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An FTP(S) NIO.2 file system

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

## Creating file systems

If the FTP file system library is available on the class path, it will register [FileSystemProviders](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/spi/FileSystemProvider.html) for schemes `ftp` and `ftps`. This allows you to create FTP and FTPS 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 classes [FTPEnvironment](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPEnvironment.html) and [FTPSEnvironment](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPSEnvironment.html) to help create the environment maps for those methods:

```java
FTPEnvironment env = new FTPEnvironment()
        .withCredentials(username, password);
FileSystem fs = FileSystems.newFileSystem(URI.create("ftp://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.

### Default directory

The default directory can be provided through the URI or trough the environment using [withDefaultDirectory](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPEnvironment.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 FTP 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("ftp://example.org"));
// created with credentials
Path path2 = Paths.get(URI.create("ftp://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 [FTPEnvironment.setDefault](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPEnvironment.html#setDefault%28com.github.robtimus.filesystems.ftp.FTPEnvironment%29) or [FTPSEnvironment.setDefault](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPSEnvironment.html#setDefault%28com.github.robtimus.filesystems.ftp.FTPSEnvironment%29) and query parameters; see usages of [QueryParam](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/class-use/FTPEnvironment.QueryParam.html) and [QueryParams](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/class-use/FTPEnvironment.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

FTP 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:`.

Attempting to set any of these attributes, either through one of the file attribute views or through a file system, will result in an [UnsupportedOperationException](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/UnsupportedOperationException.html).

### 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.

Because FTP servers do not return these values, 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`.

## FTPS support

To create an FTPS connection instead of an FTP connection, use `ftps` as the scheme. Also, use class [FTPSEnvironment](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPSEnvironment.html) instead of class [FTPEnvironment](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPEnvironment.html) to create the file system. Using an [FTPEnvironment](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPEnvironment.html) instance is still allowed, but you will not be able to specify FTPS specific properties.

## Error handling

Unfortunately, FTP servers can use the same code for multiple erroneous situations. For example, code 550 can indicate that a file does not exist, or that access to an existing file is not allowed. Because of this, most methods do 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 [FTPEnvironment](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPEnvironment.html) has method [withFileSystemExceptionFactory](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPEnvironment.html#withFileSystemExceptionFactory%28com.github.robtimus.filesystems.ftp.FileSystemExceptionFactory%29) that allows you to specify a custom [FileSystemExceptionFactory](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/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/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/DefaultFileSystemExceptionFactory.html) is used.

The `ftp-fs` library provides subclasses for [FileSystemException](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/FileSystemException.html) and several of its subclasses to allow the FTP server's reply code and string to be reserved. Instances of these classes can be returned by [FileSystemExceptionFactory](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FileSystemExceptionFactory.html) implementations as needed.

## Thread safety

The FTP protocol is fundamentally not thread safe. To overcome this limitation, FTP file systems maintain multiple connections to FTP 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 [FTPEnvironment](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPEnvironment.html) has method [withPoolConfig](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPEnvironment.html#withPoolConfig%28com.github.robtimus.filesystems.ftp.FTPPoolConfig%29) that allows you to configure the connection pool:

* The initial pool size - the number of connections that are created when an FTP 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 FTP file systems use multiple connections to an FTP server, it's possible that one or more of these connections become stale. Class [FTPFileSystemProvider](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPFileSystemProvider.html) has static method [keepAlive](https://robtimus.github.io/ftp-fs/apidocs/com.github.robtimus.filesystems.ftp/com/github/robtimus/filesystems/ftp/FTPFileSystemProvider.html#keepAlive%28java.nio.file.FileSystem%29) that, if given an instance of an FTP file system, will send a keep-alive signal (NOOP) 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

FTP file systems knows the following limitations:

* All paths use `/` as separator. `/` is not allowed inside file or directory names.
* File attributes cannot be set, either when creating files or directories or afterwards.
* 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 FTP 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.
* 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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