Android Network Management (1) ConnectivityService - 小破孩的博客

在开机过程中 systemserver 中会有如下 4 个service 启动，这4个系统服务管理和统计Android network

ConnectivityService ，提供数据连接管理服务
NetworkPolicyManagerService , 提供网络策略管理服务
NetworkManagementService , 提供对物理网络接口的管理服务
NetworkStatsService , 提供网络传输数据统计服务

其中只有 ConnectivityService 和 NetworkPolicyManagerService 对上层 app 提供 SDK 接口，其他服务通过 INetworkManagementService 和 INetworkStatsService 接口来访问这两个服务。

ConnectivityService

Android 提供了很多的网络链接通道，而 ConnectivityService 用于管理网络连接的状态与信息，通过该系统服务，我们可以查询当前可用的网络信息，发起网络链接请求，设置网络属性与参数等。以下列出部分常用网络类型:

public static final int TYPE_WIFI        = 1;
public static final int TYPE_MOBILE_MMS  = 2;	//Multimedia Messaging Service,表示网络可以通过MMSC发送MMS（彩信）
public static final int TYPE_MOBILE_SUPL = 3;	//Secure User Plane Location,网络可以使用基站进行辅助GPS定位
public static final int TYPE_MOBILE_DUN  = 4;	//Dial-Up Network,网络支持拨号的方式接入
public static final int TYPE_BLUETOOTH   = 7;
......

上层 app 可以通过 Context.getSystemService(Context.CONNECTIVITY_SERVICE) 获取到 ConnectivityManager ， ConnectivityManager 相当于 ConnectivityService 的包装，实际功能都是在 ConnectivityService 中。

以下为 android developer 中对于 ConnectivityManager 主要功能的描述 :

Monitor network connections (Wi-Fi, GPRS, UMTS, etc.)
Send broadcast intents when network connectivity changes
Attempt to “fail over” to another network when connectivity to a network is lost
Provide an API that allows applications to query the coarse-grained or fine-grained state of the available networks
Provide an API that allows applications to request and select networks for their data traffic

####ConnectivityManager常用接口
ConnectivityManager提供了很多接口用于获取系统当前的网络连接信息：

getAllNetworkInfo(): 返回所有网络信息
getActiveNetworkInfo():获取当前激活的网络连接信息
getNetworkForType: 获取指定类型的网络
requestNetwork(…) : 监听网络可用（WIFI或者移动数据）的变化
setAirplaneMode(): 开启飞行模式
addDefaultNetworkActiveListener() : 监听默认网络连接是否激活
registerNetworkCallback() : 监听某个网络请求的状态，可用时进行回调

ConnectivityService初始化

SystemServer 中会创建 ConnectivityService , 并将service 注册到 ServiceManager

	connectivity = new ConnectivityService(
			context, networkManagement, networkStats, networkPolicy);
	ServiceManager.addService(Context.CONNECTIVITY_SERVICE, connectivity);
	networkStats.bindConnectivityManager(connectivity);
	networkPolicy.bindConnectivityManager(connectivity);

ConnectivityService初始化时主要做了以下几个事情：

创建默认的移动数据请求
初始化消息队列
网络热点状态监听

先从一个问题来看 ConnectivityService , 当当前的网络连接变为不可用时，系统如何自动切换其他可用的网络连接的 , 下面以 wifi 断开自动切换到移动数据为例:
WifiStateMachine 每次连接状态变化时会调用 setNetworkDetailedState() 方法更新状态

setNetworkDetailedState(DetailedState.DISCONNECTED);

    private boolean setNetworkDetailedState(NetworkInfo.DetailedState state) {
        if (mNetworkInfo.getExtraInfo() != null && mWifiInfo.getSSID() != null
                && !mWifiInfo.getSSID().equals(WifiSsid.NONE)) {
            if (!mNetworkInfo.getExtraInfo().equals(mWifiInfo.getSSID())) {
                //更新 NetworkInfo 中的 SSID
                mNetworkInfo.setExtraInfo(mWifiInfo.getSSID());
                sendNetworkStateChangeBroadcast(null);
            }
        }

        if (state != mNetworkInfo.getDetailedState()) {
            mNetworkInfo.setDetailedState(state, null, mWifiInfo.getSSID());
            if (mNetworkAgent != null) {
                //将最新状态发送到NetworkAgent
                mNetworkAgent.sendNetworkInfo(mNetworkInfo);
            }
            sendNetworkStateChangeBroadcast(null);
            return true;
        }
        return false;
    }

这里需要了解一下 NetworkAgent ，以下为 NetworkAgent 的官方描述

A Utility class for handling for communicating between bearer-specific code and ConnectivityService.
这句话描述了NetworkAgent的作用：他是某个网络连接与ConnectivityService之间的通讯的工具

//NetworkAgent 是一个Handler的子类，并且他是一个抽象类（abstract）
public abstract class NetworkAgent extends Handler {}

        public NetworkAgent(Looper looper, Context context, String logTag, NetworkInfo ni, NetworkCapabilities nc, LinkProperties lp, int score, NetworkMisc misc) {
            super(looper);
            mContext = context;
            if (ni == null || nc == null || lp == null) {
                throw new IllegalArgumentException();
            }
            //获取ConnectivityManager对象，并向其注册自己 , 最终调用的是 ConnectivityService 的 registerNetworkAgent() 方法
            ConnectivityManager cm = (ConnectivityManager)mContext.getSystemService( Context.CONNECTIVITY_SERVICE);
            cm.registerNetworkAgent(new Messenger(this), new NetworkInfo(ni), new LinkProperties(lp), new NetworkCapabilities(nc), score, misc);
        }

