📄 query.java
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/* * Copyright (c) 2000 David Flanagan. All rights reserved. * This code is from the book Java Examples in a Nutshell, 2nd Edition. * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied. * You may study, use, and modify it for any non-commercial purpose. * You may distribute it non-commercially as long as you retain this notice. * For a commercial use license, or to purchase the book (recommended), * visit http://www.davidflanagan.com/javaexamples2. */package com.davidflanagan.examples.servlet;import javax.servlet.*;import javax.servlet.http.*;import java.sql.*;import java.io.*;/** * This class demonstrates how JDBC can be used within a servlet. It uses * initialization parameters (which come from the web.xml configuration file) * to create a single JDBC database connection, which is shared by all clients * of the servlet. ***/public class Query extends HttpServlet { Connection db; // This is the shared JDBC database connection public void init() throws ServletException { // Read initialization parameters from the web.xml file ServletConfig config = getServletConfig(); String driverClassName = config.getInitParameter("driverClassName"); String url = config.getInitParameter("url"); String username = config.getInitParameter("username"); String password = config.getInitParameter("password"); // Use those init params to establish a connection to the database // If anything goes wrong, log it, wrap the exception and re-throw it try { Class.forName(driverClassName); db = DriverManager.getConnection(url, username, password); } catch (Exception e) { log("Can't create DB connection", e); throw new ServletException("Query: can't initialize: " + e.getMessage(), e); } } /** Close the database connection when the servlet is unloaded */ public void destroy() { try { db.close(); } // Try to close the connection catch (SQLException e) {} // Ignore errors; at least we tried! } public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { response.setContentType("text/html"); // We're outputting HTML PrintWriter out = response.getWriter(); // Where to output it to // Output document header and a form for entering SQL queries // When the form is submitted, this servlet is reloaded out.println("<head><title>DB Query</title></head>\n" + "<body bgcolor=white><h1>DB Query</h1>\n" + "<form><b>Query: </b><input name='q'>" + "<input type=submit></form>"); // See if a query was specified in this request. String query = request.getParameter("q"); if (query != null) { // display the query text as a page heading out.println("<h1>" + query + "</h1>"); // Now try to execute the query and display the results in a table Statement statement = null; // An object to execute the query try { // Create a statement to use statement = db.createStatement(); // Use it to execute the specified query, and get result set ResultSet results = statement.executeQuery(query); // Ask for extra information about the results ResultSetMetaData metadata = results.getMetaData(); // How many columns are there in the results? int numcols = metadata.getColumnCount(); // Begin a table, and output a header row of column names out.println("<table border=2><tr>"); for(int i = 0; i < numcols; i++) out.print("<th>" + metadata.getColumnLabel(i+1) + "</th>"); out.println("</tr>"); // Now loop through the "rows" of the result set while(results.next()) { // For each row, display the the values for each column out.print("<tr>"); for(int i = 0; i < numcols; i++) out.print("<td>" + results.getObject(i+1) + "</td>"); out.println("</tr>"); } out.println("</table>"); // end the table } catch (SQLException e) { // If anything goes wrong (usually a SQL error) display the // error to the user so they can correct it. out.println("SQL Error: " + e.getMessage()); } finally { // Whatever happens, always close the Statement object try { statement.close(); } catch(Exception e) {} } } // Now, display the number of hits on this page by invoking the // Counter servlet and including its output in this page. // This is done with a RequestDispatcher object. RequestDispatcher dispatcher = request.getRequestDispatcher("/servlet/counter"); if (dispatcher != null) { out.println("<br>Page hits:"); // Add a request attribute that tells the servlet what to count. // Use the attribute name defined by the Counter servlet, and // use the name of this class as a unique counter name. request.setAttribute(Counter.ATTRIBUTE_NAME,Query.class.getName()); // Tell the dispatcher to invoke its servlet and include the output dispatcher.include(request, response); } // Finally, end the HTML output out.println("</body>"); }}⌨️ 快捷键说明
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