前言
在做Android开发过程中，我们经常使用bindService()绑定服务，并且通过返回的Binder对象进行多进程间通信。
ServiceConnection conn = new ServiceConnection() {
@Override
public void onServiceConnected(ComponentName name, IBinder service) {
// service是怎么来的？
media = IMedia.Stub.asInterface(service);
}
@Override
public void onServiceDisconnected(ComponentName name) {
media = null;
}
};
Intent intent = new Intent(this, MediaService.class);
bindService(intent, conn, Context.BIND_AUTO_CREATE);
但是我很好奇这个Binder对象service是怎么被返回的？带着这个疑问去系统源码里寻找答案，然后就有了这篇bindService()过程分析。
第一步 android.app.ContextImpl.java
首先bindService(intent, conn, Context.BIND_AUTO_CREATE)实际上是调用了ContextImpl里的bindService(...)方法
@Override
public boolean bindService(Intent service, ServiceConnection conn,
int flags) {
warnIfCallingFromSystemProcess();
return bindServiceCommon(service, conn, flags, mMainThread.getHandler(),
Process.myUserHandle());
}
接着又调用bindServiceCommon(...)方法
private boolean bindServiceCommon(Intent service, ServiceConnection conn, int flags, Handler
handler, UserHandle user) {
// Keep this in sync with DevicePolicyManager.bindDeviceAdminServiceAsUser.
IServiceConnection sd;
if (conn == null) {
throw new IllegalArgumentException("connection is null");
}
if (mPackageInfo != null) {
sd = mPackageInfo.getServiceDispatcher(conn, getOuterContext(), handler, flags);
}
...
int res = ActivityManager.getService().bindService(
mMainThread.getApplicationThread(), getActivityToken(), service,
service.resolveTypeIfNeeded(getContentResolver()),
sd, flags, getOpPackageName(), user.getIdentifier());
...
return res != 0;
}
通过调用ActivityManager.getService().bindService(...)将跨进程进入到ActivityManagerService的bindService(...)方法，第三步将接着这里分析。
另外，bindService(...)传入了一个重要的参数sd，那么sd是怎么获取的？它的具体类型是什么呢？请看第二步。
第二步 android.app.LoadedApk.java
首先mPackageInfo是一个LoadedApk类型对象，sd是通过调用mPackageInfo的getServiceDispatcher(...)方法获取的
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) {
if (DEBUG) Slog.d(TAG, "Returning existing dispatcher " + sd + " for conn " + c);
sd = map.get(c);
}
if (sd == null) {
sd = new ServiceDispatcher(c, context, handler, flags);
if (DEBUG) Slog.d(TAG, "Creating new dispatcher " + sd + " for conn " + c);
if (map == null) {
map = new ArrayMap<>();
mServices.put(context, map);
}
map.put(c, sd);
} else {
sd.validate(context, handler);
}
return sd.getIServiceConnection();
}
}
直接进入到LoadedApk.ServiceDispatcher的getIServiceConnection()方法
static final class ServiceDispatcher {
private final Handler mActivityThread;
ServiceDispatcher(ServiceConnection conn,
Context context, Handler activityThread, int flags) {
// mIServiceConnection的类型是InnerConnection
mIServiceConnection = new InnerConnection(this);
mConnection = conn;
mContext = context;
mActivityThread = activityThread;
mLocation = new ServiceConnectionLeaked(null);
mLocation.fillInStackTrace();
mFlags = flags;
}
IServiceConnection getIServiceConnection() {
return mIServiceConnection;
}
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, boolean dead)
throws RemoteException {
LoadedApk.ServiceDispatcher sd = mDispatcher.get();
if (sd != null) {
sd.connected(name, service, dead);
}
}
}
}
通过上面的代码可知，sd是ServiceDispatcher.InnerConnection类型的对象，这个sd后面的讲解还会用到。
第三步 com.android.server.am.ActivityManagerService.java
第一步中ContextImpl通过跨进程通信进入到了ActivityManagerService的bindService(...)方法
public int bindService(IApplicationThread caller, IBinder token, Intent service,
String resolvedType, IServiceConnection connection, int flags, String callingPackage,
int userId) throws TransactionTooLargeException {
enforceNotIsolatedCaller("bindService");
// Refuse possible leaked file descriptors
if (service != null && service.hasFileDescriptors() == true) {
throw new IllegalArgumentException("File descriptors passed in Intent");
}
if (callingPackage == null) {
throw new IllegalArgumentException("callingPackage cannot be null");
}
synchronized(this) {
return mServices.bindServiceLocked(caller, token, service,
resolvedType, connection, flags, callingPackage, userId);
}
}
然后mServices是一个ActiveServices对象、bindService (...)就直接转交给了ActiveServices的bindServiceLocked(...)
