开局一张图

源码版本：Android 11(SDK 30)

涉及到的类

• Instrumentation：负责 Application 和 Activity 的建立和生命周期控制。

• ActivityTaskManager：此类提供有关Activity及其容器（如任务、堆栈和显示）的信息并与之交互。

• ActivityTaskManagerService：用于管理Activity及其容器（任务、显示等）的系统服务。之前这个活是AMS的，现在从AMS中剥离出来了。

• ActivityStartController：用于委托Activity启动的控制器。

• ActivityStarter：专门负责一个 Activity 的启动操做。它的主要做用包括解析 Intent、建立 ActivityRecord、若是有可能还要建立 TaskRecordflex

• ActivityStack：Activity在ATMS的栈管理，用来记录已经启动的Activity的先后关系，状态信息等。通过ActivityStack决定是否需要启动新的进程。

• RootWindowContainer：设备的Root WindowContainer。

• TaskRecord：ATMS抽象出来的一个"任务"的概念，是记录ActivityRecord的栈，一个"Task"包含若干个ActivityRecord。ATMS用TaskRecord确保Activity启动和退出的顺序。

• ActivityStackSupervisor：负责所有Activity栈的管理。内部管理了mHomeStack、mFocusedStack和mLastFocusedStack三个Activity栈。其中，mHomeStack管理的是Launcher相关的Activity栈；mFocusedStack管理的是当前显示在前台Activity的Activity栈；mLastFocusedStack管理的是上一次显示在前台Activity的Activity栈。

• ActivitySupervisor：管理 activity 任务栈。

• ActivityThread：ActivityThread 运行在UI线程（主线程），App的真正入口。

• ApplicationThread：用来实现ATMS和ActivityThread之间的交互。

• ApplicationThreadProxy：ApplicationThread 在服务端的代理。ATMS就是通过该代理与ActivityThread进行通信的。

• ClientLifecycleManager：能够组合多个客户端生命周期 transaction 请求和/或回调，并将它们作为单个 transaction 执行。

• TransactionExecutor：已正确的顺序管理 transaction 执行的类。

1、Activity.java

1.1 startActivity()

@Override
public void startActivity(Intent intent) {
    //接着往里看
    this.startActivity(intent, null);
}

@Override
public void startActivity(Intent intent, @Nullable Bundle options) {
    ...
    if (options != null) {
        startActivityForResult(intent, -1, options);
    } else {
        startActivityForResult(intent, -1);
    }
}

最终调用了 startActivityForResult 方法，传入的 -1 表示不需要获取 startActivity 的结果。

1.2 startActivityForResult()

public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
    @Nullable Bundle options) {
    //mParent表示当前Activitiy的父类，
    if (mParent == null) {
        options = transferSpringboardActivityOptions(options);
        //ActivityThread mMainThread;
        Instrumentation.ActivityResult ar =
            mInstrumentation.execStartActivity(
            this, mMainThread.getApplicationThread(), mToken, this,
                intent, requestCode, options);
        ...
    } else {
        ...
    }
}

startActivityForResult 调用 Instrumentation.execStartActivity 方法。剩下的交给 Instrumentation 类去处理。

mMainThread 是 ActivityThread 类型，ActivityThread 可以理解为一个进程，这就是 Activity 所在的进程。

通过 mMainThread 获取一个 ApplicationThread 的引用，这个引用就是用来实现进程间通信的，具体来说就是 AMS 所在系统进程通知应用程序进程进行的一系列操作。

上面有Instrumentation、ActivityThread、ApplicationThread 等类的介绍。

2、Instrumentation.java

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

/**
 * Base class for implementing application instrumentation code.
 */
public class Instrumentation {
    ...
}

2.1 execStartActivity()

 public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        ...
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess(who);
            int result = ActivityTaskManager.getService()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target != null ? target.mEmbeddedID : null,
                        requestCode, 0, null, options);
            //检查启动Activity的结果
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }

在 Instrumentation 中，调用了 ActivityTaskManager.getService().startActivity() 。咱们近距离观望一下。

3、ActivityTaskManage.java

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

@TestApi
@SystemService(Context.ACTIVITY_TASK_SERVICE)
public class ActivityTaskManager {
    ...
}

3.1 getService()

    /** @hide */
    public static IActivityTaskManager getService() {
        return IActivityTaskManagerSingleton.get();
    }

3.2 IActivityTaskManagerSingleton

    @UnsupportedAppUsage(trackingBug = 129726065)
    private static final Singleton<IActivityTaskManager> IActivityTaskManagerSingleton =
            new Singleton<IActivityTaskManager>() {
                @Override
                protected IActivityTaskManager create() {
                    //代理对象（动态代理）
                    final IBinder b = ServiceManager.getService(Context.ACTIVITY_TASK_SERVICE);
                    return IActivityTaskManager.Stub.asInterface(b);
                }
            };
            

这里实际上返回的是一个 ActivityTaskManagerService 这出现了跨进程 (从应用进程(app) > system_service进程) ，然后调用其 startActivity 方法。

实际上这里就是通过 AIDL 来调用 ATMS 的 startActivity 方法，返回 IActivityTaskManager.startActivity 的结果：Activity 已正常成功启动。

至此，startActivity 的工作重心成功地从 应用进程(app) 转移到了系统进程(system_service) 的 ATMS 中。

4、ActivityTaskManagerService.java

frameworks/base/services/core/java/com/android/server/wm/ActivityTaskManagerService.java

/**
 * 用于管理 activities and their containers (task, stacks, displays,... )的 system_service(系统服务)
 */
public class ActivityTaskManagerService extends IActivityTaskManager.Stub {
    ...
}

4.1 startActivity()

@Override
public final int startActivity(IApplicationThread caller, String callingPackage,
        String callingFeatureId, Intent intent, String resolvedType, IBinder resultTo,
        String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo,
        Bundle bOptions) {
    return startActivityAsUser(caller, callingPackage, callingFeatureId, intent, resolvedType,
            resultTo, resultWho, requestCode, startFlags, profilerInfo, bOptions,
            UserHandle.getCallingUserId());
}

