Android 系统启动流程分析

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随着Android版本的升级,aosp项目中的代码也有了些变化,本文基于Android 7.0分析Android系统启动流程.

简单来说Android系统启动大体如下:
这里写图片描述


init进程

和所有Linux系统一样,Android系统的启动同样是从init进程启动.init进程会解析init.rc文件(关于init.rc中的语法,可以参见我之前写的 深入分析AIL语言及init.rc文件),加载相关目录,并启动相关服务

init进程在/system/core/init/init.c
init.rc文件在/system/core/rootdir下

在init.rc中,Zygote进程被启动.Zygote进程是Android所有进程的孵化器进程.init.rc通过include引入init.zygote.rc,这里以init.zygote64.rc为例,来看一下其中的代码:

service zygote /system/bin/app_process64 -Xzygote /system/bin --zygote --start-system-server
    class main
    priority -20
    user root
    group root readproc
    socket zygote stream 660 root system
    onrestart write /sys/android_power/request_state wake
    onrestart write /sys/power/state on
    onrestart restart audioserver
    onrestart restart cameraserver
    onrestart restart media
    onrestart restart netd
    writepid /dev/cpuset/foreground/tasks

我们队这个脚本简单分析:
1. service zygote /system/bin/app_process64 :service命令告诉init进程要创建一个名字为zygote的进程,这个zygote进程执行的程序是/system/bin/app_process64,后面是传给app_process64程序的参数.
3. socket zygote stream 660 root system:表示zygote进程需要一个名为”zygote”的socket,该socket用来实现进程间的通信.当新启动一个应用时,ActivityManagerService想向该Socket发起请求,请求zygote进程fork出一个新的进程.
4. 后面的onretart表示zygote重启时需要执行的动作.

Zygote进程启动

现在我们知道Zygote进程要执行的程序是/system/bin/app_process64,
它位于/frameworks/base/cmds/app_process/app_main.cpp当中,其入口函数是main():

int main(int argc, char* const argv[])
{
    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) {
        LOG_ALWAYS_FATAL("PR_SET_NO_NEW_PRIVS failed: %s", strerror(errno));
    }

    if (!LOG_NDEBUG) {
      String8 argv_String;
      for (int i = 0; i < argc; ++i) {
        argv_String.append("\"");
        argv_String.append(argv[i]);
        argv_String.append("\" ");
      }
      ALOGV("app_process main with argv: %s", argv_String.string());
    }

    AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
    // Process command line arguments
    // ignore argv[0]
    argc--;
    argv++;
    const char* spaced_commands[] = { "-cp", "-classpath" };

    bool known_command = false;

    int i;
    for (i = 0; i < argc; i++) {
        if (known_command == true) {
          runtime.addOption(strdup(argv[i]));
          ALOGV("app_process main add known option '%s'", argv[i]);
          known_command = false;
          continue;
        }

        for (int j = 0;
             j < static_cast<int>(sizeof(spaced_commands) / sizeof(spaced_commands[0]));
             ++j) {
          if (strcmp(argv[i], spaced_commands[j]) == 0) {
            known_command = true;
            ALOGV("app_process main found known command '%s'", argv[i]);
          }
        }

        if (argv[i][0] != '-') {
            break;
        }
        if (argv[i][1] == '-' && argv[i][2] == 0) {
            ++i; // Skip --.
            break;
        }

        runtime.addOption(strdup(argv[i]));
        ALOGV("app_process main add option '%s'", argv[i]);
    }

    // Parse runtime arguments.  Stop at first unrecognized option.
    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) {
            //init.zygote64.rc中接受的参数,表示启动SystemServer组件
            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;
        }
    }

    Vector<String8> args;
    if (!className.isEmpty()) {

        args.add(application ? String8("application") : String8("tool"));
        runtime.setClassNameAndArgs(className, argc - i, argv + i);

        if (!LOG_NDEBUG) {
          String8 restOfArgs;
          char* const* argv_new = argv + i;
          int argc_new = argc - i;
          for (int k = 0; k < argc_new; ++k) {
            restOfArgs.append("\"");
            restOfArgs.append(argv_new[k]);
            restOfArgs.append("\" ");
          }
          ALOGV("Class name = %s, args = %s", className.string(), restOfArgs.string());
        }
    } else {
        // We're in zygote mode.
        maybeCreateDalvikCache();

        if (startSystemServer) {
            args.add(String8("start-system-server"));
        }

        char prop[PROP_VALUE_MAX];
        if (property_get(ABI_LIST_PROPERTY, prop, NULL) == 0) {
            LOG_ALWAYS_FATAL("app_process: Unable to determine ABI list from property %s.",
                ABI_LIST_PROPERTY);
            return 11;
        }

        String8 abiFlag("--abi-list=");
        abiFlag.append(prop);
        args.add(abiFlag);

        // In zygote mode, pass all remaining arguments to the zygote
        // main() method.
        for (; i < argc; ++i) {
            args.add(String8(argv[i]));
        }
    }

    if (!niceName.isEmpty()) {
        runtime.setArgv0(niceName.string(), true /* setProcName */);
    }

    if (zygote) {
        //此处见到了我们熟悉的ZygoteInit,但该方法的具体实现在AndroidRuntime.start()
        runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
    } else if (className) {
        runtime.start("com.android.internal.os.RuntimeInit", args, zygote);
    } else {
        fprintf(stderr, "Error: no class name or --zygote supplied.\n");
        app_usage();
        LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
    }
}

该函数最主要的作用就是创建AppRuntime,该类同样是在该文件中定义,具体的定义如下:

class AppRuntime : public AndroidRuntime
{
public:
    AppRuntime(char* argBlockStart, const size_t argBlockLength)
        : AndroidRuntime(argBlockStart, argBlockLength)
        , mClass(NULL)
    {
    }

    void setClassNameAndArgs(const String8& className, int argc, char * const *argv) {
        mClassName = className;
        for (int i = 0; i < argc; ++i) {
             mArgs.add(String8(argv[i]));
        }
    }

    virtual void onVmCreated(JNIEnv* env)
    {
        if (mClassName.isEmpty()) {
            return; // Zygote. Nothing to do here.
        }

        char* slashClassName = toSlashClassName(mClassName.string());
        mClass = env->FindClass(slashClassName);
        if (mClass == NULL) {
            ALOGE("ERROR: could not find class '%s'\n", mClassName.string());
        }
        free(slashClassName);

        mClass = reinterpret_cast<jclass>(env->NewGlobalRef(mClass));
    }

    virtual void onStarted()
    {
        sp<ProcessState> proc = ProcessState::self();
        ALOGV("App process: starting thread pool.\n");
        proc->startThreadPool();

        AndroidRuntime* ar = AndroidRuntime::getRuntime();
        ar->callMain(mClassName, mClass, mArgs);

        IPCThreadState::self()->stopProcess();
    }

    virtual void onZygoteInit()
    {
        sp<ProcessState> proc = ProcessState::self();
        ALOGV("App process: starting thread pool.\n");
        proc->startThreadPool();
    }

    virtual void onExit(int code)
    {
        if (mClassName.isEmpty()) {
            // if zygote
            IPCThreadState::self()->stopProcess();
        }

        AndroidRuntime::onExit(code);
    }


    String8 mClassName;
    Vector<String8> mArgs;
    jclass mClass;
};

AppRuntime继承了AndroidRuntime,来继续看看AndroidRuntime.start()方法:
它在/frameworks/base/core/jni/AndroidRuntime.cpp,来看下源码:

/*
 * 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);


    //1. 启动虚拟机
    if (startVm(&mJavaVM, &env, zygote) != 0) {
        return;
    }
    onVmCreated(env);

    //2. 调用startReg()注册JNI方法
    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;

    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 {
         //3. 本质就是调用com.android.intrnal.os.ZygoteInit类的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 {
            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");
}

在start()中主要做三件事情:
1. 调用startVM函数启动虚拟机
2. 调用startReg注册JNI方法
3. 调用com.android.internal.os.ZygoteInit.Java类的main函数.

这里我们重点来ZygoteInit.java.它在
rameworks/base/core/Java/com/android/internal/os/ZygoteInit.java,我们直接来看他的main方法:

public static void main(String argv[]) {
        ZygoteServer zygoteServer = new ZygoteServer();

        // Mark zygote start. This ensures that thread creation will throw
        // an error.
        ZygoteHooks.startZygoteNoThreadCreation();

        // Zygote goes into its own process group.
        try {
            Os.setpgid(0, 0);
        } catch (ErrnoException ex) {
            throw new RuntimeException("Failed to setpgid(0,0)", ex);
        }

        try {
            Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "ZygoteInit");
            RuntimeInit.enableDdms();
            // Start profiling the zygote initialization.
            SamplingProfilerIntegration.start();

            boolean startSystemServer = false;
            String socketName = "zygote";
            String abiList = null;
            for (int i = 1; i < argv.length; i++) {
                if ("start-system-server".equals(argv[i])) {
                    startSystemServer = true;
                } else if (argv[i].startsWith(ABI_LIST_ARG)) {
                    abiList = argv[i].substring(ABI_LIST_ARG.length());
                } else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
                    socketName = argv[i].substring(SOCKET_NAME_ARG.length());
                } else {
                    throw new RuntimeException("Unknown command line argument: " + argv[i]);
                }
            }

            if (abiList == null) {
                throw new RuntimeException("No ABI list supplied.");
            }
            //创建名为zygote的socket
            zygoteServer.registerServerSocket(socketName);
            Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "ZygotePreload");
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
                SystemClock.uptimeMillis());
            preload();
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
                SystemClock.uptimeMillis());
            Trace.traceEnd(Trace.TRACE_TAG_DALVIK);

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

            // Do an initial gc to clean up after startup
            Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PostZygoteInitGC");
            gcAndFinalize();
            Trace.traceEnd(Trace.TRACE_TAG_DALVIK);

            // Disable tracing so that forked processes do not inherit stale tracing tags from
            // Zygote.
            Trace.setTracingEnabled(false);

            // Zygote process unmounts root storage spaces.
            Zygote.nativeUnmountStorageOnInit();

            ZygoteHooks.stopZygoteNoThreadCreation();

            if (startSystemServer) {
                //启动SystemServer组件
                startSystemServer(abiList, socketName, zygoteServer);
            }

            Log.i(TAG, "Accepting command socket connections");
            //等待ActivityManagerService请求
            zygoteServer.runSelectLoop(abiList);

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

这里的main()方法中主要做了三件事情
1. 通过registerServerSocket来创建Socket,用来和ActivityManagerService通信
2. 通过startSystemServer方法来启动SystemServer
3. 通过runSelectLoop方法来等待来自ActivityManagerService请求

Socket创建

下面我们分别来看看对应着几个方法的具体实现:
首先来看ZygoteServer.resiterServerSocket(),它在
/frameworks/base/core/java/com/android/internal/os/ZygoteServer.java当中:

  void registerServerSocket(String socketName) {
        if (mServerSocket == null) {
            int fileDesc;
            final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName;
            try {
                String env = System.getenv(fullSocketName);
                //从环境变量env中获取文件描述符
                fileDesc = Integer.parseInt(env);
            } catch (RuntimeException ex) {
                throw new RuntimeException(fullSocketName + " unset or invalid", ex);
            }

            try {
                //通过文件描述符创建socket,该描述符代表/dev/socket/zygote文件.
                FileDescriptor fd = new FileDescriptor();
                fd.setInt$(fileDesc);
                mServerSocket = new LocalServerSocket(fd);
            } catch (IOException ex) {
                throw new RuntimeException(
                        "Error binding to local socket '" + fileDesc + "'", ex);
            }
        }
    }

SystemServer启动

再回到ZygoteInit中看startSystemServer()方法:

/**
     * Prepare the arguments and fork for the system server process.
     */
    private static boolean startSystemServer(String abiList, String socketName, ZygoteServer zygoteServer)
            throws Zygote.MethodAndArgsCaller, RuntimeException {
        long capabilities = posixCapabilitiesAsBits(
            OsConstants.CAP_IPC_LOCK,
            OsConstants.CAP_KILL,
            OsConstants.CAP_NET_ADMIN,
            OsConstants.CAP_NET_BIND_SERVICE,
            OsConstants.CAP_NET_BROADCAST,
            OsConstants.CAP_NET_RAW,
            OsConstants.CAP_SYS_MODULE,
            OsConstants.CAP_SYS_NICE,
            OsConstants.CAP_SYS_RESOURCE,
            OsConstants.CAP_SYS_TIME,
            OsConstants.CAP_SYS_TTY_CONFIG,
            OsConstants.CAP_WAKE_ALARM
        );
        /* Containers run without this capability, so avoid setting it in that case */
        if (!SystemProperties.getBoolean(PROPERTY_RUNNING_IN_CONTAINER, false)) {
            capabilities |= posixCapabilitiesAsBits(OsConstants.CAP_BLOCK_SUSPEND);
        }
        /* Hardcoded command line to start the system server */
        String args[] = {
            "--setuid=1000",
            "--setgid=1000",
            "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,3001,3002,3003,3006,3007,3009,3010",
            "--capabilities=" + capabilities + "," + capabilities,
            "--nice-name=system_server",
            "--runtime-args",
            "com.android.server.SystemServer",
        };
        ZygoteConnection.Arguments parsedArgs = null;

        int pid;

        try {
            parsedArgs = new ZygoteConnection.Arguments(args);
            ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
            ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);

            //通过forSystemServer()方法创建第一个子进程,此处即SystemServer进程
            pid = Zygote.forkSystemServer(
                    parsedArgs.uid, parsedArgs.gid,
                    parsedArgs.gids,
                    parsedArgs.debugFlags,
                    null,
                    parsedArgs.permittedCapabilities,
                    parsedArgs.effectiveCapabilities);
        } catch (IllegalArgumentException ex) {
            throw new RuntimeException(ex);
        }

        //pid=0表示子进程,此处就是SystemServer进程
        if (pid == 0) {
            if (hasSecondZygote(abiList)) {
                waitForSecondaryZygote(socketName);
            }
            //Zygote创建的子进程(此处就是SystemServer)不需要使用Zygote中创建的Socket文件描述符,因此通过closeServerSocket()关闭它.
            zygoteServer.closeServerSocket();
            handleSystemServerProcess(parsedArgs);
        }

        return true;
    }

接着我们看一下handleSystemServerProcess()方法:

/**
     * Finish remaining work for the newly forked system server process.
     */

    private static void handleSystemServerProcess(
            ZygoteConnection.Arguments parsedArgs)
            throws Zygote.MethodAndArgsCaller {

        // set umask to 0077 so new files and directories will default to owner-only permissions.
        Os.umask(S_IRWXG | S_IRWXO);

        if (parsedArgs.niceName != null) {
            Process.setArgV0(parsedArgs.niceName);
        }

        final String systemServerClasspath = Os.getenv("SYSTEMSERVERCLASSPATH");
        if (systemServerClasspath != null) {
            performSystemServerDexOpt(systemServerClasspath);
        }

        if (parsedArgs.invokeWith != null) {
            String[] args = parsedArgs.remainingArgs;
            // If we have a non-null system server class path, we'll have to duplicate the
            // existing arguments and append the classpath to it. ART will handle the classpath
            // correctly when we exec a new process.
            if (systemServerClasspath != null) {
                String[] amendedArgs = new String[args.length + 2];
                amendedArgs[0] = "-cp";
                amendedArgs[1] = systemServerClasspath;
                System.arraycopy(parsedArgs.remainingArgs, 0, amendedArgs, 2, parsedArgs.remainingArgs.length);
            }

            WrapperInit.execApplication(parsedArgs.invokeWith,
                    parsedArgs.niceName, parsedArgs.targetSdkVersion,
                    VMRuntime.getCurrentInstructionSet(), null, args);
        } else {
            ClassLoader cl = null;
            if (systemServerClasspath != null) {
                cl = createSystemServerClassLoader(systemServerClasspath,
                                                   parsedArgs.targetSdkVersion);

                Thread.currentThread().setContextClassLoader(cl);
            }

             //调用zygoteInit()方法进一步执行启动SystemServer组件的操作
            RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
        }

    }

继续来看 RuntimeInit.zygoteInit()函数的具体代码,它在
/frameworks/base/core/java/com/android/internal/os/RuntimeInit.java文件中:

    public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
            throws Zygote.MethodAndArgsCaller {
        if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote");

        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "RuntimeInit");
        redirectLogStreams();

        commonInit();
        //native方法,用来执行一个Binder进程间同步新机制的初始化工作.之后,这个进程中的Binder对象就可以方便的进行进程间通信了
        nativeZygoteInit();
        //执行com.android.server.SystemServer类的main函数
        applicationInit(targetSdkVersion, argv, classLoader);
    }

   private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
            throws Zygote.MethodAndArgsCaller {
            //...省略众多代码
        invokeStaticMain(args.startClass, args.startArgs, classLoader);
    }

//真正反射执行SystemServer的main方法
 private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader)
            throws Zygote.MethodAndArgsCaller {
        Class<?> cl;

        try {
            cl = Class.forName(className, true, classLoader);
        } catch (ClassNotFoundException ex) {
            throw new RuntimeException(
                    "Missing class when invoking static main " + className,
                    ex);
        }

        Method m;
        try {
        // 通过反射执行SystemServer的main()方法
            m = cl.getMethod("main", new Class[] { String[].class });
        } catch (NoSuchMethodException ex) {
            //...
        } catch (SecurityException ex) {
            //...
        }
        //...
    }

接上文来看一下SystemServer.main(),
它在/frameworks/base/services/java/com/android/server/SystemServer.java

public class SystemServer{

  public static void main(String[] args) {
        new SystemServer().run();
    }


    private void run() {
        try {
            //...省略一些初始化操作

            android.os.Process.setThreadPriority(
                android.os.Process.THREAD_PRIORITY_FOREGROUND);
            android.os.Process.setCanSelfBackground(false);
            //初始化主线程Looper
            Looper.prepareMainLooper();

            //创建SystemServiceManager对象
            mSystemServiceManager = new SystemServiceManager(mSystemContext);
            LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
        }

            // 启动关键服务
            startBootstrapServices();
            //启动核心服务
            startCoreServices();
            //启动其他服务
            startOtherServices();
            //...省略多行代码

            //启动消息循环
            Looper.loop();

    }

}

Socket监听

到现在关于ZygoteInit.main()放中两个主要的方法
ZygoteServer.registerServerSocket()startSystemServer()
我们已经分析完了,他们分别完成Socket的创建以及SystemServer的启动工作,接下来回到ZygoteServer.runSelectLoop()方法:

oid runSelectLoop(String abiList) throws Zygote.MethodAndArgsCaller {
        ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
        ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();

        fds.add(mServerSocket.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);
                pollFds[i].events = (short) POLLIN;
            }
            try {
                Os.poll(pollFds, -1);
            } catch (ErrnoException ex) {
                throw new RuntimeException("poll failed", ex);
            }
            for (int i = pollFds.length - 1; i >= 0; --i) {
                if ((pollFds[i].revents & POLLIN) == 0) {
                    continue;
                }
                if (i == 0) {
                   //监听Socket链接,如果你做过Socket编程就发现此处充当了服务端Socket
                    ZygoteConnection newPeer = acceptCommandPeer(abiList);
                    peers.add(newPeer);
                    fds.add(newPeer.getFileDesciptor());
                } else {
                    //重点关注runOnce()方法
                    boolean done = peers.get(i).runOnce(this);
                    if (done) {
                        peers.remove(i);
                        fds.remove(i);
                    }
                }
            }
        }
    }

该方法非常简单就是不断的从已经创建的Socket中取出ZygoteConnection请求,继续来:ZygoteConnection.runOnce():

boolean runOnce(ZygoteServer zygoteServer) throws Zygote.MethodAndArgsCaller {

        String args[];
        Arguments parsedArgs = null;
        FileDescriptor[] descriptors;

        //...省略一堆参数解析和判断逻辑


        try {
            //...省略多行
            fd = zygoteServer.getServerSocketFileDescriptor();

            if (fd != null) {
                fdsToClose[1] = fd.getInt$();
            }

            fd = null;
            //关键:通过forkAndSpecialize()方法创建非Zygote进程,即普通应用
            pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid, parsedArgs.gids,
                    parsedArgs.debugFlags, rlimits, parsedArgs.mountExternal, parsedArgs.seInfo,
                    parsedArgs.niceName, fdsToClose, parsedArgs.instructionSet,
                    parsedArgs.appDataDir);
        } catch (ErrnoException ex) {
            //...
        } catch (IllegalArgumentException ex) {
            //...
        } catch (ZygoteSecurityException ex) {
           //...
        }

        try {
            if (pid == 0) {//此处就是普通应用的进程
                // in child
                //此处创建的进程同样用不到Zygote进程中创建的Sockt,所以关闭它
                zygoteServer.closeServerSocket();
                IoUtils.closeQuietly(serverPipeFd);
                serverPipeFd = null;
                //该方法最终调用程序的入口main方法
                handleChildProc(parsedArgs, descriptors, childPipeFd, newStderr);
                return true;
            } else {
                // in parent...pid of < 0 means failure
                IoUtils.closeQuietly(childPipeFd);
                childPipeFd = null;
                return handleParentProc(pid, descriptors, serverPipeFd, parsedArgs);
            }
        } finally {
            IoUtils.closeQuietly(childPipeFd);
            IoUtils.closeQuietly(serverPipeFd);
        }
    }

在runOnce()方法中,Zygote进程根据ActivityManagerService的请求,通过forkAndSpecialize()方法来为应用创建新进程,并执行应用的入口main()方法.

到现在我们可以用一张顺序图大体的表示上述的整个流程:
这里写图片描述

总体而言整个流程还是非常清晰的.另外关于Zygote进程及其他进程的关系,可以看如下图:


总结

系统启动时init进程会创建Zygote进程,Zygote进程负责后续Android应用框架层的其他进程的创建和启动.Zygote进程会首先创建一个SystemSever进程,然后由SystemServer负责启动系统关键服务,如ActivityManagerService或者PackageManagerService等.当需要启动一个新的应用时,ActivityManagerService会通过Socket进行通信,Zygote的Socket收到请求之后会为这个应用fork一个新进程,fork成功之后,执行的工作就交给虚拟机.