Activity的启动过程之startActivity源码解析
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http://blog.csdn.net/guangzq/article/details/74295752
本文出自祝起光的博客
一.概述
本文基于api 24
startActivity简化版流程图
二.源码解析
以startActivity为入口
方法：Activity->startActivity
@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 { // Note we want to go through this call for compatibility with // applications that may have overridden the method. startActivityForResult(intent, -1); } } public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) { startActivityForResult(intent, requestCode, null); } 最终方法都是调用的startActivityForResult，继续看：
方法：Acticity->startActivityForResult
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode, @Nullable Bundle options) { //mParent常用于ActivityGroup,但是ActivityGroup已废弃，所以一般的Activity都是走这里 if (mParent == null) { //启动一个新的Activity,核心功能位于mMainThread.getApplicationThread()。Instrumentation类是用于监视system和application交互的类。在最终启动activity //的地方会用到Instrumentation Instrumentation.ActivityResult ar = mInstrumentation.execStartActivity( this, mMainThread.getApplicationThread(), mToken, this, intent, requestCode, options); if (ar != null) { //发送调用onActivityResult的消息 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. //这段主要是讲：在result接收到之前避免acitity出现，在onCreate或者onResume期间设置可以是acticity隐藏，避免闪烁 mStartedActivity = true; } cancelInputsAndStartExitTransition(options); // TODO Consider clearing/flushing other event sources and events for child windows. } else { //ActitityGroup内部的Activity则会走这里，实现方式其实是一样的 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); } } }既然真正执行打开Activity是在execStartActivity实现的，那么继续看execStartActivity
方法：Instrumentation->execStartActivity
public ActivityResult execStartActivity( Context who, IBinder contextThread, IBinder token, Activity target, Intent intent, int requestCode, Bundle options) { IApplicationThread whoThread = (IApplicationThread) contextThread; Uri referrer = target != null ? target.onProvideReferrer() : null; if (referrer != null) { intent.putExtra(Intent.EXTRA_REFERRER, referrer); } if (mActivityMonitors != null) { synchronized (mSync) { //遍历ActivityMonitor,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)) { am.mHits++; //如果//当该monitor阻塞activity启动,也就是目标Activity无法打开就直接return if (am.isBlocking()) { return requestCode >= 0 ? am.getResult() : null; } //如果存在就跳出循环 break; } } } } try { intent.migrateExtraStreamToClipData(); intent.prepareToLeaveProcess(who); //真正的startActivity的方法，其实核心功能在whoThread的scheduleLaunchActivity完成的 int result = ActivityManagerNative.getDefault() .startActivity(whoThread, who.getBasePackageName(), intent, intent.resolveTypeIfNeeded(who.getContentResolver()), token, target != null ? target.mEmbeddedID : null, requestCode, 0, null, options); //用于检查打开Activity异常的方法，比如常见的Activity没在AndroidManifest的话， // 会抛出“...have you declared this activity in your AndroidManifest.xml?”异常 checkStartActivityResult(result, intent); } catch (RemoteException e) { throw new RuntimeException("Failure from system", e); } return null; } 补充checkStartActivityResult方法，内部实现是各种情况下需要抛出的异常信息
public 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"); case ActivityManager.START_NOT_VOICE_COMPATIBLE: throw new SecurityException( "Starting under voice control not allowed for: " + intent); case ActivityManager.START_VOICE_NOT_ACTIVE_SESSION: throw new IllegalStateException( "Session calling startVoiceActivity does not match active session"); case ActivityManager.START_VOICE_HIDDEN_SESSION: throw new IllegalStateException( "Cannot start voice activity on a hidden session"); case ActivityManager.START_CANCELED: throw new AndroidRuntimeException("Activity could not be started for " + intent); default: throw new AndroidRuntimeException("Unknown error code " + res + " when starting " + intent); } }接上文的startActivity，是定义在接口IActivityManager的方法，该方法的实现类是定义在ActivityManagerNative类的内部类ActivityManagerProxy
方法：ActivityManagerProxy->startActivity
public int startActivity(IApplicationThread caller, String callingPackage, Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo, Bundle options) throws RemoteException { Parcel data = Parcel.obtain(); Parcel reply = Parcel.obtain(); data.writeInterfaceToken(IActivityManager.descriptor); data.writeStrongBinder(caller != null ? caller.asBinder() : null); data.writeString(callingPackage); intent.writeToParcel(data, 0); data.writeString(resolvedType); data.writeStrongBinder(resultTo); data.writeString(resultWho); data.writeInt(requestCode); data.writeInt(startFlags); if (profilerInfo != null) { data.writeInt(1); profilerInfo.writeToParcel(data, Parcelable.PARCELABLE_WRITE_RETURN_VALUE); } else { data.writeInt(0); } if (options != null) { data.writeInt(1); options.writeToParcel(data, 0); } else { data.writeInt(0); } mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0); reply.readException(); int result = reply.readInt(); reply.recycle(); data.recycle(); return result; }参数众多，这里重点关注IApplicationThread参数，是个接口，关键方法scheduleLaunchActivity，实现和继承如下：
