Android源码分析-Activity的启动过程

前言

Activity是Android中一个很重要的概念，堪称四大组件之首，关于Activity有很多内容，比如生命周期和启动Flags，这二者想要说清楚，恐怕又要写两篇长文，更何况分析它们的源码呢。不过本文的侧重点不是它们，我要介绍的是一个Activity典型的启动过程，本文会从源码的角度对其进行分析。我们知道，当startActivity被调用的时候，可以启动一个Activity，但是你知道这个Activity是如何被启动的吗？每个Activity也是一个对象，你知道这个对象是啥时候被创建的吗（也就是说它的构造方法是什么时候被调用的）？为什么onCreate是Activity的执行入口？所有的这一切都被系统封装好了，对我们来说是透明的，我们使用的时候仅仅是传递一个intent然后startActivity就可以达到目的了，不过，阅读了本文以后，你将会了解它的背后到底做了哪些事情。在分析之前，我先介绍几个类：

1. Activity：这个大家都熟悉，startActivity方法的真正实现在Activity中
2. Instrumentation：用来辅助Activity完成启动Activity的过程
3. ActivityThread（包含ApplicationThread + ApplicationThreadNative + IApplicationThread）：真正启动Activity的实现都在这里

源码分析

首先看入口

code：Activity#startActivity

 @Override
 public void startActivity(Intent intent) {
     startActivity(intent, null);
 }

 @Override
 public void startActivity(Intent intent, Bundle options) {
     if (options != null) {
         startActivityForResult(intent, -1, options);
     } else {
         // Note we want to go through this call for compatibility with
         // applications that may have overridden the method.
         startActivityForResult(intent, -1);
     }
 }

 public void startActivityForResult(Intent intent, int requestCode) {
     startActivityForResult(intent, requestCode, null);
 }

说明：显然，从上往下，最终都是由startActivityForResult来实现的

接着看

code：Activity#startActivityForResult

 public void startActivityForResult(Intent intent, int requestCode, Bundle options) {
     //一般的Activity其mParent为null，mParent常用在ActivityGroup中，ActivityGroup已废弃
     if (mParent == null) {
         //这里会启动新的Activity，核心功能都在mMainThread.getApplicationThread()中完成
         Instrumentation.ActivityResult ar =
             mInstrumentation.execStartActivity(
                 this, mMainThread.getApplicationThread(), mToken, this,
                 intent, requestCode, options);
         if (ar != null) {
             //发送结果，即onActivityResult会被调用
             mMainThread.sendActivityResult(
                 mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                 ar.getResultData());
         }
         if (requestCode >= 0) {
             // If this start is requesting a result, we can avoid making
             // the activity visible until the result is received.  Setting
             // this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
             // activity hidden during this time, to avoid flickering.
             // This can only be done when a result is requested because
             // that guarantees we will get information back when the
             // activity is finished, no matter what happens to it.
             mStartedActivity = true;
         }

         final View decor = mWindow != null ? mWindow.peekDecorView() : null;
         if (decor != null) {
             decor.cancelPendingInputEvents();
         }
         // TODO Consider clearing/flushing other event sources and events for child windows.
     } else {
         //在ActivityGroup内部的Activity调用startActivity的时候会走到这里，内部处理逻辑和上面是类似的
         if (options != null) {
             mParent.startActivityFromChild(this, intent, requestCode, options);
         } else {
             // Note we want to go through this method for compatibility with
             // existing applications that may have overridden it.
             mParent.startActivityFromChild(this, intent, requestCode);
         }
     }
 }