4.2 startActivityAsUser()

@Override
public int startActivityAsUser(IApplicationThread caller, String callingPackage,
        String callingFeatureId, Intent intent, String resolvedType, IBinder resultTo,
        String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo,
        Bundle bOptions, int userId) {
    return startActivityAsUser(caller, callingPackage, callingFeatureId, intent, resolvedType,
            resultTo, resultWho, requestCode, startFlags, profilerInfo, bOptions, userId,
            true /*validateIncomingUser*/);
}

4.3 startActivityAsUser() 比上面多个参数

//重点来了
private int startActivityAsUser(IApplicationThread caller, String callingPackage,
        @Nullable String callingFeatureId, Intent intent, String resolvedType,
        IBinder resultTo, String resultWho, int requestCode, int startFlags,
        ProfilerInfo profilerInfo, Bundle bOptions, int userId, boolean validateIncomingUser) {
    assertPackageMatchesCallingUid(callingPackage);
    enforceNotIsolatedCaller("startActivityAsUser");

    userId = getActivityStartController().checkTargetUser(userId, validateIncomingUser,
            Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");

    // 在此处切换到用户app堆栈。
    return getActivityStartController().obtainStarter(intent, "startActivityAsUser")
            .setCaller(caller)
            .setCallingPackage(callingPackage)
            .setCallingFeatureId(callingFeatureId)
            .setResolvedType(resolvedType)
            .setResultTo(resultTo)
            .setResultWho(resultWho)
            .setRequestCode(requestCode)
            .setStartFlags(startFlags)
            .setProfilerInfo(profilerInfo)
            .setActivityOptions(bOptions)
            .setUserId(userId)
            .execute();

    }

4.4 getActivityStartController()

    ActivityStartController getActivityStartController() {
        return mActivityStartController;
    }

5、ActivityStartController.java

/**
 * Controller for delegating activity launches.
 */
public class ActivityStartController {
    ...
}

5.1 obtainStarter();

    /**
     * @return A starter to configure and execute starting an activity. It is valid until after
     *         {@link ActivityStarter#execute} is invoked. At that point, the starter should be
     *         considered invalid and no longer modified or used.
     */
    ActivityStarter obtainStarter(Intent intent, String reason) {
        return mFactory.obtain().setIntent(intent).setReason(reason);
    }

6、ActivityStarter.java

frameworks/base/services/core/java/com/android/server/wm/ActivityStarter.java

/**
 * Controller for interpreting how and then launching an activity.
 *
 * This class collects all the logic for determining how an intent and flags should be turned into
 * an activity and associated task and stack.
 */
class ActivityStarter {
    ...
}

6.1 execute()

    /**
     * 根据前面提供的请求参数解析必要的信息，执行开始活动的请求。
     * @return The starter result.
     */
    int execute() {
        ...
                res = executeRequest(mRequest);
        ...
    }

6.2 executeRequest()

    /**
     * Activity启动之前需要做那些准备？
     * 通常的Activity启动流程将通过 startActivityUnchecked 到 startActivityInner。
     */
    private int executeRequest(Request request) {
        ...
        //Activity的记录
        ActivityRecord sourceRecord = null;
        ActivityRecord resultRecord = null;
        ...
        //Activity stack(栈)管理
        mLastStartActivityResult = startActivityUnchecked(r, sourceRecord, voiceSession,
                request.voiceInteractor, startFlags, true /* doResume */, checkedOptions, inTask,
                restrictedBgActivity, intentGrants);

        ...
        return mLastStartActivityResult;
    }

从上面代码可以看出，经过多个方法的调用，最终通过 obtainStarter 方法获取了 ActivityStarter 类型的对象，然后调用其 execute() 方法。

在 execute() 方法中，会再次调用其内部的 executeRequest() 方法。

咱们接着看看 startActivityUnchecked() ;

6.3 startActivityUnchecked()

    private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
                IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
                int startFlags, boolean doResume, ActivityOptions options, Task inTask,
                boolean restrictedBgActivity, NeededUriGrants intentGrants) {
        ....
        try {
            ...
            result = startActivityInner(r, sourceRecord, voiceSession, voiceInteractor,
                    startFlags, doResume, options, inTask, restrictedBgActivity, intentGrants);
        } finally {
            ...
        }

        postStartActivityProcessing(r, result, startedActivityRootTask);

        return result;
    }

6.4 startActivityInner()

    int startActivityInner(final ActivityRecord r, ActivityRecord sourceRecord,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            int startFlags, boolean doResume, ActivityOptions options, Task inTask,
            boolean restrictedBgActivity, NeededUriGrants intentGrants) {
        setInitialState(r, options, inTask, doResume, startFlags, sourceRecord, voiceSession,
                voiceInteractor, restrictedBgActivity);

        //计算启动模式
        computeLaunchingTaskFlags();
        computeSourceRootTask();
        //设置启动模式
        mIntent.setFlags(mLaunchFlags);

        ...

        // 关键点来了
        mRootWindowContainer.resumeFocusedTasksTopActivities(
                        mTargetRootTask, mStartActivity, mOptions, mTransientLaunch);
        ...

        return START_SUCCESS;
    }

这个是 mRootWindowContainer 是 RootWindowContainer。

7、RootWindowContainer.java

frameworks/base/services/core/java/com/android/server/wm/RootWindowContainer.java

/** Root {@link WindowContainer} for the device. */
class RootWindowContainer extends WindowContainer<DisplayContent>
        implements DisplayManager.DisplayListener {
        ...
        }

7.1 resumeFocusedTasksTopActivities()

    boolean resumeFocusedTasksTopActivities(
            Task targetRootTask, ActivityRecord target, ActivityOptions targetOptions,
            boolean deferPause) {
        ...
        boolean result = false;
        if (targetRootTask != null && (targetRootTask.isTopRootTaskInDisplayArea()
                || getTopDisplayFocusedRootTask() == targetRootTask)) {
            result = targetRootTask.resumeTopActivityUncheckedLocked(target, targetOptions,
                    deferPause);
        }

