先来一张不专业的时序图
Service启动流程.png
基于源码9.0
要启动Service，我们要调用Context的startService方法，这是一个抽象方法，ContextWrapper类继承了Context类实现了startService方法。
public abstract ComponentName startService(Intent service);
ContextWrapper的startService方法。
@Override
public ComponentName startService(Intent service) {
//调用mBase的startService方法
return mBase.startService(service);
}
方法内部调用了mBase的startService方法。mBase的声明类型是Context，运行时类型是一个ContextImpl对象。可以参考从点击桌面应用图标到MainActivity的onResume过程分析文章。
ContextImpl的startService方法
@Override
public ComponentName startService(Intent service) {
//...
//调用startServiceCommon方法
return startServiceCommon(service, false, mUser);
}
ContextImpl的startServiceCommon方法。
private ComponentName startServiceCommon(Intent service, boolean requireForeground,
UserHandle user) {
try {
validateServiceIntent(service);
service.prepareToLeaveProcess(this);
//调用ActivityManagerService的startService方法
ComponentName cn = ActivityManager.getService().startService(
mMainThread.getApplicationThread(), service, service.resolveTypeIfNeeded(
getContentResolver()), requireForeground,
getOpPackageName(), user.getIdentifier());
//...
return cn;
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
ContextImpl的startService方法内部调用了ActivityManagerService的startService方法来启动Service。
ActivityManagerService的startService方法
@Override
public ComponentName startService(IApplicationThread caller, Intent service,
String resolvedType, boolean requireForeground, String callingPackage,
int userId) throws TransactionTooLargeException {
//...
synchronized(this) {
final int callingPid = Binder.getCallingPid();
final int callingUid = Binder.getCallingUid();
final long origId = Binder.clearCallingIdentity();
ComponentName res;
try {
//注释1. 调用ActiveServices的startServiceLocked方法
res = mServices.startServiceLocked(caller, service,
resolvedType, callingPid, callingUid,
requireForeground, callingPackage, userId);
} finally {
Binder.restoreCallingIdentity(origId);
}
return res;
}
}
在注释1处，调用了 mServices的startServiceLocked方法。 mServices的类型是ActiveServices。
ActiveServices的startServiceLocked方法
ComponentName startServiceLocked(IApplicationThread caller, Intent service,
String resolvedType, int callingPid, int callingUid, boolean fgRequired,
String callingPackage, final int userId) throws TransactionTooLargeException {
//...
//注释2
ServiceLookupResult res =retrieveServiceLocked(service, resolvedType, callingPackage,
callingPid, callingUid, userId, true, callerFg, false, false);
//...
//注释3
ComponentName cmp = startServiceInnerLocked(smap, service, r, callerFg, addToStarting);
return cmp;
}
在注释2处，调用retrieveServiceLocked来查找是否有目标service对应的ServiceRecord，查找时先在本地的ServiceMap中查询，如果没有找到，就会调用PackageManagerService去获取目标service对应的信息，并封装到ServiceRecord中，最后将ServiceRecord封装为ServiceLookupResult返回。
继续往下走注释3处，调用了ActiveServices的startServiceInnerLocked方法
ActiveServices的startServiceInnerLocked方法
ComponentName startServiceInnerLocked(ServiceMap smap, Intent service, ServiceRecord r,
boolean callerFg, boolean addToStarting) throws TransactionTooLargeException {
ServiceState stracker = r.getTracker();
//...
//注释4，调用bringUpServiceLocked方法
String error = bringUpServiceLocked(r, service.getFlags(), callerFg, false, false);
if (error != null) {
return new ComponentName("!!", error);
}
//...
return r.name;
}
ActiveServices的bringUpServiceLocked方法
private String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg,
boolean whileRestarting, boolean permissionsReviewRequired)
throws TransactionTooLargeException {
//...
ProcessRecord app;
if (!isolated) {
app = mAm.getProcessRecordLocked(procName, r.appInfo.uid, false);
//注释4，如果要启动的Service所在的进程存在
if (app != null && app.thread != null) {
try {
app.addPackage(r.appInfo.packageName, r.appInfo.longVersionCode, mAm.mProcessStats);
//注释5，调用realStartServiceLocked方法
realStartServiceLocked(r, app, execInFg);
return null;
} catch (TransactionTooLargeException e) {
throw e;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting service " + r.shortName, e);
}
}
}
//...
//注释6，如果要启动的Service所在的进程不存在，先启动进程
if (app == null && !permissionsReviewRequired) {
//注释7
if ((app=mAm.startProcessLocked(procName, r.appInfo, true, intentFlags,
hostingType, r.name, false, isolated, false)) == null) {
//...
}
//...
}
//...
