前言
此前我用较长的篇幅来介绍Android应用程序的启动过程（根Activity的启动过程），这一篇我们接着来分析Service的启动过程。建议阅读此篇文章前，请先阅读Android深入四大组件（一）应用程序启动过程（前篇）和Android深入四大组件（一）应用程序启动过程（后篇）这两篇文章。
1.ContextImpl到ActivityManageService的调用过程
要启动Service，我们会调用startService方法，它的实现在ContextWrapper中，代码如下所示。
frameworks/base/core/java/android/content/ContextWrapper.java
public class ContextWrapper extends Context {
Context mBase;
...
@Override
public ComponentName startService(Intent service) {
return mBase.startService(service);
}
...
}View Code
在startService方法中会调用mBase的startService方法，Context类型的mBase对象具体指的是什么呢？在Android深入四大组件（一）应用程序启动过程（后篇）这篇文章中我们讲过ActivityThread启动Activity时会调用如下代码创建Activity的上下文环境。
frameworks/base/core/java/android/app/ActivityThread.java
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
...
if (activity != null) {
Context appContext = createBaseContextForActivity(r, activity);//1
...
}
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor, window);
...
}
return activity;
}View Code
在注释1处创建上下文对象appContext ，并传入Activity的attach方法中，将Activity与上下文对象appContext 关联起来，这个上下文对象appContext 的具体类型是什么，我们接着查看createBaseContextForActivity方法，代码如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
private Context createBaseContextForActivity(ActivityClientRecord r, final Activity activity) {
...
ContextImpl appContext = ContextImpl.createActivityContext(
this, r.packageInfo, r.token, displayId, r.overrideConfig);
appContext.setOuterContext(activity);
Context baseContext = appContext;
...
return baseContext;
}View Code
这里可以得出结论，上下文对象appContext 的具体类型就是ContextImpl 。Activity的attach方法中将ContextImpl赋值给ContextWrapper的成员变量mBase中，因此，mBase具体指向就是ContextImpl 。
那么，我们紧接着来查看ContextImpl的startService方法，代码如下所示。
frameworks/base/core/java/android/app/ContextImpl.java
@Override
public ComponentName startService(Intent service) {
warnIfCallingFromSystemProcess();
return startServiceCommon(service, mUser);
}
private ComponentName startServiceCommon(Intent service, UserHandle user) {
try {
validateServiceIntent(service);
service.prepareToLeaveProcess(this);
/**
* 1
*/
ComponentName cn = ActivityManagerNative.getDefault().startService(
mMainThread.getApplicationThread(), service, service.resolveTypeIfNeeded(
getContentResolver()), getOpPackageName(), user.getIdentifier());
...
return cn;
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
View Code
startService方法中会return startServiceCommon方法，在startServiceCommon方法中会在注释1处调用ActivityManageService（AMS）的代理对象ActivityManagerProxy（AMP）的startService方法，最终会调用AMS的startService方法。至于注释1处的代码为何会调用AMS的startService方法，在Android深入四大组件（一）应用程序启动过程（前篇）这篇文章中已经讲过，这里不再赘述。
ContextImpl到ActivityManageService的调用过程如下面的时序图所示。
2.ActivityThread启动Service
我们接着来查看AMS的startService方法。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
Override
public ComponentName startService(IApplicationThread caller, Intent service,
String resolvedType, String callingPackage, int userId)
throws TransactionTooLargeException {
...
synchronized(this) {
...
ComponentName res = mServices.startServiceLocked(caller, service,
resolvedType, callingPid, callingUid, callingPackage, userId);//1
Binder.restoreCallingIdentity(origId);
return res;
}
}
View Code
注释1处调用mServices的startServiceLocked方法，mServices的类型是ActiveServices，ActiveServices的startServiceLocked方法代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActiveServices.java
ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
int callingPid, int callingUid, String callingPackage, final int userId)
throws TransactionTooLargeException {
...
return startServiceInnerLocked(smap, service, r, callerFg, addToStarting);
}
ComponentName startServiceInnerLocked(ServiceMap smap, Intent service, ServiceRecord r,
boolean callerFg, boolean addToStarting) throws TransactionTooLargeException {
...