        ...
        return result;
    }

当然这里看的是 targetRootTask 是 Task 对象的实例。咱们就去追踪这个方法。

8、Task.java

frameworks/base/services/core/java/com/android/server/wm/Task.java

class Task extends WindowContainer<WindowContainer> {
      ...
}

8.1 resumeTopActivityUncheckedLocked()

    @GuardedBy("mService")
    boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options,
            boolean deferPause) {
        ...
        someActivityResumed = resumeTopActivityInnerLocked(prev, options, deferPause);
        ...
        return someActivityResumed;
    }

8.2 resumeTopActivityInnerLocked()

    @GuardedBy("mService")
    private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options,
            boolean deferPause) {
        ...
            try {
                ....
            } catch (Exception e) {
                ....
                //重点
                mTaskSupervisor.startSpecificActivity(next, true, false);
                return true;
            }
        ....
        return true;
    }

mTaskSupervisor：是 ActivityTaskSupervisor 对象的实例。下面咱们看看其 startSpecificActivity() 方法。

9、ActivityTaskSupervisor.class

frameworks/base/services/core/java/com/android/server/wm/ActivityTaskSupervisor.java

public class ActivityTaskSupervisor implements RecentTasks.Callbacks {
      ...
}

9.1 startSpecificActivity()

    void startSpecificActivity(ActivityRecord r, boolean andResume, boolean checkConfig) {
        // 此 Activity 的 应用程序(进程) 是否已在运行？
        final WindowProcessController wpc =
                mService.getProcessController(r.processName, r.info.applicationInfo.uid);

        boolean knownToBeDead = false;
        if (wpc != null && wpc.hasThread()) {
            try {
                //进程已启动
                //注释1：开始启动activity
                realStartActivityLocked(r, wpc, andResume, checkConfig);
                return;
            } catch (RemoteException e) {
                
            }
        }

        r.notifyUnknownVisibilityLaunchedForKeyguardTransition();

        final boolean isTop = andResume && r.isTopRunningActivity();
        //注释2：进程未启动
        mService.startProcessAsync(r, knownToBeDead, isTop, isTop ? "top-activity" : "activity");
    }

注释1：启动activity。

注释2：进程未启动。

9.2 进程已启动

9.2.1 realStartActivityLocked()

    boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
            boolean andResume, boolean checkConfig) throws RemoteException {

        ...
                //创建 Activity 启动的 transaction
                final ClientTransaction clientTransaction = ClientTransaction.obtain(
                        proc.getThread(), r.appToken);

                final boolean isTransitionForward = r.isTransitionForward();
                clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
                        System.identityHashCode(r), r.info,
                        mergedConfiguration.getGlobalConfiguration(),
                        mergedConfiguration.getOverrideConfiguration(), r.compat,
                        r.launchedFromPackage, task.voiceInteractor, proc.getReportedProcState(),
                        r.getSavedState(), r.getPersistentSavedState(), results, newIntents,
                        r.takeOptions(), isTransitionForward,
                        proc.createProfilerInfoIfNeeded(), r.assistToken, activityClientController,
                        r.createFixedRotationAdjustmentsIfNeeded(), r.shareableActivityToken,
                        r.getLaunchedFromBubble()));

                // Set desired final state.
                final ActivityLifecycleItem lifecycleItem;
                if (andResume) {
                    lifecycleItem = ResumeActivityItem.obtain(isTransitionForward);
                } else {
                    lifecycleItem = PauseActivityItem.obtain();
                }
                clientTransaction.setLifecycleStateRequest(lifecycleItem);

                // Schedule transaction.
                mService.getLifecycleManager().scheduleTransaction(clientTransaction);
         
        ...
        return true;
    }

这的 mService 就是 ActivityTaskManagerService(ATMS) 的实例。mService.getLifecycleManager() 返回的是一个 ClientLifecycleManager 对象。想看 scheduleTransaction() 是干嘛的 只能找它了。

9.2.2 ClientLifecycleManager.scheduleTransaction()

frameworks/base/services/core/java/com/android/server/wm/ClientLifecycleManager.java

    //这里咱们传入的是一个启动 Activity 的 transaction
    void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();
        //注释1
        transaction.schedule();
        if (!(client instanceof Binder)) {
            // 如果 client 不是Binder的实例，则它是一个远程调用，此时可以安全地回收该对象。
            // 在ActivityThread中的客户端上执行transaction后，将回收用于本地调用的所有对象。
            transaction.recycle();
        }
    }

从方法中看出 transaction 是个 ClientTransaction 。那咱直接去找他。

9.2.3 ClientTransaction.schedule()

    /** Target client. */
    private IApplicationThread mClient;
    
    public void schedule() throws RemoteException {
        mClient.scheduleTransaction(this);
    }

看到 IApplicationThread ，咱们直接去找 ApplicationThread 。

9.2.4 ApplicationThread.scheduleTransaction()

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

ApplicationThread 属于 ActivityThread 的内部类。

public final class ActivityThread extends ClientTransactionHandler
        implements ActivityThreadInternal {
    ...
    private class ApplicationThread extends IApplicationThread.Stub {
        ...
        @Override
        public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
            ActivityThread.this.scheduleTransaction(transaction);
        }
        ...
    }
    ...
}

ActivityThread 类中是没有 scheduleTransaction 方法的，通常遇到这种状况，直接找其父类 ClientTransactionHandler。

9.2.5 ClientTransactionHandler.scheduleTransaction()

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

public abstract class ClientTransactionHandler {
    /** Prepare and schedule transaction for execution. */
    void scheduleTransaction(ClientTransaction transaction) {
        transaction.preExecute(this);
        sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
    }
}

从这里看到 他调用 sendMessage() 方法发送了个 ActivityThread.H.EXECUTE_TRANSACTION 消息，咱们回到其子类 ActivityThread 中查看。

