ActivityThread的main（）方法简读

一、首先，看ActivityThread 的介绍：

 * This manages the execution of the main thread in an
 * application process, scheduling and executing activities,
 * broadcasts, and other operations on it as the activity
 * manager requests.

大概的意思就是说：在应用程序里面管理和执行主线程，调度和执行Activity，广播以及在Activity上的其他业务操作。

二、直接上main()方法。

public static void main(String[] args) {
        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
        SamplingProfilerIntegration.start();

        // CloseGuard defaults to true and can be quite spammy.  We
        // disable it here, but selectively enable it later (via
        // StrictMode) on debug builds, but using DropBox, not logs.
        CloseGuard.setEnabled(false);

        Environment.initForCurrentUser();

        // Set the reporter for event logging in libcore
        EventLogger.setReporter(new EventLoggingReporter());

        // Make sure TrustedCertificateStore looks in the right place for CA certificates
        final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
        TrustedCertificateStore.setDefaultUserDirectory(configDir);

        Process.setArgV0("<pre-initialized>");

        Looper.prepareMainLooper();

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

        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }

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

        // End of event ActivityThreadMain.
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        Looper.loop();

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

三、一句一句来解读吧。
1、Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
Trace是痕迹，踪迹的意思，是在android.os包下的一个类，主要作用是将跟踪事件写入系统跟踪缓冲区，这些跟踪事件可以用Systrace可视化工具收集和使用。看一下traceBegin（）方法，是给代码段写一个跟踪信息。

public static void traceBegin(long traceTag, String methodName) {
        if (isTagEnabled(traceTag)) {
            nativeTraceBegin(traceTag, methodName);
        }
    }

public static boolean isTagEnabled(long traceTag) {
        long tags = sEnabledTags;
        if (tags == TRACE_TAG_NOT_READY) {
            tags = cacheEnabledTags();
        }
        return (tags & traceTag) != 0;
    }

private static native void nativeTraceBegin(long tag, String name);

这里传入了traceTag和methodName，首先就是判断跟踪标签是否启用，如果启用，就调用底层方法，开始跟踪事件。

2、SamplingProfilerIntegration.start();
这个类在com.android.internal.os包下，字面意思翻译就是“抽样分析集成工具”，是集成Dalvik抽样分析器框架。start()是静态方法：

    /**
     * Starts the profiler if profiling is enabled.
     */
    public static void start() {
        if (!enabled) {
            return;
        }
        if (samplingProfiler != null) {
            Log.e(TAG, "SamplingProfilerIntegration already started at " + new Date(startMillis));
            return;
        }

        ThreadGroup group = Thread.currentThread().getThreadGroup();
        SamplingProfiler.ThreadSet threadSet = SamplingProfiler.newThreadGroupThreadSet(group);
        samplingProfiler = new SamplingProfiler(samplingProfilerDepth, threadSet);
        samplingProfiler.start(samplingProfilerMilliseconds);
        startMillis = System.currentTimeMillis();
    }

首先拿到当前线程的线程组group ，threadSet ，然后开启抽样分析。

3、CloseGuard.setEnabled(false);
CloseGuard是dalvik.system包下的一个类。CloseGuard 是一种资源清理机制，资源应该被显式关闭清理。setEnabled（）明显就是使其失效，不过会在debug builds的时候重现变得有效。

 * CloseGuard is a mechanism for flagging implicit finalizer cleanup of
 * resources that should have been cleaned up by explicit close
 * methods (aka "explicit termination methods" in Effective Java).

4、Environment.initForCurrentUser();
Environment大家都很熟悉：环境。

public static void initForCurrentUser() {
        final int userId = UserHandle.myUserId();
        sCurrentUser = new UserEnvironment(userId);
    }

首先拿到当前进程的Id，然后初始化一个UserEnvironment实例。

5、EventLogger.setReporter(new EventLoggingReporter());
看源码说明，Set the reporter for event logging in libcore，简单就是说给lib库设置事件打印记者。

6、final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
确保可信证书存储在正确的地方以便于查找CA证书。

7、Process.setArgV0("<pre-initialized>");
Process是管理系统流程的工具。setArgV0（）是一个本地方法：

    /**
     * Change this process's argv[0] parameter.  This can be useful to show
     * more descriptive information in things like the 'ps' command.
     * 
     * @param text The new name of this process.
     * 
     * {@hide}
     */
    public static final native void setArgV0(String text);