第四步 com.android.server.am.ActiveServices.java
bindServiceLocked(...)的代码很长，只列出了关键部分
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();
if (bringUpServiceLocked(s, service.getFlags(), callerFg, false,
permissionsReviewRequired) != null) {
return 0;
}
}
...
ConnectionRecord c = new ConnectionRecord(b, activity,
connection, flags, clientLabel, clientIntent);
if (s.app != null && b.intent.received) {
// Service is already running, so we can immediately
// publish the connection.
try {
c.conn.connected(s.name, b.intent.binder, false);
} catch (Exception e) {
Slog.w(TAG, "Failure sending service " + s.shortName
+ " to connection " + c.conn.asBinder()
+ " (in " + c.binding.client.processName + ")", e);
}
// If this is the first app connected back to this binding,
// and the service had previously asked to be told when
// rebound, then do so.
if (b.intent.apps.size() == 1 && b.intent.doRebind) {
requestServiceBindingLocked(s, b.intent, callerFg, true);
}
} else if (!b.intent.requested) {
requestServiceBindingLocked(s, b.intent, callerFg, false);
}
...
}
从最开始的调用bindService(intent, conn, Context.BIND_AUTO_CREATE)可知传进来的参数flags的位Context.BIND_AUTO_CREATE为1，所以会执行bringUpServiceLocked (...)去启动服务，然后就直接返回了。
如果不为1会继续往下，创建一个ConnectionRecord对象c，c的conn为传入的参数connection，这个conn实际上就是第二步中的sd对象，即ServiceDispatcher.InnerConnection类型对象。
接下来判断如果服务已经在运行、则会直接调用c.conn.connected(...)发布连接，发布连接的过程将在第六步中分析。
第五步 com.android.server.am.ActiveServices.java
接着第四步的bringUpServiceLocked (...)分析
private String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg,
boolean whileRestarting, boolean permissionsReviewRequired)
throws TransactionTooLargeException
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, r.appInfo.versionCode, mAm.mProcessStats);
realStartServiceLocked(r, app, execInFg);
return null;
} catch (TransactionTooLargeException e) {
throw e;
} 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 {
...
}
}
如果进程已启动，会调用realStartServiceLocked(...)继续去启动服务
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) {
Slog.w(TAG, "Application dead when creating service " + r);
mAm.appDiedLocked(app);
throw e;
} finally {
...
}
requestServiceBindingsLocked(r, execInFg);
updateServiceClientActivitiesLocked(app, null, true);
}
app.thread是应用程序端ApplicationThread的一个远程Binder对象，是在第一步中调用ActivityManager.getService().bindService(...)传入的mMainThread.getApplicationThread()，
通过它服务端ActivityManagerService可以调用应用程序端ApplicationThread中的方法。
app.thread.scheduleCreateService这个方法通过多进程通信让应用程序去创建Service对象，并调用它的onCreate()方法。
requestServiceBindingsLocked(...)会辗转调用到r.app.thread.scheduleBindService(...)，接下来就调用到了ApplicationThread的scheduleBindService(...)方法
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);
}
发送一个Handler消息给ActivityThread.H类对象
case BIND_SERVICE:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
handleBindService((BindServiceData)msg.obj);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
接下来就调用到了ActivityThread的handleBindService()方法
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());
data.intent.prepareToEnterProcess();
try {
if (!data.rebind) {
IBinder binder = s.onBind(data.intent);
ActivityManager.getService().publishService(
data.token, data.intent, binder);
} else {
s.onRebind(data.intent);
ActivityManager.getService().serviceDoneExecuting(
data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
}
ensureJitEnabled();
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
} catch (Exception e) {
if (!mInstrumentation.onException(s, e)) {
throw new RuntimeException(
"Unable to bind to service " + s
+ " with " + data.intent + ": " + e.toString(), e);
}
}
}
}
然后通过ActivityManager.getService().publishService(...)又回到了ActivityManagerService的publishService()方法
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);
}
}
接着又回到了ActiveServices的publishServiceLocked()方法
void publishServiceLocked(ServiceRecord r, Intent intent, IBinder service) {
final long origId = Binder.clearCallingIdentity();
try {
if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "PUBLISHING " + r
+ " " + intent + ": " + service);
if (r != null) {
Intent.FilterComparison filter
= new Intent.FilterComparison(intent);
