The code for this lesson is very similar to the code you saw in Lesson 6, but additional steps (beyond converting the file access code to database access code) include setting up the environment, creating a database table, and connecting to the database. Creating a database table is a database administration task that is not part of your program code. However, establishing a database connection and the resulting database access are.

As in Lesson 6, the applet needs appropriate permissions to connect to the database. Which permissions it needs varies with the type of driver used to make the database connection.

• Database Setup
• Create Database Table
• Database Access by Applications
  • Establishing a Connection
  • Final and Private Variables
  • Writing and Reading Data
• Database Access by Applets
  • JDBC Driver
  • JDBC-ODBC Bridge with ODBC Driver
• Database Access by Servlets
• More Information

Database Setup

A database driver is software that lets a program establish a connection with a database. If you do not have the right driver for the database to which you want to connect, your program will be unable to establish the connection.

Drivers either come with the database or are available from the Web. If you install your own database, consult the documentation for the driver for information on installation and any other environment settings you need for your platform. If you are using a database at work, consult your database administrator for this information.

To show you two ways to do it, the application example uses the jdbc driver, the applet examples use the jdbc and jdbc.odbc drivers, and the servlet example uses the jdbc.odbc driver. All examples connect to an OracleOCI7.3.4 database.

Connections to other databases will involve similar steps and code. Be sure to consult your documentation or system administrator if you need help connecting to the database.

Create Database Table

TABLE DBA (
TEXTvarchar2(100),
primary key (TEXT)
)

Database Access by Applications

When you click the Click Me button, whatever is entered into the text field is saved to the database. After that, the data is retrieved from the database and displayed in the window shown on the bottom. If you write data to the table more than once, everything written is read and displayed in the window shown on the bottom, so you might have to enlarge the window to see the entire list of table items.

When Application Starts

After Writing Orange and Apple to Database

The database access application needs code to establish the database connection and do the database read and write operations.

Establishing a Database Connection

The _url string is in the form of a Uniform Resource Locator (URL). It consists of the URL, Oracle subprotcol, and Oracle data source in the form jdbc:oracle:thin, the database login username, the password, plus machine, port, and protocol information.

private Connection c;
final static private String _driver = 
"oracle.jdbc.driver.OracleDriver";
final static private String _url = 
"jdbc:oracle:thin:username/password@(description=(
address_list=(address=(protocol=tcp)
(host=developer)(port=1521)))
(source_route=yes)(connect_data=(sid=jdcsid)))";

The actionPerformed method calls the Class.forName(_driver) method to load the driver, and the DriverManager.getConnection method to establish the connection. The Exception Handling section in Lesson 6 describes try and catch blocks. The only thing different here is that this block uses two catch statements because two different errors are possible.

The call to Class.forName(_driver); throws java.lang.ClassNotFoundException, and the call to c = DriverManager.getConnection(_url); throws java.sql.SQLException. In the case of either error, the application tells the user what is wrong and exits because the program cannot operate in any meaningful way without a database driver or connection.

public void actionPerformed(ActionEvent event){
        try{
        //Load the driver
        Class.forName(_driver);
        //Establish database connection
        c = DriverManager.getConnection(_url);
        }catch (java.lang.ClassNotFoundException e){

        System.out.println("Cannot find driver class");
        System.exit(1);
        }catch (java.sql.SQLException e){

        System.out.println("Cannot get connection");
        System.exit(1);
    }

    

Final and Private Variables

final: A final variable contains a constant value that can never change once it is initialized. In the example, the user name, and password are final variables because you would not want to allow an instance of this or any other class to change this information.

private: A private variable can only be used (accessed) by the class in which it is declared. No other class can read or change private variables. In the example, the database driver, user name, and password variables are private to prevent an outside class from accessing them and jeopardizing the database connection, or compromising the secret user name and password information. You can find more information on this in the Objects and Classs lesson in The Java Tutorial

Writing and Reading Data

Object source = event.getSource();
        if(source == button){
        JTextArea displayText = new JTextArea();
            try{
            //Code to write to database
            String theText = textField.getText();
            Statement stmt = c.createStatement();
            String updateString = "INSERT INTO dba VALUES 
            			('" + theText + "')";
            int count = stmt.executeUpdate(updateString);
            

In the read operation, a ResultSet object is created from the executeQuery method of the Statement object. The ResultSet contains the data returned by the query. To retrieve the data returned, the code iterates through the ResultSet, retrieves the data, and appends the data to the text area, displayText.

//Code to read from database
            ResultSet results = stmt.executeQuery(
            	"SELECT TEXT FROM dba ");
                while(results.next()){
                String s = results.getString("TEXT");
                displayText.append(s + "\n");
            }

            stmt.close();
            } catch(java.sql.SQLException e){

            System.out.println(e.toString());
        }

        //Display text read from database
        panel.removeAll();
        panel.add("North", clicked);
        panel.add("Center", displayText);
        panel.add("South", clickButton);
        panel.validate();
        panel.repaint();
    }

    

Database Access by Applets

However, if you run the applet without a policy file, you get a stack trace indicating permission errors. The Granting Applets Permission section in Lesson 6 introduced you to policy files and how to launch an applet with the permission it needs. The Lesson 6 applet example provided the policy file and told you how to launch the applet with it. This lesson shows you how to read the stack trace to determine the permissions you need in a policy file.

