xdocs/using.xml - commons-fileupload - Git at Google

 <?xml version="1.0"?>
 <!--
    Copyright 2003-2004 The Apache Software Foundation

    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
    You may obtain a copy of the License at

        http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing, software
    distributed under the License is distributed on an "AS IS" BASIS,
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
    limitations under the License.
 -->

 <document>

   <properties>
     <title>Using FileUpload</title>
     <author email="martinc@apache.org">Martin Cooper</author>
   </properties>

 <body>

   <section name="Using FileUpload">
     <p>
       FileUpload can be used in a number of different ways, depending upon the
       requirements of your application. In the simplest case, you will call a
       single method to parse the servlet request, and then process the list of
       items as they apply to your application. At the other end of the scale,
       you might decide to customize FileUpload to take full control of the way
       in which individual items are stored; for example, you might decide to
       stream the content into a database.
     </p>
     <p>
       Here, we will describe the basic principles of FileUpload, and illustrate
       some of the simpler - and most common - usage patterns. Customization of
       FileUpload is described <a href="customizing.html">elsewhere</a>.
     </p>
   </section>

   <section name="How it works">
     <p>
       A file upload request comprises an ordered list of <em>items</em> that
       are encoded according to
       <a href="http://www.ietf.org/rfc/rfc1867.txt">RFC 1867</a>,
       "Form-based File Upload in HTML". FileUpload can parse such a request
       and provide your application with a list of the individual uploaded
       items. Each such item implements the <code>FileItem</code> interface,
       regardless of its underlying implementation.
     </p>
     <p>
       Each file item has a number of properties that might be of interest for
       your application. For example, every item has a name and a content type,
       and can provide an <code>InputStream</code> to access its data. On the
       other hand, you may need to process items differently, depending upon
       whether the item is a regular form field - that is, the data came from
       an ordinary text box or similar HTML field - or an uploaded file. The
       <code>FileItem</code> interface provides the methods to make such a
       determination, and to access the data in the most appropriate manner.
     </p>
     <p>
       FileUpload creates new file items using a <code>FileItemFactory</code>.
       This is what gives FileUpload most of its flexibility. The factory has
       ultimate control over how each item is created. The factory implementation
       that currently ships with FileUpload stores the item's data in memory or
       on disk, depending on the size of the item (i.e. bytes of data). However,
       this behavior can be customized to suit your application.
     </p>
   </section>

   <section name="Servlets and Portlets">
     <p>
       Starting with version 1.1, FileUpload supports file upload requests in
       both servlet and portlet environments. The usage is almost identical in
       the two environments, so the remainder of this document refers only to
       the servlet environment.
     </p>
     <p>
       If you are building a portlet application, the following are the two
       distinctions you should make as you read this document:
       <ul>
         <li>
           Where you see references to the <code>ServletFileUpload</code> class,
           substitute the <code>PortletFileUpload</code> class.
         </li>
         <li>
           Where you see references to the <code>HttpServletRequest</code> class,
           substitute the <code>ActionRequest</code> class.
         </li>
       </ul>
     </p>
   </section>

   <section name="Parsing the request">
     <p>
       Before you can work with the uploaded items, of course, you need to parse
       the request itself. Ensuring that the request is actually a file upload
       request is straightforward, but FileUpload makes it simplicity itself, by
       providing a static method to do just that.
     </p>
 <source><![CDATA[// Check that we have a file upload request
 boolean isMultipart = FileUpload.isMultipartContent(request);]]></source>
       <p>
         Now we are ready to parse the request into its constituent items.
       </p>
     <subsection name="The simplest case">
       <p>
         The simplest usage scenario is the following:
         <ul>
           <li>
             Uploaded items should be retained in memory as long as they are
             reasonably small.
           </li>
           <li>
             Larger items should be written to a temporary file on disk.
           </li>
           <li>
             Very large upload requests should not be permitted.
           </li>
           <li>
             The built-in defaults for the maximum size of an item to
             be retained in memory, the maximum permitted size of an upload
             request, and the location of temporary files are acceptable.
           </li>
         </ul>
       </p>
       <p>
         Handling a request in this scenario couldn't be much simpler:
       </p>
 <source><![CDATA[// Create a factory for disk-based file items
 FileItemFactory factory = new DiskFileItemFactory();