9.2.6 ActivityThread.H.handleMessage()

这里的 H 是 ActivityThread 的 内部类。

    // An executor that performs multi-step transactions.
    private final TransactionExecutor mTransactionExecutor = new TransactionExecutor(this);
    class H extends Handler {
        public void handleMessage(Message msg) {
            switch (msg.what) {
                        ...
                        case EXECUTE_TRANSACTION:
                    final ClientTransaction transaction = (ClientTransaction) msg.obj;
                    mTransactionExecutor.execute(transaction);
                    if (isSystem()) {
                        transaction.recycle();
                    }
                    break;
                    ...
            }
        }
    }

这里的 mTransactionExecutor 是 transactions 的执行者。

9.2.7 TransactionExecutor.execute()

frameworks/base/core/java/android/app/servertransaction/TransactionExecutor.java

    public void execute(ClientTransaction transaction) {
        ...
        executeCallbacks(transaction);
        ...
    }

9.2.8 TransactionExecutor.executeCallbacks()

    /** Cycle through all states requested by callbacks and execute them at proper times. */
    @VisibleForTesting
    public void executeCallbacks(ClientTransaction transaction) {
        final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
        ...
        final IBinder token = transaction.getActivityToken();
        ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
        ...
        final int size = callbacks.size();
        for (int i = 0; i < size; ++i) {
            ...
            if (postExecutionState != UNDEFINED && r != null) {
                // 重点
                final boolean shouldExcludeLastTransition =
                        i == lastCallbackRequestingState && finalState == postExecutionState;
                cycleToPath(r, postExecutionState, shouldExcludeLastTransition, transaction);
            }
        }
    }

这个方法的重点是 cycleToPath() ，咱们继续。

9.2.9 TransactionExecutor.cycleToPath()

    private void cycleToPath(ActivityClientRecord r, int finish, boolean excludeLastState,
            ClientTransaction transaction) {
        final int start = r.getLifecycleState();
        //IntArray：实现不断增长的int基元数组。
        //获取此次要执行生命周期的路径path
        final IntArray path = mHelper.getLifecyclePath(start, finish, excludeLastState);
        performLifecycleSequence(r, path, transaction);
    }

9.2.10 TransactionExecutor.performLifecycleSequence()

    /** Transition the client through previously initialized state sequence. */
    private void performLifecycleSequence(ActivityClientRecord r, IntArray path,
            ClientTransaction transaction) {
        final int size = path.size();
        for (int i = 0, state; i < size; i++) {
            state = path.get(i);
            
            switch (state) {
                case ON_CREATE:
                    mTransactionHandler.handleLaunchActivity(r, mPendingActions,
                            null /* customIntent */);
                    break;
                case ON_START:
                    mTransactionHandler.handleStartActivity(r, mPendingActions,
                            null /* activityOptions */);
                    break;
                case ON_RESUME:
                    mTransactionHandler.handleResumeActivity(r, false /* finalStateRequest */,
                            r.isForward, "LIFECYCLER_RESUME_ACTIVITY");
                    break;
                case ON_PAUSE:
                    mTransactionHandler.handlePauseActivity(r, false /* finished */,
                            false /* userLeaving */, 0 /* configChanges */, mPendingActions,
                            "LIFECYCLER_PAUSE_ACTIVITY");
                    break;
                case ON_STOP:
                    mTransactionHandler.handleStopActivity(r, 0 /* configChanges */,
                            mPendingActions, false /* finalStateRequest */,
                            "LIFECYCLER_STOP_ACTIVITY");
                    break;
                case ON_DESTROY:
                    mTransactionHandler.handleDestroyActivity(r, false /* finishing */,
                            0 /* configChanges */, false /* getNonConfigInstance */,
                            "performLifecycleSequence. cycling to:" + path.get(size - 1));
                    break;
                case ON_RESTART:
                    mTransactionHandler.performRestartActivity(r, false /* start */);
                    break;
                default:
                    throw new IllegalArgumentException("Unexpected lifecycle state: " + state);
            }
        }
    }

到这里咱们终于看到了熟悉的内容，Activity的生命周期，好激动哈哈。

这里咱们以 ON_CREATE 为例。从上面代码看出 ON_CREATE 等其实就是个int值(常量)，这里咱就不多描述了。mTransactionHandler 是 ClientTransactionHandler 的实例，直接看handleLaunchActivity() 方法。

9.2.11 ClientTransactionHandler.handleLaunchActivity()

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

public abstract class ClientTransactionHandler {
    ...
    /** Perform activity launch. */
    public abstract Activity handleLaunchActivity(@NonNull ActivityClientRecord r,
            PendingTransactionActions pendingActions, Intent customIntent);
    ...    
}

从上面看出 handleLaunchActivity() 是抽象方法，ActivityThread 做为其子类，只能找其子类 ActivityThread 看看具体实现。

9.2.12 ActivityThread.handleLaunchActivity()

    /**
     * Extended implementation of activity launch. Used when server requests a launch or relaunch.
     */
    @Override
    public Activity handleLaunchActivity(ActivityClientRecord r,
            PendingTransactionActions pendingActions, Intent customIntent) {
        // 在创建 activity 之前初始化
        if (ThreadedRenderer.sRendererEnabled
                && (r.activityInfo.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0) {
            HardwareRenderer.preload();
        }
        //初始化 Activity 的 WindowManager，每一个 Activity 都会对应一个"窗口"。
        WindowManagerGlobal.initialize();

        // 提示 GraphicsEnvironment Activity 正在进程中启动。
        GraphicsEnvironment.hintActivityLaunch();

        //这里重点：Activity 启动的核心实现。
        final Activity a = performLaunchActivity(r, customIntent);
        ...
        return a;
    }

performLaunchActivity()，Activity 启动的核心实现。具体咱们往下看

9.2.13 ActivityThread.performLaunchActivity()

    /**  Core implementation of activity launch. */
    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        ...
        ContextImpl appContext = createBaseContextForActivity(r);
        Activity activity = null;
        try {
            java.lang.ClassLoader cl = appContext.getClassLoader();
            //注释1： 新实例化的 Activity 对象。
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
            ...
        } catch (Exception e) {
            ...
        }
                //重点：调用activity.attach()。
                //建立 Activity 与 Context 之间的联系。
                //创建 PhoneWindow 对象，并与 Activity 进行关联操作。
                activity.attach(appContext, this, getInstrumentation(), r.token,
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigurationInstances, config,
                        r.referrer, r.voiceInteractor, window, r.configCallback,
                        r.assistToken, r.shareableActivityToken);
                        
