Android Activity启动流程一：从Intent到Activity创建

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目录

• 一、概览
• 二、应用内启动源码流程 (startActivity)
• • [2.1 startActivity()](#2.1 startActivity())
  • [2.2 startActivityAsUser()](#2.2 startActivityAsUser())
  • [2.3 startActivityUnchecked()](#2.3 startActivityUnchecked())
  • [2.4 resumeFocusedStackTopActivityLocked()](#2.4 resumeFocusedStackTopActivityLocked())
  • [2.5 startSpecificActivityLocked -> realStartActivityLocked()](#2.5 startSpecificActivityLocked -> realStartActivityLocked())
• [三、 Activity实例化过程](#三、 Activity实例化过程)
• [四、 setContentView](#四、 setContentView)
• [五、 后续生命周期](#五、 后续生命周期)
• [六、 推荐阅读](#六、 推荐阅读)

学习前，建议有相关知识储备：
【Android 基础】 应用（Application）启动流程

通过本文你可以学习到Activity启动流程。

一、概览

Activity 是 android 四大组件之一，很有必要知道它的启动过程，我们在上一篇文章中介绍了 APP的启动流程，里面大概讲到了Activity的

启动流程，在本文中，我们系统的再总结一下，当成一个记录。

Activity 的启动方式我们讲有两种，一种是在应用内部启动，另一种是外部启动，比如Launcher；

• 应用内启动
  通过 startActivity、startActivityForResult等方式来启动 Activity

其流程我们总结下：

1、调用 Activity 的 startActivity 方法来启动目标 Activity

2、接着就会调用到 Instrunmentation 的 execStartActivity 方法，然后调用到 AMS 的 startActivity 中去

3、调用到 AMS 中后，会执行到ActivityStarter 的 execute 方法，接着就会进行一些校验和判断权限，包括进程检查，intent检查，权限检查、是否启用新栈等

4、所有的信息存储在ActivityRecord中，ActivityRecord是Activity在system_server进程中的镜像，Activity实例与ActivityRecord实例一一对应。ActivityRecord用来存储Activity的信息，如所在的进程名称，应用的包名，所在的任务栈的taskAffinity等

5、中间再经过一系列调用，又回调到 ActivityThread 的 handleLaunchActivity 来启动 Activity。

借用一张网络图片

• 应用外启动
  通过Launcher 进程启动，Launcher 就是我们桌面程序，当系统开机后， Launcher 也随之被启动。

1、fork并调用ActivityThread的main方法创建app进程

2、然后从 ActivityThread 调用到AMS中的attachApplicationLocked，创建Application

3、Application创建完后，调用ActivityStackSupervisor的attachApplicationLocked方法，最终调用到handleLaunchActivity，进行activity的创建

二、应用内启动源码流程 (startActivity)

我们就从源码出发，一起来看看startActivity后面的流程

2.1 startActivity()

Activity.java

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

    public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
        if (mParent == null) {
            options = transferSpringboardActivityOptions(options);
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode, options);
            if (ar != null) {
                //分析启动结果
                mMainThread.sendActivityResult(
                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                    ar.getResultData());
            }

        } else {
            // 最终也是调用 execStartActivity 方法，源码如下
            if (options != null) {
                mParent.startActivityFromChild(this, intent, requestCode, options);
            } else {
                mParent.startActivityFromChild(this, intent, requestCode);
            }
        }
    }

    public void startActivityFromChild(@NonNull Activity child, @RequiresPermission Intent intent,
            int requestCode, @Nullable Bundle options) {
        options = transferSpringboardActivityOptions(options);
        Instrumentation.ActivityResult ar =
            mInstrumentation.execStartActivity(
                this, mMainThread.getApplicationThread(), mToken, child,
                intent, requestCode, options);
        if (ar != null) {
            mMainThread.sendActivityResult(
                mToken, child.mEmbeddedID, requestCode,
                ar.getResultCode(), ar.getResultData());
        }
        cancelInputsAndStartExitTransition(options);
    }

上面代码中，最终都会调用了 execStartActivity 方法，该方法会返回一个启动结果。我们一起来看看

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

    /**
     *
     * @param who              用来启动 Activity 的对象
     * @param contextThread Binder 对象，具有跨进程通信的能力，传入的是 ApplicationThread
     * @param token  Binder 对象，指向了服务端一个 ActivityRecord 对象
     * @param target 当前的 Activity
     * @param intent  Intent 对象
     * @param requestCode  请求码
     */
    public ActivityResult execStartActivity(
        Context who, IBinder contextThread, IBinder token, String target,
        Intent intent, int requestCode, Bundle options) {
        
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess(who);
            
            
            // 这里的service 就是 ActivityManagerService， 具体可以跟代码看到 
            // 这么一句 ： ServiceManager.getService(Context.ACTIVITY_SERVICE);
            
            int result = ActivityManager.getService()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target, requestCode, 0, null, options);
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            
        }
        return null;
    }

