它管理应用进程的主线程的执行(相当于普通Java程序的main入口函数),并根据AMS的要求(通过IApplicationThread接口,AMS为Client、ActivityThread.ApplicationThread为Server)负责调度和执行activities、broadcasts和其它操作。
在Android系统中,在默认情况下,一个应用程序内的各个组件(如Activity、BroadcastReceiver、Service)都会在同一个进程(Process)里执行,且由此进程的【主线程】负责执行。
在Android系统中,如果有特别指定(通过android:process),也可以让特定组件在不同的进程中运行。无论组件在哪一个进程中运行,默认情况下,他们都由此进程的【主线程】负责执行。
【主线程】既要处理Activity组件的UI事件,又要处理Service后台服务工作,通常会忙不过来。为了解决此问题,主线程可以创建多个子线程来处理后台服务工作,而本身专心处理UI画面的事件。
【主线程】的主要责任:
• 快速处理UI事件。而且只有它才处理UI事件, 其它线程还不能存取UI画面上的对象(如TextView等),此时, 主线程就叫做UI线程。基本上,Android希望UI线程能根据用户的要求做出快速响应,如果UI线程花太多时间处理后台的工作,当UI事件发生时,让用户等待时间超过5秒而未处理,Android系统就会给用户显示ANR提示信息。
只有UI线程才能执行View派生类的onDraw()函数。
• 快速处理Broadcast消息。【主线程】除了处理UI事件之外,还要处理Broadcast消息。所以在BroadcastReceiver的onReceive()函数中,不宜占用太长的时间,否则导致【主线程】无法处理其它的Broadcast消息或UI事件。如果占用时间超过10秒, Android系统就会给用户显示ANR提示信息。
注意事项:
• 尽量避免让【主线程】执行耗时的操作,让它能快速处理UI事件和Broadcast消息。
• BroadcastReceiver的子类都是无状态的,即每次启动时,才会创建其对象,然后调用它的onReceive()函数,当执行完onReceive()函数时,就立即删除此对象。由于每次调用其函数时,会重新创建一个新的对象,所以对象里的属性值,是无法让各函数所共享。
一:线程通信、ActivityThread及Thread类是理解Android线程管理的关键。
线程,作为CPU调度资源的基本单位,在Android等针对嵌入式设备的操作系统中,有着非常重要和基础的作用。本小节主要从以下三个方面进行分析:
1.《Android线程管理——线程通信》
2.《Android线程管理——ActivityThread》
3.《Android线程管理——Thread类的内部原理、休眠及唤醒》
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二、ActivityThread的主要工作及实现机制
ActivityThread是Android应用的主线程(UI线程),说起ActivityThread,不得不提到Activity的创建、启动过程以及ActivityManagerService,但本文将仅从线程管理的角度来分析ActivityThread。ActivityManagerService、ActivityStack、ApplicationThread等会在后续文章中详细分析,敬请期待喔~~不过为了说清楚ActivityThread的由来,还是需要简单介绍下。
以下引用自罗升阳大师的博客:《Android应用程序的Activity启动过程简要介绍和学习计划》
Step 1. 无论是通过Launcher来启动Activity,还是通过Activity内部调用startActivity接口来启动新的Activity,都通过Binder进程间通信进入到ActivityManagerService进程中,并且调用ActivityManagerService.startActivity接口;
Step 2. ActivityManagerService调用ActivityStack.startActivityMayWait来做准备要启动的Activity的相关信息;
Step 3. ActivityStack通知ApplicationThread要进行Activity启动调度了,这里的ApplicationThread代表的是调用ActivityManagerService.startActivity接口的进程,对于通过点击应用程序图标的情景来说,这个进程就是Launcher了,而对于通过在Activity内部调用startActivity的情景来说,这个进程就是这个Activity所在的进程了;
Step 4. ApplicationThread不执行真正的启动操作,它通过调用ActivityManagerService.activityPaused接口进入到ActivityManagerService进程中,看看是否需要创建新的进程来启动Activity;
Step 5. 对于通过点击应用程序图标来启动Activity的情景来说,ActivityManagerService在这一步中,会调用startProcessLocked来创建一个新的进程,而对于通过在Activity内部调用startActivity来启动新的Activity来说,这一步是不需要执行的,因为新的Activity就在原来的Activity所在的进程中进行启动;
Step 6. ActivityManagerServic调用ApplicationThread.scheduleLaunchActivity接口,通知相应的进程执行启动Activity的操作;
Step 7. ApplicationThread把这个启动Activity的操作转发给ActivityThread,ActivityThread通过ClassLoader导入相应的Activity类,然后把它启动起来。
大师的这段描述把ActivityManagerService、ActivityStack、ApplicationThread及ActivityThread的调用关系讲的很清楚,本文将从ActivityThread的main()方法开始分析其主要工作及实现机制。
ActivityThread源码来自:https://github.com/android/platform_frameworks_base/blob/master/core/java/android/app/ActivityThread.java
