Service有两种工作状态：
- 启动状态：执行后台计算
- 绑定状态：用于其他组件与Service交互
Service的启动过程
Service的启动从 ContextWrapper 的 startService 开始
在ContextWrapper中，大部分操作通过一个 ContextImpl 对象mBase实现
/frameworks/base/core/java/android/content/ContextWrapper.java：
@Override
public ComponentName startService(Intent service) {
return mBase.startService(service);
}
在ContextImpl中， mBase.startService() 会调用 startServiceCommon 方法，而 startServiceCommon方法又会通过 ActivityManagerNative.getDefault() （实际上就是 AMS）这个对象来启动一个服务。
/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();
ComponentName cn = ActivityManagerNative.getDefault().startService(
mMainThread.getApplicationThread(), service,
service.resolveTypeIfNeeded(getContentResolver()), 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) {
return null;
}
}
AMS会通过一个 ActiveService 对象（辅助AMS进行Service管理的类）mServices来完 成启动Service： mServices.startServiceLocked() 。
/frameworks/base/services/java/com/android/server/am/ActivityManagerService.java:
public ComponentName startService(IApplicationThread caller, Intent service,
String resolvedType, int userId) {
enforceNotIsolatedCaller("startService");
// Refuse possible leaked file descriptors
if (service != null && service.hasFileDescriptors() == true) {
throw new IllegalArgumentException("File descriptors passed in Intent");
}
if (DEBUG_SERVICE)
Slog.v(TAG, "startService: " + service + " type=" + resolvedType);
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, userId);
Binder.restoreCallingIdentity(origId);
return res;
}
}
在mServices.startServiceLocked()最后会调用 startServiceInnerLocked() 方法：将 Service的信息包装成一个 ServiceRecord 对象，通过 bringUpServiceLocked() 方法来处理，bringUpServiceLocked()又调用了 realStartServiceLocked() 方法，这才真正地去启动一个Service了。
/frameworks/base/services/java/com/android/server/am/ActiveServices.java:
ComponentName startServiceLocked(IApplicationThread caller,
...
return startServiceInnerLocked(smap, service, r, callerFg, addToStarting);
}
ComponentName startServiceInnerLocked(ServiceMap smap, Intent service,
ServiceRecord r, boolean callerFg, boolean addToStarting) {
...
String error = bringUpServiceLocked(r, service.getFlags(), callerFg, false);
...
return r.name;
}
private final String bringUpServiceLocked(ServiceRecord r,
int intentFlags, boolean execInFg, boolean whileRestarting) {
...
if (!isolated) {
app = mAm.getProcessRecordLocked(procName, r.appInfo.uid, false);
if (DEBUG_MU) Slog.v(TAG_MU, "bringUpServiceLocked: appInfo.uid=" + r.appInfo.uid + " app=" + app);
if (app != null && app.thread != null) {
try {
app.addPackage(r.appInfo.packageName, mAm.mProcessStats);
realStartServiceLocked(r, app, execInFg);
return null;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting service " + r.shortName, e);
}
// If a dead object exception was thrown -- fall through to
// restart the application.
}
} else {
// If this service runs in an isolated process, then each time
// we call startProcessLocked() we will get a new isolated
// process, starting another process if we are currently waiting
// for a previous process to come up. To deal with this, we store
// in the service any current isolated process it is running in or
// waiting to have come up.
app = r.isolatedProc;
}
...
}
private final void realStartServiceLocked(ServiceRecord r,
ProcessRecord app, boolean execInFg) throws RemoteException {
...
boolean created = false;
try {
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);
app.thread.scheduleCreateService(r, r.serviceInfo,
mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo),
app.repProcState);
r.postNotification();
created = true;
} finally {
if (!created) {
app.services.remove(r);
r.app = null;
scheduleServiceRestartLocked(r, false);
}
}
requestServiceBindingsLocked(r, execInFg);
// 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));
}
sendServiceArgsLocked(r, execInFg, true);
...
}
realStartServiceLocked()方法的工作如下：
i. app.thread.scheduleCreateService() 来创建Service并调用其onCreate()生命周期方法
ii. sendServiceArgsLocked() 调用其他生命周期方法，如onStartCommand()
iii. app.thread对象是 IApplicationThread 类型，实际上就是一个Binder，具体实现是 ApplicationThread继承ApplictionThreadNative
/frameworks/base/core/java/android/app/ActivityThread.java:
private class ApplicationThread extends ApplicationThreadNative {
...
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);
}
...
