Service 组件也是 Android 四大组件之一，它的启动过程分为显示和隐式两种。对于隐式启动的 Service 组件来说，我们只需要它的组件名称；对于显示启动的 Service 组件来说，我们需要知道它的类名称。 Service 组件可以被 Activity 组件启动，也可以被其他的 Service 组件启动。同时，它既可以在启动它的 Activity 组件或者 Service 组件所在的应用程序中启动，也可以在一个新的应用程序进程中启动。
不管 Service 组件是在 Activity 组件中启动的还是在 Service 组件中启动的，它们都是继承自ContextWrapper类的，最后调用的还是ContextImpl的startService方法。
所以直接从ContextImpl方法开始分析即可。
ContextImpl 类的 startService() 方法
@Override
public ComponentName startService(Intent service) {
warnIfCallingFromSystemProcess();
return startServiceCommon(service, mUser);
}
显然最后还是调用的startServiceCommon方法了。
ContextImpl 类的 startServiceCommon() 方法
private ComponentName startServiceCommon(Intent service, UserHandle user) {
try {
validateServiceIntent(service);
service.prepareToLeaveProcess();
// 向 ActivityManagerService 发送请求启动 Service 组件
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);
}
}
在startServiceCommon方法内又是 IPC 进程间通信了，通过ActivityManagerProxy向 Binder 驱动发送类型为START_SERVICE_TRANSACTION的消息，在ActivityManagerService响应对应消息。
ActivityManagerService 类的 startService() 方法
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");
}
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;
}
}
在 ActivityManagerService 内又是调用 ActivityServices 类的 startServiceLocked() 方法，将调用者的pid和uid传入。
ActiveServices 类的 startServiceLocked() 方法
ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
int callingPid, int callingUid, String callingPackage, int userId)
throws TransactionTooLargeException {
final boolean callerFg;
if (caller != null) {
final ProcessRecord callerApp = mAm.getRecordForAppLocked(caller);
if (callerApp == null) {
throw new SecurityException("");// 省略字符串内容
}
callerFg = callerApp.setSchedGroup != Process.THREAD_GROUP_BG_NONINTERACTIVE;
} else {
callerFg = true;
}
// 查找是否存在于 service 对应的 ServiceRecord 对象，没有则封装一个
ServiceLookupResult res =
retrieveServiceLocked(service, resolvedType, callingPackage,
callingPid, callingUid, userId, true, callerFg);
if (res == null) {
return null;
}
if (res.record == null) {
return new ComponentName("!", res.permission != null
? res.permission : "private to package");
}
ServiceRecord r = res.record;
if (!mAm.getUserManagerLocked().exists(r.userId)) { // 用户是否存在检查
Slog.d(TAG, "Trying to start service with non-existent user! " + r.userId);
return null;
}
NeededUriGrants neededGrants = mAm.checkGrantUriPermissionFromIntentLocked(
callingUid, r.packageName, service, service.getFlags(), null, r.userId);
if (unscheduleServiceRestartLocked(r, callingUid, false)) {
if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "START SERVICE WHILE RESTART PENDING: " + r);
}
r.lastActivity = SystemClock.uptimeMillis();
r.startRequested = true;
r.delayedStop = false;
r.pendingStarts.add(new ServiceRecord.StartItem(r, false, r.makeNextStartId(),
service, neededGrants));
final ServiceMap smap = getServiceMap(r.userId);
boolean addToStarting = false;
if (!callerFg && r.app == null && mAm.mStartedUsers.get(r.userId) != null) {
ProcessRecord proc = mAm.getProcessRecordLocked(r.processName, r.appInfo.uid, false);
if (proc == null || proc.curProcState > ActivityManager.PROCESS_STATE_RECEIVER) {
if (r.delayed) {
// This service is already scheduled for a delayed start; just leave
// it still waiting.
return r.name;
}
if (smap.mStartingBackground.size() >= mMaxStartingBackground) {
// Something else is starting, delay!
smap.mDelayedStartList.add(r);
r.delayed = true;
return r.name;
}
addToStarting = true;
} else if (proc.curProcState >= ActivityManager.PROCESS_STATE_SERVICE) {
addToStarting = true;
}
}
return startServiceInnerLocked(smap, service, r, callerFg, addToStarting);
}
在 startServiceLocked 方法内的调用 retrieveServiceLocked方法在 ActivityManagerService 检查是否存在与参数 service对应的一个ServiceRecord对象。如果不存在，那么 ActivityManagerService 就会到 PackageManagerService 中去获取与参数service对应的一个 Service 组件的信息，然后将这些信息封装成一个ServiceRecord对象，最后将这个ServiceRecord对象封装成一个ServiceLookupResult对象返回给调用者。
接着就是对用户是否存在进行检查，最后调用startServiceInnerLocked方法。
ActiveServices 类的 startServiceInnerLocked() 方法
ComponentName startServiceInnerLocked(ServiceMap smap, Intent service, ServiceRecord r,
boolean callerFg, boolean addToStarting) throws TransactionTooLargeException {
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 (first) {
smap.rescheduleDelayedStarts();
}
} else if (callerFg) {
smap.ensureNotStartingBackground(r);
}
return r.name;
}
该方法内调用bringUpServiceLocked方法。
ActiveServices 类的 bringUpServiceLocked() 方法
private final String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg,
boolean whileRestarting) throws TransactionTooLargeException {
if (r.app != null && r.app.thread != null) {// 满足条件，执行 service 的 onStartCommand 方法
sendServiceArgsLocked(r, execInFg, false);
return null;
}
if (!whileRestarting && r.restartDelay > 0) {
// If waiting for a restart, then do nothing.
return null;
}
// We are now bringing the service up, so no longer in the
// restarting state.
if (mRestartingServices.remove(r)) {
r.resetRestartCounter();
clearRestartingIfNeededLocked(r);
}
// Make sure this service is no longer considered delayed, we are starting it now.
if (r.delayed) {
getServiceMap(r.userId).mDelayedStartList.remove(r);
r.delayed = false;
}
// Make sure that the user who owns this service is started. If not,
// we don't want to allow it to run.
if (mAm.mStartedUsers.get(r.userId) == null) {
bringDownServiceLocked(r);
return msg;
}
// Service is now being launched, its package can't be stopped.
try {
AppGlobals.getPackageManager().setPackageStoppedState(
r.packageName, false, r.userId);
} catch (RemoteException e) {
} catch (IllegalArgumentException e) {
}
final boolean isolated = (r.serviceInfo.flags&ServiceInfo.FLAG_ISOLATED_PROCESS) != 0;
final String procName = r.processName;
ProcessRecord app;
if (!isolated) {
app = mAm.getProcessRecordLocked(procName, r.appInfo.uid, false);
if (app != null && app.thread != null) {
try {
app.addPackage(r.appInfo.packageName, r.appInfo.versionCode, mAm.mProcessStats);
realStartServiceLocked(r, app, execInFg);
return null;
} catch (TransactionTooLargeException e) {
throw e;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting service " + r.shortName, e);
}
}
} else {
app = r.isolatedProc;
}
// Not running -- get it started, and enqueue this service record
// to be executed when the app comes up.
if (app == null) {
if ((app=mAm.startProcessLocked(procName, r.appInfo, true, intentFlags,
"service", r.name, false, isolated, false)) == null) {
bringDownServiceLocked(r);
return msg;
}
if (isolated) {
r.isolatedProc = app;
}
}
if (!mPendingServices.contains(r)) {
mPendingServices.add(r);
}
if (r.delayedStop) {
// Oh and hey we've already been asked to stop!
r.delayedStop = false;
if (r.startRequested) {
stopServiceLocked(r);
}
}
return null;
}
在bringUpServiceLocked方法内先判断ServiceRecord的变量app和thread变量是否为null，也就是检查service对应的进程ProcessRecord和ApplicationThread对象是否存在，若存在，则直接执行service的onStartCommand方法。
若不存在，则将service从重启和延时启动队列中移开，因为它正在启动中了，最后再确保拥有该service的用户已经启动了。
接着 ActivityManagerService 根据进程名procName和uid调用getProcessRecordLocked方法查看对应的进程ProcessRecord是否存在。
- 若存在，则调用
realStartServiceLocked方法。 - 若不存在，则调用
startProcessLocked方法，先启动进程。
ActivityManagerService 通过startProcessLocked方法启动一个新的进程，在分析Launcher启动 Activity 组件时已经了解过了。
ActivityManagerService 创建一个新的进程，而一个新进程的入口就是ActivityThread类的main方法。在ActivityThread内会创建一个ActivityThread对象和一个ApplicationThread对象，而在main方法内又会调用ActivityThread的attach方法，用来向 ActivityManagerService 发送一个 类型为 ATTACH_APPLICATION_TRANSACTION的进程间通信，把ApplicationThread对象传递给 ActivityManagerService，以便能够 ActivityManagerService 可以和新进程进行 Binder 进程间通信。
ActivityManagerService 响应类型为ATTACH_APPLICATION_TRANSACTION的进程间通信，执行attachApplication方法，进而执行attachApplicationLocked方法。
在attachApplicationLocked方法内，首先会进行 Binder 跨进程调用，执行 ApplicationThread的bindApplication方法，然后再依次调度应用的Activity、Service、Broadcast组件。
其中，调度Service组件的代码如下：
// Find any services that should be running in this process...
if (!badApp) {
try {
// mServices 的类型为 ActivityServices
didSomething |= mServices.attachApplicationLocked(app, processName);
} catch (Exception e) {
Slog.wtf(TAG, "Exception thrown starting services in " + app, e);
badApp = true;
}
}
ActivityServices 类的 attachApplicationLocked() 方法
boolean attachApplicationLocked(ProcessRecord proc, String processName)
throws RemoteException {
boolean didSomething = false;
// Collect any services that are waiting for this process to come up.
if (mPendingServices.size() > 0) {
ServiceRecord sr = null;
try {
for (int i=0; i<mPendingServices.size(); i++) {
sr = mPendingServices.get(i);
if (proc != sr.isolatedProc && (proc.uid != sr.appInfo.uid
|| !processName.equals(sr.processName))) {
continue;
}
mPendingServices.remove(i);
i--;
proc.addPackage(sr.appInfo.packageName, sr.appInfo.versionCode,
mAm.mProcessStats);
realStartServiceLocked(sr, proc, sr.createdFromFg);
didSomething = true;
if (!isServiceNeeded(sr, false, false)) {// 不需要的服务就丢弃
bringDownServiceLocked(sr);
}
}
} catch (RemoteException e) {
throw e;
}
}
// Also, if there are any services that are waiting to restart and
// would run in this process, now is a good time to start them. It would
// be weird to bring up the process but arbitrarily not let the services
// run at this point just because their restart time hasn't come up.
if (mRestartingServices.size() > 0) {
ServiceRecord sr;
for (int i=0; i<mRestartingServices.size(); i++) {
sr = mRestartingServices.get(i);
if (proc != sr.isolatedProc && (proc.uid != sr.appInfo.uid
|| !processName.equals(sr.processName))) {
continue;
}
mAm.mHandler.removeCallbacks(sr.restarter);
mAm.mHandler.post(sr.restarter);
}
}
return didSomething;
}
attachApplicationLocked方法内启动mPendingServices队列中的服务和mRestartingServices中的服务。
真正启动服务的方法就是realStartServiceLocked。
ActivityServices 类的 realStartServiceLocked() 方法
private final void realStartServiceLocked(ServiceRecord r,
ProcessRecord app, boolean execInFg) throws RemoteException {
if (app.thread == null) {
throw new RemoteException();
}
r.app = app;
r.restartTime = r.lastActivity = SystemClock.uptimeMillis();
final boolean newService = app.services.add(r);
// 发送一个延时消息 SERVICE_TIMEOUT_MSG，Service 若没有启动超时，则回告诉 AMS 取消该消息
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);
// 进入 Service 的 onCreate
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); // 创建服务时，应用挂了
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));
}
// 进入 Service 的 onStartCommand
sendServiceArgsLocked(r, execInFg, true);
if (r.delayed) {
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) {
stopServiceLocked(r);
}
}
}
在realStartServiceLocked方法内，首先会通过bumpServiceExecutingLocked方法，然后再调用scheduleServiceTimeoutLocked方法，向 ActivityManagerService 的 MainHandler类型的 mHandler 发送一个消息 SERVICE_TIMEOUT_MSG。若 Service 在启动过程中没有及时向 ActivityManagerService 返回消息，把SERVICE_TIMEOUT_MSG消息给取消掉，那么则认为 Service 启动超时了。
标记完了时间后，就通过scheduleCreateService方法来启动 Service ，让 Service 进去onCreate状态。
新创建的进程把自己的 Binder 本地对象 ApplicationThread 传递给了 ActivityManagerService ，ActivityManagerService 就通过它来与新进程通信了，scheduleCreateService方法向 Binder 驱动发送了一个类型为 SCHEDULE_CREATE_SERVICE_TRANSACTION的消息，而在 ApplicationThread响应了该消息。
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);
}
ApplicationThread 的 scheduleCreateService方法向 ActivityThread 的主线程 mH发送了消息CREATE_SERVICE。ActivityThread 最终响应该消息。
ActivityThread 类的 handleCreateService() 方法
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();
// 通过反射创建目标服务对象
service = (Service) cl.loadClass(data.info.name).newInstance();
} catch (Exception e) {
}
try {
// 创建 ContextImpl
ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
context.setOuterContext(service);
// 创建 Application 对象
Application app = packageInfo.makeApplication(false, mInstrumentation);
service.attach(context, this, data.info.name, data.token, app,
ActivityManagerNative.getDefault());
// Service 进入 onCreate 状态
service.onCreate();
mServices.put(data.token, service);
try {
// 向 ActivityManagerService 发送消息取消 Service 启动延时消息
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
} catch (RemoteException e) {
// nothing to do.
}
} catch (Exception e) {
}
}
ActivityThread 类的handleCreateService主要是创建了 Service 对象，并且创建了ContextImpl对象和Application对象。同时调用了 Service 的onCreate方法。我们继承的 Service 的onCreate方法也是在这里被系统回调的。
随后向 ActivityManagerService 发送消息，取消 Service 延时启动的消息。
Service 进入 onCreate状态后，接下来该进入onStartCommand状态。
ActivityServices 类的 sendServiceArgsLocked() 方法
private final void sendServiceArgsLocked(ServiceRecord r, boolean execInFg,
boolean oomAdjusted) throws TransactionTooLargeException {
final int N = r.pendingStarts.size();
if (N == 0) {
return;
}
while (r.pendingStarts.size() > 0) {
Exception caughtException = null;
ServiceRecord.StartItem si;
try {
si = r.pendingStarts.remove(0);
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());
}
// 添加延时处理，类似于 onCreate
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;
}
// 跨进程 Service 进入 onStartCommand 状态
r.app.thread.scheduleServiceArgs(r, si.taskRemoved, si.id, flags, si.intent);
} catch (TransactionTooLargeException e) {
} catch (RemoteException e) {
} catch (Exception e) {
}
}
}
在sendServiceArgsLocked方法内通过跨进程scheduleServiceArgs方法，向 ActivityThread 发送消息。
ActivityThread 类的 handleServiceArgs() 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) {
// Service 进入 onStartCommand 状态
res = s.onStartCommand(data.args, data.flags, data.startId);
} else {
s.onTaskRemoved(data.args);
res = Service.START_TASK_REMOVED_COMPLETE;
}
QueuedWork.waitToFinish();
try {
// 向 ActivityManagerService 发送消息取消延时
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, SERVICE_DONE_EXECUTING_START, data.startId, res);
} catch (RemoteException e) {
// nothing to do.
}
ensureJitEnabled();
} catch (Exception e) {
}
}
}
ActivityThread 响应 Binder 驱动发送来的消息，仍然通过类型为H的mH处理消息，处理消息的方法就是handleServiceArgs。在方法内，回调了 Service 的onStartCommand方法，并且向 ActivityManagerService 发送消息取消了延时操作。
至此，Service 在新进程中的启动过程就分析结束了，从调用者的startService方法，再到 ActivityManagerService 的处理，再到 Service 进程的onCreate和onStartCommand方法。
参考
- Android 6.0 源码
- 《Android 系统源代码情景分析》
- http://gityuan.com/2016/03/06/start-service/
原创文章，转载请注明来源: Android 6.0 Service 启动过程源码分析（一）