問題背景：

android項(xiàng)目在迭代的過程中，有個獲取定位的功能模塊拦英，一直有個異常居高不下蜒什。
嘗試修改了好幾版，都沒有解決掉這個問題
日志如下：

01-04 19:55:57.770 24194 24194 E AndroidRuntime: FATAL EXCEPTION: main
1501-04 19:55:57.770 24194 24194 E AndroidRuntime: Process: com.**, PID: 24194
1601-04 19:55:57.770 24194 24194 E AndroidRuntime: java.util.ConcurrentModificationException
1701-04 19:55:57.770 24194 24194 E AndroidRuntime: at java.util.ArrayList$Itr.next(ArrayList.java:860)
1801-04 19:55:57.770 24194 24194 E AndroidRuntime: at aiv$b.a(LocationManager.java:124)
1901-04 19:55:57.770 24194 24194 E AndroidRuntime: at com.baidu.location.LocationClient.b(Unknown Source:66)
2001-04 19:55:57.770 24194 24194 E AndroidRuntime: at com.baidu.location.LocationClient.a(Unknown Source:0)
2101-04 19:55:57.770 24194 24194 E AndroidRuntime: at com.baidu.location.LocationClient$a.handleMessage(Unknown Source:171)
2201-04 19:55:57.770 24194 24194 E AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:107)
2301-04 19:55:57.770 24194 24194 E AndroidRuntime: at android.os.Looper.loop(Looper.java:227)
2401-04 19:55:57.770 24194 24194 E AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:7668)
2501-04 19:55:57.770 24194 24194 E AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)
2601-04 19:55:57.770 24194 24194 E AndroidRuntime: at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
2701-04 19:55:57.770 24194 24194 E AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:960)

該錯誤日志其實(shí)很熟悉疤估，就是 fail-fast機(jī)制（java集合(Collection)中的一種錯誤機(jī)制）灾常。

場景復(fù)現(xiàn)：

最原始代碼版本：

public final class LocationManager {
    private static final class LocationManagerInstance {
        private static final LocationManager INSTANCE = new LocationManager();
    }

    public static LocationManager getInstance() {
        return LocationManagerInstance.INSTANCE;
    }

    private LocationManager() {
        // do nothing
    }

    private static List<SoftReference<IBdLocationListener>> mWeakReferences = new ArrayList<>();
    //    mILocationListener;
    private static MyLocationListener myListener = new MyLocationListener();

    private LocationClient mLocationClient;

    /**
     * 開始申請 定位
     */
    public void startLocate(IBdLocationListener locationListener, Application application) {
        mLocationClient = new LocationClient(application);
            if (mLocationClient != null) {
                mWeakReferences.add(new SoftReference<>(locationListener));
                if (locationListener != null) {
                    locationListener.startLocation();
                }
                LogUtils.i("start collect location info: thread name:" + Thread.currentThread().getName());
                mLocationClient.start();

            }
    }

    /**
     * 取消注冊
     *
     * @param listener
     */
    public void unRegisterListener(IBdLocationListener listener) {
        synchronized (LocationManager.class) {

            if (mWeakReferences == null || mWeakReferences.isEmpty()) {
                return;
            }

            for (int i = mWeakReferences.size() - 1; i >= 0; i--) {
                SoftReference<IBdLocationListener> itemListener = mWeakReferences.get(i);
                if (itemListener != null && itemListener.get() != null && itemListener.get() == listener) {
                    mWeakReferences.remove(i);
                }
            }

        }

    }

    //BDAbstractLocationListener
    public static class MyLocationListener extends BDAbstractLocationListener {
        @Override
        public void onReceiveLocation(BDLocation location) {
                                if (mWeakReferences != null) {

                                    Iterator<SoftReference<IBdLocationListener>> iterator = mWeakReferences.iterator();
                                    while (iterator.hasNext()) {
                                        SoftReference<IBdLocationListener> next = iterator.next();
                                        if (next == null || next.get() == null) {
                                            iterator.remove();
                                            continue;
                                        }
                                        if (next.get() != null) {
                                            next.get().onReceiveLocation(location);
                                        }
                                    }
                                }
                            }
                    });

