Android深入四大组件(三)Service的绑定过程

great-hotel-service_副本.jpg
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Android深入理解四大组件系列

前言

我们可以通过调用Context的startService来启动Service,也可以通过Context的bindService来绑定Service,建议阅读此篇文章前请阅读Android深入四大组件(二)Service的启动过程这篇文章,知识点重叠的部分,本篇文章将不再赘述。

1.ContextImpl到ActivityManageService的调用过程

我们可以用bindService方法来绑定Service,它的实现在ContextWrapper中,代码如下所示。
frameworks/base/core/java/android/content/ContextWrapper.java

@Override
public boolean bindService(Intent service, ServiceConnection conn,
int flags) {
return mBase.bindService(service, conn, flags);
}

这里mBase具体指向就是ContextImpl,不明白的请查看 Android深入四大组件(二)Service的启动过程这篇文章。接着查看ContextImpl的bindService方法:
frameworks/base/core/java/android/app/ContextImpl.java

@Override
public boolean bindService(Intent service, ServiceConnection conn,
int flags) {
warnIfCallingFromSystemProcess();
return bindServiceCommon(service, conn, flags, mMainThread.getHandler(),
Process.myUserHandle());
}

在bindService方法中,又return了bindServiceCommon方法,代码如下所示。
frameworks/base/core/java/android/app/ContextImpl.java

private boolean bindServiceCommon(Intent service, ServiceConnection conn, int flags, Handler
handler, UserHandle user) {
IServiceConnection sd;
if (conn == null) {
throw new IllegalArgumentException("connection is null");
}
if (mPackageInfo != null) {
sd = mPackageInfo.getServiceDispatcher(conn, getOuterContext(), handler, flags);//1
} else {
throw new RuntimeException("Not supported in system context");
}
validateServiceIntent(service);
try {
...
/**
* 2
*/
int res = ActivityManagerNative.getDefault().bindService(
mMainThread.getApplicationThread(), getActivityToken(), service,
service.resolveTypeIfNeeded(getContentResolver()),
sd, flags, getOpPackageName(), user.getIdentifier());
...
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}

在注释1处调用了LoadedApk类型的对象mPackageInfo的getServiceDispatcher方法,它的主要作用是将ServiceConnection封装为IServiceConnection类型的对象sd,从IServiceConnection的名字我们就能得知它实现了Binder机制,这样Service的绑定就支持了跨进程。接着在注释2处我们又看见了熟悉的代码,最终会调用AMS的bindService方法。
ContextImpl到ActivityManageService的调用过程如下面的时序图所示。

绘图1_副本.png

2.Service的绑定过程

AMS的bindService方法代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

public int bindService(IApplicationThread caller, IBinder token, Intent service,
String resolvedType, IServiceConnection connection, int flags, String callingPackage,
int userId) throws TransactionTooLargeException {
enforceNotIsolatedCaller("bindService");
...
synchronized(this) {
return mServices.bindServiceLocked(caller, token, service,
resolvedType, connection, flags, callingPackage, userId);
}
}

bindService方法最后会调用ActiveServices类型的对象mServices的bindServiceLocked方法:
frameworks/base/services/core/java/com/android/server/am/ActiveServices.java

int bindServiceLocked(IApplicationThread caller, IBinder token, Intent service,
String resolvedType, final IServiceConnection connection, int flags,
String callingPackage, final int userId) throws TransactionTooLargeException {
...
if ((flags&Context.BIND_AUTO_CREATE) != 0) {
s.lastActivity = SystemClock.uptimeMillis();
/**
* 1
*/
if (bringUpServiceLocked(s, service.getFlags(), callerFg, false,
permissionsReviewRequired) != null) {
return 0;
}
}
...
if (s.app != null && b.intent.received) {//2
try {
c.conn.connected(s.name, b.intent.binder);//3
} catch (Exception e) {
...
}
if (b.intent.apps.size() == 1 && b.intent.doRebind) {//4
requestServiceBindingLocked(s, b.intent, callerFg, true);//5
}
} else if (!b.intent.requested) {//6
requestServiceBindingLocked(s, b.intent, callerFg, false);//7
}
getServiceMap(s.userId).ensureNotStartingBackground(s);
} finally {
Binder.restoreCallingIdentity(origId);
}
return 1;
}

在注释1处会bringUpServiceLocked方法,在bringUpServiceLocked方法中又会调用realStartServiceLocked方法,最终由ActivityThread来调用Service的onCreate方法启动Service,这一过程在Android深入四大组件(二)Service的启动过程这篇文章中已经讲过,这里不再赘述。
在注释2处s.app != null 表示Service已经运行,其中s是ServiceRecord类型对象,app是ProcessRecord类型对象。b.intent.received表示当前应用程序进程的Client端已经接收到绑定Service时返回的Binder,这样应用程序进程的Client端就可以通过Binder来获取要绑定的Service的访问接口。注释3处调用c.conn的connected方法,其中c.conn指的是IServiceConnection,它的具体实现为ServiceDispatcher.InnerConnection,其中ServiceDispatcher是LoadedApk的内部类,InnerConnection的connected方法内部会调用H的post方法向主线程发送消息,从而解决当前应用程序进程和Service跨进程通信的问题,在后面会详细介绍这一过程。
在注释4处如果当前应用程序进程的Client端第一次与Service进行绑定的,并且Service已经调用过onUnBind方法,则需要调用注释5的代码。
注释6处如果应用程序进程的Client端没有发送过绑定Service的请求,则会调用注释7的代码,注释7和注释5的代码区别就是最后一个参数rebind为false,表示不是重新绑定。
接着我们查看注释7的requestServiceBindingLocked方法,代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActiveServices.java

private final boolean requestServiceBindingLocked(ServiceRecord r, IntentBindRecord i,
boolean execInFg, boolean rebind) throws TransactionTooLargeException {
...
if ((!i.requested || rebind) && i.apps.size() > 0) {//1
try {
bumpServiceExecutingLocked(r, execInFg, "bind");
r.app.forceProcessStateUpTo(ActivityManager.PROCESS_STATE_SERVICE);
r.app.thread.scheduleBindService(r, i.intent.getIntent(), rebind,
r.app.repProcState);//2
...
}
...
}
return true;
}

