Platform: Rockchip
OS: Android 6.0
Kernel: 3.10.92

當有兩個或者兩個以上的已經儲存的無線網路可以連線時，系統通過attemptAutoJoin()來選擇一個最優網路。
假設只有兩個儲存的無線網路，配置檔案如下：
[email protected]:/ # cat /data/misc/wifi/wpa_supplicant.conf

network={
    ssid="RD-TA100-2.4G"
    psk="Gerrit2All"
    key_mgmt=WPA-PSK
    priority=2
}

network={
    ssid=
 
"Kris"
    psk="20122008"
    key_mgmt=WPA-PSK
    priority=3
}

來看attemptAutoJoin()：

boolean attemptAutoJoin()
{
        //獲取上一次選擇的網路配置，我上次選的是ap: "Kris"
        String lastSelectedConfiguration = mWifiConfigStore.getLastSelectedConfiguration();
        if (lastSelectedConfiguration != null) {
            age =
 
 14000;
        }
        //獲取最近配置過的網路，這裡會獲取到兩個，也就是配置檔案中的那兩個
        List<WifiConfiguration> list =
                mWifiConfigStore.getRecentConfiguredNetworks(age, false);

        for (WifiConfiguration config : list) {
            //candidate儲存當前網路配置
            if (candidate == null) {
                candidate =
 
 config;
            } else {
                //迴圈第二次時就會比較兩個網路配置
                int order = compareWifiConfigurations(candidate, config);
                if (order > 0) {
                    // Ascending : candidate < config
                    //經過一番計算後，發現order>0，那麼老的candidate網路配置就會被新的代替，
                    //如果有多餘兩個儲存的無線網路，那麼依次計算並且符合條件後替換
                    candidate = config;
                }
            }
        }
        if (mWifiStateMachine.shouldSwitchNetwork(networkDelta)) {      // !!! JNo: Here!
                //傳送自動連線event給WifiStateMachine
                mWifiStateMachine.sendMessage(WifiStateMachine.CMD_AUTO_CONNECT,
                        candidate.networkId, networkSwitchType, candidate);
                found = true;
        }
        return found;
}

compareWifiConfigurations():

int compareWifiConfigurations(WifiConfiguration a, WifiConfiguration b) 
{
    int order = 0;
    boolean linked = false;

    // ephemeral的解釋是
    // Indicate that a WifiConfiguration is temporary and should not be saved
    //nor considered by AutoJoin.
    //正常連線時WifiStateMachine在connect的時候設定為false
    //當是未信任網路時會設定為true
    if (a.ephemeral && b.ephemeral == false) {
        return 1; // b is of higher priority - ascending
    }
    if (b.ephemeral && a.ephemeral == false) {
        return -1; // a is of higher priority - descending
    }

    // Apply RSSI, in the range [-5, +5]
    // after band adjustment, +n difference roughly corresponds to +10xn dBm
    //通過RSSI也就是訊號強度計算得到一個order
    order = order + compareWifiConfigurationsRSSI(a, b, mCurrentConfigurationKey);

    // If the configurations are not linked, compare by user's choice, only a
    // very high RSSI difference can then override the choice
    //如果使用者上次有選擇過某個網路，那麼又要根據選擇的choice來重新計算最終的order
    if (!linked) {
        int choice;

        choice = getConnectChoice(a, b, false);
        if (choice > 0) {
            // a is of higher priority - descending
            order = order - choice;
            if (a.visibility != null) {
                a.visibility.lastChoiceBoost = choice;
                a.visibility.lastChoiceConfig = b.configKey();
            }
        }

        choice = getConnectChoice(b, a, false);
        if (choice > 0) {
            // a is of lower priority - ascending
            order = order + choice;
            if (b.visibility != null) {
                b.visibility.lastChoiceBoost = choice;
                b.visibility.lastChoiceConfig = a.configKey();
            }
        }
    }

    String sorder = " == ";
    if (order > 0) {
        sorder = " < ";
    } else if (order < 0) {
        sorder = " > ";
    }

    return order;
}

除錯時可以把debug log開啟，對應的log如下:

