上兩篇聊了Launcher之后的流程褪那，現(xiàn)在咱們再看一下之前的流程 --- 也就是Zygote進(jìn)程啟動之后：

無標(biāo)題.png

Zygote, 意為“受精卵”,安卓進(jìn)程孵化器
上圖可以看出隔嫡，由linux kernel 的 init （一生萬物）進(jìn)程可以啟動Zygote進(jìn)程，Zygote又啟動dalvik书在，初始化java環(huán)境灰伟，app需要的資源，系統(tǒng)服務(wù)... fork（孵化）桌面Launcher以及各個app進(jìn)程

在系統(tǒng)啟動腳本system/core/rootdir/init.rc文件中儒旬，我們可以看到啟動Zygote進(jìn)程的腳本命令：

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server  
    socket zygote stream 666   # zygote需要一個套接字
    onrestart write /sys/android_power/request_state wake  # zygote重啟的話栏账，需要執(zhí)行這個操作
    onrestart write /sys/power/state on  
    onrestart restart media  
    onrestart restart netd  

app_process對應(yīng)的源碼在frameworks/base/cmds/app_process目錄下，其入口函數(shù)main在文件app_main.cpp中：

/*
* 啟動zygote的方式為/system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
* 所以 argc == 5
*      argv里頭存的就是這5個參數(shù)argv[0]=="/system/bin/app_process" ,argv[1] == "-Xzygote"....
*/
int main(int argc, char* const argv[])
{
    ......
    // These are global variables in ProcessState.cpp
    mArgC = argc;
    mArgV = argv;

    mArgLen = 0;
    for (int i=0; i<argc; i++) {
        mArgLen += strlen(argv[i]) + 1;
    }
    mArgLen--;
    // 以上代碼主要是將參數(shù)相關(guān)信息保存到全局變量中

    AppRuntime runtime;
    const char* argv0 = argv[0];

    // Process command line arguments
    // ignore argv[0]
    argc--;
    argv++;

    // Everything up to '--' or first non '-' arg goes to the vm

    int i = runtime.addVmArguments(argc, argv); // 這個函數(shù)會返回1栈源，表示只處理了-Xzytote這一個參數(shù)挡爵，所謂的處理實(shí)際上就是將這個參數(shù)添加到了runtime對象的mOptions 變量中。

    // Parse runtime arguments.  Stop at first unrecognized option.
    bool zygote = false;
    bool startSystemServer = false;
    bool application = false;
    const char* parentDir = NULL;
    const char* niceName = NULL;
    const char* className = NULL;
    // 進(jìn)入循環(huán)之前 i == 1, argc == 4  argv 指向“-Xzygote” 
    while (i < argc) {
        const char* arg = argv[i++];
        if (!parentDir) {
            parentDir = arg;  // parentDir被賦值為"/system/bin"
        } else if (strcmp(arg, "--zygote") == 0) {  
            zygote = true;
            niceName = "zygote"; // 進(jìn)程名
        } else if (strcmp(arg, "--start-system-server") == 0) {
            startSystemServer = true;
        } else if (strcmp(arg, "--application") == 0) { // 不走這個分支
            application = true;
        } else if (strncmp(arg, "--nice-name=", 12) == 0) {  // 不走這個分支
            niceName = arg + 12;
        } else {  // 不走這個分支
            className = arg;
            break;
        }
    }

    if (niceName && *niceName) {  // 設(shè)置進(jìn)程名
        setArgv0(argv0, niceName);
        set_process_name(niceName);
    }

    runtime.mParentDir = parentDir;

    if (zygote) { // 走這個分支
        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;
    }
}

main函數(shù)主要就是創(chuàng)建了runtime實(shí)例甚垦，并且解析參數(shù)茶鹃，然后調(diào)用runtime的start函數(shù)，接著我們分析AppRuntime的start函數(shù)：

/*
 * 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.
 */
 // 首先我們明確下傳進(jìn)來的參數(shù)  className == "com.android.internal.os.ZygoteInit"  options == "start-system-server"
void AndroidRuntime::start(const char* className, const char* options)
{
    ALOGD("\n>>>>>> AndroidRuntime START %s <<<<<<\n",
            className != NULL ? className : "(unknown)");

    /*
     * 'startSystemServer == true' means runtime is obsolete and not run from
     * init.rc anymore, so we print out the boot start event here.
     */
    if (strcmp(options, "start-system-server") == 0) {
        /* 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);  //配置ANDROID_ROOT環(huán)境變量
    }

