JSP is a Java view technology running on the server machine which allows you to write template text in client side languages (like HTML, CSS, JavaScript, ect.). JSP supports taglibs, which are backed by pieces of Java code that let you control the page flow or output dynamically. A well-known taglib is JSTL. JSP also supports Expression Language, which can be used to access backend data (via attributes available in the page, request, session and application scopes), mostly in combination with taglibs.
When a JSP is requested for the first time or when the web app starts up, the servlet container will compile it into a class extending HttpServlet and use it during the web app's lifetime. You can find the generated source code in the server's work directory. In for example Tomcat, it's the /work directory. On a JSP request, the servlet container will execute the compiled JSP class and send the generated output (usually just HTML/CSS/JS) through the web server over a network to the client side, which in turn displays it in the web browser.
Servlet is a Java application programming interface (API) running on the server machine, which intercepts requests made by the client and generates/sends a response. A well-known example is the HttpServlet which provides methods to hook on HTTP requests using the popular HTTP methods such as GET and POST. You can configure HttpServlets to listen to a certain HTTP URL pattern, which is configurable in web.xml, or more recently with Java EE 6, with @WebServlet annotation.
When a Servlet is first requested or during web app startup, the servlet container will create an instance of it and keep it in memory during the web app's lifetime. The same instance will be reused for every incoming request whose URL matches the servlet's URL pattern. You can access the request data by HttpServletRequest and handle the response by HttpServletResponse. Both objects are available as method arguments inside any of the overridden methods of HttpServlet, such as doGet() and doPost().
JSF is a component based MVC framework which is built on top of the Servlet API and provides components via taglibs which can be used in JSP or any other Java based view technology such as Facelets. Facelets is much more suited to JSF than JSP. It namely provides great templating capabilities such as composite components, while JSP basically only offers the <jsp:include> for templating in JSF, so that you're forced to create custom components with raw Java code (which is a bit opaque and a lot of tedious work) when you want to replace a repeated group of components with a single component. Since JSF 2.0, JSP has been deprecated as view technology in favor of Facelets.
Note: JSP itself is NOT deprecated, just the combination of JSF with JSP is deprecated.
Note: JSP has great templating abilities by means of Taglibs, especially the (Tag File) variant. JSP templating in combination with JSF is what is lacking.
As being a MVC (Model-View-Controller) framework, JSF provides the FacesServlet as the sole request-response Controller. It takes all the standard and tedious HTTP request/response work from your hands, such as gathering user input, validating/converting them, putting them in model objects, invoking actions and rendering the response. This way you end up with basically a JSP or Facelets (XHTML) page for View and a JavaBean class as Model. The JSF components are used to bind the view with the model (such as your ASP.NET web control does) and the FacesServlet uses the JSF component tree to do all the work.
Related questions
Introduction
To browse and select a file for upload you need a HTML <input type="file"> field in the form. As stated in the HTML specification you have to use the POST method and the enctype attribute of the form has to be set to "multipart/form-data".
<form action="upload" method="post" enctype="multipart/form-data">
<input type="text" name="description" />
<input type="file" name="file" />
<input type="submit" />
</form>
After submitting such a form, the binary multipart form data is available in the request body in a different format than when the enctype isn't set.
Before Servlet 3.0, the Servlet API didn't natively support multipart/form-data. It supports only the default form enctype of application/x-www-form-urlencoded. The request.getParameter() and consorts would all return null when using multipart form data. This is where the well known Apache Commons FileUpload came into the picture.
Don't manually parse it!
You can in theory parse the request body yourself based on ServletRequest#getInputStream(). However, this is a precise and tedious work which requires precise knowledge of RFC2388. You shouldn't try to do this on your own or copypaste some homegrown library-less code found elsewhere on the Internet. Many online sources have failed hard in this, such as roseindia.net. See also uploading of pdf file. You should rather use a real library which is used (and implicitly tested!) by millions of users for years. Such a library has proven its robustness.
When you're already on Servlet 3.0 or newer, use native API
If you're using at least Servlet 3.0 (Tomcat 7, Jetty 9, JBoss AS 6, GlassFish 3, etc), then you can just use standard API provided HttpServletRequest#getPart() to collect the individual multipart form data items (most Servlet 3.0 implementations actually use Apache Commons FileUpload under the covers for this!). Also, normal form fields are available by getParameter() the usual way.
First annotate your servlet with @MultipartConfig in order to let it recognize and support multipart/form-data requests and thus get getPart() to work:
@WebServlet("/upload")
@MultipartConfig
public class UploadServlet extends HttpServlet {
// ...
}
Then, implement its doPost() as follows:
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String description = request.getParameter("description"); // Retrieves <input type="text" name="description">
Part filePart = request.getPart("file"); // Retrieves <input type="file" name="file">
String fileName = Paths.get(filePart.getSubmittedFileName()).getFileName().toString(); // MSIE fix.
InputStream fileContent = filePart.getInputStream();
// ... (do your job here)
}
Note the Path#getFileName(). This is a MSIE fix as to obtaining the file name. This browser incorrectly sends the full file path along the name instead of only the file name.
