通常我们使用Service都要和它通信，当想要与Service通信的时候，那么Service要处于绑定状态的。然后客户端可以拿到一个Binder与服务端进行通信，这个过程是很自然的。
那你真的了解过Service的绑定过程吗？为什么可以是Binder和Service通信？
同样的先看一张图大致了解一下，灰色背景框起来的是同一个类的方法，如下：
我们知道调用Context的bindService方法即可绑定一个Service，而ContextImpl是Context的实现类。那接下来就从源码的角度分析Service的绑定过程。
当然是从ContextImpl的bindService方法开始，如下：
@Override
public boolean bindService(Intent service, ServiceConnection conn,
int flags) {
warnIfCallingFromSystemProcess();
return bindServiceCommon(service, conn, flags, mMainThread.getHandler(),
Process.myUserHandle());
}
在bindService方法中又会转到bindServiceCommon方法，将Intent，ServiceConnection对象传进。
那就看看bindServiceCommon方法的实现。
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);
} 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(this);
int res = ActivityManagerNative.getDefault().bindService(
mMainThread.getApplicationThread(), getActivityToken(), service,
service.resolveTypeIfNeeded(getContentResolver()),
sd, flags, getOpPackageName(), user.getIdentifier());
if (res < 0) {
throw new SecurityException(
"Not allowed to bind to service " + service);
}
return res != 0;
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
在上述代码中，调用了mPackageInfo（LoadedApk对象）的getServiceDispatcher方法。从getServiceDispatcher方法的名字可以看出是获取一个“服务分发者”。其实是根据这个“服务分发者”获取到一个Binder对象的。
那现在就看到getServiceDispatcher方法的实现。
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();
}
}
从getServiceDispatcher方法的实现可以知道，ServiceConnection和ServiceDispatcher构成了映射关系。当存储集合不为空的时候，根据传进的key，也就是ServiceConnection，来取出对应的ServiceDispatcher对象。
当取出ServiceDispatcher对象后，最后一行代码是关键，
return sd.getIServiceConnection();调用了ServiceDispatcher对象的getIServiceConnection方法。这个方法肯定是获取一个IServiceConnection的。
IServiceConnection getIServiceConnection() {
return mIServiceConnection;
}
那么mIServiceConnection是什么？现在就可以来看下ServiceDispatcher类了。ServiceDispatcher是LoadedApk的内部类，里面封装了InnerConnection和ServiceConnection。如下：
static final class ServiceDispatcher {
private final ServiceDispatcher.InnerConnection mIServiceConnection;
private final ServiceConnection mConnection;
private final Context mContext;
private final Handler mActivityThread;
private final ServiceConnectionLeaked mLocation;
private final int mFlags;
private RuntimeException mUnbindLocation;
private boolean mForgotten;
private static class ConnectionInfo {
IBinder binder;
IBinder.DeathRecipient deathMonitor;
}
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);
}
}
}
private final ArrayMap<ComponentName, ServiceDispatcher.ConnectionInfo> mActiveConnections
= new ArrayMap<ComponentName, ServiceDispatcher.ConnectionInfo>();
ServiceDispatcher(ServiceConnection conn,
Context context, Handler activityThread, int flags) {
mIServiceConnection = new InnerConnection(this);
mConnection = conn;
mContext = context;
mActivityThread = activityThread;
mLocation = new ServiceConnectionLeaked(null);
mLocation.fillInStackTrace();
mFlags = flags;
}
//代码省略
}
先看到ServiceDispatcher的构造方法，一个ServiceDispatcher关联一个InnerConnection对象。而InnerConnection呢？，它是一个Binder，有一个很重要的connected方法。至于为什么要用Binder，因为与Service通信可能是跨进程的。
好，到了这里先总结一下：调用bindService方法绑定服务，会转到bindServiceCommon方法。
在bindServiceCommon方法中，会调用LoadedApk的getServiceDispatcher方法，并将ServiceConnection传进， 根据这个ServiceConnection取出与其映射的ServiceDispatcher对象，最后调用这个ServiceDispatcher对象的getIServiceConnection方法获取与其关联的InnerConnection对象并返回。简单点理解就是用ServiceConnection换来了InnerConnection。
现在回到bindServiceCommon方法，可以看到绑定Service的过程会转到ActivityManagerNative.getDefault()的bindService方法，其实从抛出的异常类型RemoteException也可以知道与Service通信可能是跨进程的，这个是很好理解的。
而ActivityManagerNative.getDefault()是ActivityManagerService，那么继续跟进ActivityManagerService的bindService方法即可，如下：
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);
}
}
在上述代码中，绑定Service的过程转到ActiveServices的bindServiceLocked方法，那就跟进ActiveServices的bindServiceLocked方法瞧瞧。如下：
int bindServiceLocked(IApplicationThread caller, IBinder token, Intent service,
String resolvedType, final IServiceConnection connection, int flags,
String callingPackage, final int userId) throws TransactionTooLargeException {
//代码省略
ConnectionRecord c = new ConnectionRecord(b, activity,
connection, flags, clientLabel, clientIntent);
IBinder binder = connection.asBinder();
ArrayList<ConnectionRecord> clist = s.connections.get(binder);
if (clist == null) {
clist = new ArrayList<ConnectionRecord>();
s.connections.put(binder, clist);
}
clist.add(c);
//代码省略
if ((flags&Context.BIND_AUTO_CREATE) != 0) {
s.lastActivity = SystemClock.uptimeMillis();
if (bringUpServiceLocked(s, service.getFlags(), callerFg, false,
permissionsReviewRequired) != null) {
return 0;
}
}
//代码省略
return 1;
}
将connection对象封装在ConnectionRecord中，这里的connection就是上面提到的InnerConnection对象。这一步很重要的。
然后调用了bringUpServiceLocked方法，那么就探探这个bringUpServiceLocked方法，
private String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg,
boolean whileRestarting, boolean permissionsReviewRequired)
throws TransactionTooLargeException {
//代码省略
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.