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public static void main(String[] args) { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain" ); SamplingProfilerIntegration.start(); // CloseGuard defaults to true and can be quite spammy. We // disable it here, but selectively enable it later (via // StrictMode) on debug builds, but using DropBox, not logs. CloseGuard.setEnabled( false ); Environment.initForCurrentUser(); // Set the reporter for event logging in libcore EventLogger.setReporter( new EventLoggingReporter()); AndroidKeyStoreProvider.install(); // Make sure TrustedCertificateStore looks in the right place for CA certificates final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId()); TrustedCertificateStore.setDefaultUserDirectory(configDir); Process.setArgV0( "<pre-initialized>" ); Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach( false ); if (sMainThreadHandler == null ) { sMainThreadHandler = thread.getHandler(); } if ( false ) { Looper.myLooper().setMessageLogging( new LogPrinter(Log.DEBUG, "ActivityThread" )); } // End of event ActivityThreadMain. Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); Looper.loop(); throw new RuntimeException( "Main thread loop unexpectedly exited" ); } |
上述代码中,红色部分之前的代码主要用于环境初始化、AndroidKeyStoreProvider安装等,这里不做重点说明。红色部分的代码主要分为两个功能块:1)绑定应用进程到ActivityManagerService;2)主线程Handler消息处理。
关于线程通信机制,Handler、MessageQueue、Message及Looper四者的关系请参考上一篇文章《Android线程管理——线程通信》。
2.1 应用进程绑定
main()方法通过thread.attach(false)绑定应用进程。ActivityManagerNative通过getDefault()方法返回ActivityManagerService实例,ActivityManagerService通过attachApplication将ApplicationThread对象绑定到ActivityManagerService,而ApplicationThread作为Binder实现ActivityManagerService对应用进程的通信和控制。
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private void attach( boolean system) { sCurrentActivityThread = this ; mSystemThread = system; if (!system) { …… RuntimeInit.setApplicationObject(mAppThread.asBinder()); final IActivityManager mgr = ActivityManagerNative.getDefault(); try { mgr.attachApplication(mAppThread); } catch (RemoteException ex) { // Ignore } …… } else { …… } } |
在ActivityManagerService内部,attachApplication实际是通过调用attachApplicationLocked实现的,这里采用了synchronized关键字保证同步。
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@Override public final void attachApplication(IApplicationThread thread) { synchronized ( this ) { int callingPid = Binder.getCallingPid(); final long origId = Binder.clearCallingIdentity(); attachApplicationLocked(thread, callingPid); Binder.restoreCallingIdentity(origId); } } |
attachApplicationLocked的实现较为复杂,其主要功能分为两部分:
thread.bindApplication
mStackSupervisor.attachApplicationLocked(app)
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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 { app = null ; } // …… try { // …… thread.bindApplication(processName, appInfo, providers, app.instrumentationClass, profilerInfo, app.instrumentationArguments, app.instrumentationWatcher, app.instrumentationUiAutomationConnection, testMode, enableOpenGlTrace, enableTrackAllocation, isRestrictedBackupMode || !normalMode, app.persistent, new Configuration(mConfiguration), app.compat, getCommonServicesLocked(app.isolated), mCoreSettingsObserver.getCoreSettingsLocked()); updateLruProcessLocked(app, false , null ); app.lastRequestedGc = app.lastLowMemory = SystemClock.uptimeMillis(); } catch (Exception e) { // todo: Yikes! What should we do? For now we will try to // start another process, but that could easily get us in // an infinite loop of restarting processes... Slog.wtf(TAG, "Exception thrown during bind of " + app, e); app.resetPackageList(mProcessStats); app.unlinkDeathRecipient(); startProcessLocked(app, "bind fail" , processName); return false ; } // See if the top visible activity is waiting to run in this process... if (normalMode) { try { if (mStackSupervisor.attachApplicationLocked(app)) { didSomething = true ; } } catch (Exception e) { Slog.wtf(TAG, "Exception thrown launching activities in " + app, e); badApp = true ; } } // …… } |
