Android启动过程——init.rc,Zygote,SystemServer

    /* \system\core\rootdir\init.zygote64.rc */
    service zygote /system/bin/app_process64 -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

    LOCAL_PATH:= $(call my-dir)
    include $(CLEAR_VARS)
 
    LOCAL_SRC_FILES:=         app_main.cpp
 
    LOCAL_LDFLAGS := -Wl,--version-script,art/sigchainlib/version-script.txt -Wl,--export-dynamic
 
    LOCAL_SHARED_LIBRARIES :=         libdl         libcutils         libutils         liblog         libbinder         libandroid_runtime
 
    LOCAL_WHOLE_STATIC_LIBRARIES := libsigchain
 
    LOCAL_MODULE:= app_process
    LOCAL_MULTILIB := both
    LOCAL_MODULE_STEM_32 := app_process32
    LOCAL_MODULE_STEM_64 := app_process64
    include $(BUILD_EXECUTABLE)
 
    # Create a symlink from app_process to app_process32 or 64
    # depending on the target configuration.
    include  $(BUILD_SYSTEM)/executable_prefer_symlink.mk
 
    # Build a variant of app_process binary linked with ASan runtime.
    # ARM-only at the moment.
    ifeq ($(TARGET_ARCH),arm)

/** \frameworks\base\cmds\app_process\app_main.cpp **/
int main(int argc, char* const argv[])
{
    ......
    // 注意runtime是AppRuntime对象
    AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
    /** 中间一系列代码用以解析传入参数argv  **/
 
    ++i;
    /*** 这些参数正是在脚本中传入的Arguments**/
    while (i < argc) {
        const char* arg = argv[i++];
        if (strcmp(arg, "--zygote") == 0) {
            zygote = true;
            /* 注意这里对名字进行了修改，原来的名称为app_process,这里更改为zygote
             * @value static const char ZYGOTE_NICE_NAME[] = "zygote";*/
            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;
        }
    }
 
    ......
    /** 传进来的参数 --zygote,则zygote值为true **/
    if (zygote) {
    // 下面接下来分析AppRuntime类的start方法时如何启动
        runtime.start("com.android.internal.os.ZygoteInit", args);
    } elseif (className) {
        runtime.start("com.android.internal.os.RuntimeInit", args);
    } else {
        ....
        return 10;
    }
}

 /*
 * 这里启动Android Runtime.它实现了启动虚拟机，同时通过"className"来调用
 * 该类中的main函数（"static void main(String[] args)"）;
 */
/* \frameworks\base\core\jni\AndroidRuntime.cpp */
void AndroidRuntime::start(const char* className, const Vector<String8>& options)
{
    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) {
           const int LOG_BOOT_PROGRESS_START = 3000;
           LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START,  ns2ms(systemTime(SYSTEM_TIME_MONOTONIC)));
        }
    }
 
    // 设置环境变量ANDROID_ROOT为/system
    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);
    }
 
    /*******开启虚拟机 **********/
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    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;
    }
 
    /*
     * 调用main函数
     * 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);
    }
 
    /*
     * 启动VM虚拟机，而且这个线程也会变成VM的主线程，当VM退出前该线程都不会退出
     */
    char* slashClassName = toSlashClassName(className);
    jclass startClass = env->FindClass(slashClassName);
    if (startClass == NULL) {
        ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
        /* keep going */
    } 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);
 
#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");
}

    /**
     * @path: \frameworks\base\core\java\com\android\internal\os\ZygoteInit.java
     * Prepare the arguments and fork for the system server process.
     */
    private static boolean startSystemServer(String abiList, String socketName)
            throws MethodAndArgsCaller, RuntimeException {
        ......
        /* 相关参数 */
        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",// 这里是SystemServer所在的包的完整名称
        };
        ZygoteConnection.Arguments parsedArgs = null;
 
        int pid;
 
        try {
            parsedArgs = new ZygoteConnection.Arguments(args);
            ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
            ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
 
            /* 通过此函数来生成一个新的进程，用以承载各种系统服务，
             * 该函数最终实现调用底层的fork来产生一个新的进程 */
            pid = Zygote.forkSystemServer(
                    parsedArgs.uid, parsedArgs.gid,
                    parsedArgs.gids,
                    parsedArgs.debugFlags,
                    null,
                    parsedArgs.permittedCapabilities,
                    parsedArgs.effectiveCapabilities);
        } catch (IllegalArgumentException ex) {
            thrownew RuntimeException(ex);
        }
 
        /* pid=0 表示为子进程 */
        if (pid == 0) {
            if (hasSecondZygote(abiList)) {
                waitForSecondaryZygote(socketName);
            }
 
            // 子进程即是SystemServer所在进程，通过调用该函数来实现启动各种系统服务
            handleSystemServerProcess(parsedArgs);
        }
 
        return true;
    }