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) * <p/> * 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。
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