Android存储空间讲解

http://blog.fidroid.com/post/android/ru-he-zheng-que-huo-de-androidnei-wai-sdqia-lu-jing

如何正确获得Android内外SD卡路径

外置sd卡路径，也许很多同学在平时的工作中并不会用到，因为现在很多机型都不支持外置sd卡(这也是Google目标)，所以并不用考虑外置sd卡的路径问题。除了开发文件管理类的应用之外，其他应用使用 Enviroment 这个类中的一些静态方法就能满足需要。但也有一些特殊需求需要用到外置sd卡路径，那怎么才能准确获得外置sd卡的路径呢？

方法一

//内置sd卡路径
String sdcardPath = System.getenv("EXTERNAL_STORAGE"); 
//内置sd卡路径
String sdcardPath = Environment.getExternalStorageDirectory().getAbsolutePath();
//外置置sd卡路径
String extSdcardPath = System.getenv("SECONDARY_STORAGE");

在Enviroment类的源码中获得sd卡路径其实也是通过 System.getnv() 方法来实现的，如隐藏的方法:

/** {@hide} */
public static File getLegacyExternalStorageDirectory() {
    return new File(System.getenv(ENV_EXTERNAL_STORAGE));
} 

注：更详细的内容还是去看Enviroment源码。

另外要注意的是，在API 23版本中 SECONDARY_STORAGE 被移除。

方法二

private static String getStoragePath(Context mContext, boolean is_removale) {  

      StorageManager mStorageManager = (StorageManager) mContext.getSystemService(Context.STORAGE_SERVICE);
        Class storageVolumeClazz = null;
        try {
            storageVolumeClazz = Class.forName("android.os.storage.StorageVolume");
            Method getVolumeList = mStorageManager.getClass().getMethod("getVolumeList");
            Method getPath = storageVolumeClazz.getMethod("getPath");
            Method isRemovable = storageVolumeClazz.getMethod("isRemovable");
            Object result = getVolumeList.invoke(mStorageManager);
            final int length = Array.getLength(result);
            for (int i = 0; i < length; i++) {
                Object storageVolumeElement = Array.get(result, i);
                String path = (String) getPath.invoke(storageVolumeElement);
                boolean removable = (Boolean) isRemovable.invoke(storageVolumeElement);
                if (is_removale == removable) {
                    return path;
                }
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        return null;
}

通过反射的方式使用在sdk中被 隐藏 的类 StroageVolume 中的方法getVolumeList()，获取所有的存储空间（Stroage Volume），然后通过参数is_removable控制，来获取内部存储和外部存储（内外sd卡）的路径，参数 is_removable为false时得到的是内置sd卡路径，为true则为外置sd卡路径。

在API 23 Enviroment 类中的内部类 UserEnvironment 中有一方法getExternalDirs与此一样，代码如下：

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;
}

再看Enviroment的getExternalStorageDirectory方法实现：

public static File getExternalStorageDirectory() {
    throwIfUserRequired();
    return sCurrentUser.getExternalDirs()[0];
}

可以看出，在API 23时，先是通过getExternalDirs()获取到所有存储空间的File[]数组，这个数组的第一个值：getExternalDirs()[0]，即为内置sd卡所在路径。

而在API 23 之前的版本中，并没有类似getExternalDirs()的方法通过StorageVolume直接获得存储空间(Storage Volume)，而时通过别的方式来实现的，看关键方法的源码：

public static File getExternalStorageDirectory() {
    throwIfUserRequired();
    return sCurrentUser.getExternalDirsForApp()[0];
}

这里的 getExternalDirsForApp() 和上面的 getExternalDirs() 的作用是一样的，都是得到所有存储空间的File[]数组。

public File[] getExternalDirsForApp() {
    return mExternalDirsForApp;
}

mExternalDirsForApp 是在 Enviroment 类中的内部类 UserEnvironment 的构造方法中初始化的，Enviroment#UserEnvironment构造函数源码如下：

        public UserEnvironment(int userId) {
            // See storage config details at http://source.android.com/tech/storage/
            String rawExternalStorage = System.getenv(ENV_EXTERNAL_STORAGE);
            String rawEmulatedSource = System.getenv(ENV_EMULATED_STORAGE_SOURCE);
            String rawEmulatedTarget = System.getenv(ENV_EMULATED_STORAGE_TARGET);

            String rawMediaStorage = System.getenv(ENV_MEDIA_STORAGE);
            if (TextUtils.isEmpty(rawMediaStorage)) {
                rawMediaStorage = "/data/media";
            }

            ArrayList<File> externalForVold = Lists.newArrayList();
            ArrayList<File> externalForApp = Lists.newArrayList();

            if (!TextUtils.isEmpty(rawEmulatedTarget)) {
                // Device has emulated storage; external storage paths should have
                // userId burned into them.
                final String rawUserId = Integer.toString(userId);
                final File emulatedSourceBase = new File(rawEmulatedSource);
                final File emulatedTargetBase = new File(rawEmulatedTarget);
                final File mediaBase = new File(rawMediaStorage);

                // /storage/emulated/0
                externalForVold.add(buildPath(emulatedSourceBase, rawUserId));
                externalForApp.add(buildPath(emulatedTargetBase, rawUserId));
                // /data/media/0
                mEmulatedDirForDirect = buildPath(mediaBase, rawUserId);