IntentBindRecord b = r.bindings.get(filter);
if (b != null && !b.received) {
b.binder = service;
b.requested = true;
b.received = true;
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++) {
ConnectionRecord c = clist.get(i);
if (!filter.equals(c.binding.intent.intent)) {
if (DEBUG_SERVICE) Slog.v(
TAG_SERVICE, "Not publishing to: " + c);
if (DEBUG_SERVICE) Slog.v(
TAG_SERVICE, "Bound intent: " + c.binding.intent.intent);
if (DEBUG_SERVICE) Slog.v(
TAG_SERVICE, "Published intent: " + intent);
continue;
}
if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "Publishing to: " + c);
try {
c.conn.connected(r.name, service, false);
} catch (Exception e) {
Slog.w(TAG, "Failure sending service " + r.name +
" to connection " + c.conn.asBinder() +
" (in " + c.binding.client.processName + ")", e);
}
}
}
}
serviceDoneExecutingLocked(r, mDestroyingServices.contains(r), false);
}
} finally {
Binder.restoreCallingIdentity(origId);
}
}
首先获取到ConnectionRecord类型对象c，然后调用c.conn.connected(...)发布连接。前面第四步中已提到过如果服务已经在运行，也会直接调用c.conn.connected(...)来发布连接，接下来就在第六步中具体分析c.conn.connected(...)。
第六步 android.app.LoadedApk.ServiceDispatcher.InnerConnection.java
c.conn其实就是sd，是在第一步中通过ActivityManager.getService().bindService(...)方法传入的，由于ActivityManager.getService().bindService(...)是一个跨进程通信方法，从sd到c.conn其实在native层传输时有一次转换，这样c.conn可以从服务端调用到应用端的方法。然后sd是一个ServiceDispatcher.InnerConnection类型对象。
InnerConnection继承于IServiceConnection.Stub，可知c.conn确实是一个Binder对象，调用c.conn.connected(...)，此时这个connected(...)方法已经被执行在应用端了
private static class InnerConnection extends IServiceConnection.Stub {
final WeakReference<LoadedApk.ServiceDispatcher> mDispatcher;
InnerConnection(LoadedApk.ServiceDispatcher sd) {
mDispatcher = new WeakReference<LoadedApk.ServiceDispatcher>(sd);
}
// 这个方法是被执行在应用端
public void connected(ComponentName name, IBinder service, boolean dead)
throws RemoteException {
LoadedApk.ServiceDispatcher sd = mDispatcher.get();
if (sd != null) {
sd.connected(name, service, dead);
}
}
}
接着又会调用到LoadedApk.ServiceDispatcher的connected(...)方法
public void connected(ComponentName name, IBinder service, boolean dead) {
if (mActivityThread != null) {
mActivityThread.post(new RunConnection(name, service, 0, dead));
} else {
doConnected(name, service, dead);
}
}
判断mActivityThread是否存在，注意mActivityThread是一个Handler对象，并不是ActivityThread对象，是第一步中的bindServiceCommon (...)方法传入的mMainThread.getHandler()，并创建了ServiceDispatcher以及ServiceDispatcher.InnerConnection对象。
如果mActivityThread存在，则让RunConnection的run()方法运行于UI线程，否则直接调用doConnected(...)方法
public void run() {
if (mCommand == 0) {
doConnected(mName, mService, mDead);
} else if (mCommand == 1) {
doDeath(mName, mService);
}
}
接着还是调用了doConnected(...)方法
public void doConnected(ComponentName name, IBinder service, boolean dead) {
ServiceDispatcher.ConnectionInfo old;
ServiceDispatcher.ConnectionInfo info;
synchronized (this) {
if (mForgotten) {
// We unbound before receiving the connection; ignore
// any connection received.
return;
}
old = mActiveConnections.get(name);
if (old != null && old.binder == service) {
// Huh, already have this one. Oh well!
return;
}
if (service != null) {
// A new service is being connected... set it all up.
info = new ConnectionInfo();
info.binder = service;
info.deathMonitor = new DeathMonitor(name, service);
try {
service.linkToDeath(info.deathMonitor, 0);
mActiveConnections.put(name, info);
} catch (RemoteException e) {
// This service was dead before we got it... just
// don't do anything with it.
mActiveConnections.remove(name);
return;
}
} else {
// The named service is being disconnected... clean up.
mActiveConnections.remove(name);
}
if (old != null) {
old.binder.unlinkToDeath(old.deathMonitor, 0);
}
}
// If there was an old service, it is now disconnected.
if (old != null) {
mConnection.onServiceDisconnected(name);
}
if (dead) {
mConnection.onBindingDied(name);
}
// If there is a new service, it is now connected.
if (service != null) {
mConnection.onServiceConnected(name, service);
}
}
最后调用mConnection.onServiceConnected(name, service)，service就是这样被传递回应用端的。
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