PostgreSQL connection in Java Servlet to Retrieve Information from the DB. Getting Error

问题

I am having difficulty with getting this work. I can connect to the database without problem, however I can not make it show me the html page. It does not run.

import java.io.*;
import java.sql.*;
import java.text.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class ShowBedrock extends HttpServlet 
{
    public String getServletInfo()
    {
       return "Servlet connects to PostgreSQL database and displays result of a SELECT";
    }

    private Connection dbcon;  // Connection for scope of ShowBedrock

    // "init" sets up a database connection
    public void init(ServletConfig config) throws ServletException
    {
        String loginUser = "postgres";
        String loginPasswd = "supersecret";
        String loginUrl = "jdbc:postgresql://localhost/bedrock";

        // Load the PostgreSQL driver
        try 
        {
              Class.forName("org.postgresql.Driver");
              dbcon = DriverManager.getConnection(loginUrl, loginUser, loginPasswd);
        }
        catch (ClassNotFoundException ex)
        {
               System.err.println("ClassNotFoundException: " + ex.getMessage());
               throw new ServletException("Class not found Error");
        }
        catch (SQLException ex)
        {
               System.err.println("SQLException: " + ex.getMessage());
        }
    }

    // Use http GET

    public void doGet(HttpServletRequest request, HttpServletResponse response)
        throws IOException, ServletException
    {
        response.setContentType("text/html");    // Response mime type

        // Output stream to STDOUT
        PrintWriter out = response.getWriter();

        out.println("<HTML><Head><Title>Bedrock</Title></Head>");
        out.println("<Body><H1>Bedrock</H1>");

        try
        {
                // Declare our statement
                Statement statement = dbcon.createStatement();

                String query = "SELECT name, dept, ";
                query +=       "       jobtitle ";
                query +=       "FROM   employee ";

                // Perform the query
                ResultSet rs = statement.executeQuery(query);

                out.println("<table border>");

                // Iterate through each row of rs
                while (rs.next())
                {
                   String m_name = rs.getString("name");
                   String m_dept = rs.getString("dept");
                   String m_jobtitle = rs.getString("jobtitle");
                   out.println("<tr>" + 
                               "<td>" + m_name + "</td>" +
                               "<td>" + m_dept + "</td>" +
                               "<td>" + m_jobtitle + "</td>" +
                               "</tr>");
                }

                out.println("</table></body></html>");
                statement.close();
        }
        catch(Exception ex)
        {
                out.println("<HTML>" +
                            "<Head><Title>" +
                            "Bedrock: Error" +
                            "</Title></Head>\n<Body>" +
                            "<P>SQL error in doGet: " +
                            ex.getMessage() + "</P></Body></HTML>");
                return;
        }
        out.close();
    }
}

I get the following error any body knows why?:

javax.servlet.ServletException: Class not found Error
    ShowBedrock.init(ShowBedrock.java:42)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
    org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:859)
    org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:602)
    org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
    java.lang.Thread.run(Unknown Source)


note The full stack trace of the root cause is available in the Apache Tomcat/6.0.35 logs.

回答1:

I get the following error any body knows why?

The reason why you are getting that error body is that your handler for ClassNotFoundException is throwing away the original exception stacktrace. You are writing the original exception's message to System.err but (obviously) that won't go into the normal system logs. (You'll probably find find the message in the "catalina.out" file ... depending on how Tomcat is configured / launched.)

The right way to do this is to either chain the exception; e.g.

    throw new ServletException("Class not found Error", e);

or log it explicitly at the point that you catch it. And if you do log it explicitly, make sure that you log the exception, and not just the exception message.

We can't tell you the actual root cause of the problem unless we see the original exception stacktrace ... but the two most likely causes are:

The classloader can't find a class that is needed; e.g. because the JAR file is not in the right place.
Class initialization failed for some class during the initialization of the class that you are attempting to load.

回答2:

Tomcat expects that your servlet will be in a package. Yours appears to be in the default package; please add one and recompile. I'll bet things will go better for you then.

Tomcat 6 and 7 expect to find JDBC JARs in the server /lib directory. Add your PostgreSQL JDBC JAR to the server /lib location.

回答3:

Seem you did't add Postgresql JDBC driver into classpath, as the following error suggest:

javax.servlet.ServletException: Class not found Error

Which is origin from:

throw new ServletException("Class not found Error");

You can download Postgresql JDBC driver and put in PROJECT/WEB-INF/lib folder.

Suggestion

One suggestion, your exception handling is wrong, you should change

throw new ServletException("Class not found Error");

throw new ServletException("Class not found Error", e);

来源：https://stackoverflow.com/questions/10255410/postgresql-connection-in-java-servlet-to-retrieve-information-from-the-db-getti

标签

java

postgresql

connect