 // Create a new file upload handler
 ServletFileUpload upload = new ServletFileUpload(factory);

 // Parse the request
 List /* FileItem */ items = upload.parseRequest(request);]]></source>
       <p>
         That's all that's needed. Really!
       </p>
       <p>
         The result of the parse is a <code>List</code> of file items, each of
         which implements the <code>FileItem</code> interface. Processing these
         items is discussed below.
       </p>
     </subsection>

     <subsection name="Exercising more control">
       <p>
         If your usage scenario is close to the simplest case, described above,
         but you need a little more control, you can easily customize the
         behavior of the upload handler or the file item factory or both. The
         following example shows several configuration options:
       </p>
 <source><![CDATA[// Create a factory for disk-based file items
 DiskFileItemFactory factory = new DiskFileItemFactory();

 // Set factory constraints
 factory.setSizeThreshold(yourMaxMemorySize);
 factory.setRepositoryPath(yourTempDirectory);

 // Create a new file upload handler
 ServletFileUpload upload = new ServletFileUpload(factory);

 // Set overall request size constraint
 upload.setSizeMax(yourMaxRequestSize);

 // Parse the request
 List /* FileItem */ items = upload.parseRequest(request);]]></source>
       <p>
         Of course, each of the configuration methods is independent of the
         others, but if you want to configure the factory all at once, you can
         do that with an alternative constructor, like this:
       </p>
 <source><![CDATA[// Create a factory for disk-based file items
 DiskFileItemFactory factory = new DiskFileItemFactory(
         yourMaxMemorySize, yourTempDirectory);]]></source>
       <p>
         Should you need further control over the parsing of the request, such
         as storing the items elsewhere - for example, in a database - you will
         need to look into <a href="customizing.html">customizing</a> FileUpload.
       </p>
     </subsection>
   </section>

   <section name="Processing the uploaded items">
     <p>
       Once the parse has completed, you will have a <code>List</code> of file
       items that you need to process. In most cases, you will want to handle
       file uploads differently from regular form fields, so you might process
       the list like this:
     </p>
 <source><![CDATA[// Process the uploaded items
 Iterator iter = items.iterator();
 while (iter.hasNext()) {
     FileItem item = (FileItem) iter.next();

     if (item.isFormField()) {
         processFormField(item);
     } else {
         processUploadedFile(item);
     }
 }]]></source>
     <p>
       For a regular form field, you will most likely be interested only in the
       name of the item, and its <code>String</code> value. As you might expect,
       accessing these is very simple.
     </p>
 <source><![CDATA[// Process a regular form field
 if (item.isFormField()) {
     String name = item.getFieldName();
     String value = item.getString();
     ...
 }]]></source>
     <p>
       For a file upload, there are several different things you might want to
       know before you process the content. Here is an example of some of the
       methods you might be interested in.
     </p>
 <source><![CDATA[// Process a file upload
 if (!item.isFormField()) {
     String fieldName = item.getFieldName();
     String fileName = item.getName();
     String contentType = item.getContentType();
     boolean isInMemory = item.isInMemory();
     long sizeInBytes = item.getSize();
     ...
 }]]></source>
     <p>
       With uploaded files, you generally will not want to access them via
       memory, unless they are small, or unless you have no other alternative.
       Rather, you will want to process the content as a stream, or write the
       entire file to its ultimate location. FileUpload provides simple means of
       accomplishing both of these.
     </p>
 <source><![CDATA[// Process a file upload
 if (writeToFile) {
     File uploadedFile = new File(...);
     item.write(uploadedFile);
 } else {
     InputStream uploadedStream = item.getInputStream();
     ...
     uploadedStream.close();
 }]]></source>
     <p>
       Note that, in the default implementation of FileUpload, <code>write()</code>
       will attempt to rename the file to the specified destination, if the data
       is already in a temporary file. Actually copying the data is only done if
       the the rename fails, for some reason, or if the data was in memory.
     </p>
     <p>
       If you do need to access the uploaded data in memory, you need simply
       call the <code>get()</code> method to obtain the data as an array of
       bytes.
     </p>
 <source><![CDATA[// Process a file upload in memory
 byte[] data = item.get();
 ...]]></source>
   </section>