        }
    }
}

由于是涉及到定位sdk，猜測是多線程沒加鎖導(dǎo)致的铃拇，數(shù)據(jù)不同步钞瀑。于是直接加鎖嘗試解決上線（本身概率性事件，復(fù)現(xiàn)概率不是很高）慷荔。
修改如下版本：

public final class LocationManager {
    private static final class LocationManagerInstance {
        private static final LocationManager INSTANCE = new LocationManager();
    }

    public static LocationManager getInstance() {
        return LocationManagerInstance.INSTANCE;
    }

    private LocationManager() {
        // do nothing
    }

    private static List<SoftReference<IBdLocationListener>> mWeakReferences = new ArrayList<>();
    //    mILocationListener;
    private static MyLocationListener myListener = new MyLocationListener();

    private LocationClient mLocationClient;

    /**
     * 開始申請 定位
     */
    public void startLocate(IBdLocationListener locationListener, Application application) {
        mLocationClient = new LocationClient(application);
        synchronized (LocationManager.class) {
            if (mLocationClient != null) {
                mWeakReferences.add(new SoftReference<>(locationListener));
                if (locationListener != null) {
                    locationListener.startLocation();
                }
                mLocationClient.start();
            }
        }
    }


    /**
     * 取消注冊
     *
     * @param listener
     */
    public void unRegisterListener(IBdLocationListener listener) {
        synchronized (LocationManager.class) {
            if (mWeakReferences == null || mWeakReferences.isEmpty()) {
                return;
            }
            for (int i = mWeakReferences.size() - 1; i >= 0; i--) {
                SoftReference<IBdLocationListener> itemListener = mWeakReferences.get(i);
                if (itemListener != null && itemListener.get() != null && itemListener.get() == listener) {
                    mWeakReferences.remove(i);
                }
            }

        }

    }

    //BDAbstractLocationListener
    public static class MyLocationListener extends BDAbstractLocationListener {
        @Override
        public void onReceiveLocation(BDLocation location) {
                            synchronized (LocationManager.class) {
                                if (mWeakReferences != null) {
                                    Iterator<SoftReference<IBdLocationListener>> iterator = mWeakReferences.iterator();
                                    while (iterator.hasNext()) {
                                        SoftReference<IBdLocationListener> next = iterator.next();
                                        if (next == null || next.get() == null) {
                                            iterator.remove();
                                            continue;
                                        }
                                        if (next.get() != null) {
                                            next.get().onReceiveLocation(location);
                                        }
                                    }
                                }
                            }
        }
    }
}

一直以為是線程不同步導(dǎo)致的雕什，但是發(fā)現(xiàn)加上鎖之后還是會報這個異常。

思考：

1.難道鎖對象不是同一個显晶？（確認(rèn)過確實(shí)是同一個）
2.同一個線程能同時 執(zhí)行兩處代碼监徘？（差點(diǎn)顛覆了我這么多年 理論的認(rèn)知）

哎，經(jīng)過網(wǎng)上查找資料吧碾。確實(shí)也是和自己理解的一樣：
參考資料：https://www.cnblogs.com/dolphin0520/p/3933551.html

對于該異常的結(jié)論：

1.在同一個線程中 確實(shí)也會報這個錯誤：
測試代碼如下：

public class Test {
    public static void main(String[] args)  {
        ArrayList<Integer> list = new ArrayList<Integer>();
        list.add(2);
        Iterator<Integer> iterator = list.iterator();
        while(iterator.hasNext()){
            Integer integer = iterator.next();
            if(integer==2)
                list.remove(integer);
        }
    }
}

2.多線程中更會報這個錯誤：
測試代碼如下：

public class Test {
    private static ArrayList<String> testList = new ArrayList<>();

    public static void main(String[] args) {

        for (int i = 0; i < 10; i++) {

            new Thread(new Runnable() {
                @Override
                public void run() {
                    for (int j = 0; j < 100000; j++) {
                        testList.add("" + j);