说明：上述代码关键点都有注释了，可以发现，真正打开activity的实现在Instrumentation的execStartActivity方法中，去看看

code：Instrumentation#execStartActivity

 public ActivityResult execStartActivity(
         Context who, IBinder contextThread, IBinder token, Activity target,
         Intent intent, int requestCode, Bundle options) {
     //核心功能在这个whoThread中完成，其内部scheduleLaunchActivity方法用于完成activity的打开
     IApplicationThread whoThread = (IApplicationThread) contextThread;
     if (mActivityMonitors != null) {
         synchronized (mSync) {
             //先查找一遍看是否存在这个activity
             final int N = mActivityMonitors.size();
             for (int i=0; i<N; i++) {
                 final ActivityMonitor am = mActivityMonitors.get(i);
                 if (am.match(who, null, intent)) {
                     //如果找到了就跳出循环
                     am.mHits++;
                     //如果目标activity无法打开，直接return
                     if (am.isBlocking()) {
                         return requestCode >= 0 ? am.getResult() : null;
                     }
                     break;
                 }
             }
         }
     }
     try {
         intent.migrateExtraStreamToClipData();
         intent.prepareToLeaveProcess();
         //这里才是真正打开activity的地方，核心功能在whoThread中完成。
         int result = ActivityManagerNative.getDefault()
             .startActivity(whoThread, who.getBasePackageName(), intent,
                     intent.resolveTypeIfNeeded(who.getContentResolver()),
                     token, target != null ? target.mEmbeddedID : null,
                     requestCode, 0, null, null, options);
         //这个方法是专门抛异常的，它会对结果进行检查，如果无法打开activity，
         //则抛出诸如ActivityNotFoundException类似的各种异常
         checkStartActivityResult(result, intent);
     } catch (RemoteException e) {
     }
     return null;
 }

说明：我想再说一下这个方法checkStartActivityResult，它也专业抛异常的，看代码，相信大家对下面的异常信息不陌生吧，就是它干的，其中最熟悉的非Unable to find explicit activity class莫属了，如果你在xml中没有注册目标activity，此异常将会抛出。

 /*package*/ static void checkStartActivityResult(int res, Object intent) {
     if (res >= ActivityManager.START_SUCCESS) {
         return;
     }

     switch (res) {
         case ActivityManager.START_INTENT_NOT_RESOLVED:
         case ActivityManager.START_CLASS_NOT_FOUND:
             if (intent instanceof Intent && ((Intent)intent).getComponent() != null)
                 throw new ActivityNotFoundException(
                         "Unable to find explicit activity class "
                         + ((Intent)intent).getComponent().toShortString()
                         + "; have you declared this activity in your AndroidManifest.xml?");
             throw new ActivityNotFoundException(
                     "No Activity found to handle " + intent);
         case ActivityManager.START_PERMISSION_DENIED:
             throw new SecurityException("Not allowed to start activity "
                     + intent);
         case ActivityManager.START_FORWARD_AND_REQUEST_CONFLICT:
             throw new AndroidRuntimeException(
                     "FORWARD_RESULT_FLAG used while also requesting a result");
         case ActivityManager.START_NOT_ACTIVITY:
             throw new IllegalArgumentException(
                     "PendingIntent is not an activity");
         default:
             throw new AndroidRuntimeException("Unknown error code "
                     + res + " when starting " + intent);
     }
 }

接下来我们要去看看IApplicationThread，因为核心功能由其内部的scheduleLaunchActivity方法来完成，由于IApplicationThread是个接口，所以，我们需要找到它的实现类，我已经帮大家找到了，它就是ActivityThread中的内部类ApplicationThread，看下它的继承关系：

private class ApplicationThread extends ApplicationThreadNative;

public abstract class ApplicationThreadNative extends Binder implements IApplicationThread;

可以发现，ApplicationThread还是间接实现了IApplicationThread接口，先看下这个类的结构

看完ApplicationThread的大致结构，我们应该能够猜测到，Activity的生命周期中的resume、newIntent、pause、stop等事件都是由它触发的，事实上，的确是这样的。这里，我们为了说明问题，仅仅看scheduleLaunchActivity方法

code：ApplicationThread#scheduleLaunchActivity

 public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
         ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo,
         int procState, Bundle state, List<ResultInfo> pendingResults,
         List<Intent> pendingNewIntents, boolean notResumed, boolean isForward,
         String profileName, ParcelFileDescriptor profileFd, boolean autoStopProfiler) {

     updateProcessState(procState, false);

     ActivityClientRecord r = new ActivityClientRecord();

     r.token = token;
     r.ident = ident;
     r.intent = intent;
     r.activityInfo = info;
     r.compatInfo = compatInfo;
     r.state = state;

     r.pendingResults = pendingResults;
     r.pendingIntents = pendingNewIntents;

     r.startsNotResumed = notResumed;
     r.isForward = isForward;

     r.profileFile = profileName;
     r.profileFd = profileFd;
     r.autoStopProfiler = autoStopProfiler;

     updatePendingConfiguration(curConfig);

     queueOrSendMessage(H.LAUNCH_ACTIVITY, r);
 }

说明：上述代码很好理解，构造一个activity记录，然后发送一个消息，所以，我们要看看Handler是如何处理这个消息的，现在转到这个Handler，它有个很短的名字叫做H

