理解Android的Framework层服务

前言

做为一个Android开发，Context.getSystemService是非常常用的API，但你有了解过这个API是如何实现的吗？今天，让我们一起从源码角度分析一下这个API的具体实现吧。本文源码基于Android 2.3.7_r1。

分析过程

ContextImpl.java

@Override
public Object getSystemService(String name) {
    // 省略部分代码
    if (WINDOW_SERVICE.equals(name)) {
        return WindowManagerImpl.getDefault();
    } else if (ACTIVITY_SERVICE.equals(name)) {
        return getActivityManager();
    } else if (ALARM_SERVICE.equals(name)) {
        return getAlarmManager();
    } else if (ACCOUNT_SERVICE.equals(name)) {
        return getAccountManager();
    }

    return null;
}


private AccountManager getAccountManager() {
    synchronized (mSync) {
        if (mAccountManager == null) {
            IBinder b = ServiceManager.getService(ACCOUNT_SERVICE);
            IAccountManager service = IAccountManager.Stub.asInterface(b);
            mAccountManager = new AccountManager(this, service);
        }
        return mAccountManager;
    }
}

private ActivityManager getActivityManager() {
    synchronized (mSync) {
        if (mActivityManager == null) {
            mActivityManager = new ActivityManager(getOuterContext(),
                    mMainThread.getHandler());
        }
    }
    return mActivityManager;
}

private AlarmManager getAlarmManager() {
    synchronized (sSync) {
        if (sAlarmManager == null) {
            IBinder b = ServiceManager.getService(ALARM_SERVICE);
            IAlarmManager service = IAlarmManager.Stub.asInterface(b);
            sAlarmManager = new AlarmManager(service);
        }
    }
    return sAlarmManager;
}

private ActivityManager getActivityManager() {
    synchronized (mSync) {
        if (mActivityManager == null) {
            mActivityManager = new ActivityManager(getOuterContext(),
                    mMainThread.getHandler());
        }
    }
    return mActivityManager;
}

ActivityManager.java

public List<RunningAppProcessInfo> getRunningAppProcesses() {
    try {
        return ActivityManagerNative.getDefault().getRunningAppProcesses();
    } catch (RemoteException e) {
        return null;
    }
}

我们发现ActivityManager做了一层转发，把请求转移到ActivityManagerNative去了，我们再来看下ActivityManagerNative具体做了哪些逻辑

ActivityManagerNative.java

public abstract class ActivityManagerNative extends Binder implements IActivityManager
{
    /**
     * Cast a Binder object into an activity manager interface, generating
     * a proxy if needed.
     */
    static public IActivityManager asInterface(IBinder obj)
    {
        if (obj == null) {
            return null;
        }
        IActivityManager in =
            (IActivityManager)obj.queryLocalInterface(descriptor);
        if (in != null) {
            return in;
        }
        
        return new ActivityManagerProxy(obj);
    }
    
    /**
     * Retrieve the system's default/global activity manager.
     */
    static public IActivityManager getDefault()
    {
        if (gDefault != null) {
            //if (Config.LOGV) Log.v(
            //    "ActivityManager", "returning cur default = " + gDefault);
            return gDefault;
        }
        IBinder b = ServiceManager.getService("activity");
        if (Config.LOGV) Log.v(
            "ActivityManager", "default service binder = " + b);
        gDefault = asInterface(b);
        if (Config.LOGV) Log.v(
            "ActivityManager", "default service = " + gDefault);
        return gDefault;
    }
    
    public List<ActivityManager.RunningAppProcessInfo> getRunningAppProcesses()
            throws RemoteException {
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IActivityManager.descriptor);
        mRemote.transact(GET_RUNNING_APP_PROCESSES_TRANSACTION, data, reply, 0);
        reply.readException();
        ArrayList<ActivityManager.RunningAppProcessInfo> list
        = reply.createTypedArrayList(ActivityManager.RunningAppProcessInfo.CREATOR);
        data.recycle();
        reply.recycle();
        return list;
    }
}

我们发现getDefault()方法先去ServiceManager查询到”activity”服务的句柄，然后通过asInterface方法生成一个本地代理类ActivityManagerProxy，最后该代理类再用ServiceManager查询出来的句柄去做真正的binder通信。所以当第一次去请求的时候，会有触发两次binder通信，一次是具体的业务逻辑请求，一次是ServiceManager去请求对应服务的binder，请求回来之后会保存在一个map中，所以第二次再调用getRunningAppProcesses()，只会有一次binder通信。

ServiceManager.java

public final class ServiceManager {
    private static final String TAG = "ServiceManager";

    private static IServiceManager sServiceManager;
    private static HashMap<String, IBinder> sCache = new HashMap<String, IBinder>();

    private static IServiceManager getIServiceManager() {
        if (sServiceManager != null) {
            return sServiceManager;
        }

        // Find the service manager
        sServiceManager = ServiceManagerNative.asInterface(BinderInternal.getContextObject());
        return sServiceManager;
    }

    public static IBinder getService(String name) {
        try {
            IBinder service = sCache.get(name);
            if (service != null) {
                return service;
            } else {
                return getIServiceManager().getService(name);
            }
        } catch (RemoteException e) {
            Log.e(TAG, "error in getService", e);
        }
        return null;
    }
}

总结

Stub/Proxy/Service/ServiceManager组成了整个Framework通信框架。分析系统服务之前，找到对应的Stub/Proxy/Service，不管是ActivityManager，AccountManager, AlarmManager，亦或是WindowManager等等，都是一样的套路，通过这样的方式，举一反三，触类旁通，去分析理解其他的Manager就会快很多。

参考

• https://stackoverflow.com/questions/15137247/how-does-getsystemservice-work-exactly