}
具体看app.thread.scheduleCreateService()：通过 sendMessage(H.CREATE_SERVICE , s) ， 这个过程和Activity启动过程类似，同时通过发送消息给Handler H来完成的。
H会接受这个CREATE_SERVICE消息并通过ActivityThread的 handleCreateService() 来 完成Service的最终启动。
/frameworks/base/core/java/android/app/ActivityThread.java:
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);
//通过ClassLoader创建Service对象
Service service = null;
try {
java.lang.ClassLoader cl = packageInfo.getClassLoader();
service = (Service) cl.loadClass(data.info.name).newInstance();
} 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);
//创建Service内部的Context对象
ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
context.setOuterContext(service);
//创建Application，并调用其onCreate()（只会有一次）
Application app = packageInfo.makeApplication(false, mInstrumentation);
//通过 service.attach() 方法建立Service与context的联系（与Activity类似）
service.attach(context, this, data.info.name, data.token, app,
ActivityManagerNative.getDefault());
//调用service的 onCreate() 生命周期方法，至此，Service已经启动了
service.onCreate();
//将Service对象存储到ActivityThread的一个ArrayMap中
mServices.put(data.token, service);
try {
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, 0, 0, 0);
} catch (RemoteException e) {
// nothing to do.
}
} catch (Exception e) {
if (!mInstrumentation.onException(service, e)) {
throw new RuntimeException(
"Unable to create service " + data.info.name
+ ": " + e.toString(), e);
}
}
}
除此之外，ActivityThread中还会通过handleServiceArgs方法调用Service的onStartCommand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());
}
int res;
if (!data.taskRemoved) {
//调用Service的onStartCommand方法
res = s.onStartCommand(data.args, data.flags, data.startId);
} else {
s.onTaskRemoved(data.args);
res = Service.START_TASK_REMOVED_COMPLETE;
}
QueuedWork.waitToFinish();
try {
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, 1, data.startId, res);
} catch (RemoteException e) {
// nothing to do.
}
ensureJitEnabled();
} catch (Exception e) {
if (!mInstrumentation.onException(s, e)) {
throw new RuntimeException(
"Unable to start service " + s
+ " with " + data.args + ": " + e.toString(), e);
}
}
}
}
Service的绑定过程
Service的绑定是从 ContextWrapper 的 bindService 开始
/frameworks/base/core/java/android/content/ContextWrapper.java：
public boolean bindService(Intent service, ServiceConnection conn,
int flags) {
return mBase.bindService(service, conn, flags);
}
在ContextWrapper中，交给 ContextImpl 对象 mBase.bindService()
/frameworks/base/core/java/android/app/ContextImpl.java:
@Override
public boolean bindService(Intent service, ServiceConnection conn,
int flags) {
warnIfCallingFromSystemProcess();
return bindServiceCommon(service, conn, flags, Process.myUserHandle());
}
private boolean bindServiceCommon(Intent service, ServiceConnection conn, int flags,
UserHandle user) {
IServiceConnection sd;
if (conn == null) {
throw new IllegalArgumentException("connection is null");
}
if (mPackageInfo != null) {
sd = mPackageInfo.getServiceDispatcher(conn, getOuterContext(),
mMainThread.getHandler(), flags);
} else {
throw new RuntimeException("Not supported in system context");
}
validateServiceIntent(service);
try {
IBinder token = getActivityToken();
if (token == null && (flags&BIND_AUTO_CREATE) == 0 && mPackageInfo != null
&& mPackageInfo.getApplicationInfo().targetSdkVersion
< android.os.Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
flags |= BIND_WAIVE_PRIORITY;
}
service.prepareToLeaveProcess();
int res = ActivityManagerNative.getDefault().bindService(
mMainThread.getApplicationThread(), getActivityToken(),
service, service.resolveTypeIfNeeded(getContentResolver()),
sd, flags, user.getIdentifier());
if (res < 0) {
throw new SecurityException(
"Not allowed to bind to service " + service);
}
return res != 0;
} catch (RemoteException e) {
return false;
}
}
最终会调用ContextImpl的 bindServiceCommon 方法，这个方法完成两件事：
i. 将客户端的ServiceConnection转化成 ServiceDispatcher.InnerConnection 对象。ServiceDispatcher连接ServiceConnection和InnerConnection。
这个过程通过 LoadedApk 的 getServiceDispatcher 方法来实现，将客户端的 ServiceConnection和ServiceDispatcher的映射关系存在一个ArrayMap中。
ii. 通过AMS来完成Service的具体绑定过 程 ActivityManagerNative.getDefault().bindService()
