Android系统源码--bindService()过程

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 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 mDispatcher;

        InnerConnection(LoadedApk.ServiceDispatcher sd) {
            mDispatcher = new WeakReference(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 clist = r.connections.valueAt(conni);
                    for (int i=0; i

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 mDispatcher;

    InnerConnection(LoadedApk.ServiceDispatcher sd) {
        mDispatcher = new WeakReference(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。