Android四大组件之Service工作原理

Servie的工作过程

As we all know, 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，另外一种是绑定状态，主要用于其他组件和Service交互。需要注意的是Service的这两种状态是共存的。Service启动方式也对应的分为两种
1.Context.startService

Intent intentService = new Intent(this, MyService.class);
startService(intentService);

2.Context.bindService

Intent intentService = new Intent(this, MyService.class);
bindService(intentService,mServiceConnection,BIND_AUTO_CREATE);

1.startService详解

先来看下Activity与Context之间的关系

public class Activity extends ContextThemeWrapper

public class ContextThemeWrapper extends ContextWrapper

public class ContextWrapper extends Context

public abstract class Context

在ContetWrapper里面发现了startService

public class ContextWrapper extends Context {
Context mBase;
...
public ComponentName startService(Intent service) {
return mBase.startService(service);
}

}

在前面的章节中可以了解到，Activity创建的时候回调用attach方法，将一个ContextImpl对象关联起来，这个对象就是mBase，接下来分析ContextImpl的startService方法

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()), 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) {
return null;
}
}

ActivityManagerNative.getDefault()这是个什么东东？
通过前面的章节可以了解到它是一个IPC调用，指向ActivityManagerService
ActivityManagerService.startService

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

if (DEBUG_SERVICE)
Slog.v(TAG, "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, userId);
Binder.restoreCallingIdentity(origId);
return res;
}
}

final ActiveServices mServices;

接下来分析 ActiveServices.startServiceLocked,
这个方法比较长，这里就贴出最后关键的代码

ComponentName startServiceLocked(IApplicationThread caller,
...
return startServiceInnerLocked(smap, service, r, callerFg, addToStarting);
}

ComponentName startServiceInnerLocked(ServiceMap smap, Intent service,
ServiceRecord r, boolean callerFg, boolean addToStarting) {
ProcessStats.ServiceState stracker = r.getTracker();
if (stracker != null) {
stracker.setStarted(true, mAm.mProcessStats.getMemFactorLocked(), r.lastActivity);
}
r.callStart = false;
synchronized (r.stats.getBatteryStats()) {
r.stats.startRunningLocked();
}
String error = bringUpServiceLocked(r, service.getFlags(), callerFg, false);
if (error != null) {
return new ComponentName("!!", error);
}

if (r.startRequested && addToStarting) {
boolean first = smap.mStartingBackground.size() == 0;
smap.mStartingBackground.add(r);
r.startingBgTimeout = SystemClock.uptimeMillis() + BG_START_TIMEOUT;
if (DEBUG_DELAYED_SERVICE) {
RuntimeException here = new RuntimeException("here");
here.fillInStackTrace();
Slog.v(TAG, "Starting background (first=" + first + "): " + r, here);
} else if (DEBUG_DELAYED_STARTS) {
Slog.v(TAG, "Starting background (first=" + first + "): " + r);
}
if (first) {
smap.rescheduleDelayedStarts();
}
} else if (callerFg) {
smap.ensureNotStartingBackground(r);
}

return r.name;
}

startServiceInnerLocked又会调用到bringUpServiceLocked

private final String bringUpServiceLocked(ServiceRecord r,
int intentFlags, boolean execInFg, boolean whileRestarting) {
//Slog.i(TAG, "Bring up service:");
//r.dump("  ");

if (r.app != null && r.app.thread != null) {
sendServiceArgsLocked(r, execInFg, false);
return null;
}
...

if (!isolated) {
app = mAm.getProcessRecordLocked(procName, r.appInfo.uid, false);
if (DEBUG_MU) Slog.v(TAG_MU, "bringUpServiceLocked: appInfo.uid=" + r.appInfo.uid
+ " app=" + app);
if (app != null && app.thread != null) {
try {
app.addPackage(r.appInfo.packageName, r.appInfo.versionCode, mAm.mProcessStats);
realStartServiceLocked(r, app, execInFg);
return null;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting service " + r.shortName, e);
}

// If a dead object exception was thrown -- fall through to
// restart the application.
}
}
...

return null;