                //persistableMode属性设置为： ActivityInfo.PERSIST_ACROSS_REBOOTS(persistAcrossReboots);
                //注释2：执行 Activity 的 onCreate() 方法。
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
        return activity;
    }

这里首先看看 mInstrumentation 其实就是 Instrumentation 。 在本文 startActivityForResult 部分也有用到。Instrumentation 负责 Application 和 Activity 的建立和生命周期控制。

这里用到了他的两个方法咱们一起看看。

9.2.14 Instrumentation.newActivity()

    /**
     * @param cl 用于实例化对象的类加载器(ClassLoader)。
     * @param className 实现 Activity 对象的类的名称。
     * @param intent 指定要实例化的活动类的 Intent 对象。
     * 
     * @return 新实例化的 Activity 对象。
     */
    public Activity newActivity(ClassLoader cl, String className,
            Intent intent)
            throws InstantiationException, IllegalAccessException,
            ClassNotFoundException {
        String pkg = intent != null && intent.getComponent() != null
                ? intent.getComponent().getPackageName() : null;
        return getFactory(pkg).instantiateActivity(cl, className, intent);
    }

pkg 就是 intent 所在的包名。

getFactory(pkg) 通过获取到的包名 得到 AppComponentFactory 。然后调用其 instantiateActivity() 方法获取 Activity 。

9.2.14.1 Instrumentation.getFactory()

    private AppComponentFactory getFactory(String pkg) {
        if (pkg == null) {
            ...
            return AppComponentFactory.DEFAULT;
        }
        if (mThread == null) {
            ....
            return AppComponentFactory.DEFAULT;
        }
        LoadedApk apk = mThread.peekPackageInfo(pkg, true);
        // This is in the case of starting up "android".
        if (apk == null) apk = mThread.getSystemContext().mPackageInfo;
        return apk.getAppFactory();
    }

9.2.14.2 AppComponentFactory.instantiateActivity()

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

/**
  * 用于控制 manifest 元素实例化的接口。
  *
  * @see #instantiateApplication
  * @see #instantiateActivity
  * @see #instantiateClassLoader
  * @see #instantiateService
  * @see #instantiateReceiver
  * @see #instantiateProvider
  */
public class AppComponentFactory {
      ...
}

instantiateActivity()

    /**
     * 此方法仅用于提供用于实例化的钩子。
     * 它不提供对 Activity 对象的早期访问。
     * 返回的对象还不会被初始化为 Context 并且不应该用于与其他 android API 交互。
     */
    public @NonNull Activity instantiateActivity(@NonNull ClassLoader cl, @NonNull String className,
            @Nullable Intent intent)
            throws InstantiationException, IllegalAccessException, ClassNotFoundException {
        return (Activity) cl.loadClass(className).newInstance();
    }

这里通过反射拿到 Activity 的实例。

9.2.15 Instrumentation.callActivityOnCreate()

    /**
     * 执行 Activity#onCreate方法的调用。
     *
     * @param activity 正在创建的 activity 。
     * @param icicle 将以前冻结的状态（或null）传递给 onCreate() 。
     */
    public void callActivityOnCreate(Activity activity, Bundle icicle) {
        prePerformCreate(activity);
        //重点
        activity.performCreate(icicle);
        postPerformCreate(activity);
    }

从这里可以看到即将进入 Activity 的内部。

9.2.15.1 Activity.performCreate()

    final void performCreate(Bundle icicle) {
        performCreate(icicle, null);
    }

    @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
    final void performCreate(Bundle icicle, PersistableBundle persistentState) {
        ...
        //重点
        if (persistentState != null) {
            onCreate(icicle, persistentState);
        } else {
            onCreate(icicle);
        }
        ...
    }

看到这里，startSpecificActivity() 方法中的 进程已启动的状态算是完结。startActivity 启动成功。结合 9.2.13 ActivityThread.performLaunchActivity() 查看 Activity 中的 attach() 方法调用早于 onCreate() 方法。

下面让我们回到 9.1 startSpecificActivity() 中查看进程未启动的流程情况。

9.3 进程未启动

既然进程未启动那肯定先启动进程了。咱们接着看。

    final ActivityTaskManagerService mService;
    void startSpecificActivity(ActivityRecord r, boolean andResume, boolean checkConfig) {
        ....
        mService.startProcessAsync(r, knownToBeDead, isTop, isTop ? "top-activity" : "activity");
    }

从这里看到 mService 其实就是 ATMS ，走着。

9.3.1 ATMS.startProcessAsync()

    void startProcessAsync(ActivityRecord activity, boolean knownToBeDead, boolean isTop,
            String hostingType) {
        try {
            ...
            // 发布消息以启动进程，以避免在持有 ATMS 锁的情况下调用 AMS 可能出现死锁。
            final Message m = PooledLambda.obtainMessage(ActivityManagerInternal::startProcess,
                    mAmInternal, activity.processName, activity.info.applicationInfo, knownToBeDead,
                    isTop, hostingType, activity.intent.getComponent());
            mH.sendMessage(m);
        } finally {
            Trace.traceEnd(TRACE_TAG_WINDOW_MANAGER);
        }
    }

9.3.2 ActivityManagerInternal.startProcess()

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

/**
 * Activity manager local system service interface.
 */
public abstract class ActivityManagerInternal {
    /** Starts a given process. */
    public abstract void startProcess(String processName, ApplicationInfo info,
            boolean knownToBeDead, boolean isTop, String hostingType, ComponentName hostingName);
}

