activity的启动流程
activity的启动过程分为两种，一种是根Activity的启动过程，另一种是普通Activity的启动过程。根activity的启动过程更复杂，也更具代表性，所以这里先分析根activity的启动过程。根activity的启动分为三部分：Launcher启请求AMS(ActivityManagerService)过程、AMS到ApplicationThread的调用过程以及ActivityThread启动Activity的过程。可以在这个网站在线看9.0.0_r3源码。
1，Launcher请求AMS过程
Launcher启动后会显示应用的图标于桌面上，这些应用图标就是启动根activity的入口，点击它们时就会通过Launcher类来请求AMS来启动该应用程序。Launcher的源码路径为：/packages/apps/Launcher3/src/com/android/launcher3/Launcher.java。
时序图如下：
2，接下来查看ApplicationThread的scheduleLaunchActivity方法，其中ApplicationThread是ActivityThread的内部类，应用程序进程创建后会运行代表主线程的实例ActivityThread，它管理着当前应用程序进程的线程。ApplicationThread的scheduleLaunchActivity方法如下所示。
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| @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;
...
updatePendingConfiguration(curConfig);
sendMessage(H.LAUNCH_ACTIVITY, r);
}
|
scheduleLaunchActivity方法会将启动Activity的参数封装成ActivityClientRecord ，sendMessage方法向H类发送类型为LAUNCH_ACTIVITY的消息，并将ActivityClientRecord 传递过去，sendMessage方法有多个重载方法，最终调用的sendMessage方法如下所示。
frameworks/base/core/java/android/app/ActivityThread.java:
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| private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
if (DEBUG_MESSAGES) Slog.v(
TAG, "SCHEDULE " + what + " " + mH.codeToString(what)
+ ": " + arg1 + " / " + obj);
Message msg = Message.obtain();
msg.what = what;
msg.obj = obj;
msg.arg1 = arg1;
msg.arg2 = arg2;
if (async) {
msg.setAsynchronous(true);
}
mH.sendMessage(msg);
}
|
这里mH指的是H，它是ActivityThread的内部类并继承Handler，是应用程序进程中主线程的消息管理类。H的代码如下所示。
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| 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;//1
r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);//2
handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");//3
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;
...
}
|
查看H的handleMessage方法中对LAUNCH_ACTIVITY的处理，在注释1处将传过来的msg的成员变量obj转换为ActivityClientRecord。
在注释2处通过getPackageInfoNoCheck方法获得LoadedApk类型的对象并赋值给ActivityClientRecord 的成员变量packageInfo 。应用程序进程要启动Activity时需要将该Activity所属的APK加载进来，而LoadedApk就是用来描述已加载的APK文件。
在注释3处调用handleLaunchActivity方法，代码如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
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| private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
...
WindowManagerGlobal.initialize();
//启动Activity
Activity a = performLaunchActivity(r, customIntent);//1
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
reportSizeConfigurations(r);
Bundle oldState = r.state;
//将Activity的状态置为Resume
handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);//2
if (!r.activity.mFinished && r.startsNotResumed) {
performPauseActivityIfNeeded(r, reason);
if (r.isPreHoneycomb()) {
r.state = oldState;
}
}
} else {
try {
//停止Activity启动
ActivityManager.getService()
.finishActivity(r.token, Activity.RESULT_CANCELED, null,
Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
}
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注释1处的performLaunchActivity方法用来启动Activity ，注释2处的代码用来将Activity 的状态置为Resume。如果该Activity为null则会通知AMS停止启动Activity。来查看performLaunchActivity方法做了什么：
frameworks/base/core/java/android/app/ActivityThread.java
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| private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
//获取ActivityInfo类
ActivityInfo aInfo = r.activityInfo;//1
if (r.packageInfo == null) {
//获取APK文件的描述类LoadedApk
r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
Context.CONTEXT_INCLUDE_CODE);//2
}
ComponentName component = r.intent.getComponent();//3
...
//创建要启动Activity的上下文环境
ContextImpl appContext = createBaseContextForActivity(r);//4
Activity activity = null;
try {
java.lang.ClassLoader cl = appContext.getClassLoader();
//用类加载器来创建该Activity的实例
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);//5
...
} catch (Exception e) {
...
}
try {
//创建Application
Application app = r.packageInfo.makeApplication(false, mInstrumentation);//6
...
if (activity != null) {
...
/**
*7 初始化Activity
*/
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, r.configCallback);
...
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);//8
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
...
}
r.paused = true;
mActivities.put(r.token, r);
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
...
}
return activity;
}
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注释1处用来获取ActivityInfo，ActivityInfo用于存储代码和AndroidManifes设置的Activity和receiver节点信息，比如Activity的theme和launchMode。在注释2处获取APK文件的描述类LoadedApk。注释3处获取要启动的Activity的ComponentName类，ComponentName类中保存了该Activity的包名和类名。注释4处用来创建要启动Activity的上下文环境。注释5处根据ComponentName中存储的Activity类名，用类加载器来创建该Activity的实例。注释6处用来创建Application，makeApplication方法内部会调用Application的onCreate方法。注释7处调用Activity的attach方法初始化Activity，attach方法中会创建Window对象（PhoneWindow）并与Activity自身进行关联。注释8处会调用Instrumentation的callActivityOnCreate方法来启动Activity，如下所示。
frameworks/base/core/java/android/app/Instrumentation.java
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| final void performCreate(Bundle icicle, PersistableBundle persistentState) {
restoreHasCurrentPermissionRequest(icicle);
onCreate(icicle, persistentState);
mActivityTransitionState.readState(icicle);
performCreateCommon();
}
|
performCreate方法中会调用Activity的onCreate方法，讲到这里，根Activity就启动了，即应用程序就启动了。
根Activity启动过程就讲到这里，下面我们来学习根Activity启动过程中涉及到的进程。
3,根Activity启动过程中涉及的进程