1.前言

• 最近一直在看 《Android进阶解密》 的一本书，这本书编写逻辑、流程都非常好，而且很容易看懂，非常推荐大家去看看（没有收广告费，单纯觉得作者写的很好）。
• 上一篇简单的介绍了Android进阶（三）：Application启动过程(最详细&最简单)。
• 今天就介绍Application启动之后，是如何启动应用程序中的 第一个Activity （基于Android 8.0 系统）。
• 文章中实例 linhaojian的Github

2.Activity启动过程的时序图

Activity启动流程.png

3.源码分析

3.1 ActivityManagerService：

private final boolean attachApplicationLocked(IApplicationThread thread,
      int pid, int callingUid, long startSeq) {
    //...
    //通知ActivityThread启动application
    thread.bindApplication(processName, appInfo, providers, null, profilerInfo,
                    null, null, null, testMode,
                    mBinderTransactionTrackingEnabled, enableTrackAllocation,
                    isRestrictedBackupMode || !normalMode, app.persistent,
                    new Configuration(getGlobalConfiguration()), app.compat,
                    getCommonServicesLocked(app.isolated),
                    mCoreSettingsObserver.getCoreSettingsLocked(),
                    buildSerial, isAutofillCompatEnabled);// 1
     // 启动第一个Activity
    if (normalMode) {
        try {
            //Application初始化之后，初始化&启动第一个Activity
            if (mStackSupervisor.attachApplicationLocked(app)) {// 2
                didSomething = true;
            }
        } catch (Exception e) {
            Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
            badApp = true;
        }
    }     
}

• 注释1：通知ActivityThread启动application；
• 注释2：调用ActivityStackSupervisor，启动第一个Activity；

3.2 ActivityStackSupervisor 启动Activity：

    boolean attachApplicationLocked(ProcessRecord app) throws RemoteException {
        //获取应用的进行名称
        final String processName = app.processName;
        boolean didSomething = false;
        for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
            final ActivityDisplay display = mActivityDisplays.valueAt(displayNdx);
            for (int stackNdx = display.getChildCount() - 1; stackNdx >= 0; --stackNdx) {
                final ActivityStack stack = display.getChildAt(stackNdx);
                if (!isFocusedStack(stack)) {
                    continue;
                }
                stack.getAllRunningVisibleActivitiesLocked(mTmpActivityList);
                final ActivityRecord top = stack.topRunningActivityLocked();
                final int size = mTmpActivityList.size();
                for (int i = 0; i < size; i++) {
                    final ActivityRecord activity = mTmpActivityList.get(i);
                    if (activity.app == null && app.uid == activity.info.applicationInfo.uid
                            && processName.equals(activity.processName)) {
                        try {
                            // Application初始化之后，启动第一个Activity
                            if (realStartActivityLocked(activity, app,
                                    top == activity /* andResume */, true /* checkConfig */)) { // 1
                                didSomething = true;
                            }
                        } catch (RemoteException e) {
                            Slog.w(TAG, "Exception in new application when starting activity "
                                    + top.intent.getComponent().flattenToShortString(), e);
                            throw e;
                        }
                    }
                }
            }
        }
        if (!didSomething) {
            ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
        }
        return didSomething;
    }

• 注释1：调用自身的realStartActivityLocked()，真正的启动Activity；

    final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
            boolean andResume, boolean checkConfig) throws RemoteException {
                //...
                // Create activity launch transaction.
                //通知ActivityThread创建activity的实例 ；调用Activity的OnCreate； 创建对应PhoneWindow实例
                final ClientTransaction clientTransaction = ClientTransaction.obtain(app.thread,
                        r.appToken);
                clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
                        System.identityHashCode(r), r.info,
                        // TODO: Have this take the merged configuration instead of separate global
                        // and override configs.
                        mergedConfiguration.getGlobalConfiguration(),
                        mergedConfiguration.getOverrideConfiguration(), r.compat,
                        r.launchedFromPackage, task.voiceInteractor, app.repProcState, r.icicle,
                        r.persistentState, results, newIntents, mService.isNextTransitionForward(),
                        profilerInfo)); // 2
                // Set desired final state.
                final ActivityLifecycleItem lifecycleItem;
                if (andResume) {
                    lifecycleItem = ResumeActivityItem.obtain(mService.isNextTransitionForward()); // 3
                } else {
                    lifecycleItem = PauseActivityItem.obtain(); // 4
                }
                clientTransaction.setLifecycleStateRequest(lifecycleItem);
                // 编制启动Activity实例的事务，并按顺序调用onCreate();onStart();onResume();
                mService.getLifecycleManager().scheduleTransaction(clientTransaction);// 5
    }

