/* \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參数argv  **/
 
    ++i;
    /*** 这些參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改动，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;
        }
    }
 
    ......
    /** 传进来的參数 --zygote,则zygote值为true **/
    if (zygote) {
    // 以下接下来分析AppRuntime类的start方法时怎样启动
        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，同一时候通过"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)));
        }
    }
 
    // 环境变量设置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，并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 {
        ......
        /* 保存System进程的相关启动參数 */
        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 {
        // 解析參数值
            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。
             *  该函数终于实现调用底层的fork来产生一个新的进程。
             *  这个子进程就是Android系统的System进程  **/
            pid = Zygote.forkSystemServer(
                    parsedArgs.uid, parsedArgs.gid,
                    parsedArgs.gids,
                    parsedArgs.debugFlags,
                    null,
                    parsedArgs.permittedCapabilities,
                    parsedArgs.effectiveCapabilities);
            // 由前面args參数值能够看出System进程的UID（用户ID）和SID（用户组ID）均设置为1000
        } catch (IllegalArgumentException ex) {
            throw new 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;
    }

/** @path \frameworks\base\core\java\com\android\internal\os\ZygoteInit.java*/
private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {
    ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
    ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();
    // 创建一个大小为4的Socket文件描写叙述符数组fdArray,表示Zygote进程最多能同一时候处理4个socket连接
    FileDescriptor[] fdArray = new FileDescriptor[4];
 
    // 前面创建的Server端的Socket。该变量是静态成员变量，将其加入到文件描写叙述符列表fds中
    fds.add(sServerSocket.getFileDescriptor());
    peers.add(null);
 
    int loopCount = GC_LOOP_COUNT;
    /** 消息循环处理   **/
    while (true) {
        int index;
        // gc相关
        if (loopCount <= 0) {
            gc();
            loopCount = GC_LOOP_COUNT;
        } else {
            loopCount--;
        }
  
        try {
        // 将fds中的socket转换到fdArray中
            fdArray = fds.toArray(fdArray);
            /** 调用selectReadable来监控fdArray中的Socket有数据可读
             * （有数据可读意味该socket接收到了一个连接或一个请求） */
            index = selectReadable(fdArray);
        } catch (IOException ex) {
            ......
        }
 
        if (index < 0) {
            ......
        } else if (index == 0) {// 说明AMS与Zygote进程建立了新的连接，但此时并未请求创建新进程
        // 这里仅仅是将连接描写叙述符加入到fds数组中
            ZygoteConnection newPeer = acceptCommandPeer(abiList);
            peers.add(newPeer);
            fds.add(newPeer.getFileDescriptor());
        } else { // 说明AMS向Zygote进程发送了一个创建应用程序进程的请求
            boolean done;
            /** 调用runOnce来创建一个新的应用进程*/
            done = peers.get(index).runOnce();
 
            // 处理完请求后。断开连接
            if (done) {
                peers.remove(index);
                fds.remove(index);
            }
        }
    }
}

四、System进程的启动过程

    前面3.3中分析ZygoteInit.startSystemServer中提到。系统调用handleSystemServerProcess方法来启动System进程。

/** @path: \frameworks\base\core\java\com\android\internal\os\ZygoteInit.java */
private static void handleSystemServerProcess(ZygoteConnection.Arguments parsedArgs)
        throws ZygoteInit.MethodAndArgsCaller {
    /* System进程是由System进程fork来的，因此它就会获得Zygote进程中前面创建的Server端Socket
     * 而实际System今进程中并不须要这个socket。因此将它关闭*/
    closeServerSocket();
 
    // 设置名称
    if (parsedArgs.niceName != null) {
        Process.setArgV0(parsedArgs.niceName);
    }
 
    final String systemServerClasspath = Os.getenv("SYSTEMSERVERCLASSPATH");
    if (systemServerClasspath != null) {
        performSystemServerDexOpt(systemServerClasspath);
    }
 
    if (parsedArgs.invokeWith != null) {
        ....
    } else {
        ClassLoader cl = null;
        if (systemServerClasspath != null) {
            cl = new PathClassLoader(systemServerClasspath, ClassLoader.getSystemClassLoader());
            Thread.currentThread().setContextClassLoader(cl);
        }
 
        /** 传递剩余的參数到SystemServer中，进一步启动System进程 */
        RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
    }
}

/** @path: \frameworks\base\core\java\com\android\internal\os\RuntimeInit.java */
public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
        throws ZygoteInit.MethodAndArgsCaller {
    if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote");
 
    // 将Sytem.out与err输出到log中
    redirectLogStreams();
    /** 设置System进程的时区和键盘布局等通用信息  */
    commonInit();
    /** 在System进程中创建一个Binder线程池。nativeZygoteInit会进而调用一个native方法来实现 */
    nativeZygoteInit();
    /** 转往运行SystemServer中的main函数*/
    applicationInit(targetSdkVersion, argv, classLoader);
}
 
private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
        throws ZygoteInit.MethodAndArgsCaller {
    // If the application calls System.exit(), terminate the process
    // immediately without running any shutdown hooks.  It is not possible to
    // shutdown an Android application gracefully.  Among other things, the
    // Android runtime shutdown hooks close the Binder driver, which can cause
    // leftover running threads to crash before the process actually exits.
    nativeSetExitWithoutCleanup(true);
 
    // We want to be fairly aggressive about heap utilization, to avoid
    // holding on to a lot of memory that isn‘t needed.
    VMRuntime.getRuntime().setTargetHeapUtilization(0.75f);
    VMRuntime.getRuntime().setTargetSdkVersion(targetSdkVersion);
 
    final Arguments args;
    try {
        args = new Arguments(argv);
    } catch (IllegalArgumentException ex) {
        Slog.e(TAG, ex.getMessage());
        // let the process exit
        return;
    }
 
    // Remaining arguments are passed to the start class‘s static main
    /** 因为System进程进入到SystemServer类的main函数中时，已经运行过非常多代码了
     *  因此严格意义上，main已经不是System的入口函数；
     *  为了使得main看起来像System进程的入口函数，而使用该函数来巧妙地去调用它*/
    invokeStaticMain(args.startClass, args.startArgs, classLoader);
}