07-06 15:41:00.357   557   608 D WifiAutoJoinController : attemptAutoJoin() status=wpa_state=SCANNING
07-06 15:41:00.357   557   608 D WifiAutoJoinController : p2p_device_address=b2:f1:ec:49:50:cb
07-06 15:41:00.357   557   608 D WifiAutoJoinController : address=b0:f1:ec:49:50:cb
07-06 15:41:00.357   557   608 D WifiAutoJoinController : uuid=544ae0ce-fa0a-58d2-af21-81be288cb44c
07-06 15:41:00.358   557   608 D WifiAutoJoinController : attemptAutoJoin() num recent config 2 ---> suppNetId=-1
07-06 15:41:00.358   557   608 D WifiAutoJoinController : attemptAutoJoin good candidate seen, bumped hard -> status="Kris"WPA_PSK status=0
07-06 15:41:00.358   557   608 D WifiAutoJoinController : attemptAutoJoin trying id=1 "Kris"WPA_PSK status=0
07-06 15:41:00.359   557   608 D WifiAutoJoinController : attemptAutoJoin good candidate seen, bumped hard -> status="RD-TA100-2.4G"WPA_PSK status=0
07-06 15:41:00.359   557   608 D WifiAutoJoinController : attemptAutoJoin trying id=0 "RD-TA100-2.4G"WPA_PSK status=0 current candidate "Kris"WPA_PSK
07-06 15:41:00.359   557   608 D WifiAutoJoinController : attemptAutoJoin will compare candidate  "Kris"WPA_PSK with "RD-TA100-2.4G"WPA_PSK
07-06 15:41:00.359   557   608 D WifiAutoJoinController :     compareWifiConfigurationsRSSI: "Kris"WPA_PSK rssi=-55,-127 boost=0 "RD-TA100-2.4G"WPA_PSK rssi=-47,-127 boost=0
07-06 15:41:00.359   557   608 D WifiAutoJoinController :         "Kris"WPA_PSK is5=false score=-55 "RD-TA100-2.4G"WPA_PSK is5=false score=-47
07-06 15:41:00.359   557   608 D WifiAutoJoinController :     compareWifiConfigurationsRSSI "Kris"WPA_PSK rssi=(-55,-127) num=(1,0) < "RD-TA100-2.4G"WPA_PSK rssi=(-47,-127) num=(1,0) -> 8
07-06 15:41:00.360   557   608 D WifiAutoJoinController :     compareWifiConfigurations prefers "Kris"WPA_PSK over "RD-TA100-2.4G"WPA_PSK due to user choice of 60 order -> -52
07-06 15:41:00.360   557   608 D WifiAutoJoinController : compareWifiConfigurations: "Kris"WPA_PSK > "RD-TA100-2.4G"WPA_PSK order -52
07-06 15:41:00.360   557   608 D WifiAutoJoinController : attemptAutoJoin compareWifiConfigurations returned -52
07-06 15:41:00.360   557   608 D WifiAutoJoinController : attemptAutoJoin networkSwitching candidate "Kris"WPA_PSK linked=false : delta=1000 
07-06 15:41:00.361   557   608 D WifiStateMachine: shouldSwitchNetwork  txSuccessRate=0.00 rxSuccessRate=0.00 delta 1000 -> 1000
07-06 15:41:00.361   557   608 D WifiAutoJoinController : AutoJoin auto connect with netId 1 to "Kris"WPA_PSK
07-06 15:41:00.361   557   608 D WifiAutoJoinController : attemptRoam: "Kris"WPA_PSK Found ee:29:f5:be:c5:6b rssi=-55 freq=2437
07-06 15:41:00.361   557   608 D WifiAutoJoinController : Done attemptAutoJoin status=3

總的來說,評分機制有三種：
1. ephemeral值，即是否是未信任網路
2. RSSI值
3. 使用者上次選擇，優先順序比RSSI高，除非RSSI非常大的情況