java - How to set the content type on the servlet

Question

I am using a simple servlet which sends back document contents from the database as a byte array. I would like to set a content type so that it has an appropriate extension while it is being retrieved via a doGet() call.

I do have the type of the document stored as a metadata in the database (e.g. png, gif, png, xls, docx ...).

1. What should I set as the content type so that it retains the file extension?
2. The file gets downloaded with a name of "doc", how do I set the filename on the servlet for the data being downloaded.

Answer 1

What should I set as the content type so that it retains the file extension?

Use ServletContext#getMimeType() to get the mime type based on the file name.

String mimeType = getServletContext().getMimeType(filename);

The servletcontainer usually already provides a default mime type mapping in its own web.xml. If you want to overridde or add some other, then put it as new mime mappings in webapp's web.xml. E.g.

<mime-mapping>
    <extension>docx</extension>
    <mime-type>application/vnd.openxmlformats-officedocument.wordprocessingml.document</mime-type>
</mime-mapping>
<mime-mapping>
    <extension>xlsx</extension>
    <mime-type>application/vnd.openxmlformats-officedocument.spreadsheetml.sheet</mime-type>
</mime-mapping>

Finally set it as the Content-Type response header:

response.setContentType(mimeType);

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The file gets downloaded with a name of "doc", how do I set the filename on the servlet for the data being downloaded.

Add it to the servlet URL because some browsers like MSIE ignores the filename attribute of the content disposition.

<a href="download/filename.ext">download filename.ext</a>

If the servlet is mapped on an URL pattern of /download/*, then you can obtain it as follows

String filename = request.getPathInfo().substring(1);

Finally set it in the Content-Disposition header as well to make normal browsers happy:

response.setHeader("Content-Disposition", "attachment; filename="" + filename + """);

If you don't store filenames in DB but rather IDs or something, then use it as filename instead.

<a href="download/${file.id}.${file.ext}">download ${file.id}.${file.ext}</a>

And then in the servlet

String filename = request.getPathInfo().substring(1);
String id = filename.split("\.")[0];
// Obtain from DB based on id.