作者：DeepCoder
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Service 的启动过程相对 Activity 的启动过程来说简单了很多，我们都知道怎么去创建和启动一个 Service， 那么你有没有从源码角度研究过 Service 启动后在系统层是如何运作的 ？
Activity.startService()
首先我们知道：要启动一个 Service 的时候，一般都是在 Activity 中通过 startService() 来启动：
// frameworks/base/core/java/android/app/ActivityManager.java
public class ActivityManager {
@UnsupportedAppUsage
public static IActivityManager getService() {
return IActivityManagerSingleton.get();
}
@UnsupportedAppUsage
private static final Singleton<IActivityManager> IActivityManagerSingleton =
new Singleton<IActivityManager>() {
@Override
protected IActivityManager create() {
final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
final IActivityManager am = IActivityManager.Stub.asInterface(b);
return am;
}
};
}
但是我们在 Activity 源码中并没有实现 startService() 方法，那它在哪里被调用的？找不到我们就去 Activity 的父类中找。
// frameworks/base/core/java/android/app/Activity.java
public class Activity extends ContextThemeWrapper
implements LayoutInflater.Factory2,
Window.Callback, KeyEvent.Callback,
OnCreateContextMenuListener, ComponentCallbacks2,
Window.OnWindowDismissedCallback, WindowControllerCallback,
AutofillManager.AutofillClient, ContentCaptureManager.ContentCaptureClient {
}
// frameworks/base/core/java/android/view/ContextThemeWrapper.java
public class ContextThemeWrapper extends ContextWrapper {
}
// frameworks/base/core/java/android/content/ContextWrapper.java
public class ContextWrapper extends Context {
}
Activity 继承了 ContextThemeWrapper 类， ContextThemeWrapper 又继承了 ContextWrapper类， ContextWrapper 又继承了 Context 类。
ContextWrapper.startService()
在 ContextWrapper 中实现了 startService() 方法：
// frameworks/base/core/java/android/app/ActivityManager.java
public class ActivityManager {
@UnsupportedAppUsage
public static IActivityManager getService() {
return IActivityManagerSingleton.get();
}
@UnsupportedAppUsage
private static final Singleton<IActivityManager> IActivityManagerSingleton =
new Singleton<IActivityManager>() {
@Override
protected IActivityManager create() {
final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
final IActivityManager am = IActivityManager.Stub.asInterface(b);
return am;
}
};
}
所以 startService() 方法其实是由 ContextWrapper 实现的，紧接着又调用了 mBase.startService() 方法， mBase 对象是 Context 的子类 ContextImpl ，所以调用最终进入 ContextImpl类的 startService() 方法。
ContextImpl.startService()
// frameworks/base/services/core/java/com/android/server/am/ActiveServices.java
public final class ActiveServices {
ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
int callingPid, int callingUid, boolean fgRequired, String callingPackage, final int userId)
throws TransactionTooLargeException {
return startServiceLocked(caller, service, resolvedType, callingPid, callingUid, fgRequired,
callingPackage, userId, false);
}
ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
int callingPid, int callingUid, boolean fgRequired, String callingPackage,
final int userId, boolean allowBackgroundActivityStarts)
throws TransactionTooLargeException {
... ...
// 解析 AndroidManifest.xml 文件中配置 Service 的 intent-filter 相关内容信息
ServiceLookupResult res = retrieveServiceLocked(service, null, resolvedType, callingPackage,
callingPid, callingUid, userId, true, callerFg, false, false);
if (res == null) {
return null;
}
... ...
ServiceRecord r = res.record;
... ...
// 调用 startServiceInnerLocked() 方法
ComponentName cmp = startServiceInnerLocked(smap, service, r, callerFg, addToStarting);
return cmp;
}
}
ActivityManager.getService()
// frameworks/base/core/java/android/app/ActivityManager.java
public class ActivityManager {
@UnsupportedAppUsage
public static IActivityManager getService() {
return IActivityManagerSingleton.get();
}
@UnsupportedAppUsage
private static final Singleton<IActivityManager> IActivityManagerSingleton =
new Singleton<IActivityManager>() {
@Override
protected IActivityManager create() {
final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
final IActivityManager am = IActivityManager.Stub.asInterface(b);
return am;
}
};
}
ActivityManagerService.startService()
接下来就执行到 ActivityManagerService 的 startService() 方法：
// frameworks/base/services/core/java/com/android/server/am/ActiveServices.java
public final class ActiveServices {
private final void realStartServiceLocked(ServiceRecord r,
ProcessRecord app, boolean execInFg) throws RemoteException {
... ...
try {
... ...
app.thread.scheduleCreateService(r, r.serviceInfo,
mAm.compatibilityInfoForPackage(r.serviceInfo.applicationInfo), app.getReportedProcState());
... ...
}
... ...
}
}
ActiveServices.startServiceLocked()
// frameworks/base/services/core/java/com/android/server/am/ActiveServices.java
public final class ActiveServices {
ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
int callingPid, int callingUid, boolean fgRequired, String callingPackage, final int userId)
throws TransactionTooLargeException {
return startServiceLocked(caller, service, resolvedType, callingPid, callingUid, fgRequired,
callingPackage, userId, false);
}
ComponentName startServiceLocked(IApplicationThread caller, Intent service, String resolvedType,
int callingPid, int callingUid, boolean fgRequired, String callingPackage,
final int userId, boolean allowBackgroundActivityStarts)
throws TransactionTooLargeException {
... ...
// 解析 AndroidManifest.xml 文件中配置 Service 的 intent-filter 相关内容信息
ServiceLookupResult res = retrieveServiceLocked(service, null, resolvedType, callingPackage,
callingPid, callingUid, userId, true, callerFg, false, false);
if (res == null) {
return null;
}
... ...
ServiceRecord r = res.record;
... ...
// 调用 startServiceInnerLocked() 方法
ComponentName cmp = startServiceInnerLocked(smap, service, r, callerFg, addToStarting);
return cmp;
}
}
通过 retrieveServiceLocked() 方法来解析 AndroidManifest.xml 文件中配置 Service 的 intent-filter 相关内容信息。
当解析完 Service 的 intent-filter 相关内容信息后，解析的结果会保存在 res.record 变量中。而 res 变量是一个 ServiceLookupResult 类型的对象，它的 record 变量则是一个 ServiceRecord 类型对象，用来表示一个 Service 。
ActiveServices.startServiceInnerLocked()
ActiveServices 的 startServiceLocked() 方法最后调用了 startServiceInnerLocked() 方法：
//
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.android.documentsui">
<application
android:name=".DocumentsApplication"
android:label="@string/app_label"
android:icon="@drawable/app_icon"
android:supportsRtl="true"
android:allowBackup="true"
android:backupAgent=".prefs.BackupAgent"
android:fullBackupOnly="false">
<!-- Run FileOperationService in a separate process so that we can use FileLock class to
wait until jumbo clip is done writing to disk before reading it. See ClipStorage for
details. -->
<service
android:name=".services.FileOperationService"
android:exported="false"
android:process=":com.android.documentsui.services">
</service>
</application>
</manifest>
ActiveServices.bringUpServiceLocked()
调用 bringUpServiceLocked() 方法进一步处理：
// frameworks/base/core/java/android/app/ActivityThread.java
public final class ActivityThread extends ClientTransactionHandler {
class H extends Handler {
public void handleMessage(Message msg) {
switch (msg.what) {
case CREATE_SERVICE:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceCreate: " + String.valueOf(msg.obj)));
handleCreateService((CreateServiceData)msg.obj); // 调用 handleCreateService() 方法
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
}
}
}
}
bringUpServiceLocked() 方法首先通过 getProcessRecordLocked() 方法去获取 app 对象，它是一个 ProcessRecord 类型的对象，如果它不为空，说明 Service 要运行的进程已经存在。
Service 运行的进程有两种：
（1）一种是默认的，即运行在 Activity 启动 Service 的那个进程里，也就是说在哪个进程里调用了 startService() 方法，启动的 service 就运行在哪个进程里。
（2）一种是给 Service 一个单独的进程运行，比如在 AndroidManifest 文件里配置了如下内容：
//
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.android.documentsui">
<application
android:name=".DocumentsApplication"
android:label="@string/app_label"
android:icon="@drawable/app_icon"
android:supportsRtl="true"
android:allowBackup="true"
android:backupAgent=".prefs.BackupAgent"
android:fullBackupOnly="false">
<!-- Run FileOperationService in a separate process so that we can use FileLock class to
