sftp-fs

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An SFTP NIO.2 file system

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# sftp-fs
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The `sftp-fs` library provides support for SFTP NIO.2 file systems, allowing SFTP servers to be accessed in a similar way to local file systems.

## Creating file systems

If the SFTP file system library is available on the class path, it will register a [FileSystemProvider](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/spi/FileSystemProvider.html) for scheme `sftp`. This allows you to create SFTP file systems using the `newFileSystem` methods of class [FileSystems](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/FileSystems.html). You can use class [SFTPEnvironment](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html) to help create the environment maps for those methods:

```java
SFTPEnvironment env = new SFTPEnvironment()
        .withUsername(username)
        .withUserInfo(userInfo);
FileSystem fs = FileSystems.newFileSystem(URI.create("sftp://example.org"), env);
```

### Providing credentials

Credentials can be provided either through the URI or through the environment, as shown above. For security reasons the latter is preferred.

There are a few ways to provide credentials through the environment. The easiest way is to call [withUserInfo](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html#withUserInfo%28com.jcraft.jsch.UserInfo%29) on an [SFTPEnvironment](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html) instance with a [UserInfo](https://javadoc.io/doc/com.github.mwiede/jsch/latest/com.jcraft.jsch/com/jcraft/jsch/UserInfo.html) object to provide the password, and if necessary the passphrase. The `sftp-fs` library provides class [SimpleUserInfo](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SimpleUserInfo.html) to provide a fixed password and passphrase. Note that any prompt will be answered with "no". That means that you will most likely also need to set a [HostKeyRepository](https://javadoc.io/doc/com.github.mwiede/jsch/latest/com.jcraft.jsch/com/jcraft/jsch/HostKeyRepository.html) by calling [withHostKeyRepository](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html#withHostKeyRepository%28com.jcraft.jsch.HostKeyRepository%29) on an [SFTPEnvironment](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html) instance to verify the host.

If the [UserInfo](https://javadoc.io/doc/com.github.mwiede/jsch/latest/com.jcraft.jsch/com/jcraft/jsch/UserInfo.html) object will only provide a fixed password and do nothing else, you can also call [withPassword](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html#withPassword%28char%5B%5D%29) on the [SFTPEnvironment](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html) instance instead. This too most likely requires a [HostKeyRepository](https://javadoc.io/doc/com.github.mwiede/jsch/latest/com.jcraft.jsch/com/jcraft/jsch/HostKeyRepository.html) to be set.

Instead of using a password, it's also possible to use other authentication methods using an [IdentityRepository](https://javadoc.io/doc/com.github.mwiede/jsch/latest/com.jcraft.jsch/com/jcraft/jsch/IdentityRepository.html), which can be set by calling [withIdentityRepository](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html#withIdentityRepository%28com.jcraft.jsch.IdentityRepository%29) on an [SFTPEnvironment](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html) instance. The following code snippet shows how you can use pageant to authenticate:

```java
SFTPEnvironment env = new SFTPEnvironment()
        .withUsername(username)
        .withIdentityRepository(new AgentIdentityRepository(new PageantConnector()));
FileSystem fs = FileSystems.newFileSystem(URI.create("sftp://example.org"), env);
```

### Default directory

The default directory can be provided through the URI or trough the environment using [withDefaultDirectory](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html#withDefaultDirectory%28java.lang.String%29), as follows:

| URI path | No default directory in the environment             | Default directory in the environment                |
|----------|-----------------------------------------------------|-----------------------------------------------------|
| None     | The default directory is defined by the SFTP server | The default directory is defined by the environment |
| `/`      | The default directory is `/`                        | The default directory is defined by the environment |
| Other    | The default directory is equal to the URI path      | Not allowed                                         |

## Creating paths

After a file system has been created, [Paths](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/Path.html) can be created through the file system itself using its [getPath](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/FileSystem.html#getPath%28java.lang.String,java.lang.String...%29) method. As long as the file system is not closed, it's also possible to use [Paths.get](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/Paths.html#get%28java.net.URI%29). Note that if the file system was created with credentials, the username must be part of the URL. For instance:

```java
// created without credentials
Path path1 = Paths.get(URI.create("sftp://example.org"));
// created with credentials
Path path2 = Paths.get(URI.create("sftp://username@example.org"));
```

If the username in the URI does not match the username used to create the file system, or if no file system has been created for the URI, a new file system will be created. This works like [Creating file systems](#Creating_file_systems). Since no environment can be provided this way, settings can still be provided through [SFTPEnvironment.setDefault](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html#setDefault%28com.github.robtimus.filesystems.sftp.SFTPEnvironment%29) and query parameters; see usages of [QueryParam](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/class-use/SFTPEnvironment.QueryParam.html) and [QueryParams](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/class-use/SFTPEnvironment.QueryParams.html) for the possible query parameters. If creating a new file system fails, a [FileSystemNotFoundException](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/FileSystemNotFoundException.html) will be thrown.

