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记录一次ConcurrentModificationExcept

记录一次ConcurrentModificationExcept

作者: flame_ash | 来源:发表于2021-01-05 10:55 被阅读0次

    问题背景:

    android项目在迭代的过程中,有个获取定位的功能模块,一直有个异常居高不下。
    尝试修改了好几版,都没有解决掉这个问题
    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)
    

    该错误日志其实很熟悉,就是 fail-fast机制(java集合(Collection)中的一种错误机制)。

    场景复现:

    最原始代码版本:

    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;
    
        /**
         * 开始申请 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();
    
                }
        }
    
        /**
         * 取消注册
         *
         * @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,猜测是多线程没加锁导致的,数据不同步。于是直接加锁尝试解决上线(本身概率性事件,复现概率不是很高)。
    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;
    
        /**
         * 开始申请 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();
                }
            }
        }
    
    
        /**
         * 取消注册
         *
         * @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);
                                            }
                                        }
                                    }
                                }
            }
        }
    }
    
    

    一直以为是线程不同步导致的,但是发现加上锁之后还是会报这个异常。

    思考:

    1.难道锁对象不是同一个?(确认过确实是同一个)
    2.同一个线程能同时 执行两处代码?(差点颠覆了我这么多年 理论的认知)

    哎,经过网上查找资料。确实也是和自己理解的一样:
    参考资料:https://www.cnblogs.com/dolphin0520/p/3933551.html

    对于该异常的结论:

    1.在同一个线程中 确实也会报这个错误:
    测试代码如下:

    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.多线程中更会报这个错误:
    测试代码如下:

    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:

    通过测试以及 网上查阅资料,解决方法无非就是两种(加锁 或者使用 CopyOnWriteArrayList 线程安全的集合),
    那为啥我的代码,还是报错呢???哎,苦恼....

    最终结论:

    经过重新进行了代码逻辑走查,发现:在遍历所有监听器对象并且调用onReceiveLocation方法的时候。该对象的回调中,又会调用unRegisterListener 方法,
    查看unRegisterListener方法:发现该方法中会进行 集合的删除操作。
    看到这,这不就是单线程中 产生该异常的原因嘛。我擦。。。。尼玛。。。

    这种迷惑性比较强的代码,确实有点坑爹。哎,学艺不精啊。。

    最终修复版本代码:

    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;
    
        /**
         * 开始申请 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();
    
                }
            }
        }
    
        /**
         * 取消注册
         *
         * @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) {
                        // 在此处不要删除,重置为null。在调用处,使用iterator 遍历的时候,删除
                        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);
                                            }
                                        }
                                    }
                                }
            }
        }
    }
    
    

    结论虽然是很简单,但是排查过程中确实是很痛苦的。。。
    做一下开发过程中的简单记录吧,加油!!!

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