== Using the file system with Vert.x

The Vert.x {@link io.vertx.core.file.FileSystem} object provides many operations for manipulating the file system.

There is one file system object per Vert.x instance, and you obtain it with  {@link io.vertx.core.Vertx#fileSystem()}.

A blocking and a non blocking version of each operation is provided. The non blocking versions take a handler
which is called when the operation completes or an error occurs.

Here's an example of an asynchronous copy of a file:

[source,$lang]
----
{@link examples.FileSystemExamples#example1}
----
The blocking versions are named `xxxBlocking` and return the results or throw exceptions directly. In many
cases, depending on the operating system and file system, some of the potentially blocking operations can return
quickly, which is why we provide them, but it's highly recommended that you test how long they take to return in your
particular application before using them from an event loop, so as not to break the Golden Rule.

Here's the copy using the blocking API:

[source,$lang]
----
{@link examples.FileSystemExamples#example2}
----

Many operations exist to copy, move, truncate, chmod and many other file operations. We won't list them all here,
please consult the {@link io.vertx.core.file.FileSystem API docs} for the full list.

Let's see a couple of examples using asynchronous methods:

[source,$lang]
----
{@link examples.FileSystemExamples#asyncAPIExamples}
----

=== Asynchronous files

Vert.x provides an asynchronous file abstraction that allows you to manipulate a file on the file system.

You open an {@link io.vertx.core.file.AsyncFile AsyncFile} as follows:

[source,$lang]
----
{@link examples.FileSystemExamples#example3}
----

`AsyncFile` implements `ReadStream` and `WriteStream` so you can _pipe_
files to and from other stream objects such as net sockets, http requests and responses, and WebSockets.

They also allow you to read and write directly to them.

==== Random access writes

To use an `AsyncFile` for random access writing you use the
{@link io.vertx.core.file.AsyncFile#write(io.vertx.core.buffer.Buffer,long,io.vertx.core.Handler) write} method.

The parameters to the method are:

* `buffer`: the buffer to write.
* `position`: an integer position in the file where to write the buffer. If the position is greater or equal to the size
 of the file, the file will be enlarged to accommodate the offset.
* `handler`: the result handler

Here is an example of random access writes:

[source,$lang]
----
{@link examples.FileSystemExamples#asyncFileWrite}
----

==== Random access reads

To use an `AsyncFile` for random access reads you use the
{@link io.vertx.core.file.AsyncFile#read(io.vertx.core.buffer.Buffer,int,long,int,io.vertx.core.Handler) read}
method.

The parameters to the method are:

* `buffer`: the buffer into which the data will be read.
* `offset`: an integer offset into the buffer where the read data will be placed.
* `position`: the position in the file where to read data from.
* `length`: the number of bytes of data to read
* `handler`: the result handler

Here's an example of random access reads:

[source,$lang]
----
{@link examples.FileSystemExamples#asyncFileRead}
----

==== Opening Options

When opening an `AsyncFile`, you pass an {@link io.vertx.core.file.OpenOptions OpenOptions} instance.
These options describe the behavior of the file access. For instance, you can configure the file permissions with the
{@link io.vertx.core.file.OpenOptions#setRead(boolean)}, {@link io.vertx.core.file.OpenOptions#setWrite(boolean)}
and {@link io.vertx.core.file.OpenOptions#setPerms(java.lang.String)} methods.

You can also configure the behavior if the open file already exists with
{@link io.vertx.core.file.OpenOptions#setCreateNew(boolean)} and
{@link io.vertx.core.file.OpenOptions#setTruncateExisting(boolean)}.

You can also mark the file to be deleted on
close or when the JVM is shutdown with {@link io.vertx.core.file.OpenOptions#setDeleteOnClose(boolean)}.

==== Flushing data to underlying storage.

In the `OpenOptions`, you can enable/disable the automatic synchronisation of the content on every write using
{@link io.vertx.core.file.OpenOptions#setDsync(boolean)}. In that case, you can manually flush any writes from the OS
cache by calling the {@link io.vertx.core.file.AsyncFile#flush()} method.

This method can also be called with a handler which will be called when the flush is complete.

==== Using AsyncFile as ReadStream and WriteStream

`AsyncFile` implements `ReadStream` and `WriteStream`. You can then
use them with a _pipe_ to pipe data to and from other read and write streams. For example, this would
copy the content to another `AsyncFile`:

[source,$lang]
----
{@link examples.FileSystemExamples#asyncFilePipe}
----

You can also use the _pipe_ to write file content into HTTP responses, or more generally in any
`WriteStream`.

[[classpath]]
==== Accessing files from the classpath

When vert.x cannot find the file on the filesystem it tries to resolve the
file from the class path. Note that classpath resource paths never start with
a `/`.

Due to the fact that Java does not offer async access to classpath
resources, the file is copied to the filesystem in a worker thread when the
classpath resource is accessed the very first time and served from there
asynchronously. When the same resource is accessed a second time, the file from
the filesystem is served directly from the filesystem. The original content
is served even if the classpath resource changes (e.g. in a development
system).

This caching behaviour can be set on the {@link io.vertx.core.file.FileSystemOptions#setFileCachingEnabled(boolean)}
option. The default value of this option is `true` unless the system property `vertx.disableFileCaching` is
defined.

The path where the files are cached is `/tmp/vertx-cache-UUID` by default and can be customized by setting the system
property `vertx.cacheDirBase`. When using this property, note that it should refer to a directory prefix in a process
read/writeable location, for example: `-Dvertx.cacheDirBase=/tmp/my-vertx-cache` (Note that there's no UUID).

Each vert.x process will append it's own UUID in order to keep caches independently of different applications running
on the same machine.

The whole classpath resolving feature can be disabled system-wide by setting the system
property `vertx.disableFileCPResolving` to `true`.

NOTE: these system properties are evaluated once when the `io.vertx.core.file.FileSystemOptions` class is loaded, so
these properties should be set before loading this class or as a JVM system property when launching it.

If you want to disable classpath resolving for a particular application but keep it enabled by default system-wide,
you can do so via the {@link io.vertx.core.file.FileSystemOptions#setClassPathResolvingEnabled(boolean)} option.

==== Closing an AsyncFile

To close an `AsyncFile` call the {@link io.vertx.core.file.AsyncFile#close()} method. Closing is asynchronous and
if you want to be notified when the close has been completed you can specify a handler function as an argument.
