Android Service绑定过程完整分析

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。

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