Android问题解决--“StorageManager.getVolumeList NullPointerException”空指针再现，getExternalDirs

桃扇骨 2023-01-14 04:54 76阅读 0赞

**问题：**

在获取外部存储目录时，在某些低版本Android手机上发生StorageManager.getVolumeList 空指针错误，具体如下：

Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'android.os.storage.StorageVolume[] android.os.storage.IStorageManager.getVolumeList(int, java.lang.String, int)' on a null object reference
            at android.os.storage.StorageManager.getVolumeList(StorageManager.java:1267)
            at android.os.Environment$UserEnvironment.getExternalDirs(Environment.java:109)
            at android.os.Environment.getExternalStorageState(Environment.java:1026)

**分析：**

getExternalDirs作用是获取sdcard的目录，在Android低版本（4.4以前），sdcard目录是指设备插入的真正的可插拔存储卡的目录；

在高版本（Android4.4以后），由于手机的存储空器越来越大，即使没有插入外部存储卡，系统也会默认创建sdcard，如果有卡插入，getExternalDirs就会返回多个File数组。

所以，上面的问题，应该发生在比较旧的设备上。

getExternalDirs 和getExternalDirs源码如下：

Environment.java:
         /**
         * Returns the current state of the primary shared/external storage media.
         *
         * @see #getExternalStorageDirectory()
         * @return one of {@link #MEDIA_UNKNOWN}, {@link #MEDIA_REMOVED},
         *         {@link #MEDIA_UNMOUNTED}, {@link #MEDIA_CHECKING},
         *         {@link #MEDIA_NOFS}, {@link #MEDIA_MOUNTED},
         *         {@link #MEDIA_MOUNTED_READ_ONLY}, {@link #MEDIA_SHARED},
         *         {@link #MEDIA_BAD_REMOVAL}, or {@link #MEDIA_UNMOUNTABLE}.
         */
        public static String getExternalStorageState() {
            final File externalDir = sCurrentUser.getExternalDirs()[0];
            return getExternalStorageState(externalDir);
        }
    
        /** {@hide} */
        public static class UserEnvironment {
            private final int mUserId;
    
            @UnsupportedAppUsage
            public UserEnvironment(int userId) {
                mUserId = userId;
            }
    
            @UnsupportedAppUsage
            public File[] getExternalDirs() {
                final StorageVolume[] volumes = StorageManager.getVolumeList(mUserId,
                        StorageManager.FLAG_FOR_WRITE);
                final File[] files = new File[volumes.length];
                for (int i = 0; i < volumes.length; i++) {
                    files[i] = volumes[i].getPathFile();
                }
                return files;
            }
    
            @UnsupportedAppUsage
            @Deprecated
            public File getExternalStorageDirectory() {
                return getExternalDirs()[0];
            }
    
            @UnsupportedAppUsage
            @Deprecated
            public File getExternalStoragePublicDirectory(String type) {
                return buildExternalStoragePublicDirs(type)[0];
            }
            。。。
        }

可见，getExternalDirs被注解为@UnsupportedAppUsage， 表示该api已经不被支持App使用了。

getExternalDirs调用了StorageManager.getVolumeList， 而正是在这里发生了NullPointerException。

**StorageManager.getVolumeList代码定义在StorageManager.java:**

/** {@hide} */
        @UnsupportedAppUsage
        public static @NonNull StorageVolume[] getVolumeList(int userId, int flags) {
            final IStorageManager storageManager = IStorageManager.Stub.asInterface(
                    ServiceManager.getService("mount"));
            try {
                String packageName = ActivityThread.currentOpPackageName();
                if (packageName == null) {
                    // Package name can be null if the activity thread is running but the app
                    // hasn't bound yet. In this case we fall back to the first package in the
                    // current UID. This works for runtime permissions as permission state is
                    // per UID and permission realted app ops are updated for all UID packages.
                    String[] packageNames = ActivityThread.getPackageManager().getPackagesForUid(
                            android.os.Process.myUid());
                    if (packageNames == null || packageNames.length <= 0) {
                        return new StorageVolume[0];
                    }
                    packageName = packageNames[0];
                }
                final int uid = ActivityThread.getPackageManager().getPackageUid(packageName,
                        PackageManager.MATCH_DEBUG_TRIAGED_MISSING, userId);
                if (uid <= 0) {
                    return new StorageVolume[0];
                }
                return storageManager.getVolumeList(uid, packageName, flags); //1267行
            } catch (RemoteException e) {
                throw e.rethrowFromSystemServer();
            }
        }

再对比错误信息和代码行数：

错误信息：

“at android.os.storage.StorageManager.getVolumeList(StorageManager.java:1267)”

代码行数截图：

![storageManager.getVolumeList调用][storageManager.getVolumeList]

问题解决：

1. 在api调用的时候，进行try catch异常的捕获，以及null的判空；

2. 进行sdk level的判断，在低版本和高版本进行区别对待。

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[storageManager.getVolumeList]: /images/20221022/a696de456d094173a3f11ee3fbbba57b.png