Android Service启动流程
概述
Service是android系统非常重要的四大组件之一，Service通常被用来在后台做一些耗时任务。本文从源码角度讲述启动Service流程
启动流程
1 startService
@Override
public ComponentName startService(Intent service) {
return mBase.startService(service);
}
2 [-> ContextImpl.java]
class ContextImpl extends Context {
@Override
public ComponentName startService(Intent service) {
warnIfCallingFromSystemProcess();
return startServiceCommon(service, false, mUser);
}
}
3 [-> ContextImpl.java]
private ComponentName startServiceCommon(Intent service, UserHandle user) {
try {
//检验service，当service为空则throw异常
validateServiceIntent(service);
service.prepareToLeaveProcess();
// 调用ActivityManagerNative类 【见流程3.1以及流程4】
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);
}
}
``
这里通过ActivityManager.getService().startService() 是通过binder机制发起了服务请求，这里假设startService的发起进程为A，ActivityManagerService为进程B，那么进程A通过Binder机制(采用IActivityManager接口)向进程B发起请求服务，进程B则通过Binder机制(采用IApplicationThread接口)向进程A发起请求服务。也就是说进程A和进程B通过binder机制可以互相主动发起请求来进行进程间通信
4.ActivityManager
@Override
public ComponentName startService(IApplicationThread caller, Intent service, String resolvedType, String callingPackage, int userId) throws TransactionTooLargeException {
//当调用者是孤立进程，则抛出异常。
enforceNotIsolatedCaller("startService");
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(); //调用者pid
final int callingUid = Binder.getCallingUid(); //调用者uid
final long origId = Binder.clearCallingIdentity();
//此次的mServices为ActiveServices对象 【见流程5】
ComponentName res = mServices.startServiceLocked(caller, service,
resolvedType, callingPid, callingUid, callingPackage, userId);
Binder.restoreCallingIdentity(origId);
return res;
}
}
这里mServices.startServiceLocked（） 中的mServices是ActiveServices的实例
5 ActiveServices.startServiceLocked
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(""); //抛出异常，此处省略异常字符串
callerFg = callerApp.setSchedGroup != Process.THREAD_GROUP_BG_NONINTERACTIVE;
} else {
callerFg = true;
}
//检索服务信息
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)) { //检查是否存在启动服务的user
return null;
}
NeededUriGrants neededGrants = mAm.checkGrantUriPermissionFromIntentLocked(
callingUid, r.packageName, service, service.getFlags(), null, r.userId);
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;
//对于非前台进程的调度
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) { //已计划延迟启动
return r.name;
}
if (smap.mStartingBackground.size() >= mMaxStartingBackground) {
//当超出 同一时间允许后续启动的最大服务数，则将该服务加入延迟启动的队列。
smap.mDelayedStartList.add(r);
r.delayed = true;
return r.name;
}
addToStarting = true;
} else if (proc.curProcState >= ActivityManager.PROCESS_STATE_SERVICE) {
//将新的服务加入到后台启动队列，该队列也包含当前正在运行其他services或者receivers的进程
addToStarting = true;
}
}
//【见流程6】
return startServiceInnerLocked(smap, service, r, callerFg, addToStarting);
}
这个方法最后调用的是startServiceInnerLocked
6.ActiveServices.startServiceLocked
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(); //用于耗电统计，开启运行的状态
}
//【见流程7】
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;
7.ActiveServices.bringUpServiceLocked
private final String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg, boolean whileRestarting) throws TransactionTooLargeException {
if (r.app != null && r.app.thread != null) {
//调用service.onStartCommand()过程
sendServiceArgsLocked(r, execInFg, false);
return null;
}
if (!whileRestarting && r.restartDelay > 0) {
return null; //等待延迟重启的过程，则直接返回
}
// 启动service前，把service从重启服务队列中移除
if (mRestartingServices.remove(r)) {
r.resetRestartCounter();
clearRestartingIfNeededLocked(r);
}
//service正在启动，将delayed设置为false
if (r.delayed) {
getServiceMap(r.userId).mDelayedStartList.remove(r);
r.delayed = false;
}
//确保拥有该服务的user已经启动，否则停止；
if (mAm.mStartedUsers.get(r.userId) == null) {
String msg = "";
bringDownServiceLocked(r);
return msg;
}
//服务正在启动，设置package停止状态为false
AppGlobals.getPackageManager().setPackageStoppedState(
r.packageName, false, r.userId);
final boolean isolated = (r.serviceInfo.flags&ServiceInfo.FLAG_ISOLATED_PROCESS) != 0;
