Service的工作过程

Service的工作过程

Service分为两种工作状态，一种是启动状态，主要用于执行后台计算；另一种是绑定状态，主要用于其他组件和Service的交互。这两种状态可以共存的，即Service既可以处于启动状态也可以同时处于绑定状态。

本篇博客是对Service的启动过程和绑定过程进行源码分析。



介绍一下几个重要的概念：

1. Context：/article/1390930.html

2. ActivityThread、ApplicationThread：/article/7801256.html

Service的启动过程

从ContextWrapper的startService开始：

@Override
public ComponentName startService(Intent service) {
return mBase.startService(service);
}

mBase的类型是ContextImpl。

@Override
public ComponentName startService(Intent service) {
warnIfCallingFromSystemProcess();
return startServiceCommon(service, mUser);
}

private ComponentName startServiceCommon(Intent service, UserHandle user) {
try {
validateServiceIntent(service);
service.prepareToLeaveProcess();
ComponentName cn = ActivityManagerNative.getDefault().startService(
mMainThread.getApplicationThread(), service, service.resolveTypeIfNeeded(
getContentResolver()), getOpPackageName(), user.getIdentifier());
if (cn != null) {
if (cn.getPackageName().equals("!")) {
throw new SecurityException(
"Not allowed to start service " + service
+ " without permission " + cn.getClassName());
} else if (cn.getPackageName().equals("!!")) {
throw new SecurityException(
"Unable to start service " + service
+ ": " + cn.getClassName());
}
}
return cn;
} catch (RemoteException e) {
throw new RuntimeException("Failure from system", e);
}
}

由上可知，startServiceCommon通过ActivityManagerNative.getDefault()（AMS）这个对象启动一个Service。通过AMS来启动服务的行为是一个远程调用过程。

AMS#startService

@Override
public ComponentName startService(IApplicationThread caller, Intent service,
String resolvedType, String callingPackage, int userId)
throws TransactionTooLargeException {
enforceNotIsolatedCaller("startService");
// Refuse possible leaked file descriptors
if (service != null && service.hasFileDescriptors() == true) {
throw new IllegalArgumentException("File descriptors passed in Intent");
}

if (callingPackage == null) {
throw new IllegalArgumentException("callingPackage cannot be null");
}

if (DEBUG_SERVICE) Slog.v(TAG_SERVICE,
"startService: " + service + " type=" + resolvedType);
synchronized(this) {
final int callingPid = Binder.getCallingPid();
final int callingUid = Binder.getCallingUid();
final long origId = Binder.clearCallingIdentity();
ComponentName res = mServices.startServiceLocked(caller, service,
resolvedType, callingPid, callingUid, callingPackage, userId);
Binder.restoreCallingIdentity(origId);
return res;
}
}

AMS会通过mService来完成Service后续的启动过程，mService对象的类型是ActivityServices，是一个辅助AMS进行Service管理的类，包括Service的启动、绑定和停止等。在ActivityServices的startServiceLocked方法会调用startServiceInnerLocked方法，startServiceInnerLocked方法继续调用bringUpServiceLocked方法，bringUpServiceLocked方法又调用realStartServiceLocked方法。

private final void realStartServiceLocked(ServiceRecord r,
ProcessRecord app, boolean execInFg) throws RemoteException {
if (app.thread == null) {
throw new RemoteException();
}
if (DEBUG_MU)
Slog.v(TAG_MU, "realStartServiceLocked, ServiceRecord.uid = " + r.appInfo.uid
+ ", ProcessRecord.uid = " + app.uid);
r.app = app;
r.restartTime = r.lastActivity = SystemClock.uptimeMillis();

final boolean newService = app.services.add(r);
bumpServiceExecutingLocked(r, execInFg, "create");
mAm.updateLruProcessLocked(app, false, null);
mAm.updateOomAdjLocked();

