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源码,是一个程序猿前进路上一个大的而又不得不去翻越障碍,我讨厌源码,看着一大堆。5000多行,要看完得啥时候去了啊。不过做安卓的总有这一天,自从踏上这条不归路,我就认命了。好吧,我慢慢来啃。先从Activity的启动流程开始,一个app启动的底层源码流程。那么我们该去哪里找源码呢,源码其实就在sdk的包里,并不遥远,只是平时我们很少去关注。(从应用层的入口开始,FrameWork层目前我还没那水平,看不明白),如果想从FrameWork层开始的童鞋,这里给个链接,去看看吧。,还有老罗讲Activity启动的更详细步骤,可以参考阅读:
首先找到sdk文件夹目录,打开source文件夹,我选的是android-21目录,在继续打开android文件夹,继续打开app文件夹,找到ActivityThread.java文件,这就是我们平时所说的UI线程(主线程)。
图 1
在打开Android Studio,直接把这个文件用鼠标拖入Studio界面即可。那么开始我们探索源码之路了。
首先,找到整个java文件的入口,不用问大家都知道,main函数,是所有java文件的入口。直接ctrl+F,搜索main,找到入口。开始怼他!
图2
这里不对每句代码进行详细解释,我们只撸对Activity启动有作用的代码。用过Handler的童鞋都知道,Handler是线程之间通讯的工具。第5205行代码,Looper.prepareMianLooper();就是为UI线程绑定Looper和MessageQueue,一个线程中只能有一个Looper和一个MessageQueue,可以有多个Handler这也是为什么我们平时在Activity的住线程中使用Handler传送消息时,只需要创建Handler不需要管Looper和MessageQueue的原因。
我们只需关心主线程做了哪些操作,其他的先不管。第5207行代码创建了ActivityThread的对象,同时调用了一个attach传入一个false。那么找到attach方法。
图3
初始化传入的是false,那么只看if(!system)里的代码,继续寻找和Activity有关的代码,看到5054行代码,创建了一个ActivityManager,这是Activity的管理器,那么我们在看看ActivityManagerNative这个类的getDefault()这个方法,默认返回的ActivityManager里有什么样的配置。找到源码包,搜索ActivityManagerNative,拖入Studio。
图4
这里通过gDefault对象的get()返回IActivityManager,那么看看gDefault又是什么呢。
图5
这里采用IBinder机制,通过ServiceManager获得activity的服务,调用asInterface(IBinder)方法,获得IActivityManager。再看看asInterface()方法。
图6
这里最后返回的是一个ActivityManagerProxy对象,通过代理设计模式来创建实现了IActivityManager接口的类的对象。
再回到图3ActivityThread中,获得了ActivityManager之后,第5056行调用attachApplication(),传入applicationThread类型的Binder对象。那么看一下ActivityManagerProxy的源码。
class ActivityManagerProxy implements IActivityManager{ ...... public void attachApplication(IApplicationThread app) throws RemoteException { Parcel data = Parcel.obtain(); Parcel reply = Parcel.obtain(); data.writeInterfaceToken(IActivityManager.descriptor); data.writeStrongBinder(app.asBinder()); mRemote.transact(ATTACH_APPLICATION_TRANSACTION, data, reply, 0); reply.readException(); data.recycle(); reply.recycle(); } ......}这里通过Binder驱动程序,最后进入ActivityManagerService的attachApplication函数中。那么我们看看ActivityManagerService中的attachApplication函数。
图7
在这个方法中调用了attachApplicationLocked()方法,这里将操作转发给attachApplicationLocked函数。
public final class ActivityManagerService extends ActivityManagerNative implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback { ...... private final boolean attachApplicationLocked(IApplicationThread thread, int pid) { // Find the application record that is being attached... either via // the pid if we are running in multiple processes, or just pull the // next app record if we are emulating process with anonymous threads. ProcessRecord app; if (pid != MY_PID && pid >= 0) { synchronized (mPidsSelfLocked) { app = mPidsSelfLocked.get(pid); } } else if (mStartingProcesses.size() > 0) { ...... } else { ...... } if (app == null) { ...... return false; } ...... String processName = app.processName; try { thread.asBinder().linkToDeath(new AppDeathRecipient( app, pid, thread), 0); } catch (RemoteException e) { ...... return false; } ...... 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; ...... boolean normalMode = mProcessesReady || isAllowedWhileBooting(app.info); ...... boolean badApp = false; boolean didSomething = false; // See if the top visible activity is waiting to run in this process... ActivityRecord hr = mMainStack.topRunningActivityLocked(null); if (hr != null && normalMode) { if (hr.app == null && app.info.uid == hr.info.applicationInfo.uid && processName.equals(hr.processName)) { try { if (mStackSupervisor.realStartActivityLocked(hr, app, true, true)) { didSomething = true; } } catch (Exception e) { ...... } } else { ...... } } ...... return true; } ...... }这里创建了一个ProcessRecord,首先通过pid将它取回来,放在app变量中,然后对app的其它成员进行初始化,最后调用
mStacksupervisor.realStartActivityLocked执行真正的Activity启动操作。这里要启动的Activity通过调用mStacksupervisor.topRunningActivityLocked(null)从堆栈顶端取回来,这时候在堆栈顶端的Activity就是MainActivity了。那么继续来看ActivityStackSupervisor.realStartActivityLocked()的源码.
