychord处理注册在service structure上的keychord，通常是启动service.

system/core/init/signal_handler.c中的handle_signal处理SIGCHLD signal(僵尸进程).

第二步 Zygote

Servicemanager和zygote进程就奠定了Android的基础。Zygote这个进程起来才会建立起真正的Android运行空间，初始化建立的Service都是Navtive service.在.rc脚本文件中zygote的描述：

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server

所以Zygote从frameworks/base/cmds/app_main.cpp的main(…)开始。

其中:

 AppRuntime runtime;

初始化运行时间.

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

runtime调用start方法，但是AppRuntime没有此方法，而在其父类AndroidRuntime

class AppRuntime : public AndroidRuntime

所以调用AndroidRuntime的start方法：

void AndroidRuntime::start(const char* className, const char* options)
{
    ALOGD("\n>>>>>> AndroidRuntime START %s <<<<<<\n",
            className != NULL ? className : "(unknown)");


    blockSigpipe();
...
  /* start the virtual machine */
    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;
    }
...
     env->CallStaticVoidMethod(startClass, startMeth, strArray);
...
}

调用startVM(...)新建VM：

int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv)
{
    int result = -1;
    JavaVMInitArgs initArgs;
...
 /*
     * Initialize the VM.
     *
     * The JavaVM* is essentially per-process, and the JNIEnv* is per-thread.
     * If this call succeeds, the VM is ready, and we can start issuing
     * JNI calls.
     */
    if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {
        ALOGE("JNI_CreateJavaVM failed\n");
        goto bail;
    }
...
}

其中调用到的是JNI_CreateJavaVM(...)创建VM.

onVmCreated(env)为空函数，没用！

startReg()函数用于注册JNI接口：

/*
 * Register android native functions with the VM.
 */
/*static*/ int AndroidRuntime::startReg(JNIEnv* env)
{
    /*
     * This hook causes all future threads created in this process to be
     * attached to the JavaVM.  (This needs to go away in favor of JNI
     * Attach calls.)
     */
    androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);

    ALOGV("--- registering native functions ---\n");

    /*
     * Every "register" function calls one or more things that return
     * a local reference (e.g. FindClass).  Because we haven't really
     * started the VM yet, they're all getting stored in the base frame
     * and never released.  Use Push/Pop to manage the storage.
     */
    env->PushLocalFrame(200);

    if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
        env->PopLocalFrame(NULL);
        return -1;
    }
    env->PopLocalFrame(NULL);

    //createJavaThread("fubar", quickTest, (void*) "hello");

    return 0;/fameworks/base/core/java/com/android/internal/os/ZygoteInit.java
}

AndroidRuntime的start()方法最后调用

env->CallStaticVoidMethod(startClass, startMeth, strArray)

调用/fameworks/base/core/java/com/android/internal/os/ZygoteInit.java中的main(...)函数.

    public static void main(String argv[]) {
        try {
            // Start profiling the zygote initialization.
            SamplingProfilerIntegration.start();


            registerZygoteSocket();
...
          preload();
...
          if (argv[1].equals("start-system-server")) {
startSystemServer();
} else if (!argv[1].equals("")) {
throw new RuntimeException(argv[0] + USAGE_STRING);
}
...
}

registerZygoteSocket();//来注册Socket的Listen端口，用来接受请求

  /**
     * Registers a server socket for zygote command connections
     *
     * @throws RuntimeException when open fails
     */
    private static void registerZygoteSocket() {
        if (sServerSocket == null) {
            int fileDesc;
            try {
                String env = System.getenv(ANDROID_SOCKET_ENV);
                fileDesc = Integer.parseInt(env);
            } catch (RuntimeException ex) {
                throw new RuntimeException(
                        ANDROID_SOCKET_ENV + " unset or invalid", ex);
            }

            try {
                sServerSocket = new LocalServerSocket(
                        createFileDescriptor(fileDesc));
            } catch (IOException ex) {
                throw new RuntimeException(
                        "Error binding to local socket '" + fileDesc + "'", ex);
            }
        }
    }

   preload() 主要进行预加载类和资源，以加快启动速度。preload的class列表保存在/frameworks/base/preloaded-classes文件中 。

static void preload() {
        preloadClasses();
        preloadResources();
    }

startSystemServer() ，fork进程：

   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);
        }

经过这几个步骤，Zygote就建立好了，利用Socket通讯，接收ActivityManangerService的请求

第三步 System Server

Zygote上fork的systemserver进程入口在/frameworks/base/services/java/com/android/server/SystemServer.java的main(...)。Android的所有服务循环框架都是建立SystemServer上。在SystemServer.java中看不到循环结构。

 public static void main(String[] args) {
    ...
     ...