}
在注释4处，如果要启动的Service所在的进程存在，那么直接调用realStartServiceLocked方法启动目标service。如果要启动的Service所在的进程不存在，那么在注释7处先调用ActivityManagerService的startProcessLocked启动进程。这里我们暂时只看启动的Service所在的进程存在的情况。
ActiveServices的realStartServiceLocked方法
private final void realStartServiceLocked(ServiceRecord r,
ProcessRecord app, boolean execInFg) throws RemoteException {
//...
//注释8处，调用ApplicationThread的scheduleCreateService方法
app.thread.scheduleCreateService(r, r.serviceInfo,
mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo),
app.repProcState);
//...
//注释9，
sendServiceArgsLocked(r, execInFg, true);
}
在realStartServiceLocked方法中，首先在注释8处，调用ApplicationThread的scheduleCreateService方法。
ApplicationThread的scheduleCreateService方法
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);
}
scheduleCreateService方法最终还是调用了mH.sendMessage(msg)来发送消息，这个过程就是handler发送消息和处理消息的过程，不再详细叙述，我们直接看mH的handleMessage方法即可。（这里插一句，ApplicationThread是ActivityThread的内部类，mH是ActivityThread的一个成员变量）
public void handleMessage(Message msg) {
switch (msg.what) {
case CREATE_SERVICE:
handleCreateService((CreateServiceData)msg.obj);
break;
}
}
handleMessage方法内部调用了ActivityThread的handleCreateService方法
ActivityThread的handleCreateService方法
private void handleCreateService(CreateServiceData data) {
//...
LoadedApk packageInfo = getPackageInfoNoCheck(
data.info.applicationInfo, data.compatInfo);
Service service = null;
try {
//构建Service对象
java.lang.ClassLoader cl = packageInfo.getClassLoader();
service = packageInfo.getAppFactory()
.instantiateService(cl, data.info.name, data.intent);
} catch (Exception e) {
//...
}
try {
//创建service的上下文环境
ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
context.setOuterContext(service);
Application app = packageInfo.makeApplication(false, mInstrumentation);
//service初始化
service.attach(context, this, data.info.name, data.token, app,
ActivityManager.getService());
//调用service的onCreate方法
service.onCreate();
mServices.put(data.token, service);
//...
}
}
在这个方法中创建了service的实例，创建service的上下文环境，调用service的attach方法来初始化service。并调用了service的onCreate方法了，到这里service已经启动了，那么Service的onStartCommond方法是在哪里调用的呢？
让我们回到ActiveServices的realStartServiceLocked方法中的注释9处
private final void realStartServiceLocked(ServiceRecord r,
ProcessRecord app, boolean execInFg) throws RemoteException {
//...
//注释8处，调用ApplicationThread的scheduleCreateService方法
app.thread.scheduleCreateService(r, r.serviceInfo,
mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo),
app.repProcState);
//...
//注释9处
sendServiceArgsLocked(r, execInFg, true);
}
在调用完ApplicationThread的scheduleCreateService方法以后，在注释9处调用了sendServiceArgsLocked方法。
ActiveServices的sendServiceArgsLocked方法
private final void sendServiceArgsLocked(ServiceRecord r, boolean execInFg,
boolean oomAdjusted) throws TransactionTooLargeException {
//...
//注释10处，调用ApplicationThread的scheduleServiceArgs方法
r.app.thread.scheduleServiceArgs(r, slice);
//...
}
在注释10处，调用了ApplicationThread的scheduleServiceArgs方法
ApplicationThread的scheduleServiceArgs方法
public final void scheduleServiceArgs(IBinder token, ParceledListSlice args) {
List<ServiceStartArgs> list = args.getList();
for (int i = 0; i < list.size(); i++) {
ServiceStartArgs ssa = list.get(i);
ServiceArgsData s = new ServiceArgsData();
s.token = token;
s.taskRemoved = ssa.taskRemoved;
s.startId = ssa.startId;
s.flags = ssa.flags;
s.args = ssa.args;
sendMessage(H.SERVICE_ARGS, s);
}
}
该方法内部也是调用mH.sendMessage(msg)来发送消息,接下来直接看mH的handleMessage方法即可。
public void handleMessage(Message msg) {
switch (msg.what) {
case SERVICE_ARGS:
//调用ActivityThread的handleServiceArgs方法
handleServiceArgs((ServiceArgsData)msg.obj);
break;
}
}
ActivityThread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) {
//调用Service的onStartCommand方法
res = s.onStartCommand(data.args, data.flags, data.startId);
}
}
}
在这里调用了Service的onStartCommand方法。
结尾：整篇文章看下来感觉需要叙述的地方不多，直接一路看代码就行了，哈哈。
参考
- 《Android进阶解密》
- startService的Service启动过程分析
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