17.11 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.
17.11.1 The @MultipartConfig Annotation
The @MultipartConfig annotation supports the following optional attributes.
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location: An absolute path to a directory on the file system. Thelocationattribute 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 specifiedfileSizeThresholdsetting. 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 amultipart/form-datarequest, 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>
17.11.2 The getParts and getPart Methods
The Servlet specification supports two additional HttpServletRequest methods:
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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:
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Retrieve the name, size, and content-type of the
Part -
Query the headers submitted with a
Part -
Delete a
Part -
Write a
Partout 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.
