//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。

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/0externalForVold.add(buildPath(emulatedSourceBase, rawUserId));externalForApp.add(buildPath(emulatedTargetBase, rawUserId));// /data/media/0mEmulatedDirForDirect = 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/sdcard0externalForVold.add(new File(rawExternalStorage));externalForApp.add(new File(rawExternalStorage));// /data/mediamEmulatedDirForDirect = new File(rawMediaStorage);}// Splice in any secondary storage paths, but only for ownerfinal 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 pathsfinal 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-CARDfinal 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 storagesif (!TextUtils.isEmpty(rawSecondaryStoragesStr)) {// All Secondary SD-CARDs splited into arrayfinal 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。