/frameworks/base/core/java/android/app/LoadedApk.java:
public final IServiceConnection getServiceDispatcher(ServiceConnection c,
Context context, Handler handler, int flags) {
synchronized (mServices) {
LoadedApk.ServiceDispatcher sd = null;
ArrayMap<ServiceConnection, LoadedApk.ServiceDispatcher> map = mServices.get(context);
if (map != null) {
sd = map.get(c);
}
if (sd == null) {
sd = new ServiceDispatcher(c, context, handler, flags);
if (map == null) {
map = new ArrayMap<ServiceConnection, LoadedApk.ServiceDispatcher>();
mServices.put(context, map);
}
map.put(c, sd);
} else {
sd.validate(context, handler);
}
return sd.getIServiceConnection();
}
}
mServices 定义：
private final ArrayMap<Context, ArrayMap<ServiceConnection, LoadedApk.ServiceDispatcher>> mServices = new ArrayMap<Context, ArrayMap<ServiceConnection, LoadedApk.ServiceDispatcher>>();
/frameworks/base/services/java/com/android/server/am/ActivityManagerService.java：
public int bindService(IApplicationThread caller, IBinder token,
Intent service, String resolvedType,
IServiceConnection connection, int flags, int userId) {
enforceNotIsolatedCaller("bindService");
// Refuse possible leaked file descriptors
if (service != null && service.hasFileDescriptors() == true) {
throw new IllegalArgumentException("File descriptors passed in Intent");
}
synchronized(this) {
return mServices.bindServiceLocked(caller, token, service, resolvedType,
connection, flags, userId);
}
}
AMS中，bindService()方法再调用 bindServiceLocked() ，bindServiceLocked()再调 用 bringUpServiceLocked() ，bringUpServiceLocked()又会调用 realStartServiceLocked() 。
/frameworks/base/services/java/com/android/server/am/ActiveServices.java:
private final void realStartServiceLocked(ServiceRecord r,
ProcessRecord app, boolean execInFg) throws RemoteException {
...
boolean created = false;
try {
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);
app.thread.scheduleCreateService(r, r.serviceInfo,
mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo),
app.repProcState);
r.postNotification();
created = true;
} finally {
if (!created) {
app.services.remove(r);
r.app = null;
scheduleServiceRestartLocked(r, false);
}
}
requestServiceBindingsLocked(r, execInFg);
// 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));
}
sendServiceArgsLocked(r, execInFg, true);
...
}
AMS的realStartServiceLocked()会调 用 ActiveServices 的 requrestServiceBindingLocked() 方法。
private final void requestServiceBindingsLocked(ServiceRecord r, boolean execInFg) {
for (int i=r.bindings.size()-1; i>=0; i--) {
IntentBindRecord ibr = r.bindings.valueAt(i);
if (!requestServiceBindingLocked(r, ibr, execInFg, false)) {
break;
}
}
}
private final boolean requestServiceBindingLocked(ServiceRecord r,
IntentBindRecord i, boolean execInFg, boolean rebind) {
if (r.app == null || r.app.thread == null) {
// If service is not currently running, can't yet bind.
return false;
}
if ((!i.requested || rebind) && i.apps.size() > 0) {
try {
bumpServiceExecutingLocked(r, execInFg, "bind");
r.app.forceProcessStateUpTo(ActivityManager.PROCESS_STATE_SERVICE);
r.app.thread.scheduleBindService(r, i.intent.getIntent(), rebind,
r.app.repProcState);
if (!rebind) {
i.requested = true;
}
i.hasBound = true;
i.doRebind = false;
} catch (RemoteException e) {
if (DEBUG_SERVICE) Slog.v(TAG, "Crashed while binding " + r);
return false;
}
}
return true;
}
requrestServiceBindingLocked() 方法，最终是调用了 ServiceRecord对象r的app.thread.scheduleBindService() 方法。
/frameworks/base/core/java/android/app/ActivityThread.java:
public final void scheduleBindService(IBinder token, Intent intent,
boolean rebind, int processState) {
updateProcessState(processState, false);
BindServiceData s = new BindServiceData();
s.token = token;
s.intent = intent;
s.rebind = rebind;
if (DEBUG_SERVICE)
Slog.v(TAG, "scheduleBindService token=" + token + " intent=" + intent + " uid="
+ Binder.getCallingUid() + " pid=" + Binder.getCallingPid());
sendMessage(H.BIND_SERVICE, s);
}
ApplicationThread的一系列以schedule开头的方法，内部都通过Handler H来中转： scheduleBindService()内部也是通过 sendMessage(H.BIND_SERVICE , s)。
private void handleBindService(BindServiceData data) {