}
//代码省略
return null;
}
可以看到调用了realStartServiceLocked方法，真正去启动Service了。
那么跟进realStartServiceLocked方法探探，如下：
private final void realStartServiceLocked(ServiceRecord r,
ProcessRecord app, boolean execInFg) throws RemoteException {
//代码省略
app.thread.scheduleCreateService(r, r.serviceInfo,
mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo),
app.repProcState);
r.postNotification();
created = true;
//代码省略
requestServiceBindingsLocked(r, execInFg);
updateServiceClientActivitiesLocked(app, null, true);
// 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);
//代码省略
}
这里会调用app.thread的scheduleCreateService方法去创建一个Service，然后会回调Service的生命周期方法，然而绑定Service呢？
在上述代码中，找到一个requestServiceBindingsLocked方法，从名字看是请求绑定服务的意思，那么就是它没错了。
private final void requestServiceBindingsLocked(ServiceRecord r, boolean execInFg)
throws TransactionTooLargeException {
for (int i=r.bindings.size()-1; i>=0; i--) {
IntentBindRecord ibr = r.bindings.valueAt(i);
if (!requestServiceBindingLocked(r, ibr, execInFg, false)) {
break;
}
}
}
咦，我再按住Ctrl+鼠标左键，点进去requestServiceBindingLocked方法。如下：
private final boolean requestServiceBindingLocked(ServiceRecord r, IntentBindRecord i,
boolean execInFg, boolean rebind) throws TransactionTooLargeException {
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;
}
//代码省略
return true;
}
r.app.thread调用了scheduleBindService方法来绑定服务，而r.app.thread是ApplicationThread，现在关注到 ApplicationThread即可，scheduleBindService方法如下：
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);
}
封装了待绑定的Service的信息，并发送了一个消息给主线程，
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
//代码省略
case BIND_SERVICE:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
handleBindService((BindServiceData)msg.obj);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
//代码省略
}
}
调用了handleBindService方法，即将绑定完成啦。
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);
ActivityManagerNative.getDefault().publishService(
data.token, data.intent, binder);
} else {
s.onRebind(data.intent);
ActivityManagerNative.getDefault().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);
}
}
}
}
根据token获取到Service，然后Service回调onBind方法。结束了？
可是onBind方法返回了一个binder是用来干嘛的？
我们再看看ActivityManagerNative.getDefault()调用了publishService方法做了什么工作，此时又回到了ActivityManagerService。
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);
}
}
又会交给ActiveServices处理，转到了publishServiceLocked方法，那看到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);
}
//代码省略
}
c.conn是什么？ 它是一个InnerConnection对象，对，就是那个那个Binder，上面也贴出了代码，在ActiveServices的bindServiceLocked方法中，InnerConnection对象被封装在ConnectionRecord中。那么现在它调用了connected方法，就很好理解了。
InnerConnection的connected方法如下：
public void connected(ComponentName name, IBinder service) throws RemoteException {
LoadedApk.ServiceDispatcher sd = mDispatcher.get();
if (sd != null) {
sd.connected(name, service);
}
}
会调用ServiceDispatcher 的connected方法，如下
public void connected(ComponentName name, IBinder service) {
if (mActivityThread != null) {
mActivityThread.post(new RunConnection(name, service, 0));
} else {
doConnected(name, service);
}
}
从而ActivityThread会投递一个消息到主线程，此时就解决了跨进程通信。
那么你应该猜到了RunConnection一定有在主线程回调的onServiceConnected方法，
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;
}
咦，还不出现？继续跟进doConnected方法，
public void doConnected(ComponentName name, IBinder service) {
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 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);
}
}
在最后一个if判断，终于找到了onServiceConnected方法！
总结一下，当Service回调onBind方法，其实还没有结束，会转到ActivityManagerService，然后又会在ActiveServices的publishServiceLocked方法中，从ConnectionRecord中取出InnerConnection对象。有了InnerConnection对象后，就回调了它的connected。在InnerConnection的connected方法中，又会调用ServiceDispatcher的connected方法，在ServiceDispatcher的connected方法向主线程扔了一个消息，切换到了主线程，之后就在主线程中回调了客户端传进的ServiceConnected对象的onServiceConnected方法。
至此， Service的绑定过程分析完毕。