final String procName = r.processName;
ProcessRecord app;
if (!isolated) {
//根据进程名和uid，查询ProcessRecord
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);
// 启动服务 【见流程8】
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;
}
//对于进程没有启动的情况
if (app == null) {
//启动service所要运行的进程(7.1)
if ((app=mAm.startProcessLocked(procName, r.appInfo, true, intentFlags,
"service", r.name, false, isolated, false)) == null) {
String msg = ""
bringDownServiceLocked(r); // 进程启动失败
return msg;
}
if (isolated) {
r.isolatedProc = app;
}
}
if (!mPendingServices.contains(r)) {
mPendingServices.add(r);
}
if (r.delayedStop) {
r.delayedStop = false;
if (r.startRequested) {
stopServiceLocked(r); //停止服务
}
}
return null;
}
这里判断app != null && app.thread != null 如果不为空，说明进程已经启动起来了，直接调用realStartServiceLocked启动服务，如果进程没有启动，则调用AMS.startProcessLocked启动进程
7.AMS.startProcessLocked
startProcessLocked(app, hostingType, hostingNameStr, null /* abiOverride */,
null /* entryPoint */, null /* entryPointArgs */);
AMS.startProcessLocked方法最后会调用到ActivityService.attachApplicationLocked
8.1 ActivityService.attachApplicationLocked
boolean didSomething = false;
//启动mPendingServices队列中，等待在该进程启动的服务
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--;
// 将当前服务的包信息加入到proc
proc.addPackage(sr.appInfo.packageName, sr.appInfo.versionCode,
mAm.mProcessStats);
// 启动服务，即将进入服务的生命周期 【见流程9】
realStartServiceLocked(sr, proc, sr.createdFromFg);
didSomething = true;
}
} catch (RemoteException e) {
Slog.w(TAG, "Exception in new application when starting service "
+ sr.shortName, e);
throw e;
}
}
// 对于正在等待重启并需要运行在该进程的服务，现在是启动它们的大好时机
if (mRestartingServices.size() > 0) {
ServiceRecord sr = null;
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;
9. ActivityService.AS.realStartServiceLocked
private final void realStartServiceLocked(ServiceRecord r, ProcessRecord app, boolean execInFg) throws RemoteException {
...
r.app = app;
r.restartTime = r.lastActivity = SystemClock.uptimeMillis();
final boolean newService = app.services.add(r);
//发送delay消息【见流程9.1】
bumpServiceExecutingLocked(r, execInFg, "create");
mAm.updateLruProcessLocked(app, false, null);
mAm.updateOomAdjLocked();
boolean created = false;
try {
synchronized (r.stats.getBatteryStats()) {
r.stats.startLaunchedLocked();
}
mAm.ensurePackageDexOpt(r.serviceInfo.packageName);
app.forceProcessStateUpTo(ActivityManager.PROCESS_STATE_SERVICE);
//服务进入 onCreate() 【见流程10】
app.thread.scheduleCreateService(r, r.serviceInfo,
mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo),
app.repProcState);
r.postNotification();
created = true;
} catch (DeadObjectException e) {
mAm.appDiedLocked(app); //应用死亡处理
throw e;
} finally {
if (!created) {
final boolean inDestroying = mDestroyingServices.contains(r);
serviceDoneExecutingLocked(r, inDestroying, inDestroying);
if (newService) {
app.services.remove(r);
r.app = null;
}
//尝试重新启动服务
if (!inDestroying) {
scheduleServiceRestartLocked(r, false);
}
}
}
requestServiceBindingsLocked(r, execInFg);
updateServiceClientActivitiesLocked(app, null, true);
if (r.startRequested && r.callStart && r.pendingStarts.size() == 0) {
r.pendingStarts.add(new ServiceRecord.StartItem(r, false, r.makeNextStartId(),
null, null));
}
//服务 进入onStartCommand() 【见流程17】
sendServiceArgsLocked(r, execInFg, true);
if (r.delayed) {
getServiceMap(r.userId).mDelayedStartList.remove(r);
r.delayed = false;
}
if (r.delayedStop) {
r.delayedStop = false;
if (r.startRequested) {
stopServiceLocked(r); //停止服务
}
}
}
这个方法里面调用了app.thread.scheduleCreateService的scheduleCreateService, 这里的app.thread其实是IApplicationThread接口，也就是说在AS这个服务器进程中通过binder机制 （采用IApplicationThread接口）调用app进程的scheduleCreateService
10. ApplicationThread.scheduleCreateService
private class ApplicationThread extends IApplicationThread.Stub {
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);
}
}
在ActivityThread的内部类ApplicationThread中通过Handler发送CREATE_SERVICE消息
11 ActivityThread.handleMessage
public void handleMessage(Message msg) {
switch (msg.what) {
...