   <section name="Interaction with virus scanners">
     <p>
       Virus scanners running on the same system as the web container can cause
       some unexpected behaviours for applications using FileUpload. This section
       describes some of the behaviours that you might encounter, and provides
       some ideas for how to handle them.
     </p>
     <p>
       The default implementation of FileUpload will cause uploaded items above
       a certain size threshold to be written to disk. As soon as such a file is
       closed, any virus scanner on the system will wake up and inspect it, and
       potentially quarantine the file - that is, move it to a special location
       where it will not cause problems. This, of course, will be a surprise to
       the application developer, since the uploaded file item will no longer be
       available for processing. On the other hand, uploaded items below that
       same threshold will be held in memory, and therefore will not be seen by
       virus scanners. This allows for the possibility of a virus being retained
       in some form (although if it is ever written to disk, the virus scanner
       would locate and inspect it).
     </p>
     <p>
       One commonly used solution is to set aside one directory on the system
       into which all uploaded files will be placed, and to configure the virus
       scanner to ignore that directory. This ensures that files will not be
       ripped out from under the application, but then leaves responsibility for
       virus scanning up to the application developer. Scanning the uploaded
       files for viruses can then be performed by an external process, which
       might move clean or cleaned files to an "approved" location, or by
       integrating a virus scanner within the application itself. The details of
       configuring an external process or integrating virus scanning into an
       application are outside the scope of this document.
     </p>
   </section>

   <section name="What's next">
     <p>
       Hopefully this page has provided you with a good idea of how to use
       FileUpload in your own applications. For more detail on the methods
       introduced here, as well as other available methods, you should refer
       to the <a href="apidocs/index.html">JavaDocs</a>.
     </p>
     <p>
       The usage described here should satisfy a large majority of file upload
       needs. However, should you have more complex requirements, FileUpload
       should still be able to help you, with it's flexible
       <a href="customizing.html">customization</a> capabilities.
     </p>
   </section>

 </body>
 </document>
	<?xml version="1.0"?>
	<!--
	Copyright 2003-2004 The Apache Software Foundation

	Licensed under the Apache License, Version 2.0 (the "License");
	you may not use this file except in compliance with the License.
	You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
	-->

	<document>

	<properties>
	<title>Using FileUpload</title>
	<author email="martinc@apache.org">Martin Cooper</author>
	</properties>

	<body>

	<section name="Using FileUpload">
	<p>
	FileUpload can be used in a number of different ways, depending upon the
	requirements of your application. In the simplest case, you will call a
	single method to parse the servlet request, and then process the list of
	items as they apply to your application. At the other end of the scale,
	you might decide to customize FileUpload to take full control of the way
	in which individual items are stored; for example, you might decide to
	stream the content into a database.
	</p>
	<p>
	Here, we will describe the basic principles of FileUpload, and illustrate
	some of the simpler - and most common - usage patterns. Customization of
	FileUpload is described <a href="customizing.html">elsewhere</a>.
	</p>
	</section>

	<section name="How it works">
	<p>
	A file upload request comprises an ordered list of <em>items</em> that
	are encoded according to
	<a href="http://www.ietf.org/rfc/rfc1867.txt">RFC 1867</a>,
	"Form-based File Upload in HTML". FileUpload can parse such a request
	and provide your application with a list of the individual uploaded
	items. Each such item implements the <code>FileItem</code> interface,
	regardless of its underlying implementation.
	</p>
	<p>
	Each file item has a number of properties that might be of interest for
	your application. For example, every item has a name and a content type,
	and can provide an <code>InputStream</code> to access its data. On the
	other hand, you may need to process items differently, depending upon
	whether the item is a regular form field - that is, the data came from
	an ordinary text box or similar HTML field - or an uploaded file. The
	<code>FileItem</code> interface provides the methods to make such a
	determination, and to access the data in the most appropriate manner.
	</p>
	<p>
	FileUpload creates new file items using a <code>FileItemFactory</code>.
	This is what gives FileUpload most of its flexibility. The factory has
	ultimate control over how each item is created. The factory implementation
	that currently ships with FileUpload stores the item's data in memory or
	on disk, depending on the size of the item (i.e. bytes of data). However,
	this behavior can be customized to suit your application.
	</p>
	</section>

