如何在 JSP 页面中从数据库中检索和显示图像? [英] How to retrieve and display images from a database in a JSP page?

问题描述

如何在 JSP 页面中从数据库中检索和显示图像?

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

推荐答案

让我们分步骤看看会发生什么:

Let's see in steps what should happen:

• JSP 基本上是一种视图技术，应该生成 HTML 输出.
• 要以 HTML 格式显示图像，您需要 HTML 元素.
• 要让它定位图像，您需要指定其src 属性.
• src 属性需要指向有效的 http:// URL，因此不是本地磁盘文件系统路径 file:// 因为当服务器和客户端在物理上不同的机器上运行时，这将永远无法工作.
• 图像 URL 需要在请求路径(例如 http://example.com/context/images/foo.png)或请求参数(例如 >http://example.com/context/images?id=1).
• 在 JSP/Servlet 世界中，你可以让一个 Servlet 监听某个 URL 模式，比如 /images/*，这样你就可以在特定的 URL 上执行一些 Java 代码.
• 图像是二进制数据，将从数据库中以 byte[] 或 InputStream 的形式获取，JDBC API 提供了 ResultSet#getBytes() 和 ResultSet#getBinaryStream() 为此，JPA API 提供了 @Lob 为此.莉>
• 在 Servlet 中，您可以将这个 byte[] 或 InputStream 写入响应的 OutputStream 中，通常的 Java IO 方式.
• 需要指示客户端将数据作为图像处理，因此至少 Content-Type 响应头也需要设置.您可以通过 ServletContext#getMimeType() 基于图像文件扩展名，您可以通过 在 web.xml.

• 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 (e.g. http://example.com/context/images/foo.png) or as request parameter (e.g. 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.

应该是这样.它几乎自己编写代码.让我们从 HTML 开始(在 JSP 中):

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">

如有必要，您还可以在使用 EL 动态设置 src="https://stackoverflow.com/tags/jstl/info">JSTL:

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>

然后定义/创建一个 servlet，它监听 /images/*<的 URL 模式的 GET 请求/code>，下面的例子使用普通的 JDBC 来完成这项工作:

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);
        }
    }

}

就是这样.如果您担心 HEAD 和缓存标头并正确响应这些请求，请使用此 静态资源servlet的抽象模板.

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.

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