String error = bringUpServiceLocked(r, service.getFlags(), callerFg, false, false);
...
return r.name;
}
View Code
startServiceLocked方法的末尾return了startServiceInnerLocked方法，而startServiceInnerLocked方法中又调用了bringUpServiceLocked方法：
frameworks/base/services/core/java/com/android/server/am/ActiveServices.java
private String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg,
boolean whileRestarting, boolean permissionsReviewRequired)
throws TransactionTooLargeException {
...
final String procName = r.processName;//1
ProcessRecord app;
if (!isolated) {
app = mAm.getProcessRecordLocked(procName, r.appInfo.uid, false);//2
if (DEBUG_MU) Slog.v(TAG_MU, "bringUpServiceLocked: appInfo.uid=" + r.appInfo.uid
+ " app=" + app);
if (app != null && app.thread != null) {//3
try {
app.addPackage(r.appInfo.packageName, r.appInfo.versionCode,
mAm.mProcessStats);
realStartServiceLocked(r, app, execInFg);//4
return null;
} catch (TransactionTooLargeException e) {
throw e;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting service " + r.shortName, e);
}
}
} else {
app = r.isolatedProc;
}
if (app == null && !permissionsReviewRequired) {//5
if ((app=mAm.startProcessLocked(procName, r.appInfo, true, intentFlags,
"service", r.name, false, isolated, false)) == null) {//6
...
}
if (isolated) {
r.isolatedProc = app;
}
}
...
}
View Code
在注释1处得到ServiceRecord的processName的值赋值给procName ，其中processName用来描述Service想要在哪个进程运行，默认是当前进程，我们也可以在AndroidManifes配置文件中设置android:process属性来新开启一个进程运行Service。注释2处将procName和Service的uid传入到AMS的getProcessRecordLocked方法中，来查询是否存在一个与Service对应的ProcessRecord类型的对象app，ProcessRecord主要用来记录运行的应用程序进程的信息。注释5处需要判断两个条件，一个是用来运行Service的应用程序进程不存在，另一个是应用程序之间的组件调用不需要检查权限，满足这两个条件则调用注释6处的AMS的startProcessLocked方法来创建对应的应用程序进程。关于创建应用程序进程请查看Android应用程序进程启动过程（前篇） 和Android应用程序进程启动过程（后篇）这两篇文章。注释3处判断如果用来运行Service的应用程序进程存在，则调用注释4处的realStartServiceLocked方法：
frameworks/base/services/core/java/com/android/server/am/ActiveServices.java
private final void realStartServiceLocked(ServiceRecord r,
ProcessRecord app, boolean execInFg) throws RemoteException {
...
try {
...
app.thread.scheduleCreateService(r, r.serviceInfo,
mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo),
app.repProcState);
r.postNotification();
created = true;
} catch (DeadObjectException e) {
...
}
...
}
View Code
在realStartServiceLocked方法中调用了app.thread的scheduleCreateService方法。其中app.thread是IApplicationThread类型的，它的实现是ActivityThread的内部类ApplicationThread，其中ApplicationThread继承了ApplicationThreadNative，而ApplicationThreadNative继承了Binder并实现了IApplicationThread接口。ApplicationThread的scheduleCreateService方法如下所示。
frameworks/base/core/java/android/app/ActivityThread.java
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);
}
View Code
首先将要启动的信息封装成CreateServiceData 对象并传给sendMessage方法，sendMessage方法向H发送CREATE_SERVICE消息，H是ActivityThread的内部类并继承Handler。这个过程和应用程序的启动过程（根Activity启动过程）是类似的。我们接着查看H的handleMessage方法。
frameworks/base/core/java/android/app/ActivityThread.java
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
...
case CREATE_SERVICE:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER,
("serviceCreate: " + String.valueOf(msg.obj)));
handleCreateService((CreateServiceData)msg.obj);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
...
}
...
}
...
}View Code
handleMessage方法根据消息类型，调用了handleCreateService方法：
frameworks/base/core/java/android/app/ActivityThread.java
private void handleCreateService(CreateServiceData data) {
unscheduleGcIdler();
LoadedApk packageInfo = getPackageInfoNoCheck(
data.info.applicationInfo, data.compatInfo);//1
Service service = null;
try {
java.lang.ClassLoader cl = packageInfo.getClassLoader();//2
service = (Service) cl.loadClass(data.info.name).newInstance();//3
} catch (Exception e) {
...
}
}
try {
if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);
ContextImpl context = ContextImpl.createAppContext(this, packageInfo);//4
context.setOuterContext(service);
Application app = packageInfo.makeApplication(false, mInstrumentation);
service.attach(context, this, data.info.name, data.token, app,
ActivityManagerNative.getDefault());//5
service.onCreate();//6
mServices.put(data.token, service);//7
...
} catch (Exception e) {
...
}
}View Code
注释1处获取要启动Service的应用程序的LoadedApk，LoadedApk是一个APK文件的描述类。注释2处通过调用LoadedApk的getClassLoader方法来获取类加载器。接着在注释3处根据CreateServiceData对象中存储的Service信息，将Service加载到内存中。注释4处创建Service的上下文环境ContextImpl对象。注释5处通过Service的attach方法来初始化Service。注释6处调用Service的onCreate方法，这样Service就启动了。在注释7处将启动的Service加入到ActivityThread的成员变量mServices中，其中mServices是ArrayMap类型。
最后给出这一节的时序图。