	<section name="Servlets and Portlets">
	<p>
	Starting with version 1.1, FileUpload supports file upload requests in
	both servlet and portlet environments. The usage is almost identical in
	the two environments, so the remainder of this document refers only to
	the servlet environment.
	</p>
	<p>
	If you are building a portlet application, the following are the two
	distinctions you should make as you read this document:
	<ul>
	<li>
	Where you see references to the <code>ServletFileUpload</code> class,
	substitute the <code>PortletFileUpload</code> class.
	</li>
	<li>
	Where you see references to the <code>HttpServletRequest</code> class,
	substitute the <code>ActionRequest</code> class.
	</li>
	</ul>
	</p>
	</section>

	<section name="Parsing the request">
	<p>
	Before you can work with the uploaded items, of course, you need to parse
	the request itself. Ensuring that the request is actually a file upload
	request is straightforward, but FileUpload makes it simplicity itself, by
	providing a static method to do just that.
	</p>
	<source><![CDATA[// Check that we have a file upload request
	boolean isMultipart = FileUpload.isMultipartContent(request);]]></source>
	<p>
	Now we are ready to parse the request into its constituent items.
	</p>
	<subsection name="The simplest case">
	<p>
	The simplest usage scenario is the following:
	<ul>
	<li>
	Uploaded items should be retained in memory as long as they are
	reasonably small.
	</li>
	<li>
	Larger items should be written to a temporary file on disk.
	</li>
	<li>
	Very large upload requests should not be permitted.
	</li>
	<li>
	The built-in defaults for the maximum size of an item to
	be retained in memory, the maximum permitted size of an upload
	request, and the location of temporary files are acceptable.
	</li>
	</ul>
	</p>
	<p>
	Handling a request in this scenario couldn't be much simpler:
	</p>
	<source><![CDATA[// Create a factory for disk-based file items
	FileItemFactory factory = new DiskFileItemFactory();

	// Create a new file upload handler
	ServletFileUpload upload = new ServletFileUpload(factory);

	// Parse the request
	List /* FileItem */ items = upload.parseRequest(request);]]></source>
	<p>
	That's all that's needed. Really!
	</p>
	<p>
	The result of the parse is a <code>List</code> of file items, each of
	which implements the <code>FileItem</code> interface. Processing these
	items is discussed below.
	</p>
	</subsection>

	<subsection name="Exercising more control">
	<p>
	If your usage scenario is close to the simplest case, described above,
	but you need a little more control, you can easily customize the
	behavior of the upload handler or the file item factory or both. The
	following example shows several configuration options:
	</p>
	<source><![CDATA[// Create a factory for disk-based file items
	DiskFileItemFactory factory = new DiskFileItemFactory();

	// Set factory constraints
	factory.setSizeThreshold(yourMaxMemorySize);
	factory.setRepositoryPath(yourTempDirectory);

	// Create a new file upload handler
	ServletFileUpload upload = new ServletFileUpload(factory);

	// Set overall request size constraint
	upload.setSizeMax(yourMaxRequestSize);

	// Parse the request
	List /* FileItem */ items = upload.parseRequest(request);]]></source>
	<p>
	Of course, each of the configuration methods is independent of the
	others, but if you want to configure the factory all at once, you can
	do that with an alternative constructor, like this:
	</p>
	<source><![CDATA[// Create a factory for disk-based file items
	DiskFileItemFactory factory = new DiskFileItemFactory(
	yourMaxMemorySize, yourTempDirectory);]]></source>
	<p>
	Should you need further control over the parsing of the request, such
	as storing the items elsewhere - for example, in a database - you will
	need to look into <a href="customizing.html">customizing</a> FileUpload.
	</p>
	</subsection>
	</section>

