Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial

## Uploading Files with Java Servlet Technology

Supporting file uploads is a very basic and common requirement for many web applications. In prior versions of the Servlet specification, implementing file upload required the use of external libraries or complex input processing. The Java Servlet specification now helps to provide a viable solution to the problem in a generic and portable way. Java Servlet technology now supports file upload out of the box, so any web container that implements the specification can parse multipart requests and make mime attachments available through the `HttpServletRequest` object.

A new annotation, `javax.servlet.annotation.MultipartConfig`, is used to indicate that the servlet on which it is declared expects requests to be made using the `multipart/form-data` MIME type. Servlets that are annotated with `@MultipartConfig` can retrieve the `Part` components of a given `multipart/form-data` request by calling the `request.getPart(String name)` or `request.getParts()` method.

### The @MultipartConfig Annotation

The `@MultipartConfig` annotation supports the following optional attributes.

- `location`: An absolute path to a directory on the file system. The `location` attribute does not support a path relative to the application context. This location is used to store files temporarily while the parts are processed or when the size of the file exceeds the specified `fileSizeThreshold` setting. The default location is `""`.

- `fileSizeThreshold`: The file size in bytes after which the file will be temporarily stored on disk. The default size is 0 bytes.

- `MaxFileSize`: The maximum size allowed for uploaded files, in bytes. If the size of any uploaded file is greater than this size, the web container will throw an exception (`IllegalStateException`). The default size is unlimited.

- `maxRequestSize`: The maximum size allowed for a `multipart/form-data` request, in bytes. The web container will throw an exception if the overall size of all uploaded files exceeds this threshold. The default size is unlimited.

For, example, the `@MultipartConfig` annotation could be constructed as follows:

    @MultipartConfig(location="/tmp", fileSizeThreshold=1024*1024,
        maxFileSize=1024*1024*5, maxRequestSize=1024*1024*5*5)

Instead of using the `@MultipartConfig` annotation to hard-code these attributes in your file upload servlet, you could add the following as a child element of the servlet configuration element in the `web.xml` file:

    <multipart-config>
        <location>/tmp</location>
        <max-file-size>20848820</max-file-size>
        <max-request-size>418018841</max-request-size>
        <file-size-threshold>1048576</file-size-threshold>
    </multipart-config>

### The getParts and getPart Methods

The Servlet specification supports two additional `HttpServletRequest` methods:

- `Collection<Part> getParts()`

- `Part getPart(String name)`

The `request.getParts()` method returns collections of all `Part` objects. If you have more than one input of type file, multiple `Part` objects are returned. Because `Part` objects are named, the `getPart(String name)` method can be used to access a particular `Part`. Alternatively, the `getParts()` method, which returns an `Iterable<Part>`, can be used to get an `Iterator` over all the `Part` objects.

The `javax.servlet.http.Part` interface is a simple one, providing methods that allow introspection of each `Part`. The methods do the following:

- Retrieve the name, size, and content-type of the `Part`

- Query the headers submitted with a `Part`

- Delete a `Part`

- Write a `Part` out to disk

For example, the `Part` interface provides the `write(String filename)` method to write the file with the specified name. The file can then be saved in the directory that is specified with the `location` attribute of the `@MultipartConfig` annotation or, in the case of the `fileupload` example, in the location specified by the Destination field in the form.

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