boolean created = false;
try {
if (LOG_SERVICE_START_STOP) {
String nameTerm;
int lastPeriod = r.shortName.lastIndexOf('.');
nameTerm = lastPeriod >= 0 ? r.shortName.substring(lastPeriod) : r.shortName;
EventLogTags.writeAmCreateService(
r.userId, System.identityHashCode(r), nameTerm, r.app.uid, r.app.pid);
}
synchronized (r.stats.getBatteryStats()) {
r.stats.startLaunchedLocked();
}
mAm.ensurePackageDexOpt(r.serviceInfo.packageName);
app.forceProcessStateUpTo(ActivityManager.PROCESS_STATE_SERVICE);
app.thread.scheduleCreateService(r, r.serviceInfo,
mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo),
app.repProcState);
r.postNotification();
created = true;
} catch (DeadObjectException e) {
Slog.w(TAG, "Application dead when creating service " + r);
mAm.appDiedLocked(app);
throw e;
} finally {
if (!created) {
// Keep the executeNesting count accurate.
final boolean inDestroying = mDestroyingServices.contains(r);
serviceDoneExecutingLocked(r, inDestroying, inDestroying);

// Cleanup.
if (newService) {
app.services.remove(r);
r.app = null;
}

// Retry.
if (!inDestroying) {
scheduleServiceRestartLocked(r, false);
}
}
}

requestServiceBindingsLocked(r, execInFg);

updateServiceClientActivitiesLocked(app, null, true);

// If the service is in the started state, and there are no
// pending arguments, then fake up one so its onStartCommand() will
// be called.
if (r.startRequested && r.callStart && r.pendingStarts.size() == 0) {
r.pendingStarts.add(new ServiceRecord.StartItem(r, false, r.makeNextStartId(),
null, null));
}

sendServiceArgsLocked(r, execInFg, true);

if (r.delayed) {
if (DEBUG_DELAYED_STARTS) Slog.v(TAG_SERVICE, "REM FR DELAY LIST (new proc): " + r);
getServiceMap(r.userId).mDelayedStartList.remove(r);
r.delayed = false;
}

if (r.delayedStop) {
// Oh and hey we've already been asked to stop!
r.delayedStop = false;
if (r.startRequested) {
if (DEBUG_DELAYED_STARTS) Slog.v(TAG_SERVICE,
"Applying delayed stop (from start): " + r);
stopServiceLocked(r);
}
}
}

在realStartServiceLocked方法中，首先通过app.thread的scheduleCreateService方法来创建Service对象并调用其onCreate，接着调用sendServiceArgsLocked方法来调用Service的其他方法，比如onStartCommand，这两个过程均是进程间的通信。具体实现是ApplicationThread。

ApplicationThread#scheduleCreateService

public final void scheduleCreateService(IBinder token,
ServiceInfo info, CompatibilityInfo compatInfo, int processState) {
updateProcessState(processState, false);
CreateServiceData s = new CreateServiceData();
s.token = token;
s.info = info;
s.compatInfo = compatInfo;

sendMessage(H.CREATE_SERVICE, s);
}

这个过程和Activity的启动类似。发送消息给Handler H，H接受消息H.CREATE_SERVICE并通过ActivityThread的handleCreateService方法来完成Service的启动。

ActivityThread#handleCreateService

private void handleCreateService(CreateServiceData data) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();

LoadedApk packageInfo = getPackageInfoNoCheck(
data.info.applicationInfo, data.compatInfo);
Service service = null;
try {
java.lang.ClassLoader cl = packageInfo.getClassLoader();
service = (Service) cl.loadClass(data.info.name).newInstance();
} catch (Exception e) {
if (!mInstrumentation.onException(service, e)) {
throw new RuntimeException(
"Unable to instantiate service " + data.info.name
+ ": " + e.toString(), e);
}
}

try {
if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);

ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
context.setOuterContext(service);

Application app = packageInfo.makeApplication(false, mInstrumentation);
service.attach(context, this, data.info.name, data.token, app,
ActivityManagerNative.getDefault());
service.onCreate();
mServices.put(data.token, service);
try {
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
} catch (RemoteException e) {
// nothing to do.
}
} catch (Exception e) {
if (!mInstrumentation.onException(service, e)) {
throw new RuntimeException(
"Unable to create service " + data.info.name
+ ": " + e.toString(), e);
}
}
}