这里是个抽象类，咱们去找其实现类 ActivityManagerService.LocalService。

9.3.3 ActivityManagerService.LocalService.startProcess()

frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

public class ActivityManagerService extends IActivityManager.Stub
        implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback, ActivityManagerGlobalLock {   
    ...
    @VisibleForTesting
    public final class LocalService extends ActivityManagerInternal
            implements ActivityManagerLocal {
        ...
        @Override
        public void startProcess(String processName, ApplicationInfo info, boolean knownToBeDead,
                boolean isTop, String hostingType, ComponentName hostingName) {
            try {
                synchronized (ActivityManagerService.this) {
                    // 如果该进程称为top app，请设置一个提示，以便在启动该进程时，可以立即应用最高优先级，以避免在附加top app进程之前cpu被其他进程抢占。
                    
                    startProcessLocked(processName, info, knownToBeDead, 0 /* intentFlags */,
                            new HostingRecord(hostingType, hostingName, isTop),
                            ZYGOTE_POLICY_FLAG_LATENCY_SENSITIVE, false /* allowWhileBooting */,
                            false /* isolated */);
                }
            } finally {
            }
        }
    }
}    

9.3.4 ActivityManagerService.startProcessLocked()

    /**
     * Process management.
     */
    final ProcessList mProcessList;
    @GuardedBy("this")
    final ProcessRecord startProcessLocked(String processName,
            ApplicationInfo info, boolean knownToBeDead, int intentFlags,
            HostingRecord hostingRecord, int zygotePolicyFlags, boolean allowWhileBooting,
            boolean isolated, boolean keepIfLarge) {
        return mProcessList.startProcessLocked(processName, info, knownToBeDead, intentFlags,
                hostingRecord, zygotePolicyFlags, allowWhileBooting, isolated, 0 /* isolatedUid */,
                keepIfLarge, null /* ABI override */, null /* entryPoint */,
                null /* entryPointArgs */, null /* crashHandler */);
    }

可以看出，这里的 mProcessList 就是 ProcessList。

9.3.5 ProcessList.startProcessLocked()

frameworks/base/services/core/java/com/android/server/am/ProcessList.java

    @GuardedBy("mService")
    final ProcessRecord startProcessLocked(String processName, ApplicationInfo info,
            boolean knownToBeDead, int intentFlags, HostingRecord hostingRecord,
            int zygotePolicyFlags, boolean allowWhileBooting, boolean isolated, int isolatedUid,
            boolean keepIfLarge, String abiOverride, String entryPoint, String[] entryPointArgs,
            Runnable crashHandler) {
        ...
        final boolean success =
                startProcessLocked(app, hostingRecord, zygotePolicyFlags, abiOverride);
        checkSlow(startTime, "startProcess: done starting proc!");
        return success ? app : null;
    }
    ------------分割线-------------
    @GuardedBy("mService")
    final boolean startProcessLocked(ProcessRecord app, HostingRecord hostingRecord,
            int zygotePolicyFlags, String abiOverride) {
        return startProcessLocked(app, hostingRecord, zygotePolicyFlags,
                false /* disableHiddenApiChecks */, false /* disableTestApiChecks */,
                false /* mountExtStorageFull */, abiOverride);
    }
    ------------分割线-------------
    /**
     * @return {@code true} if process start is successful, false otherwise.
     */
    @GuardedBy("mService")
    boolean startProcessLocked(ProcessRecord app, HostingRecord hostingRecord,
            int zygotePolicyFlags, boolean disableHiddenApiChecks, boolean disableTestApiChecks,
            boolean mountExtStorageFull, String abiOverride) {
            ...
            return startProcessLocked(hostingRecord, entryPoint, app, uid, gids,
                    runtimeFlags, zygotePolicyFlags, mountExternal, seInfo, requiredAbi,
                    instructionSet, invokeWith, startTime);
    }
    ------------分割线-------------
    @GuardedBy("mService")
    boolean startProcessLocked(HostingRecord hostingRecord, String entryPoint, ProcessRecord app,
            int uid, int[] gids, int runtimeFlags, int zygotePolicyFlags, int mountExternal,
            String seInfo, String requiredAbi, String instructionSet, String invokeWith,
            long startTime) {
                ...
                //终于到重点了。
                final Process.ProcessStartResult startResult = startProcess(hostingRecord,
                        entryPoint, app,
                        uid, gids, runtimeFlags, zygotePolicyFlags, mountExternal, seInfo,
                        requiredAbi, instructionSet, invokeWith, startTime);
                
                handleProcessStartedLocked(app, startResult.pid, startResult.usingWrapper,
                        startSeq, false);
                ...
    }

反复调用，终于看到了关键代码 startProcess() 。

9.3.6 ProcessList.startProcess()

    private Process.ProcessStartResult startProcess(HostingRecord hostingRecord, String entryPoint,
            ProcessRecord app, int uid, int[] gids, int runtimeFlags, int zygotePolicyFlags,
            int mountExternal, String seInfo, String requiredAbi, String instructionSet,
            String invokeWith, long startTime) {

            ...
            final Process.ProcessStartResult startResult;
            if (hostingRecord.usesWebviewZygote()) {
                //进程是否应该从 webview zygote 产生
                startResult = startWebView(entryPoint,
                        app.processName, uid, uid, gids, runtimeFlags, mountExternal,
                        app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
                        app.info.dataDir, null, app.info.packageName, app.mDisabledCompatChanges,
                        new String[]{PROC_START_SEQ_IDENT + app.startSeq});
            } else if (hostingRecord.usesAppZygote()) {
                //进程是否应该从应用程序 zygote 中产生
                final AppZygote appZygote = createAppZygoteForProcessIfNeeded(app);

                // 我们无法隔离应用程序数据和存储数据，因为父 zygote 已经这样做了。
                startResult = appZygote.getProcess().start(entryPoint,
                        app.processName, uid, uid, gids, runtimeFlags, mountExternal,
                        app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
                        app.info.dataDir, null, app.info.packageName,
                        /*zygotePolicyFlags=*/ ZYGOTE_POLICY_FLAG_EMPTY, isTopApp,
                        app.mDisabledCompatChanges, pkgDataInfoMap, whitelistedAppDataInfoMap,
                        false, false,
                        new String[]{PROC_START_SEQ_IDENT + app.startSeq});
            } else {
                startResult = Process.start(entryPoint,
                        app.processName, uid, uid, gids, runtimeFlags, mountExternal,
                        app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
                        app.info.dataDir, invokeWith, app.info.packageName, zygotePolicyFlags,
                        isTopApp, app.mDisabledCompatChanges, pkgDataInfoMap,
                        whitelistedAppDataInfoMap, bindMountAppsData, bindMountAppStorageDirs,
                        new String[]{PROC_START_SEQ_IDENT + app.startSeq});
            }
            ...
   }