## Attributes

### File attributes

SFTP file systems fully support read-access to the following attributes:

* Attributes defined in [BasicFileAttributeView](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/attribute/BasicFileAttributeView.html) and [BasicFileAttributes](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/attribute/BasicFileAttributes.html), available both with and without prefixes `basic:` or `posix:`.
* Attributes defined in [FileOwnerAttributeView](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/attribute/FileOwnerAttributeView.html), available with prefixes `owner:` or `posix:`.
* Attributes defined in [PosixFileAttributeView](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/attribute/PosixFileAttributeView.html) and [PosixFileAttributes](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/attribute/PosixFileAttributes.html), available with prefix `posix:`.

It's not possible to set the last access time or creation time, either through one of the file attribute views or through a file system. Attempting to do so will result in an [UnsupportedOperationException](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/UnsupportedOperationException.html). All other attributes are supported, although when setting the owner or group the name must be the UID/GID.

### File store attributes

When calling [getAttribute](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/FileStore.html#getAttribute%28java.lang.String%29) on a file store, the following attributes are supported:

* `totalSpace`: returns the same value as the [getTotalSpace](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/FileStore.html#getTotalSpace%28%29) method.
* `usableSpace`: returns the same value as the [getUsableSpace](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/FileStore.html#getUsableSpace%28%29) method.
* `unallocatedSpace`: returns the same value as the [getUnallocatedSpace](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/FileStore.html#getUnallocatedSpace%28%29) method.

These values are only supported by SFTP servers that support the `statvfs@openssh.com` extension. If this extension is not supported, these methods will all return `Long.MAX_VALUE`.

There is no support for [FileStoreAttributeView](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/attribute/FileStoreAttributeView.html). Calling [getFileStoreAttributeView](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/FileStore.html#getFileStoreAttributeView%28java.lang.Class%29) on a file store will simply return `null`.

## Error handling

Not all SFTP servers support the same set of error codes. Because of this, most methods may not throw the correct exception ([NoSuchFileException](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/NoSuchFileException.html), [AccessDeniedException](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/AccessDeniedException.html), etc).

To allow this behaviour to be modified, class [SFTPEnvironment](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html) has method [withFileSystemExceptionFactory](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html#withFileSystemExceptionFactory%28com.github.robtimus.filesystems.sftp.FileSystemExceptionFactory%29) that allows you to specify a custom [FileSystemExceptionFactory](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/FileSystemExceptionFactory.html) implementation which will be used to create exceptions based on the reply code and string of the command that triggered the error. By default, an instance of class [DefaultFileSystemExceptionFactory](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/DefaultFileSystemExceptionFactory.html) is used.

## Thread safety

The SFTP protocol is fundamentally not thread safe. To overcome this limitation, SFTP file systems maintain multiple connections to SFTP servers. The number of connections determines the number of concurrent operations that can be executed. If all connections are busy, a new operation will block until a connection becomes available. Class [SFTPEnvironment](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html) has method [withPoolConfig](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPEnvironment.html#withPoolConfig%28com.github.robtimus.filesystems.sftp.SFTPPoolConfig%29) that allows you to configure the connection pool:

* The initial pool size - the number of connections that are created when an SFTP file system is created. The default is `1`.
* The maximum pool size - the maximum number of concurrent operations. The default is `5`.
* The maximum wait time - this determines how long to wait until a connection is available. The default is to wait indefinitely.
* The maximum time that connections can be idle. The default is indefinitely.

When a stream or channel is opened for reading or writing, the connection will block because it will wait for the download or upload to finish. This will not occur until the stream or channel is closed. It is therefore advised to close streams and channels as soon as possible.

## Connection management

Because SFTP file systems use multiple connections to an SFTP server, it's possible that one or more of these connections become stale. Class [SFTPFileSystemProvider](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPFileSystemProvider.html) has static method [keepAlive](https://robtimus.github.io/sftp-fs/apidocs/com.github.robtimus.filesystems.sftp/com/github/robtimus/filesystems/sftp/SFTPFileSystemProvider.html#keepAlive%28java.nio.file.FileSystem%29) that, if given an instance of an SFTP file system, will send a keep-alive signal over each of its idle connections. You should ensure that this method is called on a regular interval. An alternative is to set a maximum idle time (see [Thread safety](#Thread_safety)).

## Limitations

SFTP file systems knows the following limitations:

* All paths use `/` as separator. `/` is not allowed inside file or directory names.
* File attributes cannot be set when creating files or directories.
* Symbolic links can be read and traversed, but not created.
* There is no support for hard links.
* Files can be marked as executable if the SFTP server indicates it is. That does not mean the file can be executed in the local JVM.
* [SeekableByteChannel](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/channels/SeekableByteChannel.html) is supported because it's used by [Files.createFile](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/Files.html#createFile%28java.nio.file.Path,java.nio.file.attribute.FileAttribute...%29). However, these channels do not support seeking specific positions or truncating.
* [FileSystem.getFileStores()](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/FileSystem.html#getFileStores%28%29) will only return a [FileStore](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/FileStore.html) for the root path, even if the SFTP server actually has several mount points.
* Although [FileSystemProvider.getFileStore(Path)](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/spi/FileSystemProvider.html#getFileStore%28java.nio.file.Path%29) will return a [FileStore](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/FileStore.html) for the actual [Path](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/Path.html), its name will always be `/`, even if the file or directory is located on a different mount point.
* There is no support for [UserPrincipalLookupService](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/attribute/UserPrincipalLookupService.html).
* There is no support for [WatchService](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/WatchService.html).

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