	<section name="Processing the uploaded items">
	<p>
	Once the parse has completed, you will have a <code>List</code> of file
	items that you need to process. In most cases, you will want to handle
	file uploads differently from regular form fields, so you might process
	the list like this:
	</p>
	<source><![CDATA[// Process the uploaded items
	Iterator iter = items.iterator();
	while (iter.hasNext()) {
	FileItem item = (FileItem) iter.next();

	if (item.isFormField()) {
	processFormField(item);
	} else {
	processUploadedFile(item);
	}
	}]]></source>
	<p>
	For a regular form field, you will most likely be interested only in the
	name of the item, and its <code>String</code> value. As you might expect,
	accessing these is very simple.
	</p>
	<source><![CDATA[// Process a regular form field
	if (item.isFormField()) {
	String name = item.getFieldName();
	String value = item.getString();
	...
	}]]></source>
	<p>
	For a file upload, there are several different things you might want to
	know before you process the content. Here is an example of some of the
	methods you might be interested in.
	</p>
	<source><![CDATA[// Process a file upload
	if (!item.isFormField()) {
	String fieldName = item.getFieldName();
	String fileName = item.getName();
	String contentType = item.getContentType();
	boolean isInMemory = item.isInMemory();
	long sizeInBytes = item.getSize();
	...
	}]]></source>
	<p>
	With uploaded files, you generally will not want to access them via
	memory, unless they are small, or unless you have no other alternative.
	Rather, you will want to process the content as a stream, or write the
	entire file to its ultimate location. FileUpload provides simple means of
	accomplishing both of these.
	</p>
	<source><![CDATA[// Process a file upload
	if (writeToFile) {
	File uploadedFile = new File(...);
	item.write(uploadedFile);
	} else {
	InputStream uploadedStream = item.getInputStream();
	...
	uploadedStream.close();
	}]]></source>
	<p>
	Note that, in the default implementation of FileUpload, <code>write()</code>
	will attempt to rename the file to the specified destination, if the data
	is already in a temporary file. Actually copying the data is only done if
	the the rename fails, for some reason, or if the data was in memory.
	</p>
	<p>
	If you do need to access the uploaded data in memory, you need simply
	call the <code>get()</code> method to obtain the data as an array of
	bytes.
	</p>
	<source><![CDATA[// Process a file upload in memory
	byte[] data = item.get();
	...]]></source>
	</section>

	<section name="Interaction with virus scanners">
	<p>
	Virus scanners running on the same system as the web container can cause
	some unexpected behaviours for applications using FileUpload. This section
	describes some of the behaviours that you might encounter, and provides
	some ideas for how to handle them.
	</p>
	<p>
	The default implementation of FileUpload will cause uploaded items above
	a certain size threshold to be written to disk. As soon as such a file is
	closed, any virus scanner on the system will wake up and inspect it, and
	potentially quarantine the file - that is, move it to a special location
	where it will not cause problems. This, of course, will be a surprise to
	the application developer, since the uploaded file item will no longer be
	available for processing. On the other hand, uploaded items below that
	same threshold will be held in memory, and therefore will not be seen by
	virus scanners. This allows for the possibility of a virus being retained
	in some form (although if it is ever written to disk, the virus scanner
	would locate and inspect it).
	</p>
	<p>
	One commonly used solution is to set aside one directory on the system
	into which all uploaded files will be placed, and to configure the virus
	scanner to ignore that directory. This ensures that files will not be
	ripped out from under the application, but then leaves responsibility for
	virus scanning up to the application developer. Scanning the uploaded
	files for viruses can then be performed by an external process, which
	might move clean or cleaned files to an "approved" location, or by
	integrating a virus scanner within the application itself. The details of
	configuring an external process or integrating virus scanning into an
	application are outside the scope of this document.
	</p>
	</section>

	<section name="What's next">
	<p>
	Hopefully this page has provided you with a good idea of how to use
	FileUpload in your own applications. For more detail on the methods
	introduced here, as well as other available methods, you should refer
	to the <a href="apidocs/index.html">JavaDocs</a>.
	</p>
	<p>
	The usage described here should satisfy a large majority of file upload
	needs. However, should you have more complex requirements, FileUpload
	should still be able to help you, with it's flexible
	<a href="customizing.html">customization</a> capabilities.
	</p>
	</section>

	</body>
	</document>