}

bringUpServiceLocked主要调用了realStartServiceLocked

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();

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

boolean created = false;
try {
String nameTerm;
int lastPeriod = r.shortName.lastIndexOf('.');
nameTerm = lastPeriod >= 0 ? r.shortName.substring(lastPeriod) : r.shortName;
if (LOG_SERVICE_START_STOP) {
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);
} finally {
if (!created) {
app.services.remove(r);
r.app = null;
scheduleServiceRestartLocked(r, false);
return;
}
}

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, "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, "Applying delayed stop (from start): " + r);
stopServiceLocked(r);
}
}
}

其中 app.thread.scheduleCreateService 就是IPC调用客户端启动服务，H类的实例会发送消息并处理该消息
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, 0, 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);
}
}
}

这个方法比较重要，有必要总结一下，handleCreateService主要做了以下几个事情
1.通过ClassLoader加载service的实例
2.创建Application对象并调用其onCreate方法
3.创建ContextImpl对象，并通过Service的attach方法关联起来（与Activity类似）
4.最后调用Service的onCreate方法并将Service对象ActivityThread的一个mServies列表中

realStartServiceLocked除了调用了上面的方法外，还会调用sendServiceArgsLocked

sendServiceArgsLocked

private final void sendServiceArgsLocked(ServiceRecord r, boolean execInFg,
boolean oomAdjusted) {
final int N = r.pendingStarts.size();
if (N == 0) {
return;
}

while (r.pendingStarts.size() > 0) {
try {
ServiceRecord.StartItem si = r.pendingStarts.remove(0);
if (DEBUG_SERVICE) Slog.v(TAG, "Sending arguments to: "
+ r + " " + r.intent + " args=" + si.intent);
if (si.intent == null && N > 1) {
// If somehow we got a dummy null intent in the middle,
// then skip it.  DO NOT skip a null intent when it is
// the only one in the list -- this is to support the
// onStartCommand(null) case.
continue;
}
si.deliveredTime = SystemClock.uptimeMillis();
r.deliveredStarts.add(si);
si.deliveryCount++;
if (si.neededGrants != null) {
mAm.grantUriPermissionUncheckedFromIntentLocked(si.neededGrants,
si.getUriPermissionsLocked());
}
bumpServiceExecutingLocked(r, execInFg, "start");
if (!oomAdjusted) {
oomAdjusted = true;
mAm.updateOomAdjLocked(r.app);
}
int flags = 0;
if (si.deliveryCount > 1) {
flags |= Service.START_FLAG_RETRY;
}
if (si.doneExecutingCount > 0) {
flags |= Service.START_FLAG_REDELIVERY;
}
r.app.thread.scheduleServiceArgs(r, si.taskRemoved, si.id, flags, si.intent);
} catch (RemoteException e) {
// Remote process gone...  we'll let the normal cleanup take
// care of this.
if (DEBUG_SERVICE) Slog.v(TAG, "Crashed while scheduling start: " + r);
break;
} catch (Exception e) {
Slog.w(TAG, "Unexpected exception", e);
break;
}
}
}

r.app.thread.scheduleServiceArgs这又是一个跨进程调用，AMS通过ApplicationThread来通知进程

ActivityThread.ApplicationThread.scheduleServiceArgs

public final void scheduleServiceArgs(IBinder token, boolean taskRemoved, int startId,
int flags ,Intent args) {
ServiceArgsData s = new ServiceArgsData();
s.token = token;
s.taskRemoved = taskRemoved;
s.startId = startId;
s.flags = flags;
s.args = args;

sendMessage(H.SERVICE_ARGS, s);
}

实际上就是通过H这个handler实例发送了一个消息

private void sendMessage(int what, Object obj) {
sendMessage(what, obj, 0, 0, false);
}

private void sendMessage(int what, Object obj, int arg1) {
sendMessage(what, obj, arg1, 0, false);
}

private void sendMessage(int what, Object obj, int arg1, int arg2) {
sendMessage(what, obj, arg1, arg2, false);
}

private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
if (DEBUG_MESSAGES) Slog.v(
TAG, "SCHEDULE " + what + " " + mH.codeToString(what)
+ ": " + arg1 + " / " + obj);
Message msg = Message.obtain();
msg.what = what;
msg.obj = obj;
msg.arg1 = arg1;
msg.arg2 = arg2;
if (async) {
msg.setAsynchronous(true);
}
mH.sendMessage(msg);
}