code：ActivityThread#H

 //这个类太长，我只帖出了我们用到的部分
 private class H extends Handler {

     public void handleMessage(Message msg) {
         if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
         switch (msg.what) {
             //这里处理LAUNCH_ACTIVITY消息类型
             case LAUNCH_ACTIVITY: {
                 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
                 ActivityClientRecord r = (ActivityClientRecord)msg.obj;

                 r.packageInfo = getPackageInfoNoCheck(
                         r.activityInfo.applicationInfo, r.compatInfo);
                 //这里处理startActivity消息
                 handleLaunchActivity(r, null);
                 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
             } break;
             case RELAUNCH_ACTIVITY: {
                 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart");
                 ActivityClientRecord r = (ActivityClientRecord)msg.obj;
                 handleRelaunchActivity(r);
                 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
             } break;
             case PAUSE_ACTIVITY:
                 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
                 handlePauseActivity((IBinder)msg.obj, false, msg.arg1 != 0, msg.arg2);
                 maybeSnapshot();
                 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                 break;
             ...
         }
 }

说明：看来还要看handleLaunchActivity

code：ActivityThread#handleLaunchActivity

 private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
     // If we are getting ready to gc after going to the background, well
     // we are back active so skip it.
     unscheduleGcIdler();

     if (r.profileFd != null) {
         mProfiler.setProfiler(r.profileFile, r.profileFd);
         mProfiler.startProfiling();
         mProfiler.autoStopProfiler = r.autoStopProfiler;
     }

     // Make sure we are running with the most recent config.
     handleConfigurationChanged(null, null);

     if (localLOGV) Slog.v(
         TAG, "Handling launch of " + r);
     //终于到底了，大家都有点不耐烦了吧，从方法名可以看出，
     //performLaunchActivity真正完成了activity的调起,
     //同时activity会被实例化，并且onCreate会被调用
     Activity a = performLaunchActivity(r, customIntent);

     if (a != null) {
         r.createdConfig = new Configuration(mConfiguration);
         Bundle oldState = r.state;
         //看到没，目标activity的onResume会被调用
         handleResumeActivity(r.token, false, r.isForward,
                 !r.activity.mFinished && !r.startsNotResumed);

         if (!r.activity.mFinished && r.startsNotResumed) {
             // The activity manager actually wants this one to start out
             // paused, because it needs to be visible but isn't in the
             // foreground.  We accomplish this by going through the
             // normal startup (because activities expect to go through
             // onResume() the first time they run, before their window
             // is displayed), and then pausing it.  However, in this case
             // we do -not- need to do the full pause cycle (of freezing
             // and such) because the activity manager assumes it can just
             // retain the current state it has.
             try {
                 r.activity.mCalled = false;
                 //同时，由于新activity被调起了，原activity的onPause会被调用
                 mInstrumentation.callActivityOnPause(r.activity);
                 // We need to keep around the original state, in case
                 // we need to be created again.  But we only do this
                 // for pre-Honeycomb apps, which always save their state
                 // when pausing, so we can not have them save their state
                 // when restarting from a paused state.  For HC and later,
                 // we want to (and can) let the state be saved as the normal
                 // part of stopping the activity.
                 if (r.isPreHoneycomb()) {
                     r.state = oldState;
                 }
                 if (!r.activity.mCalled) {
                     throw new SuperNotCalledException(
                         "Activity " + r.intent.getComponent().toShortString() +
                         " did not call through to super.onPause()");
                 }

             } catch (SuperNotCalledException e) {
                 throw e;

             } catch (Exception e) {
                 if (!mInstrumentation.onException(r.activity, e)) {
                     throw new RuntimeException(
                             "Unable to pause activity "
                             + r.intent.getComponent().toShortString()
                             + ": " + e.toString(), e);
                 }
             }
             r.paused = true;
         }
     } else {
         // If there was an error, for any reason, tell the activity
         // manager to stop us.
         try {
             ActivityManagerNative.getDefault()
                 .finishActivity(r.token, Activity.RESULT_CANCELED, null);
         } catch (RemoteException ex) {
             // Ignore
         }
     }
 }

说明：关于原activity和新activity之间的状态同步，如果大家感兴趣可以自己研究下，因为逻辑太复杂，我没法把所有问题都说清楚，否则就太深入细节而淹没了整体逻辑，研究源码要的就是清楚整体逻辑。下面看最后一个方法，这个方法是activity的启动过程的真正实现。
code：ActivityThread#performLaunchActivity

 private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
     // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");

