Android 之ActivityThead、ActivityManagerService 与activity的管理和创建

在android中，Activity是四大组件中比较重要的一个（当然其他的也比较重要），那么android中是怎样管理这些activity的？应用的进程和主线程是怎么创建的，应用的消息循环又是在什么时候创建的？在这篇文章中将详细介绍：

先来看下涉及到的类，通过以下类图对整体先有个大概的印象：



ActivityThread:
ActivityThread主要用来启动应用程序的主线程，并且管理在应用端跟用户打交道的activity。在应用端的activity信息全部被存储在ActivityThread的成员变量mActivities中。

[java]
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final HashMap<IBinder, ActivityRecord> mActivities=
new HashMap<IBinder, ActivityRecord>();

[java]
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/*** 应用程序的启动入口 . ，主线程在启动的时候系统会自动建立消息循环机制。*/public
static final
void main(String[] args) { SamplingProfilerIntegration.start(); Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); ActivityThread thread =
new ActivityThread(); thread.attach(false); Looper.loop();
if (Process.supportsProcesses()) {
throw new RuntimeException("Main thread loop unexpectedly exited"); } thread.detach(); String name = (thread.mInitialApplication !=
null) ? thread.mInitialApplication.getPackageName() :
"<unknown>";Slog.i(TAG, "Main thread of " + name +
" is now exiting"); }

/*** 应用程序的启动入口 . ，主线程在启动的时候系统会自动建立消息循环机制。*/public static final void main(String[] args) {  SamplingProfilerIntegration.start();  Process.setArgV0("<pre-initialized>");  Looper.prepareMainLooper();  ActivityThread thread = new ActivityThread();  thread.attach(false);  Looper.loop();    if (Process.supportsProcesses()) {     throw new RuntimeException("Main thread loop unexpectedly exited");   }   thread.detach();   String name = (thread.mInitialApplication != null)            ? thread.mInitialApplication.getPackageName()            : "<unknown>";Slog.i(TAG, "Main thread of " + name + " is now exiting");    }

在建立消息循环之前，会通过thread.attach(false)来初始化应用程序的运行环境，并建立activityThread和ActivityManagerService之间的桥mAppThread，
mAppThread是IApplicationThread的一个实例。

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android.ddm.DdmHandleAppName.setAppName("<pre-initialized>"); RuntimeInit.setApplicationObject(mAppThread.asBinder()); IActivityManager mgr = ActivityManagerNative.getDefault();
try { mgr.attachApplication(mAppThread); }
catch (RemoteException ex) { }

[c-sharp]
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HistoryRecord r = new HistoryRecord(this, callerApp, callingUid,intent, resolvedType, aInfo, mConfiguration,resultRecord, resultWho, requestCode,
componentSpecified);

HistoryRecord r = new HistoryRecord(this, callerApp, callingUid,intent, resolvedType, aInfo, mConfiguration,resultRecord, resultWho, requestCode, componentSpecified);

把r加入到mHistory中。

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mHistory.add(addPos, r);

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app.thread.scheduleLaunchActivity(new Intent(r.intent), r,System.identityHashCode(r), r.info, r.icicle, results, newIntents, !andResume, isNextTransitionForward());

app.thread.scheduleLaunchActivity(new Intent(r.intent), r,System.identityHashCode(r),                    r.info, r.icicle, results, newIntents, !andResume,                    isNextTransitionForward());

涉及的主要类图：



再来看下ApplicationThread中的scheduleLaunchActivity方法：

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public final void scheduleLaunchActivity(Intent intent, IBinder token,
int ident, ActivityInfo info, Bundle state, List<ResultInfo> pendingResults, List<Intent> pendingNewIntents, boolean notResumed, boolean isForward) { ActivityRecord r =
new ActivityRecord(); r.token = token; r.ident = ident; r.intent = intent; r.activityInfo = info; r.state = state; r.pendingResults = pendingResults; r.pendingIntents = pendingNewIntents; r.startsNotResumed = notResumed;
r.isForward = isForward; queueOrSendMessage(H.LAUNCH_ACTIVITY, r); }

public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident, ActivityInfo info, Bundle state, List<ResultInfo> pendingResults, List<Intent> pendingNewIntents, boolean notResumed, boolean isForward) { ActivityRecord r = new ActivityRecord(); r.token = token; r.ident = ident; r.intent = intent; r.activityInfo = info; r.state = state; r.pendingResults = pendingResults; r.pendingIntents = pendingNewIntents; r.startsNotResumed = notResumed; r.isForward = isForward; queueOrSendMessage(H.LAUNCH_ACTIVITY, r); }

在这个里面主要是根据服务端返回回来的信息创建客户端activity记录ActivityRecord.
并通过Handler发送消息到消息队列，进入消息循环。在ActivityThread.handleMessage()中处理消息。最终在handleLaunchActivity方法中把ActivityRecord记录加入到mActivities（mActivities.put(r.token,r)）中，并启动activity（涉及到window、view、windowManager，详情请看handleResumeActivity（）方法和上一篇关于window、WindowManager的介绍）

总结：

在客户端和服务端分别有一个管理activity的地方，服务端是在mHistory中，处于mHistory栈顶的就是当前处于running状态的activity，客户端是在mActivities中。

在startActivity时，首先会在ActivityManagerService中建立HistoryRecord，并加入到mHistory中，然后通过scheduleLaunchActivity在客户端创建ActivityRecord记录并加入到mActivities中。最终在ActivityThread发起请求，进入消息循环，完成activity的启动和窗口的管理等