Android Activity的启动过程源码解析

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。 也希望大家多多支持编程小技巧。

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