thread对象其实是ActivityThread里ApplicationThread对象在ActivityManagerService的代理对象,故此执行thread.bindApplication,最终会调用ApplicationThread的bindApplication方法。该bindApplication方法的实质是通过向ActivityThread的消息队列发送BIND_APPLICATION消息,消息的处理调用handleBindApplication方法,handleBindApplication方法比较重要的是会调用如下方法:
mInstrumentation.callApplicationOnCreate(app);
callApplicationOnCreate即调用应用程序Application的onCreate()方法,说明Application的onCreate()方法会比所有activity的onCreate()方法先调用。
mStackSupervisor为ActivityManagerService的成员变量,类型为ActivityStackSupervisor。
/** Run all ActivityStacks through this */
ActivityStackSupervisor mStackSupervisor;
从注释可以看出,mStackSupervisor为Activity堆栈管理辅助类实例。ActivityStackSupervisor的attachApplicationLocked()方法的调用了realStartActivityLocked()方法,在realStartActivityLocked()方法中,会调用scheduleLaunchActivity()方法:
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final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app, boolean andResume, boolean checkConfig) throws RemoteException { //... try { //... app.thread.scheduleLaunchActivity( new Intent(r.intent), r.appToken, System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration), r.compat, r.icicle, results, newIntents, !andResume, mService.isNextTransitionForward(), profileFile, profileFd, profileAutoStop); //... } catch (RemoteException e) { //... } //... return true ; } |
app.thread也是ApplicationThread对象在ActivityManagerService的一个代理对象,最终会调用ApplicationThread的scheduleLaunchActivity方法。
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// we use token to identify this activity without having to send the // activity itself back to the activity manager. (matters more with ipc) @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(); …… sendMessage(H.LAUNCH_ACTIVITY, r); } |
同bindApplication()方法,最终是通过向ActivityThread的消息队列发送消息,在ActivityThread完成实际的LAUNCH_ACTIVITY的操作。
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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; r.packageInfo = getPackageInfoNoCheck( r.activityInfo.applicationInfo, r.compatInfo); handleLaunchActivity(r, null ); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); } break ; …… } |
handleLaunchActivity()用于启动Activity。具体的启动流程不在这里详述了,这里重点说明ApplicationThread及ActivityThread的线程通信机制。
2.2 主线程消息处理
在《Android线程管理——线程通信》中谈到了普通线程中Handler、MessageQueue、Message及Looper四者的关系,那么,ActivityThread中的线程通信又有什么不同呢?不同之处主要表现为两点:1)Looper的初始化方式;2)Handler生成。
首先,ActivityThread通过Looper.prepareMainLooper()初始化Looper,为了直观比较ActivityThread与普通线程初始化Looper的区别,把两种初始化方法放在一起:
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/** Initialize the current thread as a looper. * This gives you a chance to create handlers that then reference * this looper, before actually starting the loop. Be sure to call * {@link #loop()} after calling this method, and end it by calling * {@link #quit()}. */ public static void prepare() { prepare( true ); } private static void prepare( boolean quitAllowed) { if (sThreadLocal.get() != null ) { throw new RuntimeException( "Only one Looper may be created per thread" ); } sThreadLocal.set( new Looper(quitAllowed)); } /** * Initialize the current thread as a looper, marking it as an * application's main looper. The main looper for your application * is created by the Android environment, so you should never need * to call this function yourself. See also: {@link #prepare()} */ public static void prepareMainLooper() { prepare( false ); synchronized (Looper. class ) { if (sMainLooper != null ) { throw new IllegalStateException( "The main Looper has already been prepared." ); } sMainLooper = myLooper(); } } |
