Android 数据连接分析
把网络接入过程简单分为三个阶段
触发阶段
—-该阶段是由各种不同事件触发的，比如SIM载入完毕、PS域Attach成功、通话结束、APN改变等，该阶段的最终都是要调用setupDataOnConnectableApns()方法；
准备连接阶段
—-该阶段是指，在DcTracker收到建立连接的请求之后，需要进行一系列有效性检测，比如APN是否已经激活、PS是否已经就绪、用户是否打开网络开关等，然后创建/找到核实的DataConnection对象，准备发起连接请求。
发送连接命令阶段
—-该阶段是指，在DataConnection收到DcTracker的请求之后，将请求转交给RILJ的过程，RILJ通过socket将请求发送到了RILD，RILD根据请求类型再将AT指令发送到Modem层，由底层完成信令的发送和接收。简单概括就是如下步骤，此处创建的DcAsyncChannel是双向的。 
1、触发条件
//漫游打开关闭（EVENT_ROAMING_ON/EVENT_ROAMING_OFF事件）
static final String REASON_ROAMING_ON = "roamingOn";
static final String REASON_ROAMING_OFF = "roamingOff";
//PS attach（STATE_IN_SERVICE）
static final String REASON_DATA_ATTACHED = "dataAttached";
//APN Changed（EVENT_APN_CHANGED）
static final String REASON_APN_CHANGED = "apnChanged";
//通话结束
static final String REASON_VOICE_CALL_ENDED = "2GVoiceCallEnded";
//SIM载入完毕 （EVENT_RECORDS_LOADED 事件）
static final String REASON_SIM_LOADED = "simLoaded";
//网络模式改变 （EVENT_DATA_RAT_CHANGED 事件）
static final String REASON_NW_TYPE_CHANGED = "nwTypeChanged"; - 1
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2、有效性及状态检查
在触发条件满足后都会调用setupDataOnConnectableApns(String reason)
之后会调用trySetupData,在trySetupData里面会进行有效性检测
private boolean trySetupData(ApnContext apnContext, ArrayList waitingApns) {
// Allow SETUP_DATA request for E-APN to be completed during emergency call and MOBILE DATA On/Off cases as well.
boolean isEmergencyApn = apnContext.getApnType().equals(PhoneConstants.APN_TYPE_EMERGENCY);
final ServiceStateTracker sst = mPhone.getServiceStateTracker();
boolean desiredPowerState = sst.getDesiredPowerState();
boolean checkUserDataEnabled = !(apnContext.getApnType().equals(PhoneConstants.APN_TYPE_IMS));
if (apnContext.isConnectable() && (isEmergencyApn ||
(isDataAllowed(apnContext) &&
getAnyDataEnabled(checkUserDataEnabled) && !isEmergency()))) {
if (apnContext.getState() == DctConstants.State.FAILED) {
apnContext.setState(DctConstants.State.IDLE);
}
int radioTech = mPhone.getServiceState().getRilDataRadioTechnology();
apnContext.setConcurrentVoiceAndDataAllowed(sst.isConcurrentVoiceAndDataAllowed());
if (apnContext.getState() == DctConstants.State.IDLE) {
if (waitingApns == null) {
waitingApns = buildWaitingApns(apnContext.getApnType(), radioTech);
}
if (waitingApns.isEmpty()) {
notifyNoData(DcFailCause.MISSING_UNKNOWN_APN, apnContext);
notifyOffApnsOfAvailability(apnContext.getReason());
return false;
} else {
apnContext.setWaitingApns(waitingApns);
}
}
boolean retValue = setupData(apnContext, radioTech);
notifyOffApnsOfAvailability(apnContext.getReason());
return retValue;
}
} - 1
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条件1：apnContext.isConnectable()
public boolean isConnectable() {
return isReady() && ((mState == DctConstants.State.IDLE)
|| (mState == DctConstants.State.SCANNING)
|| (mState == DctConstants.State.RETRYING)
|| (mState == DctConstants.State.FAILED));
} - 1
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其中isReady()就是检查APN type有没有使能，所以在数据链接之前一定要调用ApnContext的setEnabled()将要打开的数据类型对应的APN使能；然后就是检查APN的状态，这个状态会在DataConnection状态机随着联网状态的变迁而改变。