        System.loadLibrary("android_servers");
        init1(args);
    }

加载一个叫android_servers的本地库，他提供本 地方法的接口（源程序在framework/base/services/jni/目录中）。然后调用本地方法设置服务。具体执行设置的代码在 frameworks/base/cmds/system_server/library/system_init.cpp中。

Init1()是在Native空间实现的（com_andoird_server_SystemServer.cpp）

JNI如下：

static JNINativeMethod gMethods[] = {
    /* name, signature, funcPtr */
    { "init1", "([Ljava/lang/String;)V", (void*) android_server_SystemServer_init1 },
};

static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz)
{
    system_init();
}

system_init.cpp中的system_init()实现如下：

extern "C" status_t system_init()
{
...
property_get("system_init.startsurfaceflinger", propBuf, "1");
if (strcmp(propBuf, "1") == 0) {
// Start the SurfaceFlinger
SurfaceFlinger::instantiate();
}

property_get("system_init.startsensorservice", propBuf, "1");
if (strcmp(propBuf, "1") == 0) {
// Start the sensor service
SensorService::instantiate();
}
...
ALOGI("System server: starting Android services.\n");
JNIEnv* env = runtime->getJNIEnv();
if (env == NULL) {
return UNKNOWN_ERROR;
}
jclass clazz = env->FindClass("com/android/server/SystemServer");
if (clazz == NULL) {
return UNKNOWN_ERROR;
}
jmethodID methodId = env->GetStaticMethodID(clazz, "init2", "()V");
if (methodId == NULL) {
return UNKNOWN_ERROR;
}
env->CallStaticVoidMethod(clazz, methodId);


ALOGI("System server: entering thread pool.\n");
ProcessState::self()->startThreadPool();
IPCThreadState::self()->joinThreadPool();
ALOGI("System server: exiting thread pool.\n");


return NO_ERROR;
}

可以看到等初始化传感器，视频，音频等服务后通过env->CallStaticVoidMethod(clazz, methodId)回调到了SystemServer的init2().

SystemServer.java有这么一段话：

 /**
     * This method is called from Zygote to initialize the system. This will cause the native
     * services (SurfaceFlinger, AudioFlinger, etc..) to be started. After that it will call back
     * up into init2() to start the Android services.
     */
    native public static void init1(String[] args);

init2会在init1的实现中回调到！

system_init()开启了线程池，join_threadpool() 将当前线程挂起，等待binder的请求,启动了循环状态。

SystemServer.java中init2()建立了Android中主要的系统服务(WindowManagerServer（Wms）、ActivityManagerSystemService（AmS）、PackageManagerServer（PmS)......).

public static final void init2() {
        Slog.i(TAG, "Entered the Android system server!");
        Thread thr = new ServerThread();
        thr.setName("android.server.ServerThread");
        thr.start();
    }

这个init2()建立了一个线程 thr，ServerThread线程 中New Service和AddService来建立服务：

class ServerThread extends Thread {
    private static final String TAG = "SystemServer";
    private static final String ENCRYPTING_STATE = "trigger_restart_min_framework";
    private static final String ENCRYPTED_STATE = "1";


    ContentResolver mContentResolver;
...
            Slog.i(TAG, "Entropy Mixer");
            ServiceManager.addService("entropy", new EntropyMixer());


            Slog.i(TAG, "Power Manager");
            power = new PowerManagerService();
            ServiceManager.addService(Context.POWER_SERVICE, power);