• 注释2：封装启动Activity的相关数据与Activity的启动流程（LaunchActivityItem 下面会介绍该类）；
• 注释3：封装Activity启动之后生命周期变化为Resume的流程（ResumeActivityItem下面会介绍该类）；
• 注释4：封装Activity启动之后生命周期变化为Pause的流程；
• 注释5：真正开始启动Activity与调用相关的生命周期方法；

3.3 ClientLifecycleManager管理ClientTransaction：

    void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();// 1
        transaction.schedule();// 2
        if (!(client instanceof Binder)) {
            // If client is not an instance of Binder - it's a remote call and at this point it is
            // safe to recycle the object. All objects used for local calls will be recycled after
            // the transaction is executed on client in ActivityThread.
            transaction.recycle();
        }
    }

• 注释1：获取需要启动Activity进程的代理对象IApplicationThread；
• 注释2：调用ClientTransaction中的schedule()；

3.4 ClientTransaction中schedule函数：

    public void schedule() throws RemoteException {
        mClient.scheduleTransaction(this);// 1
    }

• 注释1：mClient就是与ActivityThread通讯的代理对象(IApplicationThread)，所以这里其实是调用ActivityThread类中ApplicationThread内部类的scheduleTransaction()；

3.5 ApplicationThread的scheduleTransaction函数：

    private class ApplicationThread extends IApplicationThread.Stub {
        @Override
        public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
            ActivityThread.this.scheduleTransaction(transaction);// 1
        }    
    }

• 注释1：这里其实最后还是调用了ActivityThread的scheduleTransaction()，而这个scheduleTransaction()其实真正的实现是ClientTransactionHandler，因为ActivityThread是继承ClientTransactionHandler类的；

public abstract class ClientTransactionHandler {
    /** Prepare and schedule transaction for execution. */
    void scheduleTransaction(ClientTransaction transaction) {
        transaction.preExecute(this);
        sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);// 2
    }
}

• 注释2：调用ActivityThread的sendMessage()，将ClientTransaction参数通过Handler机制切换至主线程进行处理；

3.6 ActivityThread的H类：

    class H extends Handler {
        //...
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case EXECUTE_TRANSACTION: // 1
                    final ClientTransaction transaction = (ClientTransaction) msg.obj;// 2
                    mTransactionExecutor.execute(transaction);// 3
                    if (isSystem()) {
                        // Client transactions inside system process are recycled on the client side
                        // instead of ClientLifecycleManager to avoid being cleared before this
                        // message is handled.
                        transaction.recycle();
                    }
                    // TODO(lifecycler): Recycle locally scheduled transactions.
                    break;              
            }        
        }
    }

• 注释1：接收ClientTransactionHandler发送的EXECUTE_TRANSACTION事件；
• 注释2：接收ClientTransactionHandler发送的ClientTransaction参数；
• 注释3：通过TransactionExecutor真正处理ClientTransaction中封装的Activity相关信息；

3.7 TransactionExecutor处理ClientTransaction：

    public void execute(ClientTransaction transaction) {
        final IBinder token = transaction.getActivityToken();
        log("Start resolving transaction for client: " + mTransactionHandler + ", token: " + token);
        executeCallbacks(transaction);
        executeLifecycleState(transaction);
        mPendingActions.clear();
        log("End resolving transaction");
    }

