Environment.getExternalStorageState();
该方法用于Returns the current state of the primary "external" storage device.
返回值有以下常量：
`MEDIA_BAD_REMOVAL` Sd卡被卸载前已经被移除 。Storage state if the media was removed before it was unmounted.
`MEDIA_CHECKING` Sd卡正在进行磁盘检查。 Storage state if the media is present and being disk-checked.
`MEDIA_EJECTING` Storage state if the media is in the process of being ejected.
`MEDIA_MOUNTED` Sd卡存在，且具有读写权限。Storage state if the media is present and mounted at its mount point with read/write access.
`MEDIA_MOUNTED_READ_ONLY` Sd卡存在，仅具有读权限。Storage state if the media is present and mounted at its mount point with read-only access.
`MEDIA_NOFS` Sd卡存在，但是空白，或者正在使用不受支持的文件系统。Storage state if the media is present but is blank or is using an unsupported filesystem.
`MEDIA_REMOVED` Sd卡被移除。Storage state if the media is not present.
`MEDIA_SHARED` Sd卡未安装，通过USB大容量分享。Storage state if the media is present not mounted, and shared via USB mass storage.
`MEDIA_UNKNOWN` 未知状态。Unknown storage state, such as when a path isn't backed by known storage media.
`MEDIA_UNMOUNTABLE` Sd卡存在，但不能被安装。Storage state if the media is present but cannot be mounted.
`MEDIA_UNMOUNTED` Sd卡存在，但未安装。Storage state if the media is present but not mounted.
Environment.getExternalStorageState(new File(new URI("123")));
该方法用于Returns the current state of the storage device that provides the given path.
Environment.getStorageState(new File(new URI("123")));
该方法已过时，用上一个方法代替。
Environment.getDataDirectory();
该方法用于Return the user data directory.返回值为File类型。
Environment.getDownloadCacheDirectory();
该方法用于Return the download/cache content directory.返回值为File类型。
非常重要Environment.getExternalStorageDirectory();

该方法用于Return the primary external storage directory.返回值为File类型。 当用户将手机和电脑连接在一起，此时存储设备被移除了，或者其他问题，返回的目录有可能是不能用的，需判定当前状态。此时不要对External一词感到疑惑，仅指该目录可以更好的存储用于分享的内容媒介。 它是一个可以持有想当多的数据的文件系统，该系统可以被所有应用访问但不需要任何权限。一般来说它是Sd卡，但“外部存储”也可以使内置的存储设备来实现，此时“外部存储”是区别于受保护的内部存储的， 并且“外部存储”可以作为一个文件系统安装在电脑上。

在具有多用户的设备上(由UserManager描述)，每一个用户有它们自己独立的外部存储。应用程序们在运行的时候可以仅访问用户的外部存储。具有多个外部存储目录的设备，一个外部存储目录意味着一个用户可以交互的主要的外部存储。为了避免玷污该用户的根命名空间，因此应用程序不应该直接使用这个顶级目录，可以访问(使用)次级存储目录。对于一个应用程序而言，任何私有的文件都应该存储在由Context.getExternalFilesDir方法返回的目录下，当该应用被卸载的时候，系统将考虑删除这个文件。其他共享型的文件则应该存放于由Environment类的getExternalStoragePublicDirectory(String)方法返回的目录下。拥有写的权限就默认拥有读的权限。从Android的KITKAT版本开始，如果我们的应用程序仅需存储内部数据，应该使用Context的getExternalFilesDir(String)方法与getExternalCacheDir()方法，这不需要读与写的权限。这两个方法返回的路径在不同的设备平台上可能会发生改变，因此应用程序应该仅仅持有相对路径。

On devices with multiple users (as described by UserManager), each user has their own isolated external storage. Applications only have access to the external storage for the user they're running as.

非常重要Environment.getExternalStoragePublicDirectory(Environment.TYPE);

该方法用于Get a top-level public external storage directory for placing files of a particular type. 返回值为File类型。

获得一个用于存储一个特殊类型文件的顶级公开外部存储目录。用户通常在这个目录存储管理它们自己的文件。因此我们应该考虑放在此处的文件内容，以确保我们不会删除他们的文件或者阻碍他们自己的管理。

参数主要有:

DIRECTORY_ALARMS 闹钟铃声文件类型。Standard directory in which to place any audio files that should be in the list of alarms that the user can select (not as regular music).
DIRECTORY_DCIM 相机的图片或视频类型。The traditional location for pictures and videos when mounting the device as a camera.
DIRECTORY_DOCUMENTS 文档类型。Standard directory in which to place documents that have been created by the user.
DIRECTORY_DOWNLOADS 用户下载的文件。Standard directory in which to place files that have been downloaded by the user.
DIRECTORY_MOVIES 用户可用的电影。Standard directory in which to place movies that are available to the user.
DIRECTORY_MUSIC 标准音乐文件。Standard directory in which to place any audio files that should be in the regular list of music for the user.
DIRECTORY_NOTIFICATIONS 音频文件通知铃声。Standard directory in which to place any audio files that should be in the list of notifications that the user can select (not as regular music).
DIRECTORY_PICTURES 用户可用的图片Standard directory in which to place pictures that are available to the user.
DIRECTORY_PODCASTS 播客音频。Standard directory in which to place any audio files that should be in the list of podcasts that the user can select (not as regular music).
DIRECTORY_RINGTONES 手机铃声音频。Standard directory in which to place any audio files that should be in the list of ringtones that the user can select (not as regular music).
Environment.getRootDirectory();
该方法用于Return root of the "system" partition holding the core Android OS. 返回值为File类型。