接着看ActivityThread.H.handleMessage

case SERVICE_ARGS:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceStart");
handleServiceArgs((ServiceArgsData)msg.obj);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;

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, 1, 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);
}
}
}
}

其中s.onStartCommand就调用了Service的onStartCommand方法。
到此StartService的分析告一段路。

2.bindService详解

通startService一样，开始也是Activity.bindService->ContextWrapper.bindService->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, user.getIdentifier());
if (res < 0) {
throw new SecurityException(
"Not allowed to bind to service " + service);
}
return res != 0;
} catch (RemoteException e) {
return false;
}
}

sd = mPackageInfo.getServiceDispatcher这里在客户端先ServiceConnection类型的conn转化成了IServiceConnection类型的sd，IServiceConnection实为接口

这里的mPackageInfo是一个LoadedApk类型，看看这个函数到底做了些神马东东

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();
}
}

这里将客户端的ServiceConnection对象实际上是转换成了ServieDispatcher.InnerConnection对象。为啥要转呢，原因很简单，因为服务的绑定是要跨进程的必须借助Binder，而ServiceDispatcher.InnerConnection对象就充当了这个角色。ServiceDispatcher就把这两种对象连接起来了。

接着看bindServiceCommon后面的代码，有了上面startService分析的经验很容易知道ActivityManagerNative.getDefault().bindService这里是个IPC调用，直接指向了AMS.bindService

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

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

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

ActiveServices.bindServiceLocked-> bringUpServiceLocked->realStartServiceLocked
realStartServiceLocked又回到了startService分析的那里，与startService不同的是bindService会调用ActiveServices的requestServiceBindingLocked方法

ActiveServices.requestServiceBindingLocked

private final boolean requestServiceBindingLocked(ServiceRecord r,
IntentBindRecord i, boolean execInFg, boolean rebind) {
if (r.app == null || r.app.thread == null) {
// If service is not currently running, can't yet bind.
return false;
}
if ((!i.requested || rebind) && i.apps.size() > 0) {
try {
bumpServiceExecutingLocked(r, execInFg, "bind");
r.app.forceProcessStateUpTo(ActivityManager.PROCESS_STATE_SERVICE);
r.app.thread.scheduleBindService(r, i.intent.getIntent(), rebind,
r.app.repProcState);
if (!rebind) {
i.requested = true;
}
i.hasBound = true;
i.doRebind = false;
} catch (RemoteException e) {
if (DEBUG_SERVICE) Slog.v(TAG, "Crashed while binding " + r);
return false;
}
}
return true;
}

接着分析ActivityThread.ApplicationThread.sheduleBindService

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);
}

再熟悉不过了，直接看处理方法吧

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);
<span style="color:#ff6666;"> ActivityManagerNative.getDefault().publishService(
data.token, data.intent, binder);</span>
} else {
s.onRebind(data.intent);
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, 0, 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);
}
}
}
}

客户端进程再次IPC调用AMS的publishService方法

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);
}
}

ActiveServices.publishServiceLocked

void publishServiceLocked(ServiceRecord r, Intent intent, IBinder service) {
...
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);
}
...
}

这个方法很长这里就截取了最关键的一句代码，其中c的类型就是Connectionrecord，而c.conn的类型就是ServiceDispatcher.InnerConnection对象

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);
}
}
}

从LoadedApk.ServiceDispatcher.InnerConnection的定义可以看出InnerConnection.connected->ServiceDispacher.connected

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

这里ServiceDispatcher的mActivityThread就是一个handler，对应ActivityThread中的H类，这样RunConnection经过H.post运行在主线程中，ServiceConnection的方法就是在主线程中被回调的。接着看RunConnection的定义

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;
}

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);
}
}

ServiceConnected的onServiceConnected执行之后，bindService的分析也告一段落。