•普通线程的prepare()方法默认quitAllowed参数为true,表示允许退出,ActivityThread在prepareMainLooper()方法中调用prepare()方法,参数为false,表示主线程不允许退出。
•普通线程只调用prepare()方法,ActivityThread在调用完prepare()方法之后,会通过myLooper()方法将本地线程<ThreadLocal>的Looper对象的引用交给sMainLooper。myLooper()其实就是调用sThreadLocal的get()方法实现的。
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/** * Return the Looper object associated with the current thread. Returns * null if the calling thread is not associated with a Looper. */ public static Looper myLooper() { return sThreadLocal.get(); } |
•之所以要通过sMainLooper指向ActivityThread的Looper对象,就是希望通过getMainLooper()方法将主线程的Looper对象开放给其他线程。
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/** Returns the application's main looper, which lives in the main thread of the application. */ public static Looper getMainLooper() { synchronized (Looper. class ) { return sMainLooper; } } |
其次,ActivityThread与普通线程的Handler生成方式也不一样。普通线程生成一个与Looper绑定的Handler即可,ActivityThread通过sMainThreadHandler指向getHandler()的返回值,而getHandler()方法返回的其实是一个继承Handler的H对象。。
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private class H extends Handler { …… } final H mH = new H(); final Handler getHandler() { return mH; } |
真正实现消息机制“通”信的其实是Looper的loop()方法,loop()方法的核心实现如下:
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/** * Run the message queue in this thread. Be sure to call * {@link #quit()} to end the loop. */ public static void loop() { final Looper me = myLooper(); if (me == null ) { throw new RuntimeException( "No Looper; Looper.prepare() wasn't called on this thread." ); } final MessageQueue queue = me.mQueue; // Make sure the identity of this thread is that of the local process, // and keep track of what that identity token actually is. Binder.clearCallingIdentity(); final long ident = Binder.clearCallingIdentity(); for (;;) { Message msg = queue.next(); // might block if (msg == null ) { // No message indicates that the message queue is quitting. return ; } // This must be in a local variable, in case a UI event sets the logger Printer logging = me.mLogging; if (logging != null ) { logging.println( ">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } msg.target.dispatchMessage(msg); if (logging != null ) { logging.println( "<<<<< Finished to " + msg.target + " " + msg.callback); } // Make sure that during the course of dispatching the // identity of the thread wasn't corrupted. final long newIdent = Binder.clearCallingIdentity(); if (ident != newIdent) { Log.wtf(TAG, "Thread identity changed from 0x" + Long.toHexString(ident) + " to 0x" + Long.toHexString(newIdent) + " while dispatching to " + msg.target.getClass().getName() + " " + msg.callback + " what=" + msg.what); } msg.recycle(); } } |
大致流程如下:
•首先通过上述myLooper()方法获取Looper对象,取出Looper持有的MessageQueue;
•然后从MessageQueue取出Message,如果Message为null,说明线程正在退出;
•Message不为空,则调用Message的target handler对该Message进行分发,具体分发、处理流程可参考《Android线程管理——线程通信》;
•消息处理完毕,调用recycle()方法进行回收。