Android线程管理（二）——ActivityThread
上文分析了Handler、MessageQueue、Message及Looper四者的关系，本文将对Android中最重要的线程——ActivityThread进行说明。
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。（如有需要，可前往ActivityThread源码）
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，从Looper.prepareMainLooper()到Looper.loop()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。
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。
@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)
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：
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。
// 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。
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：
/** 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。
/**
* 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。
/** 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。
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：
/**
* 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。