通过 Binder 调用 AMS 启动 Activity，我们接着往下看

2.2 startActivityAsUser()

ActivityManagerService.java

public class ActivityManagerService extends IActivityManager.Stub


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

    public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
                                         Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
                                         int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId,
                                         boolean validateIncomingUser) {
        enforceNotIsolatedCaller("startActivity");
  
        // 首先 检查调用者权限
        userId = mActivityStartController.checkTargetUser(userId, validateIncomingUser,
                Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");
  
        // TODO: Switch to user app stacks here.
        return mActivityStartController.obtainStarter(intent, "startActivityAsUser")
                .setCaller(caller)
                .setCallingPackage(callingPackage)
                .setResolvedType(resolvedType)
                .setResultTo(resultTo)
                .setResultWho(resultWho)
                .setRequestCode(requestCode)
                .setStartFlags(startFlags)
                .setProfilerInfo(profilerInfo)
                .setActivityOptions(bOptions)
                .setMayWait(userId)
                .execute();

    }
    
    最后调用的是 ActivityStarter.execute();
    

上面代码最终调用到了 startActivityAsUser 方法，在内部将所有点的参数都交给了 ActivityStarter

2.3 startActivityUnchecked()

ActivityStarter 该类包含了启动的所有逻辑，比如 Intent 解析以及任务栈等。

ActivityStarter.java

   int execute() {
        try {
            if (mRequest.mayWait) {
                return startActivityMayWait(mRequest.caller, ...);
            } else {
                // todo 
                return startActivity(mRequest.caller, ...);  
            }
        } finally {
            onExecutionComplete();
        }
    }

   private int startActivity(IApplicationThread caller, Intent intent, ...) {
        
        // 再次检查调用者权限，包括进程检查，intent检查，权限检查等
        boolean abort = !mSupervisor.checkStartAnyActivityPermission(intent, aInfo, ...);
        abort |= !mService.mIntentFirewall.checkStartActivity(intent, ...);


        // 每个 Activity 都会对应一个 ActivityRecord 对象
        ActivityRecord r = new ActivityRecord(mService, ...);


        // todo 
        result = startActivityUnchecked(r, sourceRecord ...);
   }


    // Note: This method should only be called from {@link startActivity}.
    private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord ...) {

        //设置初始化状态
        setInitialState(r, options, inTask, ...);
        
        //判断启动模式，并且在 mLaunchFlags 上追加对应标记
        computeLaunchingTaskFlags();
        
        //设置 Activity 的栈
        computeSourceStack();
        
        //设置 LaunchFlags 到 intent 上
        mIntent.setFlags(mLaunchFlags);

        //决定是否用新的栈
        ActivityRecord reusedActivity = getReusableIntentActivity();
        
        ...

        // Should this be considered a new task?
        int result = START_SUCCESS;
        if (mStartActivity.resultTo == null && mInTask == null && !mAddingToTask
             && (mLaunchFlags & FLAG_ACTIVITY_NEW_TASK) != 0) {
          newTask = true;
          // 创建一个新的task来启动
          result = setTaskFromReuseOrCreateNewTask(taskToAffiliate, topStack);
        } else


        // todo 
        mSupervisor.resumeFocusedStackTopActivityLocked();    
    
    }

上面代码中会进行一些校验和判断权限，包括进程检查，intent检查，权限检查等

2.4 resumeFocusedStackTopActivityLocked()

ActivityStackSupervisor.java

    boolean resumeFocusedStackTopActivityLocked(ActivityStack targetStack...) {


        return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);

        return false;
      }

resumeTopActivityUncheckedLocked -> resumeTopActivityInnerLocked -> startSpecificActivityLocked()

ActivityStack.java

    @GuardedBy("mService")
    boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
        if (mStackSupervisor.inResumeTopActivity) {

            Activity 存在则resume
            transaction.setLifecycleStateRequest( ResumeActivityItem.obtain(next.app.repProcState,。。。);
            mService.getLifecycleManager().scheduleTransaction(transaction);

            不存在则调用下面这个
            result = resumeTopActivityInnerLocked(prev, options);

        } finally {
        }

        return result;
    }

    @GuardedBy("mService")
    private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {

            mStackSupervisor.startSpecificActivityLocked(next, true, false);
            
            return true;
    }

2.5 startSpecificActivityLocked -> realStartActivityLocked()

ActivityStackSupervisor.java

startSpecificActivityLocked -> realStartActivityLocked(); 到这个地方，我们就可以看到真的开始启动 activity，

后面就跟Application 里面一样了

    void startSpecificActivityLocked(ActivityRecord r ...) {
        if (app != null && app.thread != null) {
          try {
            // 真的开始启动 activity  ,看下面的方法
            realStartActivityLocked(r, app, andResume, checkConfig);
            return;
          } catch (RemoteException e) {
          }
  
