Android Activity的启动过程过程分析
前言
在了解Activity的启动的过程的时候，我们需要先了解下面几个东西是什么，这将更加有助于你深入了解android 活动的启动机制
老司机绕道
-
Activity
这个类大家再熟悉不过了，android最重要的组件之一，实际上它就是一个Java对象，它会被创建，同时也会被垃圾回收就只销毁，只不过它受AMS(ActivityManagerService)管理，所以它才有生命周期。 -
ActivityResult
Activity 管理服务端中，Activity的记录和缓存，换句话说：就是客户端启动一个Activity，AMS(ActivityManagerService)会对它进行缓存，而缓存的Activity的类型就是ActivityResult。 -
AMS - ActivityManagerService
android系统服务，Android管理的服务端，用于管理Activity的各种行为，控制Activity的生命周期，派发消息事件，低内存管理等等。实现了IBinder接口，可以用于进程间的通信。 -
ApplicationThread
该类实现了IBinder接口，Activity整个框架中客户端和服务端AMS（ActivityManagerService）通信的接口。同时也是ActivityThread的内部类。这样就有效的把AMS(ActivityManagerService）和ActivityThread绑在了一起。 -
ActivityThread
ApplicationThread所绑定的客户端就是ActivityThread，ActivityThread这个类在Activity客户端中的作用举足轻重。
- 它是应用程序的入口，大家都知道，Java程序的入口为main()方法，同样，当AMS（ActivityManagerService）拉起了一个新的进程，同时启动一个主线程的时候，主线程就从ActivityThread.main方法开始执行，他回初始化一些对象，然后自己进入消息等待队列，也就是Looper.looper();一旦进入loop()方法，线程就进入了死循环，再也不会退出；一直等待别人给它发消息，然后执行这个消息。这也是EDT(事件驱动模型)的原理。
- 它是Activity客户端的管理类，由它来决定什么时候调用onCreate()，什么时候调用onResume()方法，当Activity发起一个请求时，比如startActivity()或者finish()的时候，它就会来除了这个请求，然后调用其它的人来做具体的事。
-
Launcher
Android系统启动后，加载的第一个程序，是其他应用程序的入口。
也可以简单的理解为我们看到的android桌面 -
Instrumentation
这个类在Activity启动的时候以及广播注册的时候都会调用，除了跟android的测试有关之外，还是Activity管理中实际做事的人，比如说startActivity()，在某种情况下，就是调用这个类，然后调用AMS(ActivityManagerService),但是有的时候也是通过ApplicationThread去访问AMS的。 -
PackageManagerService
是Android系统中最常用的服务之一。它负责系统中Package的管理，应用程序的安装、卸载、信息查询等。这只是一个很笼统的说法。
PackageManagerService详解点我
正文
Activity作为android 四大组件之一,也是最基本的组件，负责用户的交互的所有功能。Activity的启动过程也并非是一件神秘的事情，接下来就简单的从源码的角度分析一下Activity的启动过程。
根Activity一般就是指我们项目中的MainActivity，代表了一个android应用程序，一般也是在一个新的进程中启动起来。在android系统中所有的Activity组件都保存在堆栈中，我们启动一个新的Activity组件就位于上一个Activity的上面，那么我们从桌面（Launcher）打开一个APP的 是一个怎么样的过程呢，如下所示：
1. Launcher向ActivityManagerService发送一个启动MainActivity的请求；
2. ActivityManagerService首先将MainActivity的相关信息保存下来，然后向Launcher发送一个实质进入暂停的状态的请求；
3. Launcher收到暂停状态之后，就会向ActivityManagerService发送一个已经进入暂停的状态的请求，便于ActivityManagerService继续执行启动MainActivity的操作；
4. ActivityManagerService检查用于运行MainActivity的进程，如果不存在，则会启动一个新的进程
5. ActivityManagerService将第（2）步保存下来的MainActivity相关信息发送个新的进程，便于该进程启动MainActivity组件
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Launcher.startActivitySafely
boolean startActivitySafely(Intent intent, Object tag) { intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK); try { startActivity(intent); return true; } catch (ActivityNotFoundException e) {} }当我们在Launcher上点击应用图标时，startActivitySafely方法会被调用。需要启动的Activity信息保存在Intent中，包括action、category等等。那么Launcher是如何获得Intent里面的这些信息呢？首先，系统在启动时会启动一个叫做PackageManagerService的管理服务，并且通过它来安装系统中的应用程序，这个过程中，PackageManagerService会对应用程序的配置文件androidManifest.xml进行解析，从而得到程序里面的组件信息（包括Activity、Service、Broadcast等），然后PackageManagerService去查询所有的action为
android.intent.action.MAIN并且category为“android.intent.category.LAUNCHER”的Activity，然后为每一个应用程序创建一个快捷图标，并把程序信息与之关联。上述代码中，Activity的启动标志位位置为Intent.FLAG_ACTIVITY_NEW_TASK,便于它可以在一个新的任务战中启动。 -
Activity.startActivity
@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); } }调用startActivityForResult,第二个参数requestCode为-1，则表示Activity关闭时不需要将结果传回来。
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Activity.startActivityForResult
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode, @Nullable Bundle options) { if (mParent == null) { options = transferSpringboardActivityOptions(options); Instrumentation.ActivityResult ar = mInstrumentation.execStartActivity( this, mMainThread.getApplicationThread(), mToken, this, intent, requestCode, options); if (ar != null) { 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; } cancelInputsAndStartExitTransition(options); // TODO Consider clearing/flushing other event sources and events for child windows. } else { 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); } } }实际上是调用的
mInstrumentation.execStartActivity来启动Activity，mInstrumentation类型为Instrumentation, Instrumentation用于监控程序和系统之间的交互操作。mInstrumentation代为执行Activity的启动操作，便于它可以监控这个交互过程。mMainThread的类型为ActivityThread，用于描述一个应用程序进程，系统每启动一个程序都会在它里面加载一个ActivityThread的实例，并且将实例保存在Activity的成员变量mMainThread中，而mMainThread.getApplicationThread()则用于获取内部为一个ApplicationThread的本地Binder对象。mToken的类型为IBinder，它是一个Binder的代理对象，指向了ActivityManagerService中一个类型为ActivityResult的本地对象。每一个已经启动的Activity在ActivityManagerService中都有一个对应的ActivityRecord对象，用于维护Activity的运行状态以及信息。 -
