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(xiě)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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
|
@Overridepublic void startActivity(Intent intent) { startActivity(intent, null); } @Overridepublic 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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
|
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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
|
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。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
|
/*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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
|
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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
|
//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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
|
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
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
|
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 id="codetool">
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服務(wù)器之家。 原文鏈接：https://blog.csdn.net/singwhatiwanna/article/details/18154335 延伸 · 閱讀
精彩推薦
最近更新 編輯推薦
閱讀排行
熱門標(biāo)簽 preferenceactivity 拉伸效果 適配 橫豎屏切換 圖片翻轉(zhuǎn)動(dòng)畫(huà) binder 程序退出 圖片居中 自定義標(biāo)題 響應(yīng)接口 觸屏震動(dòng) 退出程序 Andorid descendantFocusability PopupWindow dialog 分包 SharedPreferences RelativeLayout NotificationManager Notification asynctask TextView 縮放動(dòng)畫(huà) Gallery ImageView OnCreate 顏色編輯器 RadioGroup tablelayout |