case CREATE_SERVICE:
handleCreateService((CreateServiceData)msg.obj); //【见流程15】
break;
case BIND_SERVICE:
handleBindService((BindServiceData)msg.obj);
break;
case UNBIND_SERVICE:
handleUnbindService((BindServiceData)msg.obj);
break;
case SERVICE_ARGS:
handleServiceArgs((ServiceArgsData)msg.obj); // serviceStart
break;
case STOP_SERVICE:
handleStopService((IBinder)msg.obj);
maybeSnapshot();
break;
...
}
}
12 ActivityThread.handleCreateService
//当应用处于后台即将进行GC，而此时被调回到活动状态，则跳过本次gc。
unscheduleGcIdler();
LoadedApk packageInfo = getPackageInfoNoCheck(data.info.applicationInfo, data.compatInfo);
java.lang.ClassLoader cl = packageInfo.getClassLoader();
//通过反射创建目标服务对象
Service service = (Service) cl.loadClass(data.info.name).newInstance();
...
try {
//创建ContextImpl对象
ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
context.setOuterContext(service);
//创建Application对象,如果已创建，则不会再创建
Application app = packageInfo.makeApplication(false, mInstrumentation);
service.attach(context, this, data.info.name, data.token, app,
ActivityManagerNative.getDefault());
//调用服务onCreate()方法 【见流程13.1】
service.onCreate();
mServices.put(data.token, service);
//调用服务创建完成
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
} catch (Exception e) {
...
}
13 Service.onCreate
public abstract class Service extends ContextWrapper implements ComponentCallbacks2 {
public void onCreate(){ }
}
至此Service的生命周期方法onCreate就开始执行了
总结
根据以上源代码流程分析可以得出，service启动流程
从关键代码流程来看:
1.ContextImpl.startService()
2.ActivityManagerProxy.startService() (ActivityManagerProxy和ActivityManangerService都实现了IActivityManager接口)
3.ActivityManagerNative.transact() => 通过Binder方式(采用IActivityManager接口调用ActivityManangerService)
4.ActivityManangerService.startService()
5.ApplicationThreadProxy.scheduleCreateService()
6.ApplicationThreadNative.onTransact
7.ApplicationThreadNative. scheduleCreateService
8.ApplicationThread. scheduleCreateService=>通过Binder方式(采用IApplicaitonThread接口)
9.ActivityThread.mH.sendMessage()
10.ActivityThread.handleCreateService();
11.创建application(只会创建一次)
12.反射创建Service,调用其onCreate方法
从进程方面来看
在整个startService启动过程中，共涉及到以下几个进程
- Process A进程，是指调用startService命令所在的进程，也就是启动服务所在的进程，例如点击桌面app图标，此处Process A便是Launch所在进程
- system_server进程：系统进程,例如ApplicationThreadProxy,ActivityManagerService都运行在system_server进程的不同线程中
- Zygote进程:由init进程孵化而来
- Remote Service进程: 远程服务所在进程,由system_server进程孵化而来,主线程负责Activity/Service/BroadcastReceiver 以及ui操作等的生命周期调用，另外每个app进程至少会有两个binder线程ApplicationThread(简称AT)和ActivityManagerProxy（简称AMP）
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