主要完成了以下四件事：

1. 首先通过类加载器创建Service的实例

2. 然后创建Appliction对象并调用其onCreate

3. 接着调用ContextImpl对象并通过Service的attach方法建立二者的关系，和Activity的类似。

4. 最后调用Service的onCreate方法并将Service对象存储到ActivityThread中的一个列表中。

由于Service的onCreate被执行，意味着Service已经启动，除此之外，ActivityThread中还会通过handleServiceArgs方法调用Service的onStartCommand方法。

private void handleServiceArgs(ServiceArgsData data) {
Service s = mServices.get(data.token);
if (s != null) {
try {
if (data.args != null) {
data.args.setExtrasClassLoader(s.getClassLoader());
data.args.prepareToEnterProcess();
}
int res;
if (!data.taskRemoved) {
res = s.onStartCommand(data.args, data.flags, data.startId);
} else {
s.onTaskRemoved(data.args);
res = Service.START_TASK_REMOVED_COMPLETE;
}

QueuedWork.waitToFinish();

try {
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, SERVICE_DONE_EXECUTING_START, data.startId, res);
} catch (RemoteException e) {
// nothing to do.
}
ensureJitEnabled();
} catch (Exception e) {
if (!mInstrumentation.onException(s, e)) {
throw new RuntimeException(
"Unable to start service " + s
+ " with " + data.args + ": " + e.toString(), e);
}
}
}
}

Service的绑定过程

Service的绑定过程也是从ContextWrapper开始。

@Override
public boolean bindService(Intent service, ServiceConnection conn,
int flags) {
return mBase.bindService(service, conn, flags);
}

ContextImpl#bindServiceCommon

private boolean bindServiceCommon(Intent service, ServiceConnection conn, int flags,
UserHandle user) {
IServiceConnection sd;
if (conn == null) {
throw new IllegalArgumentException("connection is null");
}
if (mPackageInfo != null) {
sd = mPackageInfo.getServiceDispatcher(conn, getOuterContext(),
mMainThread.getHandler(), flags);
} else {
throw new RuntimeException("Not supported in system context");
}
validateServiceIntent(service);
try {
IBinder token = getActivityToken();
if (token == null && (flags&BIND_AUTO_CREATE) == 0 && mPackageInfo != null
&& mPackageInfo.getApplicationInfo().targetSdkVersion
< android.os.Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
flags |= BIND_WAIVE_PRIORITY;
}
service.prepareToLeaveProcess();
int res = ActivityManagerNative.getDefault().bindService(
mMainThread.getApplicationThread(), getActivityToken(), service,
service.resolveTypeIfNeeded(getContentResolver()),
sd, flags, getOpPackageName(), user.getIdentifier());
if (res < 0) {
throw new SecurityException(
"Not allowed to bind to service " + service);
}
return res != 0;
} catch (RemoteException e) {
throw new RuntimeException("Failure from system", e);
}
}

首先将客户端的ServiceConnection对象转化为ServiceDispather.InnerConnection对象。因为服务的绑定有可能是跨进程的，ServiceConnection必须借助Binder才能让远程服务端回调自己的方法，ServiceDispather.InnerConnection刚好充当了Binder。ServiceDispather起连接ServiceConnection和InnerConnection的作用。此过程由LoadedApk的getServiceDispather方法完成。

LoadedApk#getServiceDispather

public final IServiceConnection getServiceDispatcher(ServiceConnection c,
Context context, Handler handler, int flags) {
synchronized (mServices) {
LoadedApk.ServiceDispatcher sd = null;
ArrayMap<ServiceConnection, LoadedApk.ServiceDispatcher> map = mServices.get(context);
if (map != null) {
sd = map.get(c);
}
if (sd == null) {
sd = new ServiceDispatcher(c, context, handler, flags);
if (map == null) {
map = new ArrayMap<ServiceConnection, LoadedApk.ServiceDispatcher>();
mServices.put(context, map);
}
map.put(c, sd);
} else {
sd.validate(context, handler);
}
return sd.getIServiceConnection();
}
}

mService是一个ArrayMap存储了一个应用当前活动的ServiceConnection和ServiceDispatcher的映射关系。

系统首先会查找是否存在相同的ServiceConnection，若不存在，则重新创建一个ServiceDispatcher对象并将其存储在mService中，在ServiceDispatcher内部又保存了ServiceConnection和InnerConnection对象。当Service和客户端建立连接后，系统会通过InnerConnection来调用ServiceConnection中的onServiceConnected方法，这个过程可能跨进程。当ServiceDispatcher创建好以后，getServiceDispatcher会返回其保存的InnerConnection对象。