注释1处i.requested表示是否发送过绑定Service的请求,从前面的代码得知是没有发送过,因此,!i.requested为true。从前面的代码得知rebind值为false,那么(!i.requested || rebind)的值为true。如果IntentBindRecord中的应用程序进程记录大于0,则会调用注释2的代码,r.app.thread的类型为IApplicationThread,它的实现我们已经很熟悉了,是ActivityThread的内部类ApplicationThread,scheduleBindService方法如下所示。
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);
}

首先将Service的信息封装成BindServiceData对象,需要注意的BindServiceData的成员变量rebind的值为false,后面会用到它。接着将BindServiceData传入到sendMessage方法中。sendMessage向H发送消息,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 BIND_SERVICE:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
handleBindService((BindServiceData)msg.obj);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
...
}
...
}
...
}

H在接收到BIND_SERVICE类型消息时,会在handleMessage方法中会调用handleBindService方法:
frameworks/base/core/java/android/app/ActivityThread.java

private void handleBindService(BindServiceData data) {
Service s = mServices.get(data.token);//1
if (DEBUG_SERVICE)
Slog.v(TAG, "handleBindService s=" + s + " rebind=" + data.rebind);
if (s != null) {
try {
data.intent.setExtrasClassLoader(s.getClassLoader());
data.intent.prepareToEnterProcess();
try {
if (!data.rebind) {//2
IBinder binder = s.onBind(data.intent);//3
ActivityManagerNative.getDefault().publishService(
data.token, data.intent, binder);//4
} else {
s.onRebind(data.intent);//5
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
}
ensureJitEnabled();
}
...
}
...
}
}

注释1处获取要绑定的Service 。注释2处的BindServiceData的成员变量rebind的值为false,这样会调用注释3处的代码来调用Service的onBind方法,这样Service处于绑定状态了。如果rebind的值为true就会调用注释5处的Service的onRebind方法,结合前文的bindServiceLocked方法的注释4处,我们得知如果当前应用程序进程的Client端第一次与Service进行绑定,并且Service已经调用过onUnBind方法,则会调用Service的onRebind方法。
接着查看注释4的代码,实际上是调用AMS的publishService方法。
讲到这,先给出这一部分的代码时序图(不包括Service启动过程)
绘图2_副本.png

我们接着来查看AMS的publishService方法,代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

public void publishService(IBinder token, Intent intent, IBinder service) {
...
synchronized(this) {
if (!(token instanceof ServiceRecord)) {
throw new IllegalArgumentException("Invalid service token");
}
mServices.publishServiceLocked((ServiceRecord)token, intent, service);
}
}

publishService方法中,调用了ActiveServices类型的mServices对象的publishServiceLocked方法:
frameworks/base/services/core/java/com/android/server/am/ActiveServices.java

void publishServiceLocked(ServiceRecord r, Intent intent, IBinder service) {
final long origId = Binder.clearCallingIdentity();
try {
...
for (int conni=r.connections.size()-1; conni>=0; conni--) {
ArrayList<ConnectionRecord> clist = r.connections.valueAt(conni);
for (int i=0; i<clist.size(); i++) {
...
try {
c.conn.connected(r.name, service);//1
} catch (Exception e) {
...
}
}
}
}
serviceDoneExecutingLocked(r, mDestroyingServices.contains(r), false);
}
} finally {
Binder.restoreCallingIdentity(origId);
}
}

注释1处的代码,我在前面介绍过,c.conn指的是IServiceConnection,它的具体实现为ServiceDispatcher.InnerConnection,其中ServiceDispatcher是LoadedApk的内部类,ServiceDispatcher.InnerConnectiond的connected方法的代码如下所示。
frameworks/base/core/java/android/app/LoadedApk.java

static final class ServiceDispatcher {
...
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);//1
}
}
}
...
}

在注释1处调用了ServiceDispatcher 类型的sd对象的connected方法,代码如下所示。
frameworks/base/core/java/android/app/LoadedApk.java

public void connected(ComponentName name, IBinder service) {
if (mActivityThread != null) {
mActivityThread.post(new RunConnection(name, service, 0));//1
} else {
doConnected(name, service);
}
}

注释1处调用Handler类型的对象mActivityThread的post方法,mActivityThread实际上指向的是H。因此,通过调用H的post方法将RunConnection对象的内容运行在主线程中。RunConnection的定义如下所示。
frameworks/base/core/java/android/app/LoadedApk.java

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;
}

在RunConnection的run方法中调用了doConnected方法:
frameworks/base/core/java/android/app/LoadedApk.java

public void doConnected(ComponentName name, IBinder service) {
...
// If there was an old service, it is not disconnected.
if (old != null) {
mConnection.onServiceDisconnected(name);
}
// If there is a new service, it is now connected.
if (service != null) {
mConnection.onServiceConnected(name, service);//1
}
}

在注释1处调用了ServiceConnection类型的对象mConnection的onServiceConnected方法,这样在客户端中实现了ServiceConnection接口的类的onServiceConnected方法就会被执行。至此,Service的绑定过程就分析到这。
最后给出剩余部分的代码时序图。

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