Android Activity的啟動(dòng)過(guò)程源碼解析
前言
Activity是Android中一個(gè)很重要的概念,堪稱四大組件之首,關(guān)于Activity有很多內(nèi)容,比如生命周期和啟動(dòng)Flags,這二者想要說(shuō)清楚,恐怕又要寫兩篇長(zhǎng)文,更何況分析它們的源碼呢。不過(guò)本文的側(cè)重點(diǎn)不是它們,我要介紹的是一個(gè)Activity典型的啟動(dòng)過(guò)程,本文會(huì)從源碼的角度對(duì)其進(jìn)行分析。我們知道,當(dāng)startActivity被調(diào)用的時(shí)候,可以啟動(dòng)一個(gè)Activity,但是你知道這個(gè)Activity是如何被啟動(dòng)的嗎?每個(gè)Activity也是一個(gè)對(duì)象,你知道這個(gè)對(duì)象是啥時(shí)候被創(chuàng)建的嗎(也就是說(shuō)它的構(gòu)造方法是什么時(shí)候被調(diào)用的)?為什么onCreate是Activity的執(zhí)行入口?所有的這一切都被系統(tǒng)封裝好了,對(duì)我們來(lái)說(shuō)是透明的,我們使用的時(shí)候僅僅是傳遞一個(gè)intent然后startActivity就可以達(dá)到目的了,不過(guò),閱讀了本文以后,你將會(huì)了解它的背后到底做了哪些事情。在分析之前,我先介紹幾個(gè)類:
- Activity:這個(gè)大家都熟悉,startActivity方法的真正實(shí)現(xiàn)在Activity中
- Instrumentation:用來(lái)輔助Activity完成啟動(dòng)Activity的過(guò)程
- ActivityThread(包含ApplicationThread + ApplicationThreadNative + IApplicationThread):真正啟動(dòng)Activity的實(shí)現(xiàn)都在這里
源碼分析
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); }
說(shuō)明:顯然,從上往下,最終都是由startActivityForResult來(lái)實(shí)現(xiàn)的
接著看
code:Activity#startActivityForResult
public void startActivityForResult(Intent intent, int requestCode, Bundle options) { //一般的Activity其mParent為null,mParent常用在ActivityGroup中,ActivityGroup已廢棄 if (mParent == null) { //這里會(huì)啟動(dòng)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) { //發(fā)送結(jié)果,即onActivityResult會(huì)被調(diào)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內(nèi)部的Activity調(diào)用startActivity的時(shí)候會(huì)走到這里,內(nèi)部處理邏輯和上面是類似的 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); } } }
說(shuō)明:上述代碼關(guān)鍵點(diǎn)都有注釋了,可以發(fā)現(xiàn),真正打開(kāi)activity的實(shí)現(xiàn)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) { //核心功能在這個(gè)whoThread中完成,其內(nèi)部scheduleLaunchActivity方法用于完成activity的打開(kāi) IApplicationThread whoThread = (IApplicationThread) contextThread; if (mActivityMonitors != null) { synchronized (mSync) { //先查找一遍看是否存在這個(gè)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環(huán) am.mHits++; //如果目標(biāo)activity無(wú)法打開(kāi),直接return if (am.isBlocking()) { return requestCode >= 0 ? am.getResult() : null; } break; } } } } try { intent.migrateExtraStreamToClipData(); intent.prepareToLeaveProcess(); //這里才是真正打開(kāi)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); //這個(gè)方法是專門拋異常的,它會(huì)對(duì)結(jié)果進(jìn)行檢查,如果無(wú)法打開(kāi)activity, //則拋出諸如ActivityNotFoundException類似的各種異常 checkStartActivityResult(result, intent); } catch (RemoteException e) { } return null; }
說(shuō)明:我想再說(shuō)一下這個(gè)方法checkStartActivityResult,它也專業(yè)拋異常的,看代碼,相信大家對(duì)下面的異常信息不陌生吧,就是它干的,其中最熟悉的非Unable to find explicit activity class莫屬了,如果你在xml中沒(méi)有注冊(cè)目標(biāo)activity,此異常將會(huì)拋出。
/*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); } }
接下來(lái)我們要去看看IApplicationThread,因?yàn)楹诵墓δ苡善鋬?nèi)部的scheduleLaunchActivity方法來(lái)完成,由于IApplicationThread是個(gè)接口,所以,我們需要找到它的實(shí)現(xiàn)類,我已經(jīng)幫大家找到了,它就是ActivityThread中的內(nèi)部類ApplicationThread,看下它的繼承關(guān)系:
private class ApplicationThread extends ApplicationThreadNative;
public abstract class ApplicationThreadNative extends Binder implements IApplicationThread;
可以發(fā)現(xiàn),ApplicationThread還是間接實(shí)現(xiàn)了IApplicationThread接口,先看下這個(gè)類的結(jié)構(gòu)
看完ApplicationThread的大致結(jié)構(gòu),我們應(yīng)該能夠猜測(cè)到,Activity的生命周期中的resume、newIntent、pause、stop等事件都是由它觸發(fā)的,事實(shí)上,的確是這樣的。這里,我們?yōu)榱苏f(shuō)明問(wèn)題,僅僅看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); }
說(shuō)明:上述代碼很好理解,構(gòu)造一個(gè)activity記錄,然后發(fā)送一個(gè)消息,所以,我們要看看Handler是如何處理這個(gè)消息的,現(xiàn)在轉(zhuǎn)到這個(gè)Handler,它有個(gè)很短的名字叫做H
code:ActivityThread#H
//這個(gè)類太長(zhǎng),我只帖出了我們用到的部分 private class H extends Handler { public void handleMessage(Message msg) { if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what)); switch (msg.what) { //這里處理LAUNCH_ACTIVITY消息類型 case LAUNCH_ACTIVITY: { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart"); ActivityClientRecord r = (ActivityClientRecord)msg.obj; r.packageInfo = getPackageInfoNoCheck( r.activityInfo.applicationInfo, r.compatInfo); //這里處理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; ... } }
說(shuō)明:看來(lái)還要看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); //終于到底了,大家都有點(diǎn)不耐煩了吧,從方法名可以看出, //performLaunchActivity真正完成了activity的調(diào)起, //同時(shí)activity會(huì)被實(shí)例化,并且onCreate會(huì)被調(diào)用 Activity a = performLaunchActivity(r, customIntent); if (a != null) { r.createdConfig = new Configuration(mConfiguration); Bundle oldState = r.state; //看到?jīng)],目標(biāo)activity的onResume會(huì)被調(diào)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; //同時(shí),由于新activity被調(diào)起了,原activity的onPause會(huì)被調(diào)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 } } }
說(shuō)明:關(guān)于原activity和新activity之間的狀態(tài)同步,如果大家感興趣可以自己研究下,因?yàn)檫壿嬏珡?fù)雜,我沒(méi)法把所有問(wèn)題都說(shuō)清楚,否則就太深入細(xì)節(jié)而淹沒(méi)了整體邏輯,研究源碼要的就是清楚整體邏輯。下面看最后一個(gè)方法,這個(gè)方法是activity的啟動(dòng)過(guò)程的真正實(shí)現(xiàn)。
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); } //首先從intent中解析出目標(biāo)activity的啟動(dòng)參數(shù) 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(類加載器)將目標(biāo)activity的類通過(guò)類名加載進(jìn)來(lái)并調(diào)用newInstance來(lái)實(shí)例化一個(gè)對(duì)象 //其實(shí)就是通過(guò)Activity的無(wú)參構(gòu)造方法來(lái)new一個(gè)對(duì)象,對(duì)象就是在這里new出來(lái)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; //目標(biāo)activity的onCreate被調(diào)用了,到此為止,Activity被啟動(dòng)了,接下來(lái)的流程就是Activity的生命周期了, //本文之前已經(jīng)提到,其生命周期的各種狀態(tài)的切換由ApplicationThread內(nèi)部來(lái)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; }
總結(jié)
相信當(dāng)你看到這里的時(shí)候,你對(duì)Activity的啟動(dòng)過(guò)程應(yīng)該有了一個(gè)感性的認(rèn)識(shí)。Activity很復(fù)雜,特性很多,本文沒(méi)法對(duì)各個(gè)細(xì)節(jié)進(jìn)行深入分析,而且就算真的對(duì)各個(gè)細(xì)節(jié)都進(jìn)行了深入分析,那文章要有多長(zhǎng)啊,還有人有耐心看下去嗎?希望本文能夠給大家?guī)?lái)一些幫助,謝謝大家閱讀。 也希望大家多多支持腳本之家。
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