AMS#bindService

public int bindService(IApplicationThread caller, IBinder token, Intent service,
String resolvedType, IServiceConnection connection, int flags, String callingPackage,
int userId) throws TransactionTooLargeException {
enforceNotIsolatedCaller("bindService");

// Refuse possible leaked file descriptors
if (service != null && service.hasFileDescriptors() == true) {
throw new IllegalArgumentException("File descriptors passed in Intent");
}

if (callingPackage == null) {
throw new IllegalArgumentException("callingPackage cannot be null");
}

synchronized(this) {
return mServices.bindServiceLocked(caller, token, service,
resolvedType, connection, flags, callingPackage, userId);
}
}

接下来AMS会调用ActivityServices的startServiceLocked方法。在ActivityServices的startServiceLocked方法会调用startServiceInnerLocked方法，startServiceInnerLocked方法继续调用bringUpServiceLocked方法，bringUpServiceLocked方法又调用realStartServiceLocked方法。最终通过ApplicationThread来完成Service实例的创建并执行onCreate方法，和启动Service不同的是，Service的绑定过程会调用app.thread的scheduleBindService方法。

public final void scheduleBindService(IBinder token, Intent intent,
boolean rebind, int processState) {
updateProcessState(processState, false);
BindServiceData s = new BindServiceData();
s.token = token;
s.intent = intent;
s.rebind = rebind;

if (DEBUG_SERVICE)
Slog.v(TAG, "scheduleBindService token=" + token + " intent=" + intent + " uid="
+ Binder.getCallingUid() + " pid=" + Binder.getCallingPid());
sendMessage(H.BIND_SERVICE, s);
}

在H内部收到BIND_SERVICE，会交给ActivityThread的handleBindService方法来处理。

ActivityThread#handleBindService

private void handleBindService(BindServiceData data) {
Service s = mServices.get(data.token);
if (DEBUG_SERVICE)
Slog.v(TAG, "handleBindService s=" + s + " rebind=" + data.rebind);
if (s != null) {
try {
data.intent.setExtrasClassLoader(s.getClassLoader());
data.intent.prepareToEnterProcess();
try {
if (!data.rebind) {
IBinder binder = s.onBind(data.intent);
ActivityManagerNative.getDefault().publishService(
data.token, data.intent, binder);
} else {
s.onRebind(data.intent);
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
}
ensureJitEnabled();
} catch (RemoteException ex) {
}
} catch (Exception e) {
if (!mInstrumentation.onException(s, e)) {
throw new RuntimeException(
"Unable to bind to service " + s
+ " with " + data.intent + ": " + e.toString(), e);
}
}
}
}

在handleBindService中，首先根据Service的token取出Service对象，然后调用Service的onBind方法，Service的onBind方法返回一个Binder对象给客户端使用。原则上来说，Service的onBind方法调用后Service就被绑定成功，但是onBind方法是Service的方法，这个时候客户端并不知道已经成功连接Service，所以还要调用客户端的ServiceConnection中的onServiceConnected，这是由AMS的publishService方法完成。

Service有一个特性，当多次绑定同一个Service时，Service的onBind方法只会执行一次，除非Service被终止。当Service的onBind执行以后，系统还需要告知客户端已经成功连接Service了。由AMS的publishService方法完成。

AMS#publishService

public void publishService(IBinder token, Intent intent, IBinder service) {
// Refuse possible leaked file descriptors
if (intent != null && intent.hasFileDescriptors() == true) {
throw new IllegalArgumentException("File descriptors passed in Intent");
}

synchronized(this) {
if (!(token instanceof ServiceRecord)) {
throw new IllegalArgumentException("Invalid service token");
}
mServices.publishServiceLocked((ServiceRecord)token, intent, service);
}
}

