How to retrieve and display images from a database in a JSP page?

2018-06-16 07:40:19

如何从JSP页面中的数据库中检索和显示图像？

Let's see in steps what should happen:

JSP is basically a view technology which is supposed to generate HTML output.

To display an image in HTML, you need the HTML <img> element.

To let it locate an image, you need to specify its src attribute.

The src attribute needs to point to a valid http:// URL and thus not a local disk file system path file:// as that would never work when the server and client run at physically different machines.

The image URL needs to have the image identifier in either the request path (eg http://example.com/context/images/foo.png ) or as request parameter (eg http://example.com/context/images?id=1 ).

In JSP/Servlet world, you can let a Servlet listen on a certain URL pattern like /images/* , so that you can just execute some Java code on specific URL's.

Images are binary data and are to be obtained as either a byte[] or InputStream from the DB, the JDBC API offers the ResultSet#getBytes() and ResultSet#getBinaryStream() for this, and JPA API offers @Lob for this.

In the Servlet you can just write this byte[] or InputStream to the OutputStream of the response the usual Java IO way.

The client side needs to be instructed that the data should be handled as an image, thus at least the Content-Type response header needs to be set as well. You can obtain the right one via ServletContext#getMimeType() based on image file extension which you can extend and/or override via <mime-mapping> in web.xml .

That should be it. It almost writes code itself. Let's start with HTML (in JSP):

<img src="${pageContext.request.contextPath}/images/foo.png">
<img src="${pageContext.request.contextPath}/images/bar.png">
<img src="${pageContext.request.contextPath}/images/baz.png">

You can if necessary also dynamically set src with EL while iterating using JSTL:

<c:forEach items="${imagenames}" var="imagename">
    <img src="${pageContext.request.contextPath}/images/${imagename}">
</c:forEach>

Then define/create a servlet which listens on GET requests on URL pattern of /images/* , the below example uses plain vanilla JDBC for the job:

@WebServlet("/images/*")
public class ImageServlet extends HttpServlet {

    // content=blob, name=varchar(255) UNIQUE.
    private static final String SQL_FIND = "SELECT content FROM Image WHERE name = ?";

    @Resource(name="jdbc/yourDB") // For Tomcat, define as <Resource> in context.xml and declare as <resource-ref> in web.xml.
    private DataSource dataSource;

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String imageName = request.getPathInfo().substring(1); // Returns "foo.png".

        try (Connection connection = dataSource.getConnection(); PreparedStatement statement = connection.prepareStatement(SQL_FIND)) {
            statement.setString(1, imageName);

            try (ResultSet resultSet = statement.executeQuery()) {
                if (resultSet.next()) {
                    byte[] content = resultSet.getBytes("content");
                    response.setContentType(getServletContext().getMimeType(imageName));
                    response.setContentLength(content.length);
                    response.getOutputStream().write(content);
                } else {
                    response.sendError(HttpServletResponse.SC_NOT_FOUND); // 404.
                }
            }
        } catch (SQLException e) {
            throw new ServletException("Something failed at SQL/DB level.", e);
        }
    }

}

That's it. In case you worry about HEAD and caching headers and properly responding on those requests, use this abstract template for static resource servlet.

How to retrieve and display images from a database in a JSP page?

See also: