android 一个应用程序的启动

    android 程序启动过程，这次主要研究的是具有Activity的android app！

package dalvik.system;
class NativeStart implements Runnable {
    private NativeStart() {}

    private static native void main(String[] dummy);

    public native void run();
}

由此部分代码通过JNI进一步激活ZygoteInit 中的main（）由此一个新的Linux进程就启动了，但是在android中已经把Linux这些特新封装起来了，隐藏了linux的一些特性。只流下了：Activity、Service、ContentProvider、BroadcastReceiver。

   public static void main(String argv[]) {
        try {
            registerZygoteSocket();
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
                SystemClock.uptimeMillis());
            preloadClasses();
            preloadResources();
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
                SystemClock.uptimeMillis());

            // 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(
                        "ZygoteInit.main expects two arguments");
            }

            if (argv[1].equals("true")) {
                startSystemServer();
            }

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

            if (ZYGOTE_FORK_MODE) {
                runForkMode();
            } else {
                runSelectLoopMode();
            }

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

    /**
     * Helper exception class which holds a method and arguments and
     * can call them. This is used as part of a trampoline to get rid of
     * the initial process setup stack frames.
     */
    public static class MethodAndArgsCaller extends Exception
            implements Runnable {
        /** method to call */
        private final Method mMethod;

        /** argument array */
        private final String[] mArgs;

        public MethodAndArgsCaller(Method method, String[] args) {
            mMethod = method;
            mArgs = args;
        }

        public void run() {
            try {
                mMethod.invoke(null, new Object[] { mArgs });
            } catch (IllegalAccessException ex) {
                throw new RuntimeException(ex);
            } catch (InvocationTargetException ex) {
                Throwable cause = ex.getCause();
                if (cause instanceof RuntimeException) {
                    throw (RuntimeException) cause;
                } else if (cause instanceof Error) {
                    throw (Error) cause;
                }
                throw new RuntimeException(ex);
            }
        }
    }

当抛出MethodAndArgsCaller caller 异常时，执行：caller.run();
mMethod.invoke(null, new Object[] { mArgs });激活了ActivityThread中的main函数，因为main函数为static ，所以invoke第一个参数为null
此异常是间接由startSystemServer();throw的。如下代码：

    /**
     * Prepare the arguments and fork for the system server process.
     */
    private static boolean startSystemServer() 
            throws MethodAndArgsCaller, RuntimeException {
        /* Hardcoded command line to start the system server */
        String args[] = {
            "--setuid=1000",
            "--setgid=1000",
            "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,3001,3002,3003",
            "--capabilities=130104352,130104352",
            "--runtime-init",
            "--nice-name=system_server",
            "com.android.server.SystemServer",
        };
        ZygoteConnection.Arguments parsedArgs = null;

        int pid;

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

            /*
             * Enable debugging of the system process if *either* the command line flags
             * indicate it should be debuggable or the ro.debuggable system property
             * is set to "1"
             */
            int debugFlags = parsedArgs.debugFlags;
            if ("1".equals(SystemProperties.get("ro.debuggable")))
                debugFlags |= Zygote.DEBUG_ENABLE_DEBUGGER;

            /* Request to fork the system server process */
            pid = Zygote.forkSystemServer(
                    parsedArgs.uid, parsedArgs.gid,
                    parsedArgs.gids, debugFlags, null);
        } catch (IllegalArgumentException ex) {
            throw new RuntimeException(ex);
        } 
 
        /* For child process */
        if (pid == 0) {
            handleSystemServerProcess(parsedArgs);
        }

        return true;
    }

    /**
     * Finish remaining work for the newly forked system server process.
     */
    private static void handleSystemServerProcess(
            ZygoteConnection.Arguments parsedArgs)
            throws ZygoteInit.MethodAndArgsCaller {
        /*
         * First, set the capabilities if necessary
         */

        if (parsedArgs.uid != 0) {
            try {
                setCapabilities(parsedArgs.permittedCapabilities,
                                parsedArgs.effectiveCapabilities);
            } catch (IOException ex) {
                Log.e(TAG, "Error setting capabilities", ex);
            }
        }

        closeServerSocket();

        /*
         * Pass the remaining arguments to SystemServer.
         * "--nice-name=system_server com.android.server.SystemServer"
         */
        RuntimeInit.zygoteInit(parsedArgs.remainingArgs);
        /* should never reach here */
    }

    /**
     * The main function called when started through the zygote process. This
     * could be unified with main(), if the native code in finishInit()
     * were rationalized with Zygote startup.<p>
     *
     * Current recognized args:
     * <ul>
     *   <li> --nice-name=<i>nice name to appear in ps</i>
     *   <li> <code> [--] &lt;start class name&gt;  &lt;args&gt;
     * </ul>
     *
     * @param argv arg strings
     */
    public static final void zygoteInit(String[] argv)
            throws ZygoteInit.MethodAndArgsCaller {
        // TODO: Doing this here works, but it seems kind of arbitrary. Find
        // a better place. The goal is to set it up for applications, but not
        // tools like am.
        System.setOut(new AndroidPrintStream(Log.INFO, "System.out"));
        System.setErr(new AndroidPrintStream(Log.WARN, "System.err"));

        commonInit();
        zygoteInitNative();

        int curArg = 0;
        for ( /* curArg */ ; curArg < argv.length; curArg++) {
            String arg = argv[curArg];

            if (arg.equals("--")) {
                curArg++;
                break;
            } else if (!arg.startsWith("--")) {
                break;
            } else if (arg.startsWith("--nice-name=")) {
                String niceName = arg.substring(arg.indexOf('=') + 1);
                Process.setArgV0(niceName);
            }
        }

        if (curArg == argv.length) {
            Slog.e(TAG, "Missing classname argument to RuntimeInit!");
            // let the process exit
            return;
        }

        // Remaining arguments are passed to the start class's static main

        String startClass = argv[curArg++];
        String[] startArgs = new String[argv.length - curArg];

        System.arraycopy(argv, curArg, startArgs, 0, startArgs.length);
        invokeStaticMain(startClass, startArgs);
    }

    /**
     * Invokes a static "main(argv[]) method on class "className".
     * Converts various failing exceptions into RuntimeExceptions, with
     * the assumption that they will then cause the VM instance to exit.
     *
     * @param className Fully-qualified class name
     * @param argv Argument vector for main()
     */
    private static void invokeStaticMain(String className, String[] argv)
            throws ZygoteInit.MethodAndArgsCaller {

        // 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);

        Class<?> cl;

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

        Method m;
        try {
            m = cl.getMethod("main", new Class[] { String[].class });
        } catch (NoSuchMethodException ex) {
            throw new RuntimeException(
                    "Missing static main on " + className, ex);
        } catch (SecurityException ex) {
            throw new RuntimeException(
                    "Problem getting static main on " + className, ex);
        }

        int modifiers = m.getModifiers();
        if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
            throw new RuntimeException(
                    "Main method is not public and static on " + className);
        }

        /*
         * This throw gets caught in ZygoteInit.main(), which responds
         * by invoking the exception's run() method. This arrangement
         * clears up all the stack frames that were required in setting
         * up the process.
         */
        throw new ZygoteInit.MethodAndArgsCaller(m, argv);
    }