条件2：isEmergencyApn 是否是紧急APN
条件3：isDataAllowed(apnContext)
private boolean isDataAllowed(ApnContext apnContext) {
return apnContext.isReady() && isDataAllowed();
}
protected boolean isDataAllowed() {
final boolean internalDataEnabled;
synchronized (mDataEnabledLock) {
internalDataEnabled = mInternalDataEnabled;
}
boolean attachedState = mAttached.get();
boolean desiredPowerState = mPhone.getServiceStateTracker().getDesiredPowerState();
IccRecords r = mIccRecords.get();
boolean recordsLoaded = false;
if (r != null) {
recordsLoaded = r.getRecordsLoaded();
}
//FIXME always attach
boolean psRestricted = mIsPsRestricted;
int phoneNum = TelephonyManager.getDefault().getPhoneCount();
if (phoneNum > 1) {
attachedState = true;
psRestricted = false;
}
int dataSub = SubscriptionManager.getDefaultDataSubId();
boolean defaultDataSelected = SubscriptionManager.isValidSubscriptionId(dataSub);
PhoneConstants.State state = PhoneConstants.State.IDLE;
if (mPhone.getCallTracker() != null) {
state = mPhone.getCallTracker().getState();
}
boolean allowed =
(attachedState || mAutoAttachOnCreation) &&
recordsLoaded &&
(state == PhoneConstants.State.IDLE ||
mPhone.getServiceStateTracker().isConcurrentVoiceAndDataAllowed()) &&
internalDataEnabled &&
defaultDataSelected &&
(!mPhone.getServiceState().getDataRoaming() || getDataOnRoamingEnabled()) &&
//!mIsPsRestricted &&
!psRestricted &&
desiredPowerState;- 1
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—-判断是否已经ATTACH成功，SIM是否初始化完毕，当前手机服务是否支持，漫游下是否允许上网等
条件4：getAnyDataEnabled(checkUserDataEnabled)
—-该条件主要判断用户是否打开了数据开关
这部分代码很混乱，如果出现问题得一一排查
3、找到可用的APN
其中waitingApns = buildWaitingApns(apnContext.getApnType(), radioTech)便是找到可用的APN
buildWaitingApns中主要的逻辑有两个：
1、如果用户是否设置了Preferred，该值通过以下代码读取的：
usePreferred = ! mPhone.getContext().getResources().getBoolean(com.android.
internal.R.bool.config_dontPreferApn);
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如果用户设置了Preferred APN，并且mPreferredApn不为空（mPreferredApn来自于数据业务APN参数的创建createAllApnList中保存的，例如用户手动选择了APN TYPE那么这个APN就会被保留下来）因此此时waitingApns就会等于mPreferredApn
2、如果用户没有设置Preferred APN，那么就会在mAllApnSettings（来自于createAllApnList）中去寻找可用的APN，并将所有的可用的APN放在waitingApns中
最终联网采用的是waitingApns0
4、找到/创建DataConnection状态机
private boolean setupData(ApnContext apnContext, int radioTech) {
if (dcac == null) {
dcac = findFreeDataConnection();
if (dcac == null) {
dcac = createDataConnection();
}
}
apnContext.setDataConnectionAc(dcac);
apnContext.setApnSetting(apnSetting);
apnContext.setState(DctConstants.State.CONNECTING);
mPhone.notifyDataConnection(apnContext.getReason(), apnContext.getApnType());
Message msg = obtainMessage();
msg.what = DctConstants.EVENT_DATA_SETUP_COMPLETE;
msg.obj = apnContext;
dcac.bringUp(apnContext, getInitialMaxRetry(), profileId, radioTech, mAutoAttachOnCreation,msg);
return true;- 1
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这个方法内部主要完成了两个任务：
1、更新当前APN参数的状态并把状态发送到系统中(还是通过notifyDataConnection()来完成)