public abstract class ApplicationThreadNative extends Binder implements IApplicationThreadprivate class ApplicationThread extends ApplicationThreadNative方法：ActivityThread->ApplicationThread->scheduleLaunchActivity
@Override public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident, ActivityInfo info, Configuration curConfig, Configuration overrideConfig, CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor, int procState, Bundle state, PersistableBundle persistentState, List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents, boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) { updateProcessState(procState, false); ActivityClientRecord r = new ActivityClientRecord(); r.token = token; r.ident = ident; r.intent = intent; r.referrer = referrer; r.voiceInteractor = voiceInteractor; r.activityInfo = info; r.compatInfo = compatInfo; r.state = state; r.persistentState = persistentState; r.pendingResults = pendingResults; r.pendingIntents = pendingNewIntents; r.startsNotResumed = notResumed; r.isForward = isForward; r.profilerInfo = profilerInfo; r.overrideConfig = overrideConfig; updatePendingConfiguration(curConfig); sendMessage(H.LAUNCH_ACTIVITY, r); } 这个方法主要是用来记录activity的各种参数，然后发送启动activity的消息，接着看这一消息的实现
private class H extends Handler { ... public void handleMessage(Message msg) { if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what)); switch (msg.what) { case LAUNCH_ACTIVITY: { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart"); final ActivityClientRecord r = (ActivityClientRecord) msg.obj; r.packageInfo = getPackageInfoNoCheck( r.activityInfo.applicationInfo, r.compatInfo); //处理LAUNCH_ACTIVITY的消息 handleLaunchActivity(r, null, "LAUNCH_ACTIVITY"); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); } break; ... }真正的处理启动Activity的逻辑来了
方法：ActvityThread->performLaunchActivity
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) { // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")"); //解析启动Activity的相关信息，包括component和packageInfo ActivityInfo aInfo = r.activityInfo; if (r.packageInfo == null) { r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo, Context.CONTEXT_INCLUDE_CODE); } 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); } //通过ClassLoader将目标activity加载进来，并实例化一个activity对象 Activity activity = null; try { java.lang.ClassLoader cl = r.packageInfo.getClassLoader(); activity = mInstrumentation.newActivity( cl, component.getClassName(), r.intent); StrictMode.incrementExpectedActivityCount(activity.getClass()); r.intent.setExtrasClassLoader(cl); r.intent.prepareToEnterProcess(); 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 { //创建Application对象 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) { //创建activity的上下文信息 Context appContext = createBaseContextForActivity(r, activity); CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager()); Configuration config = new Configuration(mCompatConfiguration); if (r.overrideConfig != null) { config.updateFrom(r.overrideConfig); } if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity " + r.activityInfo.name + " with config " + config); Window window = null; if (r.mPendingRemoveWindow != null && r.mPreserveWindow) { window = r.mPendingRemoveWindow; r.mPendingRemoveWindow = null; r.mPendingRemoveWindowManager = null; } //将context和activity的相关信息通过attach设置到目标activity中去 activity.attach(appContext, this, getInstrumentation(), r.token, r.ident, app, r.intent, r.activityInfo, title, r.parent, r.embeddedID, r.lastNonConfigurationInstances, config, r.referrer, r.voiceInteractor, window); 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; if (r.isPersistable()) { //通过mInstrumentation的callActivityOnCreate来间接调用目标activity的onCreate方法 mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState); } else { 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.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); } } if (!r.activity.mFinished) { activity.mCalled = false; if (r.isPersistable()) { mInstrumentation.callActivityOnPostCreate(activity, r.state, r.persistentState); } else { 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; }performLaunchActivity的主要逻辑有三点：
- 解析启动Activity的相关信息，包括component和packageInfo
- 通过ClassLoader将目标activity加载进来，并实例化一个activity对象
- 通过mInstrumentation的callActivityOnCreate来间接调用目标activity的onCreate方法，然后就是走activity的声明周期了
3.总结
整个startActivity的源码解析到此结束，是不是对经常用的这一api有了更深的认识
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