                    }
                }
            }).start();
        }


        for (int i = 0; i < 10; i++) {

            new Thread(new Runnable() {
                @Override
                public void run() {

                    for (int j = 0; j < 10000; j++) {

                        for (String s : testList) {

                            System.out.println(s);
                        }
                    }
                }
            }).start();
        }

    }
}

思考：

通過測試以及 網(wǎng)上查閱資料凰盔，解決方法無非就是兩種（加鎖 或者使用 CopyOnWriteArrayList 線程安全的集合），
那為啥我的代碼倦春，還是報錯呢户敬？？睁本？哎尿庐，苦惱....

最終結(jié)論：

經(jīng)過重新進(jìn)行了代碼邏輯走查，發(fā)現(xiàn)：在遍歷所有監(jiān)聽器對象并且調(diào)用onReceiveLocation方法的時候呢堰。該對象的回調(diào)中抄瑟，又會調(diào)用unRegisterListener 方法，
查看unRegisterListener方法：發(fā)現(xiàn)該方法中會進(jìn)行 集合的刪除操作枉疼。
看到這皮假，這不就是單線程中 產(chǎn)生該異常的原因嘛撮抓。我擦犬辰。侠畔。毛萌。。尼瑪褪测。猴誊。。

這種迷惑性比較強(qiáng)的代碼侮措，確實(shí)有點(diǎn)坑爹懈叹。哎，學(xué)藝不精啊分扎。澄成。

最終修復(fù)版本代碼：

public final class LocationManager {
    private static final class LocationManagerInstance {
        private static final LocationManager INSTANCE = new LocationManager();
    }

    public static LocationManager getInstance() {
        return LocationManagerInstance.INSTANCE;
    }

    private LocationManager() {
        // do nothing
    }

    private static List<SoftReference<IBdLocationListener>> mWeakReferences = new ArrayList<>();
    //    mILocationListener;
    private static MyLocationListener myListener = new MyLocationListener();

    private LocationClient mLocationClient;

    /**
     * 開始申請 定位
     */
    public void startLocate(IBdLocationListener locationListener, Application application) {
        mLocationClient = new LocationClient(application);
      
        synchronized (LocationManager.class) {
            if (mLocationClient != null) {
                mWeakReferences.add(new SoftReference<>(locationListener));
                if (locationListener != null) {
                    locationListener.startLocation();
                }
                mLocationClient.start();

            }
        }
    }

    /**
     * 取消注冊
     *
     * @param listener
     */
    public void unRegisterListener(IBdLocationListener listener) {
        synchronized (LocationManager.class) {

            if (mWeakReferences == null || mWeakReferences.isEmpty()) {
                return;
            }
            for (int i = mWeakReferences.size() - 1; i >= 0; i--) {
                SoftReference<IBdLocationListener> itemListener = mWeakReferences.get(i);
                if (itemListener != null && itemListener.get() != null && itemListener.get() == listener) {
                    // 在此處不要刪除，重置為null笆包。在調(diào)用處，使用iterator 遍歷的時候略荡，刪除
                    mWeakReferences.set(i, null);
                }
            }

        }

    }

    //BDAbstractLocationListener
    public static class MyLocationListener extends BDAbstractLocationListener {
        @Override
        public void onReceiveLocation(BDLocation location) {
                            synchronized (LocationManager.class) {
                                if (mWeakReferences != null) {

                                    Iterator<SoftReference<IBdLocationListener>> iterator = mWeakReferences.iterator();
                                    while (iterator.hasNext()) {
                                        SoftReference<IBdLocationListener> next = iterator.next();
                                        if (next == null || next.get() == null) {
                                            iterator.remove();
                                            continue;
                                        }
                                        if (next.get() != null) {
                                            next.get().onReceiveLocation(location);
                                        }
                                    }
                                }
                            }
        }
    }
}

結(jié)論雖然是很簡單庵佣，但是排查過程中確實(shí)是很痛苦的。汛兜。巴粪。
做一下開發(fā)過程中的簡單記錄吧，加油Ｖ嗝８馗！