            Slog.i(TAG, "Activity Manager");
            context = ActivityManagerService.main(factoryTest);
...
        // We now tell the activity manager it is okay to run third party
        // code.  It will call back into us once it has gotten to the state
        // where third party code can really run (but before it has actually
        // started launching the initial applications), for us to complete our
        // initialization.
        ActivityManagerService.self().systemReady(new Runnable() {
            public void run() {
                Slog.i(TAG, "Making services ready");


                if (!headless) startSystemUi(contextF);
                try {
                    if (mountServiceF != null) mountServiceF.systemReady();
                } catch (Throwable e) {
                    reportWtf("making Mount Service ready", e);
                }
...
}

执行完systemReady()后，会相继启动相关联服务的systemReady()函数，完成整体初始化。

第三步 Home启动

上面的ServerThread调用到ActivityManagerService的systemReady()然后回调到/frameworks/base/services/java/com/android/server/am/ActivityManagerServcie.java的systemReady(...).

public final class ActivityManagerService extends ActivityManagerNative
implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
......

public void systemReady(final Runnable goingCallback) {
......

synchronized (this) {
......

mMainStack.resumeTopActivityLocked(null);
}
}

......
}

调用/frameworks/base/services/java/com/android/server/am/ActivityStack.java中:

 final boolean resumeTopActivityLocked(ActivityRecord prev, Bundle options) {
 // Find the first activity that is not finishing.
        ActivityRecord next = topRunningActivityLocked(null);
...

        if (next == null) {

         Log.d(TAG,"jscese start Launcher");
            // There are no more activities!  Let's just start up the
            // Launcher...
            if (mMainStack) {
                ActivityOptions.abort(options);
                return mService.startHomeActivityLocked(mCurrentUser);
            }
        }
...
}

next为当前系统Activity堆栈最顶端的Activity，如果没有，next == null那么就启动home！

回调到ActivityManagerServcie.java的：

boolean startHomeActivityLocked(int userId) {
...
        Intent intent = new Intent(
            mTopAction,
            mTopData != null ? Uri.parse(mTopData) : null);
        intent.setComponent(mTopComponent);
        if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) {
            intent.addCategory(Intent.CATEGORY_HOME);
        }
        ActivityInfo aInfo =
            resolveActivityInfo(intent, STOCK_PM_FLAGS, userId);
        if (aInfo != null) {
            intent.setComponent(new ComponentName(
                    aInfo.applicationInfo.packageName, aInfo.name));

            Log.d(TAG,"jscese first packageName==   "+aInfo.applicationInfo.packageName);

            // Don't do this if the home app is currently being
            // instrumented.
            aInfo = new ActivityInfo(aInfo);
            aInfo.applicationInfo = getAppInfoForUser(aInfo.applicationInfo, userId);
            ProcessRecord app = getProcessRecordLocked(aInfo.processName,
                    aInfo.applicationInfo.uid);
            if (app == null || app.instrumentationClass == null) {
                intent.setFlags(intent.getFlags() | Intent.FLAG_ACTIVITY_NEW_TASK);
                mMainStack.startActivityLocked(null, intent, null, aInfo,
                        null, null, 0, 0, 0, 0, null, false, null);
            }
        }


        return true;
    }

首先创建一个CATEGORY_HOME类型的Intent，然后通过Intent.resolveActivityInfo函数向PackageManagerService查询Category类型为HOME的Activity!

launcher的AndroidManifest.xml文件中可见：

  <activity
            android:name="com.android.mslauncher.LauncherActivity"
 ...

            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.HOME" />
                <category android:name="android.intent.category.DEFAULT" />
                <category android:name="android.intent.category.MONKEY"/>
            </intent-filter>
        </activity>
...

跑到ActivityStack.java的：

final int startActivityLocked(IApplicationThread caller,
    Intent intent, String resolvedType,
    Uri[] grantedUriPermissions,
    int grantedMode, ActivityInfo aInfo, IBinder resultTo,

......


ActivityRecord r = new ActivityRecord(mService, this, callerApp, callingUid,
intent, resolvedType, aInfo, mService.mConfiguration,
resultRecord, resultWho, requestCode, componentSpecified);


......


  err = startActivityUncheckedLocked(r, sourceRecord, startFlags, true, options);
...
}

往后走就是启动一个activity的流程了，最终启动的是launcher的onCreate方法！

至此启动完成！

此博文图片模型来自http://blog.csdn.net/maxleng/article/details/5508372

撰写不易，转载请注明出处http://blog.csdn.net/jscese/article/details/17115395