    //const char* kernelHack = getenv("LD_ASSUME_KERNEL");
    //ALOGD("Found LD_ASSUME_KERNEL='%s'\n", kernelHack);
    //1：調(diào)用startVm函數(shù)創(chuàng)建虛擬機(jī)艰亮；
    /* start the virtual machine */
    JniInvocation jni_invocation;
    jni_invocation.Init(NULL);
    JNIEnv* env;
    if (startVm(&mJavaVM, &env) != 0) { // 創(chuàng)建虛擬機(jī)
        return;
    }
    onVmCreated(env);
    //2：調(diào)用startReg函數(shù)注冊Android Natvie函數(shù)闭翩；
    /*
     * Register android functions.
     */
    if (startReg(env) < 0) {
        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;
    jstring optionsStr;

    stringClass = env->FindClass("java/lang/String");
    assert(stringClass != NULL);
    strArray = env->NewObjectArray(2, stringClass, NULL);
    assert(strArray != NULL);
    classNameStr = env->NewStringUTF(className);
    assert(classNameStr != NULL);
    env->SetObjectArrayElement(strArray, 0, classNameStr);
    optionsStr = env->NewStringUTF(options);
    env->SetObjectArrayElement(strArray, 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 {
        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 {
            //3：讓虛擬機(jī)去執(zhí)行com.android.internal.os.ZygoteInit的main函數(shù)。
            /* 調(diào)用com.android.internal.os.ZygoteInit的main函數(shù)垃杖，strArray是參數(shù)男杈，數(shù)組里面有兩個元素,
            className == "com.android.internal.os.ZygoteInit"  options == "start-system-server" */
            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");
}

start函數(shù)主要做了以下幾件事情：
1：調(diào)用startVm函數(shù)創(chuàng)建虛擬機(jī)；
2：調(diào)用startReg函數(shù)注冊Android Natvie函數(shù)调俘；
3：讓虛擬機(jī)去執(zhí)行com.android.internal.os.ZygoteInit的main函數(shù)伶棒。

public static void main(String argv[]) {
        try {
            // Start profiling the zygote initialization.
            SamplingProfilerIntegration.start();
            // 1、創(chuàng)建一個套接字彩库，用于監(jiān)聽ams發(fā)過來的fork請求
            registerZygoteSocket(); 
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
                SystemClock.uptimeMillis());
            preload(); 
            // 2肤无、加載classes 和resources, 后面會詳細(xì)分析
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
                SystemClock.uptimeMillis());

            // Finish profiling the zygote initialization.
            SamplingProfilerIntegration.writeZygoteSnapshot();

            // Do an initial gc to clean up after startup
            gc();

            // If requested, start system server directly from Zygote
            if (argv.length != 2) {
                throw new RuntimeException(argv[0] + USAGE_STRING);
            }

            if (argv[1].equals("start-system-server")) {
                //3、 創(chuàng)建system server進(jìn)程骇钦，ams wms pms等常見service都在該進(jìn)程里面
                startSystemServer(); 
            } else if (!argv[1].equals("")) {
                throw new RuntimeException(argv[0] + USAGE_STRING);
            }

            Log.i(TAG, "Accepting command socket connections");

            if (ZYGOTE_FORK_MODE) {
                runForkMode();
            } else {
                // 4宛渐、進(jìn)入循環(huán)監(jiān)聽模式，監(jiān)聽外來請求
                runSelectLoopMode(); 
            }

            closeServerSocket();
        } catch (MethodAndArgsCaller caller) {
            caller.run();
        } catch (RuntimeException ex) {
            Log.e(TAG, "Zygote died with exception", ex);
            closeServerSocket();
            throw ex;
        }
    }

com.android.internal.os.ZygoteInit的main函數(shù)主要做了四件事情：

1：調(diào)用registerZygoteSocket()創(chuàng)建一個套接字眯搭，用于監(jiān)聽ams發(fā)過來的fork請求窥翩，如下：
private static void registerZygoteSocket(String socketName) {
    if (sServerSocket == null) {
        int fileDesc;
        //此處的socket name，就是zygote
        final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName;
        try {
            //記得么鳞仙？在init.zygote.rc被加載時寇蚊，就會創(chuàng)建一個名為zygote的socket
            String env = System.getenv(fullSocketName);
            fileDesc = Integer.parseInt(env);
        } catch (RuntimeException ex) {
            throw new RuntimeException(fullSocketName + " unset or invalid", ex);
        }

        try {
            FileDescriptor fd = new FileDescriptor();
            //獲取zygote socket的文件描述符
            fd.setInt$(fileDesc);
            //將zygote socket包裝成一個server socket
            sServerSocket = new LocalServerSocket(fd);
        } catch (IOException ex) {
            throw new RuntimeException("Error binding to local socket '" + fileDesc + "'", ex);
        }
    }
}

2：調(diào)用preload()預(yù)加載classes 和resources，如下棍好；

static void preload() {
    Log.d(TAG, "begin preload");
    //讀取文件framework/base/preloaded-classes仗岸，然后通過反射加載對應(yīng)的類
    //需要加載數(shù)千個類允耿，啟動慢的原因之一
    preloadClasses();
    //負(fù)載加載一些常用的系統(tǒng)資源
    preloadResources();
    //圖形相關(guān)的
    preloadOpenGL();
    //一些必要庫
    preloadSharedLibraries();
    //好像是語言相關(guān)的字符信息
    preloadTextResources();
    // Ask the WebViewFactory to do any initialization that must run in the zygote process, for memory sharing purposes.
    WebViewFactory.prepareWebViewInZygote();
    Log.d(TAG, "end preload");
}