In case you want to upload multiple files via either multiple="true",
<input type="file" name="files" multiple="true" />
or the old-fashioned way with multiple inputs,
<input type="file" name="files" />
<input type="file" name="files" />
<input type="file" name="files" />
...
then you can collect them as below (unfortunately there is no such method as request.getParts("files")):
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// ...
List<Part> fileParts = request.getParts().stream().filter(part -> "files".equals(part.getName()) && part.getSize() > 0).collect(Collectors.toList()); // Retrieves <input type="file" name="files" multiple="true">
for (Part filePart : fileParts) {
String fileName = Paths.get(filePart.getSubmittedFileName()).getFileName().toString(); // MSIE fix.
InputStream fileContent = filePart.getInputStream();
// ... (do your job here)
}
}
When you're not on Servlet 3.1 yet, manually get submitted file name
Note that Part#getSubmittedFileName() was introduced in Servlet 3.1 (Tomcat 8, Jetty 9, WildFly 8, GlassFish 4, etc). If you're not on Servlet 3.1 yet, then you need an additional utility method to obtain the submitted file name.
private static String getSubmittedFileName(Part part) {
for (String cd : part.getHeader("content-disposition").split(";")) {
if (cd.trim().startsWith("filename")) {
String fileName = cd.substring(cd.indexOf('=') + 1).trim().replace("\"", "");
return fileName.substring(fileName.lastIndexOf('/') + 1).substring(fileName.lastIndexOf('\\') + 1); // MSIE fix.
}
}
return null;
}
String fileName = getSubmittedFileName(filePart);
Note the MSIE fix as to obtaining the file name. This browser incorrectly sends the full file path along the name instead of only the file name.
When you're not on Servlet 3.0 yet, use Apache Commons FileUpload
If you're not on Servlet 3.0 yet (isn't it about time to upgrade?), the common practice is to make use of Apache Commons FileUpload to parse the multpart form data requests. It has an excellent User Guide and FAQ (carefully go through both). There's also the O'Reilly ("cos") MultipartRequest, but it has some (minor) bugs and isn't actively maintained anymore for years. I wouldn't recommend using it. Apache Commons FileUpload is still actively maintained and currently very mature.
In order to use Apache Commons FileUpload, you need to have at least the following files in your webapp's /WEB-INF/lib:
Your initial attempt failed most likely because you forgot the commons IO.
Here's a kickoff example how the doPost() of your UploadServlet may look like when using Apache Commons FileUpload:
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
try {
List<FileItem> items = new ServletFileUpload(new DiskFileItemFactory()).parseRequest(request);
for (FileItem item : items) {
if (item.isFormField()) {
// Process regular form field (input type="text|radio|checkbox|etc", select, etc).
String fieldName = item.getFieldName();
String fieldValue = item.getString();
// ... (do your job here)
} else {
// Process form file field (input type="file").
String fieldName = item.getFieldName();
String fileName = FilenameUtils.getName(item.getName());
InputStream fileContent = item.getInputStream();
// ... (do your job here)
}
}
} catch (FileUploadException e) {
throw new ServletException("Cannot parse multipart request.", e);
}
// ...
}
It's very important that you don't call getParameter(), getParameterMap(), getParameterValues(), getInputStream(), getReader(), etc on the same request beforehand. Otherwise the servlet container will read and parse the request body and thus Apache Commons FileUpload will get an empty request body. See also a.o. ServletFileUpload#parseRequest(request) returns an empty list.
Note the FilenameUtils#getName(). This is a MSIE fix as to obtaining the file name. This browser incorrectly sends the full file path along the name instead of only the file name.
Alternatively you can also wrap this all in a Filter which parses it all automagically and put the stuff back in the parametermap of the request so that you can continue using request.getParameter() the usual way and retrieve the uploaded file by request.getAttribute(). You can find an example in this blog article.
Workaround for GlassFish3 bug of getParameter() still returning null
Note that Glassfish versions older than 3.1.2 had a bug wherein the getParameter() still returns null. If you are targeting such a container and can't upgrade it, then you need to extract the value from getPart() with help of this utility method:
private static String getValue(Part part) throws IOException {
BufferedReader reader = new BufferedReader(new InputStreamReader(part.getInputStream(), "UTF-8"));
StringBuilder value = new StringBuilder();
char[] buffer = new char[1024];
for (int length = 0; (length = reader.read(buffer)) > 0;) {
value.append(buffer, 0, length);
}
return value.toString();
}
String description = getValue(request.getPart("description")); // Retrieves <input type="text" name="description">
Saving uploaded file (don't use getRealPath() nor part.write()!)
Head to the following answers for detail on properly saving the obtained InputStream (the fileContent variable as shown in the above code snippets) to disk or database:
Serving uploaded file
Head to the following answers for detail on properly serving the saved file from disk or database back to the client:
Ajaxifying the form
Head to the following answers how to upload using Ajax (and jQuery). Do note that the servlet code to collect the form data does not need to be changed for this! Only the way how you respond may be changed, but this is rather trivial (i.e. instead of forwarding to JSP, just print some JSON or XML or even plain text depending on whatever the script responsible for the Ajax call is expecting).
Hope this all helps :)
Best Answer
The fact that exists just one instance doesn't means it isn't multithread. More thread can concurrently call tha same method of the same instance. Servlets are absolutly multithread.