Android四大组件-Service工作过程

一.前言

通过上篇IPC之Binder连接池机制Binder连接池机制
，我们知道通过bindService方法能完成整个服务的绑定操作，并且通过onBind回调方法返回IBinder实例，在客户端通过ServiceConnection#onServiceConnected方法得到IBinder这个实例。接下来我们从源码的角度来分析下绑定服务的工作过程。

二.工作过程

大致流程：首先调用Context#bindService抽象方法，然后我们看Context类的实现类ContextImpl#bindService，然后跨进程调用到ActivityManagerService中，然后通过IApplicationThread回调到ActivityThread.ApplicationThread#scheduleBindService方法中通过mH(Hander)切换到主线程中ActivityThread#handleBindService处理。下面详细的分析工作过程。

Intent intent = new Intent(context, BinderPoolService.class);
context.bindService(intent, serviceConnection, Context.BIND_AUTO_CREATE);

以上调用流程如下：

ZW_bind_service.png

1.实现Context的ContextImpl

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

然后直接调用bindServiceCommon方法：

private boolean bindServiceCommon(Intent service, ServiceConnection conn, int flags, Handler
            handler, UserHandle user) {
     //省略。。。
        if (mPackageInfo != null) {
            sd = mPackageInfo.getServiceDispatcher(conn, getOuterContext(), handler, flags);
        } else {
            throw new RuntimeException("Not supported in system context");
        }
        //省略。。。
            int res = ActivityManager.getService().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();
        }
    }

bindServiceCommon方法主要工作是对ServiceConnection实例做一些封装成可跨进程的实例为后面回调做铺垫，相应校验以及通过IActivityManager接口跨进程调用到ActivityManagerService(AMS)中。

2.AMS#bindService

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);
        }
    }

3.ActiveServices#bindServiceLocked

通过上面2可以看到直接调用ActiveServices#bindServiceLocked方法。
bindServiceLocked方法逻辑比较多，这里重点关注bind过程就可以了，所以我们直接看bindServiceLocked方法中调用requestServiceBindingLocked的逻辑

 //省略。。。
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);
            }
 //省略。。。

接下来看看requestServiceBindingLocked方法

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 (DEBUG_SERVICE) Slog.d(TAG_SERVICE, "requestBind " + i + ": requested=" + i.requested
                + " rebind=" + rebind);
        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;
            } catch (TransactionTooLargeException e) {
                // Keep the executeNesting count accurate.
                if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "Crashed while binding " + r, e);
                final boolean inDestroying = mDestroyingServices.contains(r);
                serviceDoneExecutingLocked(r, inDestroying, inDestroying);
                throw e;
            } catch (RemoteException e) {
                if (DEBUG_SERVICE) Slog.v(TAG_SERVICE, "Crashed while binding " + r);
                // Keep the executeNesting count accurate.
                final boolean inDestroying = mDestroyingServices.contains(r);
                serviceDoneExecutingLocked(r, inDestroying, inDestroying);
                return false;
            }
        }
        return true;
    }

重点关注 r.app.thread.scheduleBindService(r, i.intent.getIntent(), rebind,
r.app.repProcState);这句逻辑。r.app.thread其实就是ApplicationThread实例，在客户端通过mMainThread.getApplicationThread()传给ActivityServices中ServiceRecord记录的。这里其实就通过IApplicationThread跨进程回调到ActivityThread#ApplicationThread#scheduleBindService方法中。回调到4.

4.ActivityThread#ApplicationThread

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);
        }

通过mH切换到主线程处理

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) {
                        //1.调用Service中的onBind方法获取IBinder接口
                        IBinder binder = s.onBind(data.intent);
                       //2.通过IActivityManager接口跨进程调用到AMS中
                        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);
                }
            }
        }
    }

handleBindService方法中调用Service中的onBind方法获取IBinder接口，然后跨进程发布服务到AMS中。

5.AMS

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);
        }
    }

直接调用到ActiveServices#publishServiceLocked

6.ActiveServices#publishServiceLocked

void publishServiceLocked(ServiceRecord r, Intent intent, IBinder service) {
       //省略。。。
        try {
    
                if (b != null && !b.received) {
                  
                    for (int conni=r.connections.size()-1; conni>=0; conni--) {
                    
                           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);
                            }
                        }
                    }
       
//省略。。。
            }
        } finally {
            Binder.restoreCallingIdentity(origId);
        }
    }

重点看c.conn.connected(r.name, service, false)方法。调用到LoadApk#connected方法

7.LoadApk#connected

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doConnected方法最终会回调到前面当成功绑定一个服务的时候onServiceConnected方法中。这个时候整个绑定服务过程就完成了。

public void doConnected(ComponentName name, IBinder service, boolean dead) {
          。。。
            // 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 viable service, it is now connected.
            if (service != null) {
                mConnection.onServiceConnected(name, service);
            } else {
                // The binding machinery worked, but the remote returned null from onBind().
                mConnection.onNullBinding(name);
            }
        }

三.总结
服务组件的启动分为startService和bindService两种方式，这里bind启动服务就上面这几步过程。startService过程差不多，这里就不分析了。