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：

• Activity：这个大家都熟悉，startActivity方法的真正实现在Activity中
• Instrumentation：用来辅助Activity完成启动Activity的过程
• 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

image.png

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。