boolean runOnce() throws ZygoteInit.MethodAndArgsCaller {
        String args[];
        Arguments parsedArgs = null;
        FileDescriptor[] descriptors;

        try {
            args = readArgumentList();
            descriptors = mSocket.getAncillaryFileDescriptors();
        } catch (IOException ex) {
            Log.w(TAG, "IOException on command socket " + ex.getMessage());
            closeSocket();
            return true;
        }

        if (args == null) {
            // EOF reached.
            closeSocket();
            return true;
        }

        /** the stderr of the most recent request, if avail */
        PrintStream newStderr = null;

        if (descriptors != null && descriptors.length >= 3) {
            newStderr = new PrintStream(
                    new FileOutputStream(descriptors[2]));
        }

        int pid = -1;
        FileDescriptor childPipeFd = null;
        FileDescriptor serverPipeFd = null;

        try {
            parsedArgs = new Arguments(args);

            applyUidSecurityPolicy(parsedArgs, peer);
            applyRlimitSecurityPolicy(parsedArgs, peer);
            applyCapabilitiesSecurityPolicy(parsedArgs, peer);
            applyInvokeWithSecurityPolicy(parsedArgs, peer);

            applyDebuggerSystemProperty(parsedArgs);
            applyInvokeWithSystemProperty(parsedArgs);

            int[][] rlimits = null;

            if (parsedArgs.rlimits != null) {
                rlimits = parsedArgs.rlimits.toArray(intArray2d);
            }

            if (parsedArgs.runtimeInit && parsedArgs.invokeWith != null) {
                FileDescriptor[] pipeFds = Libcore.os.pipe();
                childPipeFd = pipeFds[1];
                serverPipeFd = pipeFds[0];
                ZygoteInit.setCloseOnExec(serverPipeFd, true);
            }

            pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid,
                    parsedArgs.gids, parsedArgs.debugFlags, rlimits);
        } catch (IOException ex) {
            logAndPrintError(newStderr, "Exception creating pipe", ex);
        } catch (ErrnoException ex) {
            logAndPrintError(newStderr, "Exception creating pipe", ex);
        } catch (IllegalArgumentException ex) {
            logAndPrintError(newStderr, "Invalid zygote arguments", ex);
        } catch (ZygoteSecurityException ex) {
            logAndPrintError(newStderr,
                    "Zygote security policy prevents request: ", ex);
        }

        try {
            if (pid == 0) {
                // in child
                IoUtils.closeQuietly(serverPipeFd);
                serverPipeFd = null;
                handleChildProc(parsedArgs, descriptors, childPipeFd, newStderr);

                // should never get here, the child is expected to either
                // throw ZygoteInit.MethodAndArgsCaller or exec().
                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);
        }
    }
}

调用readArgumentList函数来获取即将创建的应用进程的启动参数
调用Zygote.forkAndSpecialize(...) 通过函数fork创建一个子进程
调用handleChildProc成员函数 启动子进程

五. 应用程序主线程消息循环的创建过程

当一个应用程序创建(即第四部分)完成后,会去调用RuntimeInit类的静态成员函数invokeStaticMain,将ActivityThread类的静态成员函数main设置为新创建的应用程序进程的入口

frameworks/base/core/java/com/android/internal/os/RuntimeInit.java

private static void invokeStaticMain(String className, String[] argv) throws ZygoteInit.MethodAndArgsCaller {
        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);
        }

        throw new ZygoteInit.MethodAndArgsCaller(m, argv);
}

参数className的值为"android.app.ActivityThread"
执行m = cl.getMethod("main", new Class[] { String[].class }); 将main函数构造到Method对象中
执行throw new ZygoteInit.MethodAndArgsCaller(m, argv); 抛出异常

frameworks/base/core/java/com/android/internal/os/ZygoteInit.java

public static void main(String argv[]) {
        try {
            // Start profiling the zygote initialization.
            SamplingProfilerIntegration.start();

            registerZygoteSocket();
        ......

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

        .......

            if (argv[1].equals("start-system-server")) {
                startSystemServer();
            } else if (!argv[1].equals("")) {
                throw new RuntimeException(argv[0] + USAGE_STRING);
            }

            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;
        }
}

捕获异常,执行caller.run();

frameworks/base/core/java/com/android/internal/os/ZygoteInit.java

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);
            }
    }
}

成员变量mMethod指向ActivityThread类的静态成员函数main
成员变量mArgs指向新创建的应用程序的启动参数
执行mMethod.invoke(null, new Object[] { mArgs }); 调用ActivityThread类的静态成员函数main

frameworks/base/core/java/android/app/ActivityThread.java

public static void main(String[] args) {
    ......
        Looper.prepareMainLooper();
        if (sMainThreadHandler == null) {
            sMainThreadHandler = new Handler();
        }

        ActivityThread thread = new ActivityThread();
        thread.attach(false);

        AsyncTask.init();

        if (false) {
            Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread"));
        }

        Looper.loop();
        throw new RuntimeException("Main thread loop unexpectedly exited");
}

执行Looper.prepareMainLooper(); 在当前应用程序的主线程中创建一个消息循环(即创建一个消息队列)
执行Looper.loop(); 启动消息循环

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