        }
    }

    真的开始启动 activity
    final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
            boolean andResume, boolean checkConfig) throws RemoteException {

        // Create activity launch transaction.
        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));
            
        // Schedule transaction.
        mService.getLifecycleManager().scheduleTransaction(clientTransaction);
    }

ActivityManagerService

    ClientLifecycleManager getLifecycleManager() {
        return mLifecycleManager;
    }
    

ClientLifecycleManager

    通过代码，我们可以看到，获取的client就是 ActivityThread，IApplicationThread是一个AIDL文件
    void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();
        transaction.schedule();
        
    }

ClientTransaction

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

ClientTransactionHandler

    //ActivityThread中没有复写scheduleTransaction，会执行到父类的方法
    //public final class ActivityThread extends ClientTransactionHandler
    //ClientTransactionHandler.java
    public abstract class ClientTransactionHandler {
    
        void scheduleTransaction(ClientTransaction transaction) {
            transaction.preExecute(this);
            //发送消息
            sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
        }
    }

ActivityThread.java

    case EXECUTE_TRANSACTION:
        final ClientTransaction transaction = (ClientTransaction) msg.obj;
        mTransactionExecutor.execute(transaction);
        break;
        

这里其实就是执行LaunchActivityItem的execute方法，

其赋值的地方在realStartActivityLocked()方法，大家可以回头看看，前面有这么一句代码

clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),

TransactionExecutor.java

    public void execute(ClientTransaction transaction) {
        final IBinder token = transaction.getActivityToken();

        executeCallbacks(transaction);

        executeLifecycleState(transaction);
        mPendingActions.clear();
        log("End resolving transaction");
    }

    /** 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 ActivityLifecycleItem finalStateRequest = transaction.getLifecycleStateRequest();
        
        final int size = callbacks.size();
        for (int i = 0; i < size; ++i) {
            final ClientTransactionItem item = callbacks.get(i);
            
            item.execute(mTransactionHandler, token, mPendingActions);
            item.postExecute(mTransactionHandler, token, mPendingActions);
            
        }
    }

到这里就调用到我们熟悉的handleLaunchActivity了

LaunchActivityItem.java

    @Override
    public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        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 */);
    }

最后调用到ActivityThread

三、 Activity实例化过程

ActivityThread.java

    /**
     * Extended implementation of activity launch. Used when server requests a launch or relaunch.
     */
    @Override
    public Activity handleLaunchActivity(ActivityClientRecord r,
            PendingTransactionActions pendingActions, Intent customIntent) {

        final Activity a = performLaunchActivity(r, customIntent);
        
        return a;
    }

    Activity实例化过程
    
    /**  Core implementation of activity launch. */
    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {

        Activity activity = null;
        try {
            // 通过ClassLoader去加载需要启动的activity， 反射实例化Activity对象
            java.lang.ClassLoader cl = appContext.getClassLoader();
            activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);
        } catch (Exception e) {
        }

        // 在该方法内部创建window，并设置window回调，
        activity.attach(appContext, this, getInstrumentation() ...);
        
            
        theme
        
        //当实例化Activity对象后，继续执行callActivityOnCreate， 继而调用Activity的onCreate，
        // 这样就完成了Activity生命周期的第一个回调onCreate方法
        if (r.isPersistable()) {
            mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
        } else {
            mInstrumentation.callActivityOnCreate(activity, r.state);
        }
    }

下面的代码比较简单，就不贴了

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

四、 setContentView

这里内容太多，我们另外写一篇文章。

五、 后续生命周期

接下来就是执行 Activity 其他生命周期函数

ActivityThread.java

    @Override
    public void handleStartActivity(ActivityClientRecord r,
            PendingTransactionActions pendingActions) {
        final Activity activity = r.activity;

        // Restore instance state
        if (pendingActions.shouldRestoreInstanceState()) {
            if (r.isPersistable()) {
                if (r.state != null || r.persistentState != null) {
                    mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,
                            r.persistentState);
                }
            } else if (r.state != null) {
                mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
            }
        }
        
        // Call postOnCreate()
        if (pendingActions.shouldCallOnPostCreate()) {
            activity.mCalled = false;
            if (r.isPersistable()) {
                mInstrumentation.callActivityOnPostCreate(activity, r.state,
                        r.persistentState);
            } else {
                mInstrumentation.callActivityOnPostCreate(activity, r.state);
            }
        }
    }


    @Override
    public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
            String reason) {
        
    }

写在最后：

在应用进程创建 activity 后，activity 将执行以下操作：

1. 初始化值。
2. 调用构造函数。
3. 根据 activity 的当前生命周期状态，相应地调用回调方法，如 Activity.onCreate()。
   通常，onCreate() 方法对加载时间的影响最大，因为它执行工作的开销最高：加载和渲染视图，以及初始化运行 activity 所需的对象。

六、 推荐阅读

Java 专栏

SQL 专栏

数据结构与算法

Android学习专栏