final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app, boolean andResume, boolean checkConfig) throws RemoteException { .... r.app = app; .... int idx = app.activities.indexOf(r); if (idx < 0) { app.activities.add(r); } ... try { if (app.thread == null) { throw new RemoteException(); } List这里最终通过app.thread进入到ApplicationThreadProxy的scheduleLaunchActivity函数中,注意,这里的第二个参数r,是一个ActivityRecord类型的Binder对象,用来作来这个Activity的token值。app.thread.scheduleLaunchActivity调用的是ApplicationThreadProxy的方法,那么再看看ApplicationThreadProxy。results = null; List newIntents = null; ... app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken, System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration), r.compat, r.task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results, newIntents, !andResume, mService.isNextTransitionForward(), profilerInfo); ..... } catch (RemoteException e) { .. } ... return true; }
class ApplicationThreadProxy implements IApplicationThread { ...... public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident, ActivityInfo info, Bundle state, List函数首先创建一个ActivityClientRecord实例,并且初始化它的成员变量,然后调用ActivityThread类的queueOrSendMessage函数进一步处理。那么再来看看ActivityThread源码的queueOrSendMessage。pendingResults, List pendingNewIntents, boolean notResumed, boolean isForward) throws RemoteException { Parcel data = Parcel.obtain(); data.writeInterfaceToken(IApplicationThread.descriptor); intent.writeToParcel(data, 0); data.writeStrongBinder(token); data.writeInt(ident); info.writeToParcel(data, 0); data.writeBundle(state); data.writeTypedList(pendingResults); data.writeTypedList(pendingNewIntents); data.writeInt(notResumed ? 1 : 0); data.writeInt(isForward ? 1 : 0); mRemote.transact(SCHEDULE_LAUNCH_ACTIVITY_TRANSACTION, data, null, IBinder.FLAG_ONEWAY); data.recycle(); } ...... }
public final class ActivityThread { ...... private final class ApplicationThread extends ApplicationThreadNative { ...... // if the thread hasn't started yet, we don't have the handler, so just // save the messages until we're ready. private final void queueOrSendMessage(int what, Object obj) { queueOrSendMessage(what, obj, 0, 0); } ...... private final void queueOrSendMessage(int what, Object obj, int arg1, int arg2) { synchronized (this) { ...... Message msg = Message.obtain(); msg.what = what; msg.obj = obj; msg.arg1 = arg1; msg.arg2 = arg2; mH.sendMessage(msg); } } ...... } ...... }函数把消息内容放在msg中,然后通过mH把消息分发出去,这里的成员变量mH我们在前面已经见过,消息分发出去后,最后会调用H类的handleMessage函数。 这里最后调用ActivityThread类的handleLaunchActivity函数进一步处理。再看看handleLaunchActivity();
public final class ActivityThread { ...... private final void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) { ...... Activity a = performLaunchActivity(r, customIntent); if (a != null) { r.createdConfig = new Configuration(mConfiguration); Bundle oldState = r.state; handleResumeActivity(r.token, false, r.isForward); ...... } else { ...... } } ...... }这里首先调用performLaunchActivity函数来加载这个Activity类,即activity.MainActivity,然后调用它的onCreate函数,最后回到handleLaunchActivity函数时,再调用handleResumeActivity函数来使这个Activity进入Resumed状态,即会调用这个Activity的onResume函数,这是遵循Activity的生命周期的。再看performLaunchActivity的源码。
public final class ActivityThread { ...... private final Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) { ActivityInfo aInfo = r.activityInfo; if (r.packageInfo == null) { r.packageInfo = getPackageInfo(aInfo.applicationInfo, 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); r.intent.setExtrasClassLoader(cl); if (r.state != null) { r.state.setClassLoader(cl); } } catch (Exception e) { ...... } try { Application app = r.packageInfo.makeApplication(false, mInstrumentation); ...... if (activity != null) { ContextImpl appContext = new ContextImpl(); appContext.init(r.packageInfo, r.token, this); appContext.setOuterContext(activity); CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager()); Configuration config = new Configuration(mConfiguration); ...... activity.attach(appContext, this, getInstrumentation(), r.token, r.ident, app, r.intent, r.activityInfo, title, r.parent, r.embeddedID, r.lastNonConfigurationInstance, r.lastNonConfigurationChildInstances, config); if (customIntent != null) { activity.mIntent = customIntent; } r.lastNonConfigurationInstance = null; r.lastNonConfigurationChildInstances = null; activity.mStartedActivity = false; int theme = r.activityInfo.getThemeResource(); if (theme != 0) { activity.setTheme(theme); } activity.mCalled = false; mInstrumentation.callActivityOnCreate(activity, r.state); ...... r.activity = activity; r.stopped = true; if (!r.activity.mFinished) { activity.performStart(); r.stopped = false; } if (!r.activity.mFinished) { if (r.state != null) { mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state); } } if (!r.activity.mFinished) { activity.mCalled = false; 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) { ...... } catch (Exception e) { ...... } return activity; } ...... }从上面可以看到,mInrumentation.callActivityOncreate(),那么在这里进正式进入了Activity的onCreate()方法了。之后会依次调用Activity的生命周期方法。调用生命周期方法的类都是Intrumentation。
到这里就差不多了,应用层从入口到Activity启动调用Activity的生命周期,当然中间也省略了一些不太重要的步骤。最后附一张流程图。