Android的Activity启动流程详解
Activity启动分为很多种情况，这里说的是打开新的应用程序第一个Activity的流程。
1. AcitivityManager产生新进程，新进程从android.app.ActivityThread.main开始运行。这里就是一般意义上的程序入口点，类似于C的main函数。
ActivityManagerService.java
private final void startProcessLocked(ProcessRecord app, String hostingType, String hostingNameStr) {
// Process里面通知Zygote服务，Zygote真正产生新的进程（准确说是复制一个）
int pid = Process.start("android.app.ActivityThread", mSimpleProcessManagement ? app.processName : null, uid, uid,
}
2.ActivityThread的main函数中将Looper准备起来，prepareMainLooper标志着这是应用程序主线程的Looper对象。
ActivityThread.java
public static final void main(String[] args) {
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread(); thread.attach(false); // 这里闭合消息循环 Looper.loop();
}
//3. 接下来调用attach，参数为false，表明这不是系统进程，是给普通应用程序用使用的进程。
ActivityThread.java
private final void attach(boolean system) {
ViewRoot.addFirstDrawHandler(new Runnable() {
public void run() {
ensureJitEnabled();
}
});
RuntimeInit.setApplicationObject(mAppThread.asBinder());
IActivityManager mgr = ActivityManagerNative.getDefault();
mgr.attachApplication(mAppThread);
}
//mAppThread是ApplicationThread对象，是提供给ActivityManagerService控制ActivityThread的回调接口。
private final class ApplicationThread extends ApplicationThreadNative {
public final void schedulePauseActivity(IBinder token, boolean finished,boolean userLeaving, int configChanges){
queueOrSendMessage(finished ? H.PAUSE_ACTIVITY_FINISHING : H.PAUSE_ACTIVITY, token, (userLeaving ? 1 : 0), configChanges); }
public final void scheduleStopActivity(IBinder token, boolean showWindow, int configChanges) { queueOrSendMessage(showWindow ? H.STOP_ACTIVITY_SHOW : H.STOP_ACTIVITY_HIDE, token, 0, configChanges); }
.........
}
ActivityManagerService要Pause当前Activity，就会调用schedulePauseActivity想本地的消息循环 中加入一个H.PAUSE_ACTIVITY的消息，然后立即返回以避免ActivityManagerService的阻塞。
4.现在又回到了ActivityManagerService中
ActivityManagerService.java
// ActivityManagerService中XXXLocked函数才是真正干活的地方，XXX只是个套
public final void attachApplication(IApplicationThread thread) { int callingPid = Binder.getCallingPid(); attachApplicationLocked(thread, callingPid); }
private final boolean attachApplicationLocked(IApplicationThread thread, int pid) {
// 这里取出对应该Pid的ProcessRecord对象，如果取不出来，你就等着out吧
app = mPidsSelfLocked.get(pid)
// 为ProcessRecord对象补充信息
app.thread = thread; app.curAdj = app.setAdj = -100; app.curSchedGroup = Process.THREAD_GROUP_DEFAULT; app.setSchedGroup = Process.THREAD_GROUP_BG_NONINTERACTIVE; app.forcingToForeground = null; app.foregroundServices = false; app.debugging = false; // 清除timeout监测 mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);
// 回调之前的ActivityThread，让它记住自己在ActivityManagerService中的相关信息，传这么大坨东西，真给力
thread.bindApplication(processName, app.instrumentationInfo != null ? app.instrumentationInfo : app.info, providers, app.instrumentationClass, app.instrumentationProfileFile, app.instrumentationArguments, app.instrumentationWatcher, testMode, isRestrictedBackupMode || !normalMode, mConfiguration, getCommonServicesLocked());
// topRunningActivityLocked的意思没看太明白
HistoryRecord hr = topRunningActivityLocked(null);
// 启动Activity
realStartActivityLocked(hr, app, true, true);
}
private final boolean realStartActivityLocked(HistoryRecord r, ProcessRecord app, boolean andResume, boolean checkConfig){
// 这里又跑到ActivityThread中去了
app.thread.scheduleLaunchActivity(new Intent(r.intent), r, System.identityHashCode(r), r.info, r.icicle, results, newIntents, !andResume, isNextTransitionForward());
}
5.ActivityThread开始调度用户的Activity启动了
ActivityThread.java
private final void handleLaunchActivity(ActivityRecord r, Intent customIntent) { Activity a = performLaunchActivity(r, customIntent);
......
}
private final void performLaunchActivity(ActivityRecord r, Intent customIntent) {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader(); // 产生Activity activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);
// 将新生的Activity与当前应用关联
activity.attach(appContext, this, getInstrumentation(), r.token, r.ident, app, r.intent, r.activityInfo, title, r.parent, r.embeddedID, r.lastNonConfigurationInstance, r.lastNonConfigurationChildInstances, config);
.....
}
6.Acivity与当前App关联，直到这里，平时应用所见的Activity才真正被构建
Actiivty.java
final void attach(Context context, ActivityThread aThread, Instrumentation instr, IBinder token, int ident, Application application, Intent intent, ActivityInfo info, CharSequence title, Activity parent, String id, Object lastNonConfigurationInstance, HashMap<String,Object> lastNonConfigurationChildInstances, Configuration config) {
// 记录传入ContexImpl实例，这就是平时所见的Activity Context attachBaseContext(context); // 创建与Activity关联的Window，可以简单理解为显示窗口 mWindow = PolicyManager.makeNewWindow(this); mUiThread = Thread.currentThread(); mMainThread = aThread; mInstrumentation = instr; mToken = token; mIdent = ident; mApplication = application; mIntent = intent; mComponent = intent.getComponent(); mActivityInfo = info; mTitle = title; mParent = parent; mEmbeddedID = id; // 构建WindowManager的代理LocalWindowManager mWindow.setWindowManager(null, mToken, mComponent.flattenToString()); if (mParent != null) { // 难道说整个Activity栈中的Activity都有同一个Container mWindow.setContainer(mParent.getWindow()); } mWindowManager = mWindow.getWindowManager(); }
7.ActivityThread
ActivityThread.java
private final void performLaunchActivity(ActivityRecord r, Intent customIntent) {
......
// 回调Activity::onCreate
mInstrumentation.callActivityOnCreate(activity, r.state);
// 记录新产生的Activity mActivities.put(r.token, r);
}
private final void handleLaunchActivity(ActivityRecord r, Intent customIntent) {
......
handleResumeActivity(r.token, false, r.isForward);
}
final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward) { ActivityRecord r = performResumeActivity(token, clearHide);
}
public final ActivityRecord performResumeActivity(IBinder token, boolean clearHide) {
if (r.pendingIntents != null) { deliverNewIntents(r, r.pendingIntents); r.pendingIntents = null; } if (r.pendingResults != null) { deliverResults(r, r.pendingResults); r.pendingResults = null; } //回调Activity::onResume r.activity.performResume();
}
到这里，Activity从产生到初始化的过程就全部结束了。之后就是等待用户界面的消息，根据消息进行相应的处理。整个过程中，在 ActivityManagerServer、ActivityThread的互相协作下构建出Activity，并在相应的时机回调Activity的 相应接口，完成Activity的初始化。