无论那种情况都会调用 Process.start() ;

9.3.7 Process.start()

frameworks/base/core/java/android/os/Process.java

    /**
     * State associated with the zygote process.
     */
    public static final ZygoteProcess ZYGOTE_PROCESS = new ZygoteProcess();
    public static ProcessStartResult start(@NonNull final String processClass,
                                           @Nullable final String niceName,
                                           int uid, int gid, @Nullable int[] gids,
                                           int runtimeFlags,
                                           int mountExternal,
                                           int targetSdkVersion,
                                           @Nullable String seInfo,
                                           @NonNull String abi,
                                           @Nullable String instructionSet,
                                           @Nullable String appDataDir,
                                           @Nullable String invokeWith,
                                           @Nullable String packageName,
                                           int zygotePolicyFlags,
                                           boolean isTopApp,
                                           @Nullable long[] disabledCompatChanges,
                                           @Nullable Map<String, Pair<String, Long>>
                                                   pkgDataInfoMap,
                                           @Nullable Map<String, Pair<String, Long>>
                                                   whitelistedDataInfoMap,
                                           boolean bindMountAppsData,
                                           boolean bindMountAppStorageDirs,
                                           @Nullable String[] zygoteArgs) {
        return ZYGOTE_PROCESS.start(processClass, niceName, uid, gid, gids,
                    runtimeFlags, mountExternal, targetSdkVersion, seInfo,
                    abi, instructionSet, appDataDir, invokeWith, packageName,
                    zygotePolicyFlags, isTopApp, disabledCompatChanges,
                    pkgDataInfoMap, whitelistedDataInfoMap, bindMountAppsData,
                    bindMountAppStorageDirs, zygoteArgs);
    }

这里的 ZYGOTE_PROCESS 就是 ZygoteProcess 。调用其 start() 方法。

9.3.8 ZygoteProcess.start()

frameworks/base/core/java/android/os/ZygoteProcess.java

    /**
     * 开始一个新的进程。
     * 如果启用了进程，则会创建一个新进程并在那里执行processClass 的静态main() 函数。
     * 该函数返回后进程将继续运行。
     * 
     * 如果未启用进程，则会在调用者进程中创建一个新线程并在那里调用 processclass 的 main()。
     * 
     * niceName 参数，如果不是空字符串，则是一个自定义名称，用于提供给进程而不是使用 processClass。
     * 这允许您创建易于识别的进程，即使您使用相同的基本 processClass 来启动它们。
     *
     * 当 invokeWith 不为 null 时，进程将作为一个新的 app 而不是 zygote fork 启动。
     * 请注意，这仅适用于 uid 0 或当runtimeFlags 包含 DEBUG_ENABLE_DEBUGGER 时。
     *
     * @param processClass 用作进程主入口点的类。
     * @param niceName 用于进程的更易读的名称。
     * @param uid 进程将在其下运行的用户 ID。
     * @param gid 进程将在其下运行的组 ID。
     * @param gids 与进程关联的附加组 ID。
     * @param runtimeFlags 附加标志。
     * @param targetSdkVersion 应用的目标 SDK 版本。
     * @param seInfo null-ok 新进程的 SELinux 信息。
     * @param abi 非空此应用程序应使用的 ABI。
     * @param instructionsSet null-确定要使用的指令集。
     * @param appDataDir null-ok 应用程序的数据目录。
     * @param invokeWith null-ok 要调用的命令。
     * @param packageName null-ok 这个进程所属的包名。
     * @param zygotePolicyFlags 用于确定如何启动应用程序的标志。
     * @param isTopApp 进程是否为高优先级应用程序启动。
     * @param disabledCompatChanges 正在启动的进程的禁用兼容更改的 null-ok 列表。
     * @param pkgDataInfoMap 从相关包名称映射到私有数据目录卷 UUID 和 inode 编号。
     * @param allowlistedDataInfoList 从允许的包名称映射到私有数据目录卷 UUID 和 inode 编号。
     * @param bindMountAppsData zygote 是否需要挂载 CE 和 DE 数据。
     * @param bindMountAppStorageDirs zygote 是否需要挂载Android/obb 和Android/data。
     *
     * @param zygoteArgs 提供给 Zygote 进程的附加参数。
     * @return 一个对象，描述尝试启动进程的结果。
     * @throws RuntimeException 出现致命启动失败
     */
    //这里从上一个方法传来一大堆参数就不多描述了
    public final Process.ProcessStartResult start(...) {
        ...
            return startViaZygote(processClass, niceName, uid, gid, gids,
                    runtimeFlags, mountExternal, targetSdkVersion, seInfo,
                    abi, instructionSet, appDataDir, invokeWith, /*startChildZygote=*/ false,
                    packageName, zygotePolicyFlags, isTopApp, disabledCompatChanges,
                    pkgDataInfoMap, allowlistedDataInfoList, bindMountAppsData,
                    bindMountAppStorageDirs, zygoteArgs);
        ...
    }

9.3.9 ZygoteProcess.startViaZygote()

    //这里从上一个方法传来一大堆参数就不多描述了
    private Process.ProcessStartResult startViaZygote(...)
                                                      throws ZygoteStartFailedEx {
        ...
        synchronized(mLock) {
            // The USAP pool can not be used if the application will not use the systems graphics
            // driver.  If that driver is requested use the Zygote application start path.
            return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi),
                                              zygotePolicyFlags,
                                              argsForZygote);
        }
    }