由上可知，AMS的publishService方法将具体的工作交给了ActiveServices类型的mService对象来处理。

void publishServiceLocked(ServiceRecord r, Intent intent, IBinder service) {
final long origId = Binder.clearCallingIdentity();
try {
if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "PUBLISHING " + r
+ " " + intent + ": " + service);
if (r != null) {
Intent.FilterComparison filter
= new Intent.FilterComparison(intent);
IntentBindRecord b = r.bindings.get(filter);
if (b != null && !b.received) {
b.binder = service;
b.requested = true;
b.received = true;
for (int conni=r.connections.size()-1; conni>=0; conni--) {
ArrayList<ConnectionRecord> clist = r.connections.valueAt(conni);
for (int i=0; i<clist.size(); i++) {
ConnectionRecord c = clist.get(i);
if (!filter.equals(c.binding.intent.intent)) {
if (DEBUG_SERVICE) Slog.v(
TAG_SERVICE, "Not publishing to: " + c);
if (DEBUG_SERVICE) Slog.v(
TAG_SERVICE, "Bound intent: " + c.binding.intent.intent);
if (DEBUG_SERVICE) Slog.v(
TAG_SERVICE, "Published intent: " + intent);
continue;
}
if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "Publishing to: " + c);
try {
c.conn.connected(r.name, service);
} catch (Exception e) {
Slog.w(TAG, "Failure sending service " + r.name +
" to connection " + c.conn.asBinder() +
" (in " + c.binding.client.processName + ")", e);
}
}
}
}

serviceDoneExecutingLocked(r, mDestroyingServices.contains(r), false);
}
} finally {
Binder.restoreCallingIdentity(origId);
}

核心代码

c.conn.connected(r.name, service);

其中c是ConnectionRecord;c.conn是ServiceDispatcher.InnerConnection,Service就是Service的onBind返回的Binder对象。

LoadedApk#ServiceDispatcher#InnerConnection

private static class InnerConnection extends IServiceConnection.Stub {
final WeakReference<LoadedApk.ServiceDispatcher> mDispatcher;

InnerConnection(LoadedApk.ServiceDispatcher sd) {
mDispatcher = new WeakReference<LoadedApk.ServiceDispatcher>(sd);
}

public void connected(ComponentName name, IBinder service) throws RemoteException {
LoadedApk.ServiceDispatcher sd = mDispatcher.get();
if (sd != null) {
sd.connected(name, service);
}
}
}

InnerConnection的connected方法调用ServiceDispatcher的connected方法。

LoadedApk#ServiceDispatcher#connected

public void connected(ComponentName name, IBinder service) {
if (mActivityThread != null) {
mActivityThread.post(new RunConnection(name, service, 0));
} else {
doConnected(name, service);
}
}

ServiceDispatcher的mActivityThread是ActivityThread中的Handler H。如此，RunConnection就可以经由H的post方法从而运行在主线程中，因此客户端的ServiceConnection中的方法是在主线程被回调的。

private final class RunConnection implements Runnable {
RunConnection(ComponentName name, IBinder service, int command) {
mName = name;
mService = service;
mCommand = command;
}

public void run() {
if (mCommand == 0) {
doConnected(mName, mService);
} else if (mCommand == 1) {
doDeath(mName, mService);
}
}

final ComponentName mName;
final IBinder mService;
final int mCommand;
}

显然，RunConnection的润方法也是简单的调用了ServiceDispatcher的doConnected方法，由于ServiceDispatcher内部保存了客户端的ServiceConnection对象，因此它可以很方便地调用ServiceConnection对象的onServiceConnected方法。

LoadedApk#ServiceDispatcher#doConnected

public void doConnected(ComponentName name, IBinder service) {
ServiceDispatcher.ConnectionInfo old;
ServiceDispatcher.ConnectionInfo info;

synchronized (this) {
if (mForgotten) {
// We unbound before receiving the connection; ignore
// any connection received.
return;
}
old = mActiveConnections.get(name);
if (old != null && old.binder == service) {
// Huh, already have this one.  Oh well!
return;
}

if (service != null) {
// A new service is being connected... set it all up.
mDied = false;
info = new ConnectionInfo();
info.binder = service;
info.deathMonitor = new DeathMonitor(name, service);
try {
service.linkToDeath(info.deathMonitor, 0);
mActiveConnections.put(name, info);
} catch (RemoteException e) {
// This service was dead before we got it...  just
// don't do anything with it.
mActiveConnections.remove(name);
return;
}

} else {
// The named service is being disconnected... clean up.
mActiveConnections.remove(name);
}

if (old != null) {
old.binder.unlinkToDeath(old.deathMonitor, 0);
}
}

// If there was an old service, it is not disconnected.
if (old != null) {
mConnection.onServiceDisconnected(name);
}
// If there is a new service, it is now connected.
if (service != null) {
mConnection.onServiceConnected(name, service);
}
}

客户端的onServiceConnected方法执行后，Service的绑定过程也就完成了。