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) {
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++;
if (am.isBlocking()) {
return requestCode >= 0 ? am.getResult() : null;
}
break;
}
}
}
}
try {
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess(who);
int result = ActivityManagerNative.getDefault()
.startActivity(whoThread, who.getBasePackageName(), intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
token, target != null ? target.mEmbeddedID : null,
requestCode, 0, null, options);
checkStartActivityResult(result, intent);
} catch (RemoteException e) {
throw new RuntimeException("Failure from system", e);
}
return null;
}
```
> ActivityManagerNative.getDefault()获取一个ActivityManagerService的代理对象，然后调用它的startActivity方法来通知ActivityManagerService去启动Activity。中间还有一系列的过程，最后是调用ActivityThread中的私有内部类ApplicationThread的scheduleLauncherActivity来进行Activity的启动。
-
AcivityThread.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);
}
```
> 构造了一个ActivityClientRecord，然后调用sendMessage发送一个消息。在引用程序对应的进程中。每一个Activity组件都是使用一个ActivityClientRecord对象来描述的，他们保存在ActivityThread类的成员变量mActivities中。
-
ActivityThread.H.handleMessage
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); handleLaunchActivity(r, null, "LAUNCH_ACTIVITY"); 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"); SomeArgs args = (SomeArgs) msg.obj; handlePauseActivity((IBinder) args.arg1, false, (args.argi1 & USER_LEAVING) != 0, args.argi2, (args.argi1 & DONT_REPORT) != 0, args.argi3); maybeSnapshot(); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); } break; // ………… 此处有代码省略
}
> 首先将msg里面的obj转化成一个ActivityClientRecord对象，然后调用来获取一个LoaderAPK对象并保存在ActivityClientRecord对象的成员变量packageInfo中。loader对象用于藐视一个已经加载的APK文件。最后调用handleLauncherActivity来启动Activity组件。
7. ActivityThread.handleLaunchActivity
```java
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();
mSomeActivitiesChanged = true;
if (r.profilerInfo != null) {
mProfiler.setProfiler(r.profilerInfo);
mProfiler.startProfiling();
}
// Make sure we are running with the most recent config.
handleConfigurationChanged(null, null);
if (localLOGV) Slog.v(
TAG, "Handling launch of " + r);
// Initialize before creating the activity
WindowManagerGlobal.initialize();
Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
reportSizeConfigurations(r);
Bundle oldState = r.state;
handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);
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.
performPauseActivityIfNeeded(r, reason);
// 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;
}
}
} 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,
Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
}
Activity a = performLaunchActivity(r ,customIntent);真正完成Activity的吊起，Activity被实例化，onCreate被调用。performLaunchActivity函数加载用户自定义的Activity的派生类，并执行其onCreate函数，它将返回此Activity对象。·``handleResumeActivity(r.token, false, r.isForward,!r.activity.mFinished && !r.startsNotResumed);```同理，再调用Activity实例的Resume(用户界面可见)
-
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); } 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(); 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 { 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 (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; } 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()) { 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; }java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent)将Activity类文件加载到内存中,创建Activity实例mInstrumentation.callActivityOnCreate(activity, r.state);至此，Activity启动过程就结束了，其生命周期由ApplicationThread来管理;
ActivityRecord里面的token，是一个Binder的代理对象，和ActivityClientRecord对象一样，都是用来藐视所启动的Activity组件，只不过前者是在Activityma中使用，后者是在引用程序进程中使用。
到这里，Activity启动过程就完了。
子Activity的启动过程和应用程序的启动过程是非常类似的，过程如下：
1. 已启动的Activity向ActivityManagerService发送一个启动ChildActivity的请求；
2. ActivityManagerService首先将ChildActivity的信息保存下来，再向Activity发送一个中止的请求；
3. Activity收到请求进入中止状态，告诉ActivityManagerService，便于ActivityManagerService继续执行启动ChildActivity的操作。
4. ActivityManagerService检查ChildActivity所运行的进程是否存在，存在就发送ChildActivity信息给它，以便进行启动，
总结
经过了应用进程和系统进程一系列操作，完成Activity的启动，并且也找到了Activity的生命周期方法到底是怎么调用的，只是很多方法太长，其中的一些逻辑还不是很理解，后面有时间再仔细看看
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