8、Looper.prepareMainLooper();
终于开始干正事了，这里初始化了主线程。首先是在prepare（）里面判断当前线程是否为空，因为Looper只允许有一个主线程。

public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

    public static void prepare() {
        prepare(true);
    }

    private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

9、ActivityThread thread = new ActivityThread();
thread.attach(false);
这里直接实例化了一个ActivityThread 对象，并调用了attach(false)。

private void attach(boolean system) {
        sCurrentActivityThread = this;
        mSystemThread = system;
        if (!system) {
            ViewRootImpl.addFirstDrawHandler(new Runnable() {
                @Override
                public void run() {
                    ensureJitEnabled();
                }
            });
            android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
                                                    UserHandle.myUserId());
            RuntimeInit.setApplicationObject(mAppThread.asBinder());
            final IActivityManager mgr = ActivityManagerNative.getDefault();
            try {
                mgr.attachApplication(mAppThread);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
            // Watch for getting close to heap limit.
            BinderInternal.addGcWatcher(new Runnable() {
                @Override public void run() {
                    if (!mSomeActivitiesChanged) {
                        return;
                    }
                    Runtime runtime = Runtime.getRuntime();
                    long dalvikMax = runtime.maxMemory();
                    long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();
                    if (dalvikUsed > ((3*dalvikMax)/4)) {
                        if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024)
                                + " total=" + (runtime.totalMemory()/1024)
                                + " used=" + (dalvikUsed/1024));
                        mSomeActivitiesChanged = false;
                        try {
                            mgr.releaseSomeActivities(mAppThread);
                        } catch (RemoteException e) {
                            throw e.rethrowFromSystemServer();
                        }
                    }
                }
            });
        } else {
            // Don't set application object here -- if the system crashes,
            // we can't display an alert, we just want to die die die.
            android.ddm.DdmHandleAppName.setAppName("system_process",
                    UserHandle.myUserId());
            try {
                mInstrumentation = new Instrumentation();
                ContextImpl context = ContextImpl.createAppContext(
                        this, getSystemContext().mPackageInfo);
                mInitialApplication = context.mPackageInfo.makeApplication(true, null);
                mInitialApplication.onCreate();
            } catch (Exception e) {
                throw new RuntimeException(
                        "Unable to instantiate Application():" + e.toString(), e);
            }
        }

        // add dropbox logging to libcore
        DropBox.setReporter(new DropBoxReporter());

        ViewRootImpl.addConfigCallback(new ComponentCallbacks2() {
            @Override
            public void onConfigurationChanged(Configuration newConfig) {
                synchronized (mResourcesManager) {
                    // We need to apply this change to the resources
                    // immediately, because upon returning the view
                    // hierarchy will be informed about it.
                    if (mResourcesManager.applyConfigurationToResourcesLocked(newConfig, null)) {
                        updateLocaleListFromAppContext(mInitialApplication.getApplicationContext(),
                                mResourcesManager.getConfiguration().getLocales());

                        // This actually changed the resources!  Tell
                        // everyone about it.
                        if (mPendingConfiguration == null ||
                                mPendingConfiguration.isOtherSeqNewer(newConfig)) {
                            mPendingConfiguration = newConfig;

                            sendMessage(H.CONFIGURATION_CHANGED, newConfig);
                        }
                    }
                }
            }
            @Override
            public void onLowMemory() {
            }
            @Override
            public void onTrimMemory(int level) {
            }
        });
    }

有耐心的就看看。

10、if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
判断当前主线程的Handler是否为空。

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

12、Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
跟前面的Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");相呼应，结束跟踪。

13、Looper.loop();
最重要的一句来了，前面Looper.prepareMainLooper()开启主线程，而这里的loop()就是处理主线程程序的关键段，所有通过Handler传到主线程的Message都会缓存在Looper的消息队列Queue里面，loop()里有个for(;;)循环，不断的从消息队列里面拿出消息进行处理，然后就有了事件的信息交流啦。

14、最后一句throw new RuntimeException("Main thread loop unexpectedly exited");
必要的异常处理语句。

最后说一句，若有任何问题和疑问，欢迎指正。