wait until jumbo clip is done writing to disk before reading it. See ClipStorage for
details. -->
<service
android:name=".services.FileOperationService"
android:exported="false"
android:process=":com.android.documentsui.services">
</service>
</application>
</manifest>
在这段配置里有 android:process=“xxx” 声明，这个声明用来实现 service 单独运行在 “xxx” 进程里。这样做的好处是， 即使应用程序无法工作，由于 service 单独运行在一个进程里，所以会继续工作 。
回到前面的方法，如果变量 app 为空，就代表 service 要运行的进程还没有启动，于是调用 startProcessLocked() 方法去启动一个新的进程。
如果 app 不为空，采用默认的方式启动 Service，最终调用到 realStartServiceLocked() 方法：
ActiveServices.realStartServiceLocked()
// frameworks/base/services/core/java/com/android/server/am/ActiveServices.java
public final class ActiveServices {
private final void realStartServiceLocked(ServiceRecord r,
ProcessRecord app, boolean execInFg) throws RemoteException {
... ...
try {
... ...
app.thread.scheduleCreateService(r, r.serviceInfo,
mAm.compatibilityInfoForPackage(r.serviceInfo.applicationInfo), app.getReportedProcState());
... ...
}
... ...
}
}
在这个方法中，app 对象的 thread 变量是一个 ApplicationThread Binder 对象，调用它的scheduleCreateService() 方法之后，会进入客户端的 ActivityThread 中。
ActivityThread.scheduleCreateService()
// frameworks/base/core/java/android/app/ActivityThread.java
public final class ActivityThread extends ClientTransactionHandler {
// ApplicationThread 是一个 Binder
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);
}
}
}
ApplicationThread 的 scheduleCreateService() 方法通过调用 sendMessage() 方法来发送一个 msg 消息 ，当 Handler 接收到了 msg 消息之后，它会调用 handleCreateService() 方法来做进一步处理。
H.handleMessage()
// frameworks/base/core/java/android/app/ActivityThread.java
public final class ActivityThread extends ClientTransactionHandler {
class H extends Handler {
public void handleMessage(Message msg) {
switch (msg.what) {
case CREATE_SERVICE:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceCreate: " + String.valueOf(msg.obj)));
handleCreateService((CreateServiceData)msg.obj); // 调用 handleCreateService() 方法
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
}
}
}
}
ActivityThread.handleCreateService()
// frameworks/base/core/java/android/app/ActivityThread.java
public final class ActivityThread extends ClientTransactionHandler {
private void handleCreateService(CreateServiceData data) {
... ...
LoadedApk packageInfo = getPackageInfoNoCheck(data.info.applicationInfo, data.compatInfo);
Service service = null;
try {
java.lang.ClassLoader cl = packageInfo.getClassLoader();
service = packageInfo.getAppFactory().instantiateService(cl, data.info.name, data.intent);
} 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, ActivityManager.getService());
service.onCreate(); // 进入 Service.onCreate() 方法
mServices.put(data.token, service);
try {
ActivityManager.getService().serviceDoneExecuting(
data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
} catch (Exception e) {
if (!mInstrumentation.onException(service, e)) {
throw new RuntimeException(
"Unable to create service " + data.info.name
+ ": " + e.toString(), e);
}
}
}
}
在 ActivityThread 类的 handleCreateService() 方法中，首先通过 ClassLoader 类把 Service 加载进来，而参数 data.info.name 表示这个 Service 的名字， instantiateService() 方法是创建一个 Service 实例。接着， 创建一个 Context 对象 ，作为上下文环境之用。
handleCreateService() 方法最后调用了 service 的 onCreate() 方法 ，当这个方法被调用之后， 就会进入应用程序里 Service 的 onCreate() 方法 。
至此，Service 的启动就分析完毕，这个过程与启动 Activity 相比简单了很多。
总结
看到最后，希望这篇文章能帮你梳理清楚 “Service 的启动流程” 。
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