Service s = mServices.get(data.token);
if (DEBUG_SERVICE)
Slog.v(TAG, "handleBindService s=" + s + " rebind=" + data.rebind);
if (s != null) {
try {
data.intent.setExtrasClassLoader(s.getClassLoader());
try {
if (!data.rebind) {
IBinder binder = s.onBind(data.intent);
ActivityManagerNative.getDefault().publishService(
data.token, data.intent, binder);
} else {
s.onRebind(data.intent);
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, 0, 0, 0);
}
ensureJitEnabled();
} catch (RemoteException ex) {
}
} catch (Exception e) {
if (!mInstrumentation.onException(s, e)) {
throw new RuntimeException(
"Unable to bind to service " + s
+ " with " + data.intent + ": " + e.toString(), e);
}
}
}
}
在H内部接收到BIND_SERVICE这类消息时就交给 ActivityThread 的 handleBindService() 方法处理：
i. 根据Servcie的token取出Service对象
ii. 调用Service的 onBind() 方法，至此，Service就处于绑定状态了。
iii. 这时客户端还不知道已经成功连接Service，需要调用客户端的binder对象来调用客户端的ServiceConnection中的 onServiceConnected() 方法，这个通过 ActivityManagerNative.getDefault().publishService() 进行。
/frameworks/base/services/java/com/android/server/am/ActivityManagerService.java:
public void publishService(IBinder token, Intent intent, IBinder service) {
// Refuse possible leaked file descriptors
if (intent != null && intent.hasFileDescriptors() == true) {
throw new IllegalArgumentException("File descriptors passed in Intent");
}
synchronized(this) {
if (!(token instanceof ServiceRecord)) {
throw new IllegalArgumentException("Invalid service token");
}
mServices.publishServiceLocked((ServiceRecord)token, intent, service);
}
}
AMS的publishService()交给ActivityService对象 mServices 的 publishServiceLocked() 来 处理
/frameworks/base/services/java/com/android/server/am/ActiveServices.java:
void publishServiceLocked(ServiceRecord r, Intent intent, IBinder service) {
final long origId = Binder.clearCallingIdentity();
...
try {
c.conn.connected(r.name, service);
} catch (Exception e) {
Slog.w(TAG, "Failure sending service " + r.name +
" to connection " + c.conn.asBinder() +
" (in " + c.binding.client.processName + ")", e);
}
...
}
核心代码就一句话 c.conn.connected(r.name,service) 。对象c的类型 是 ConnectionRecord ，c.conn就是ServiceDispatcher.InnerConnection对象，service就是 Service的onBind方法返回的Binder对象。
/frameworks/base/core/java/android/app/LoadedApk.java
private static class InnerConnection extends IServiceConnection.Stub {
final WeakReference<LoadedApk.ServiceDispatcher> mDispatcher;
InnerConnection(LoadedApk.ServiceDispatcher sd) {
mDispatcher = new WeakReference<LoadedApk.ServiceDispatcher>(sd);
}
public void connected(ComponentName name, IBinder service) throws RemoteException {
LoadedApk.ServiceDispatcher sd = mDispatcher.get();
if (sd != null) {
sd.connected(name, service);
}
}
}
public void connected(ComponentName name, IBinder service) {
if (mActivityThread != null) {
mActivityThread.post(new RunConnection(name, service, 0));
} else {
doConnected(name, service);
}
}
c.conn.connected(r.name,service)内部实现是交给了 mActivityThread.post(new RunnConnection(name ,service,0)); 实现。
ServiceDispatcher的mActivityThread是一个 Handler，其实就是ActivityThread中的H。这样一来RunConnection就经由H的post方法从而运行在主线程中，因此客户端ServiceConnection中的方法是在主线程中被回调 的。
private final class RunConnection implements Runnable {
RunConnection(ComponentName name, IBinder service, int command) {
mName = name;
mService = service;
mCommand = command;
}
public void run() {
if (mCommand == 0) {
doConnected(mName, mService);
} else if (mCommand == 1) {
doDeath(mName, mService);
}
}
final ComponentName mName;
final IBinder mService;
final int mCommand;
}
public void doConnected(ComponentName name, IBinder service) {
...
// If there is a new service, it is now connected.
if (service != null) {
mConnection.onServiceConnected(name, service);
}
}
RunConnection的定义如下：
i. 继承Runnable接口， run() 方法的实现也是简单调用了ServiceDispatcher 的 doConnected 方法。
ii. 由于ServiceDispatcher内部保存了客户端的ServiceConntion对象，可以很方便地调 用ServiceConntion对象的 onServiceConnected 方法。
iii. 客户端的onServiceConnected方法执行后，Service的绑定过程也就完成了。
iv. service绑定后通过ServiceDispatcher通知客户端的过程可以说明 ServiceDispatcher起着连接ServiceConnection和InnerConnection的作用。 至于Service的停止和解除绑定的过程，系统流程都是类似的。