2、通过DcAsyncChannel的bringUp()方法发起连接请求
这里显示通过findFreeDataConnection()方法搜索可用的DcAsyncChannel，找不到的话就通过createDataConnection()创建，如果没有找到，就需要创建新的DcAsyncChannel
/* Return the DC AsyncChannel for the new data connection /
private DcAsyncChannel createDataConnection() {
int id = mUniqueIdGenerator.getAndIncrement();
DataConnection conn = DataConnection.makeDataConnection(mPhone, id,this, mDcTesterFailBringUpAll, mDcc);
mDataConnections.put(id, conn);
DcAsyncChannel dcac = new DcAsyncChannel(conn, LOG_TAG);
int status = dcac.fullyConnectSync(mPhone.getContext(), this, conn.getHandler());
if (status == AsyncChannel.STATUS_SUCCESSFUL) {
mDataConnectionAcHashMap.put(dcac.getDataConnectionIdSync(), dcac); //将创建好的DcAsyncChannel保存起来，以便下一次链接的时候可以找到可用的DcAsyncChannel
}
return dcac;
}
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创建DcAsyncChannel通道，是链接DcTracker和DataConnection的通道，但是他们不在同一个线程中，在DcTrackerBase的构造中new了一个新的HandlerThread，并将looper给了DcController和DataConnection。
为什么要闯将DcAsyncChannel通道，而不直接将DcTracker消息传到DataConnection中去呢？
因为在DcTracker中有需要同步获取DataConnection中的情况，比如在findFreeDataConnection中调用了
dcac.isInactiveSync()
private DcAsyncChannel findFreeDataConnection() {
for (DcAsyncChannel dcac : mDataConnectionAcHashMap.values()) {
if (dcac.isInactiveSync() && dataConnectionNotInUse(dcac)) {
return dcac;
}
}
return null;
}
public boolean isInactiveSync() {
boolean value;
if (isCallerOnDifferentThread()) {
Message response = sendMessageSynchronously(REQ_IS_INACTIVE);
if ((response != null) && (response.what == RSP_IS_INACTIVE)) {
value = rspIsInactive(response);
} else {
value = false;
}
} else {
value = mDc.getIsInactive();
}
return value;
} - 1
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其中的sendMessageSynchronously(REQ_IS_INACTIVE);即需要获取DataConnection中的是否处于INACTIVE状态，这不能是异步的。
5、通过DcAsyncChannel将消息传入状态机
将apnContext通过dcac传入DataConnection状态机进行数据连接
dcac.bringUp(apnContext, getInitialMaxRetry(), profileId, radioTech, mAutoAttachOnCreation,msg);- 1
6、状态机状态轮转
DataConnection默认状态是DcInactiveState,首先会在DcInactiveState调用onConnect去调用setupDataCall
private void onConnect(ConnectionParams cp) {
// msg.obj will be returned in AsyncResult.userObj;
Message msg = obtainMessage(EVENT_SETUP_DATA_CONNECTION_DONE, cp);
msg.obj = cp;
int authType = mApnSetting.authType;
if (authType == -1) {
authType = TextUtils.isEmpty(mApnSetting.user) ? RILConstants.SETUP_DATA_AUTH_NONE
: RILConstants.SETUP_DATA_AUTH_PAP_CHAP;
}
String protocol;
if (mPhone.getServiceState().getDataRoaming()) {
protocol = mApnSetting.roamingProtocol;
} else {
protocol = mApnSetting.protocol;
}
mPhone.mCi.setupDataCall(
Integer.toString(cp.mRilRat + 2),
Integer.toString(cp.mProfileId),
mApnSetting.apn, mApnSetting.user, mApnSetting.password,
Integer.toString(authType),
protocol, msg);
}
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在onConnect会包装一个EVENT_SETUP_DATA_CONNECTION_DONE消息在msg中，这个消息会一并发往RILJ->RILD，当RILD返回响应时会携带这消息并发出来，DataConnection状态机的DcActivatingState会扑捉到此消息进行状态机轮状，然后这里面全是状态机轮状来维护网络的链接状态： 
至此数据连接的发起过程完毕，但是理上网还有很远，后续工作还要更新路由表等等