在看 zygoteSendArgsAndGetResult() 之前咱们看看 openZygoteSocketIfNeeded(abi) 。

9.3.10 ZygoteProcess.openZygoteSocketIfNeeded()

    /**
     * 尝试打开一个 session socket到具有兼容 ABI 的 Zygote 进程（如果尚未打开）。 如果兼容的 session socket已经打开，则返回该 session socket。
     * 
     * 此功能可能会阻塞，并且可能必须尝试连接到多个 Zygotes 才能找到合适的 Zygotes。 需要保持 mLock。
     */
    @GuardedBy("mLock")
    private ZygoteState openZygoteSocketIfNeeded(String abi) throws ZygoteStartFailedEx {
        try {
            attemptConnectionToPrimaryZygote();

            ...
    }
    ------------分割线-------------
    /**
     * 如果Primary Zygote不存在或已断开连接，则为它创建一个 ZygoteState。
     */
    @GuardedBy("mLock")
    private void attemptConnectionToPrimaryZygote() throws IOException {
        if (primaryZygoteState == null || primaryZygoteState.isClosed()) {
            primaryZygoteState =
                    ZygoteState.connect(mZygoteSocketAddress, mUsapPoolSocketAddress);
            ...
        }
    }
    ------------分割线-------------
    static ZygoteState connect(@NonNull LocalSocketAddress zygoteSocketAddress,
            @Nullable LocalSocketAddress usapSocketAddress)
            throws IOException {
        ...
        //创建LocalSocket
        final LocalSocket zygoteSessionSocket = new LocalSocket();
            //将zygoteSessionSocket连接到端点。
            //只能在尚未连接的实例上调用。
            zygoteSessionSocket.connect(zygoteSocketAddress);
        ...
    }

打开了 ZygoteSocket 之后咱们看看 zygoteSendArgsAndGetResult();

9.4.11 ZygoteProcess.zygoteSendArgsAndGetResult()

    @GuardedBy("mLock")
    private Process.ProcessStartResult zygoteSendArgsAndGetResult(
            ZygoteState zygoteState, int zygotePolicyFlags, @NonNull ArrayList<String> args)
            throws ZygoteStartFailedEx {
        ...
        String msgStr = args.size() + "\n" + String.join("\n", args) + "\n";        
        return attemptZygoteSendArgsAndGetResult(zygoteState, msgStr);
    }

9.4.12 ZygoteProcess.attemptZygoteSendArgsAndGetResult()

    private Process.ProcessStartResult attemptZygoteSendArgsAndGetResult(
            ZygoteState zygoteState, String msgStr) throws ZygoteStartFailedEx {
        try {
            final BufferedWriter zygoteWriter = zygoteState.mZygoteOutputWriter;
            final DataInputStream zygoteInputStream = zygoteState.mZygoteInputStream;

            //写，调用zygote(ZygoteInit)。
            zygoteWriter.write(msgStr);
            zygoteWriter.flush();

            // Always read the entire result from the input stream to avoid leaving
            // bytes in the stream for future process starts to accidentally stumble
            // upon.
            Process.ProcessStartResult result = new Process.ProcessStartResult();
            //读，造成阻塞
            result.pid = zygoteInputStream.readInt();
            result.usingWrapper = zygoteInputStream.readBoolean();

            if (result.pid < 0) {
                throw new ZygoteStartFailedEx("fork() failed");
            }

            return result;
        } catch (IOException ex) {
            zygoteState.close();
            Log.e(LOG_TAG, "IO Exception while communicating with Zygote - "
                    + ex.toString());
            throw new ZygoteStartFailedEx(ex);
        }
    }

9.4.13 ZygoteInit.main()

    @UnsupportedAppUsage
    public static void main(String[] argv) {
            ...
            // zygote 永久循环。
            caller = zygoteServer.runSelectLoop(abiList);
    }

9.4.14 ZygoteService.runSelectLoop()

    /**
     * 运行 zygote 进程的选择循环。 在新连接发生时接受新连接，并从连接中读取一次一个 spawn-request 值的命令。
     * @param abiList 此 zygote 支持的 ABI 列表。
     */
    Runnable runSelectLoop(String abiList) {
        ...
                            //Session socket accepted from the Zygote server socket
                            ZygoteConnection connection = peers.get(pollIndex);
                            //通过 fork 启动 子进程(应用程序)
                            final Runnable command = connection.processOneCommand(this);
        ...                                   
    }

9.4.15 ZygoteConnection.processOneCommand()

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

    Runnable processOneCommand(ZygoteServer zygoteServer) {
        ...
        pid = Zygote.forkAndSpecialize(parsedArgs.mUid, parsedArgs.mGid, parsedArgs.mGids,
                parsedArgs.mRuntimeFlags, rlimits, parsedArgs.mMountExternal, parsedArgs.mSeInfo,
                parsedArgs.mNiceName, fdsToClose, fdsToIgnore, parsedArgs.mStartChildZygote,
                parsedArgs.mInstructionSet, parsedArgs.mAppDataDir, parsedArgs.mIsTopApp,
                parsedArgs.mPkgDataInfoList, parsedArgs.mWhitelistedDataInfoList,
                parsedArgs.mBindMountAppDataDirs, parsedArgs.mBindMountAppStorageDirs);
        ...
    }

fork 出一个新的进程(APP)，一个APP的程序入口就是ActivityThread.main()。

9.4.15 ActivityThread.main()

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

在 ActivityThread 初始化的时候，就已经创建消息循环了，所以在主线程里面创建 Handler 不需要指定 Looper，而如果在其他线程使用Handler，则需要 单独使用Looper.prepare()和Looper.loop() 创建消息循环。

    public static void main(String[] args) {
        ...
        //初始化当前进程的 Looper 对象
        Looper.prepareMainLooper();
        ...
        ActivityThread thread = new ActivityThread();
        //此处创建Application
        thread.attach(false, startSeq);

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

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

        // 调用 Looper 的 loop 方法开启无限循环。
        Looper.loop();

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

Looper ：从 MessageQueue 中取出 Message，然后处理 Message 中指定的任务(startActivity)。

至此，不管是调用 startActivity() 启动页面，还是调用 startActivity() 开启其他进程的界面都算完成了。

如有问题麻烦指正。