3：調(diào)用startSystemServer()創(chuàng)建system server進(jìn)程，ams wms pms等常見service都在該進(jìn)程里面扒怖，如下较锡；

private static boolean startSystemServer(String abiList, String socketName) {
    //準(zhǔn)備capabilities參數(shù)
    ........
    String args[] = {
        "--setuid=1000",
        "--setgid=1000",
        "--setgroups=.........",
        "--capabilities=" + capabilities + "," + capabilities,
        "--nice-name=system_server",
        "--runtime-args",
        "com.android.server.SystemServer",
    };
    ZygoteConnection.Arguments parsedArgs = null;

    int pid;

    try {
        //將上面準(zhǔn)備的參數(shù)，按照ZygoteConnection的風(fēng)格進(jìn)行封裝
        parsedArgs = new ZygoteConnection.Arguments(args);
        ...........

        //通過fork"分裂"出system server
        /* 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);
    }

    if (pid == 0) {
        ............
        //pid = 0, 在進(jìn)程system server中
        //system server進(jìn)程處理自己的工作
        handleSystemServerProcess(parsedArgs);
    }

    return true;
}

4：調(diào)用runSelectLoopMode()進(jìn)入循環(huán)監(jiān)聽模式盗痒，監(jiān)聽外來請求蚂蕴，如下。

private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {
    ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
    ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();

    //首先將server socket加入到fds
    fds.add(sServerSocket.getFileDescriptor());
    peers.add(null);

    while (true) {
        StructPollfd[] pollFds = new StructPollfd[fds.size()];
        for (int i = 0; i < pollFds.length; ++i) {
            pollFds[i] = new StructPollfd();
            pollFds[i].fd = fds.get(i);
            //關(guān)注事件到來
            pollFds[i].events = (short) POLLIN;
        }
        try {
            //等待事件到來
            Os.poll(pollFds, -1);
        } catch (ErrnoException ex) {
            throw new RuntimeException("poll failed", ex);
        }
        //注意這里是倒序的
        for (int i = pollFds.length - 1; i >= 0; --i) {
            if ((pollFds[i].revents & POLLIN) == 0) {
                continue;
            }
            //server socket最先加入fds积糯， 因此這里是server socket收到數(shù)據(jù)
            if (i == 0) {
                //收到新的建立通信的請求掂墓，建立通信連接
                ZygoteConnection newPeer = acceptCommandPeer(abiList);
                //加入到peers和fds
                peers.add(newPeer);
                fds.add(newPeer.getFileDesciptor());
             } else {
                //其它通信連接收到數(shù)據(jù)，runOnce執(zhí)行對應(yīng)命令
                boolean done = peers.get(i).runOnce();
                if (done) {
                    //對應(yīng)通信連接不再需要執(zhí)行其它命令看成，關(guān)閉并移除
                    peers.remove(i);
                    fds.remove(i);
                }
            }
        }
    }
}

從上面代碼可知君编，初始時，fds中僅有server socket川慌，因此當(dāng)有數(shù)據(jù)到來時吃嘿，將執(zhí)行i ==0 的部分。
此時梦重，顯然是需要創(chuàng)建新的通信連接兑燥，因此acceptCommandPeer將被調(diào)用。

private static ZygoteConnection acceptCommandPeer(String abiList) {
    try {
        return new ZygoteConnection(sServerSocket.accept(), abiList);
    } catch (IOException ex) {
        throw new RuntimeException("IOException during accept()", ex);
    }
}
總結(jié)
1：acceptCommandPeer封裝了socket的accpet函數(shù)琴拧。于是我們知道降瞳，對應(yīng)的新的連接，zygote將會創(chuàng)建出一個新的socket與其通信蚓胸，并將該socket加入到fds中挣饥。因此，一旦通信連接建立后沛膳，fds中將會包含有多個socket扔枫。
2：當(dāng)poll監(jiān)聽到這一組sockets上有數(shù)據(jù)到來時，就會從阻塞中恢復(fù)锹安。于是短荐，我們需要判斷到底是哪個socket收到了數(shù)據(jù)。
3：在runSelectLoop中采用倒序的方式輪詢叹哭，由于server socket第一個被加入到fds忍宋，因此最后輪詢到的socket才需要處理新建連接的操作；其它socket收到數(shù)據(jù)時风罩，僅需要調(diào)用zygoteConnection的runonce函數(shù)執(zhí)行數(shù)據(jù)對應(yīng)的操作讶踪。
4：若一個連接處理完所有對應(yīng)消息后，該連接對應(yīng)的socket和連接等將被移除泊交。

那么從 system server 到 ActivityManager 到 Launcher 又是如何溝通的呢乳讥，也就是第一篇提到的
AMS.startProcessLocked()-->Process.start()-->zygoteSendArgsAndGetResult()-->ZygoteInit.invokeStaticMain(cloader, className, mainArgs)(ActivityThread.main)
system server，具體的過程在介紹system server時再分析