3.8 executeCallbacks()启动Activity：

    @VisibleForTesting
    public void executeCallbacks(ClientTransaction transaction) {
        final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
        if (callbacks == null) {
            // No callbacks to execute, return early.
            return;
        }
        log("Resolving callbacks");
        final IBinder token = transaction.getActivityToken();
        ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
        // In case when post-execution state of the last callback matches the final state requested
        // for the activity in this transaction, we won't do the last transition here and do it when
        // moving to final state instead (because it may contain additional parameters from server).
        final ActivityLifecycleItem finalStateRequest = transaction.getLifecycleStateRequest();
        final int finalState = finalStateRequest != null ? finalStateRequest.getTargetState()
                : UNDEFINED;
        // Index of the last callback that requests some post-execution state.
        final int lastCallbackRequestingState = lastCallbackRequestingState(transaction);
        final int size = callbacks.size();
        for (int i = 0; i < size; ++i) {
            final ClientTransactionItem item = callbacks.get(i);// 1
            log("Resolving callback: " + item);
            final int postExecutionState = item.getPostExecutionState();
            final int closestPreExecutionState = mHelper.getClosestPreExecutionState(r,
                    item.getPostExecutionState());
            if (closestPreExecutionState != UNDEFINED) {
                cycleToPath(r, closestPreExecutionState);
            }
            item.execute(mTransactionHandler, token, mPendingActions);// 2
            item.postExecute(mTransactionHandler, token, mPendingActions);
            if (r == null) {
                // Launch activity request will create an activity record.
                r = mTransactionHandler.getActivityClient(token);
            }
            if (postExecutionState != UNDEFINED && r != null) {
                // Skip the very last transition and perform it by explicit state request instead.
                final boolean shouldExcludeLastTransition =
                        i == lastCallbackRequestingState && finalState == postExecutionState;
                cycleToPath(r, postExecutionState, shouldExcludeLastTransition);
            }
        }
    }

• 注释1：获取ClientTransaction中的ClientTransactionItem对象（其实这里的对象就上面3.2介绍中的LaunchActivityItem）；
• 注释2：调用LaunchActivityItem类的execute()，下面我们看看LaunchActivityItem；

public class LaunchActivityItem extends ClientTransactionItem {
    @Override
    public void execute(ClientTransactionHandler client, IBinder token,// 1
            PendingTransactionActions pendingActions) {
        Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
        ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,
                mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,
                mPendingResults, mPendingNewIntents, mIsForward,
                mProfilerInfo, client);
        client.handleLaunchActivity(r, pendingActions, null /* customIntent */);// 2
        Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }
}

• 注释1：刚刚说了ClientTransactionHandler是被ActivityThread继承，所以这里传入进来就是ActivityThread;
• 注释2：看到了吧，经过多次不同层次相互调用，最终真正调用ActivityThread的handleLaunchActivity()，启动应用的第一个Activity;

3.8 executeLifecycleState()改变Activity的生命周期：

    private void executeLifecycleState(ClientTransaction transaction) {
        final ActivityLifecycleItem lifecycleItem = transaction.getLifecycleStateRequest();// 1
        if (lifecycleItem == null) {
            // No lifecycle request, return early.
            return;
        }
        log("Resolving lifecycle state: " + lifecycleItem);
        final IBinder token = transaction.getActivityToken();
        final ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
        if (r == null) {
            // Ignore requests for non-existent client records for now.
            return;
        }
        // Cycle to the state right before the final requested state.
        cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */);
        // Execute the final transition with proper parameters.
        lifecycleItem.execute(mTransactionHandler, token, mPendingActions); // 2
        lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
    }

• 注释1：获取ClientTransaction中的ActivityLifecycleItem 对象（其实这里的对象就上面3.2介绍中的ResumeActivityItem）；
• 注释2：调用ResumeActivityItem的execute()，下面我们看看ResumeActivityItem；

public class ResumeActivityItem extends ActivityLifecycleItem {
    //...
    @Override
    public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityResume");
        client.handleResumeActivity(token, true /* finalStateRequest */, mIsForward, // 1
                "RESUME_ACTIVITY");
        Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }
}

• 注释1：其实就是调用Activity的handleResumeActivity()，最终就会调用Activity的onStart()与onResume()；

4.类关系

Activity启动类结构.png

• 通过上图发现，Activity启动过程就是AMS与ActivityThread不同的进程交互实现的。

5.总结

• 到此，Activity启动过程介绍完毕。
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• linhaojian的Github

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