     ActivityInfo aInfo = r.activityInfo;
     if (r.packageInfo == null) {
         r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                 Context.CONTEXT_INCLUDE_CODE);
     }
     //首先从intent中解析出目标activity的启动参数
     ComponentName component = r.intent.getComponent();
     if (component == null) {
         component = r.intent.resolveActivity(
             mInitialApplication.getPackageManager());
         r.intent.setComponent(component);
     }

     if (r.activityInfo.targetActivity != null) {
         component = new ComponentName(r.activityInfo.packageName,
                 r.activityInfo.targetActivity);
     }

     Activity activity = null;
     try {
         java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
         //用ClassLoader（类加载器）将目标activity的类通过类名加载进来并调用newInstance来实例化一个对象
         //其实就是通过Activity的无参构造方法来new一个对象，对象就是在这里new出来的。
         activity = mInstrumentation.newActivity(
                 cl, component.getClassName(), r.intent);
         StrictMode.incrementExpectedActivityCount(activity.getClass());
         r.intent.setExtrasClassLoader(cl);
         if (r.state != null) {
             r.state.setClassLoader(cl);
         }
     } catch (Exception e) {
         if (!mInstrumentation.onException(activity, e)) {
             throw new RuntimeException(
                 "Unable to instantiate activity " + component
                 + ": " + e.toString(), e);
         }
     }

     try {
         Application app = r.packageInfo.makeApplication(false, mInstrumentation);

         if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
         if (localLOGV) Slog.v(
                 TAG, r + ": app=" + app
                 + ", appName=" + app.getPackageName()
                 + ", pkg=" + r.packageInfo.getPackageName()
                 + ", comp=" + r.intent.getComponent().toShortString()
                 + ", dir=" + r.packageInfo.getAppDir());

         if (activity != null) {
             Context appContext = createBaseContextForActivity(r, activity);
             CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
             Configuration config = new Configuration(mCompatConfiguration);
             if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
                     + r.activityInfo.name + " with config " + config);
             activity.attach(appContext, this, getInstrumentation(), r.token,
                     r.ident, app, r.intent, r.activityInfo, title, r.parent,
                     r.embeddedID, r.lastNonConfigurationInstances, config);

             if (customIntent != null) {
                 activity.mIntent = customIntent;
             }
             r.lastNonConfigurationInstances = null;
             activity.mStartedActivity = false;
             int theme = r.activityInfo.getThemeResource()
             if (theme != 0) {
                 activity.setTheme(theme);
             }

             activity.mCalled = false;
             //目标activity的onCreate被调用了，到此为止，Activity被启动了，接下来的流程就是Activity的生命周期了，
             //本文之前已经提到，其生命周期的各种状态的切换由ApplicationThread内部来完成
             mInstrumentation.callActivityOnCreate(activity, r.state);
             if (!activity.mCalled) {
                 throw new SuperNotCalledException(
                     "Activity " + r.intent.getComponent().toShortString() +
                     " did not call through to super.onCreate()");
             }
             r.activity = activity;
             r.stopped = true;
             if (!r.activity.mFinished) {
                 activity.performStart();
                 r.stopped = false;
             }
             if (!r.activity.mFinished) {
                 if (r.state != null) {
                     mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
                 }
             }
             if (!r.activity.mFinished) {
                 activity.mCalled = false;
                 mInstrumentation.callActivityOnPostCreate(activity, r.state);
                 if (!activity.mCalled) {
                     throw new SuperNotCalledException(
                         "Activity " + r.intent.getComponent().toShortString() +
                         " did not call through to super.onPostCreate()");
                 }
             }
         }
         r.paused = true;

         mActivities.put(r.token, r);

     } catch (SuperNotCalledException e) {
         throw e;

     } catch (Exception e) {
         if (!mInstrumentation.onException(activity, e)) {
             throw new RuntimeException(
                 "Unable to start activity " + component
                 + ": " + e.toString(), e);
         }
     }

     return activity;
 }

总结

相信当你看到这里的时候，你对Activity的启动过程应该有了一个感性的认识。Activity很复杂，特性很多，本文没法对各个细节进行深入分析，而且就算真的对各个细节都进行了深入分析，那文章要有多长啊，还有人有耐心看下去吗？希望本文能够给大家带来一些帮助，谢谢大家阅读。