            } else {
                // Device has physical external storage; use plain paths.
                if (TextUtils.isEmpty(rawExternalStorage)) {
                    Log.w(TAG, "EXTERNAL_STORAGE undefined; falling back to default");
                    rawExternalStorage = "/storage/sdcard0";
                }

                // /storage/sdcard0
                externalForVold.add(new File(rawExternalStorage));
                externalForApp.add(new File(rawExternalStorage));
                // /data/media
                mEmulatedDirForDirect = new File(rawMediaStorage);
            }

            // Splice in any secondary storage paths, but only for owner
            final String rawSecondaryStorage = System.getenv(ENV_SECONDARY_STORAGE);
            if (!TextUtils.isEmpty(rawSecondaryStorage) && userId == UserHandle.USER_OWNER) {
                for (String secondaryPath : rawSecondaryStorage.split(":")) {
                    externalForVold.add(new File(secondaryPath));
                    externalForApp.add(new File(secondaryPath));
                }
            }

            mExternalDirsForVold = externalForVold.toArray(new File[externalForVold.size()]);
            mExternalDirsForApp = externalForApp.toArray(new File[externalForApp.size()]);
        }

也可以根据这个方法得到一个获取所有存储空间的路径的方法getStorageDirectories()：

/**
     * Returns all available SD-Cards in the system (include emulated)
     * 

     * Warning: Hack! Based on Android source code of version 4.3 (API 18)
     * Because there is no standard way to get it.
     * TODO: Test on future Android versions 4.4+
     *
     * @return paths to all available SD-Cards in the system (include emulated)
     */
    private static final Pattern DIR_SEPARATOR = Pattern.compile("/");
    public List<String> getStorageDirectories() {
        // Final set of paths
        final ArrayList<String> rv = new ArrayList<String>();
        // Primary physical SD-CARD (not emulated)
        final String rawExternalStorage = System.getenv("EXTERNAL_STORAGE");
        // All Secondary SD-CARDs (all exclude primary) separated by ":"
        final String rawSecondaryStoragesStr = System.getenv("SECONDARY_STORAGE");
        // Primary emulated SD-CARD
        final String rawEmulatedStorageTarget = System.getenv("EMULATED_STORAGE_TARGET");
        if (TextUtils.isEmpty(rawEmulatedStorageTarget)) {
            // Device has physical external storage; use plain paths.
            if (TextUtils.isEmpty(rawExternalStorage)) {
                // EXTERNAL_STORAGE undefined; falling back to default.
                rv.add("/storage/sdcard0");
            } else {
                rv.add(rawExternalStorage);
            }
        } else {
            // Device has emulated storage; external storage paths should have
            // userId burned into them.
            final String rawUserId;
            if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR1) {
                rawUserId = "";
            } else {
                final String path = Environment.getExternalStorageDirectory().getAbsolutePath();
                final String[] folders = DIR_SEPARATOR.split(path);
                final String lastFolder = folders[folders.length - 1];
                boolean isDigit = false;
                try {
                    Integer.valueOf(lastFolder);
                    isDigit = true;
                } catch (NumberFormatException ignored) {
                }
                rawUserId = isDigit ? lastFolder : "";
            }
            // /storage/emulated/0[1,2,...]
            if (TextUtils.isEmpty(rawUserId)) {
                rv.add(rawEmulatedStorageTarget);
            } else {
                rv.add(rawEmulatedStorageTarget + File.separator + rawUserId);
            }
        }
        // Add all secondary storages
        if (!TextUtils.isEmpty(rawSecondaryStoragesStr)) {
            // All Secondary SD-CARDs splited into array
            final String[] rawSecondaryStorages = rawSecondaryStoragesStr.split(File.pathSeparator);
            Collections.addAll(rv, rawSecondaryStorages);
        }
        rootmode = Sp.getBoolean("rootmode", false);
        if (rootmode)
            rv.add("/");
        File usb = getUsbDrive();
        if (usb != null && !rv.contains(usb.getPath())) rv.add(usb.getPath());

        return rv;
    }

public File getUsbDrive() {
        File parent;
        parent = new File("/storage");

        try {
            for (File f : parent.listFiles()) {
                if (f.exists() && f.getName().toLowerCase().contains("usb") && f.canExecute()) {
                    return f;
                }
            }
        } catch (Exception e) {
        }
        parent = new File("/mnt/sdcard/usbStorage");
        if (parent.exists() && parent.canExecute())
            return (parent);
        parent = new File("/mnt/sdcard/usb_storage");
        if (parent.exists() && parent.canExecute())
            return parent;

        return null;
    }

综上分析，通过方法一和方法二都可以正确的获取内外sd卡路径，但方法一会存在以下问题：

1、API>=23 时方法一无效（暂未测试）

2、有些厂商的Rom改动太多，对相关原生API的支持存在问题，这时方法一可能会存在问题。

3、其他一些情况造成的原因（基本与2差不多，是ROM等因素造成的）

所以，在使用时建议使用方法二来获取内外置sd卡路径，在API 23（Android 6.0）之前使用getStorageDirectories() 应该也是OK的。