来看 ConnectivityService 中的 registerNetworkAgent()

@ConnectivityService.java 
public void registerNetworkAgent(Messenger messenger, NetworkInfo networkInfo, LinkProperties linkProperties, NetworkCapabilities networkCapabilities, int currentScore, NetworkMisc networkMisc) {  
    //权限检查  
    enforceConnectivityInternalPermission();  
    //创建NetworkAgentInfo对象  
    NetworkAgentInfo nai = new NetworkAgentInfo(messenger, new AsyncChannel(),  
            new NetworkInfo(networkInfo), new LinkProperties(linkProperties),  
            new NetworkCapabilities(networkCapabilities), currentScore, mContext, mTrackerHandler,  
            new NetworkMisc(networkMisc));  
    synchronized (this) {  
        nai.networkMonitor.systemReady = mSystemReady;  
    }  
    //向自己Handler发送EVENT_REGISTER_NETWORK_AGENT消息  
    mHandler.sendMessage(mHandler.obtainMessage(EVENT_REGISTER_NETWORK_AGENT, nai));  
}  

mHandler 对 EVENT_REGISTER_NETWORK_AGENT 的处理是 handleRegisterNetworkAgent

    private void handleRegisterNetworkAgent(NetworkAgentInfo na) {
        //将 na 放入 mNetworkAgentInfos 中
        mNetworkAgentInfos.put(na.messenger, na);
        synchronized (mNetworkForNetId) {
            mNetworkForNetId.put(na.network.netId, na);
        }
        //向NetworkAgentInfo的asyncChannel对象发起连接请求 , 后续连接的消息由 mTrackerHandler 来处理 
        na.asyncChannel.connect(mContext, mTrackerHandler, na.messenger);
        NetworkInfo networkInfo = na.networkInfo;
        na.networkInfo = null;
        updateNetworkInfo(na, networkInfo);
    }

这里可以看到将刚才创建的 NetworkAgentInfo 加入到了 mNetworkAgentInfos 中，后面还会和获取到的 NetworkAgent 对象创建AsyncChannel双向通道，后面也就是通过该通道来通信。

回头在看一下 WifiStateMachine 中 NetworkAgent 的创建，可以看到是在 wifi 连接建立之后创建好：

@WifiStateMachine.java
    class L2ConnectedState extends State {
        @Override
        public void enter() {
            mNetworkAgent = new WifiNetworkAgent(getHandler().getLooper(), mContext,
                    "WifiNetworkAgent", mNetworkInfo, mNetworkCapabilitiesFilter,
                    mLinkProperties, 60, mNetworkMisc); 

在来继续看wifi 断开时的流程 setNetworkDetailedState(DetailedState.DISCONNECTED) -> mNetworkAgent.sendNetworkInfo(mNetworkInfo)

@NetworkAgent.java 
public void sendNetworkInfo(NetworkInfo networkInfo) {  
    queueOrSendMessage(EVENT_NETWORK_INFO_CHANGED, new NetworkInfo(networkInfo));  
}

@ConnectivityService.java  
    private class NetworkStateTrackerHandler extends Handler {

        @Override
        public void handleMessage(Message msg) {
            if (!maybeHandleAsyncChannelMessage(msg) &&
                    !maybeHandleNetworkMonitorMessage(msg) &&
                    !maybeHandleNetworkAgentInfoMessage(msg)) {
                maybeHandleNetworkAgentMessage(msg);
            }
        }

        private void maybeHandleNetworkAgentMessage(Message msg) {
	   //从 mNetworkAgentInfos 中找到对应的 NetworkAgentInfo 
            NetworkAgentInfo nai = mNetworkAgentInfos.get(msg.replyTo);
                case NetworkAgent.EVENT_NETWORK_INFO_CHANGED: {
                    NetworkInfo info = (NetworkInfo) msg.obj;
                    //通过updateNetworkInfo来进行更新  
                    updateNetworkInfo(nai, info);
                    break;
                }
    }

private void updateNetworkInfo(NetworkAgentInfo networkAgent, NetworkInfo newInfo) {  
    NetworkInfo.State state = newInfo.getState();  
    NetworkInfo oldInfo = null;  
    synchronized (networkAgent) {  
        oldInfo = networkAgent.networkInfo;  
        //将最新的networkInfo更新到ConnectivityService  
        networkAgent.networkInfo = newInfo;  
    }  
  
  
    if (state == NetworkInfo.State.CONNECTED && !networkAgent.created) {  
    } else if (state == NetworkInfo.State.DISCONNECTED || state == NetworkInfo.State.SUSPENDED) {  
        //断开WIFI的NetworkAgent中的AsyncChannel  
        networkAgent.asyncChannel.disconnect();  
    }  
}

在updateNetworkInfo()中，ConnectivityService将最新的NetworkInfo保存在networkAgent中，等待其他应用来获取。然后就向NetworkAgent的AsyncChannel发起disconnect()的请求，该请求将会在ConnectivityService中收到CMD_CHANNEL_DISCONNECTED的回应

    private class NetworkStateTrackerHandler extends Handler {
        private boolean maybeHandleAsyncChannelMessage(Message msg) {
                case AsyncChannel.CMD_CHANNEL_DISCONNECTED: {
                    handleAsyncChannelDisconnected(msg);
                    break;
                }

ConnectivityService

ConnectivityService初始化

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