To keep things interesting, this lesson has two versions of the database access applet: one uses the JDBC driver, and the other uses the the JDBC-ODBC bridge with an Open DataBase Connectivity (ODBC) driver.

Both applets do the same operations to the same database table using different drivers. Each applet has its own policy file with different permission lists and has different requirements for locating the database driver

JDBC Driver

Starting the Applet: To successfully run, the DbaAppl.java applet needs an available database driver and a policy file. This section walks through the steps to get everything set up. Here is the DbaAppl.html file for running the DbaAppl applet:

<HTML>
    <BODY>
    <APPLET CODE=DbaAppl.class
    WIDTH=200
    HEIGHT=100>
    </APPLET>
    </BODY>
    </HTML>
    

And here is how to start the applet with appletviewer:

 appletviewer DbaAppl.html
    

Locating the Database Driver: Assuming the driver is not available to the DriverManager for some reason, the following error generates when you click the Click Me button.

 cannot find driver
    

Once you have the driver in place, launch the applet again.

 appletviewer DbaAppl.html
    

java.security.AccessControlException: access denied 
    (java.net.SocketPermission developer resolve)
    

You can use Policy tool to create the policy file you need, or you can create it with an ASCII editor. Here is the policy file with the permission indicated by the stack trace:

grant {
        permission java.net.SocketPermission "developer", 
        "resolve";
        "accessClassInPackage.sun.jdbc.odbc";
    };

    

Run the applet again, this time with a policy file named DbaApplPol that has the above permission in it:

appletviewer -J-Djava.security.policy=DbaApplPol 
    DbaAppl.html
    

 java.security.AccessControlException: access denied 
    (java.net.SocketPermission
    129.144.176.176:1521 connect,resolve)
    

Here is the DbaApplPol policy file with the permission indicated by the stack trace added to it:

grant {
        permission java.net.SocketPermission "developer", 
        "resolve";
        permission java.net.SocketPermission 
        "129.144.176.176:1521", "connect,resolve";
    };

    

 appletviewer -J-Djava.security.policy=DbaApplPol 
    DbaAppl.html
    

JDBC-ODBC Bridge with ODBC Driver

1. Use ODBC from a Java program

    

2. Load ODBC drivers as JDBC drivers. This example uses the JDBC-ODBC bridge to load an ODBC driver to connect to the database. The applet has no ODBC code, however.

The DriverManager uses environment settings to locate and load the database driver. For this example, the driver file does not need to be locally accessible.

Start the Applet: Here is the DbaOdb.html file for running the DbaOdbAppl applet:

<HTML>
    <BODY>
    <APPLET CODE=DbaOdbAppl.class 
    WIDTH=200 
    HEIGHT=100>
    </APPLET>
    </BODY>
    </HTML>
    

 appletviewer DbaOdb.html
    

Reading a Stack Trace: If the DbaOdbAppl.java applet is launched without a policy file, the following stack trace is generated when the end user clicks the Click Me button.

 java.security.AccessControlException: access denied
    (java.lang.RuntimePermission 
    accessClassInPackage.sun.jdbc.odbc )
    

You can use Policy tool to create the policy file you need, or you can create it with an ASCII editor. Here is the policy file with the permission indicated by the stack trace:

grant {
        permission java.lang.RuntimePermission
        "accessClassInPackage.sun.jdbc.odbc";
    };

    

Run the applet again, this time with a policy file named DbaOdbPol that has the above permission in it:

 appletviewer -J-Djava.security.policy=DbaOdbPol 
    DbaOdb.html
    

 java.security.AccessControlException:
    access denied (java.lang.RuntimePermission
    file.encoding read)
    

 grant {
        permission java.lang.RuntimePermission
        "accessClassInPackage.sun.jdbc.odbc";
        permission java.util.PropertyPermission
        "file.encoding", "read";
    };

    

 appletviewer -J-Djava.security.policy=DbaOdbPol 
    DbaOdb.html
    

Database Access by Servlets

The web server has to be configured to locate the database. Consult your web server documentation or database administrator for help. With Java WebServer 1.1.1, the configuration setup involves editing the startup scripts with such things as environment settings for loading the ODBC driver, and locating and connecting to the database.

import java.io.*;
    import javax.servlet.*;
    import javax.servlet.http.*;
    import java.sql.*;
    import java.net.*;
    import java.io.*;
        public class DbaServlet extends HttpServlet {
        private Connection c;
        final static private String _driver = 
        	"sun.jdbc.odbc.JdbcOdbcDriver";
        final static private String _user = "username";
        final static private String _pass = "password";
        final static private String 
        			_url = "jdbc:odbc:jdc";
        public void doPost(HttpServletRequest request,
        	HttpServletResponse response)
            	throws ServletException, IOException{
            response.setContentType("text/html");
            PrintWriter out = response.getWriter();
            out.println("<title>Example<title>" +
            "<body bgcolor=FFFFFF>");
            out.println("<h2>Button Clicked</h2>");
            String DATA = request.getParameter("DATA");
                if(DATA != null){
                out.println("<STRONG>Text from 
                			form:</STRONG>");
                out.println(DATA);
                } else {

                out.println("No text entered.");
            }

            //Establish database connection

                try{
                Class.forName (_driver);
                c = DriverManager.getConnection(_url, 
                				_user,
                				_pass);
                } catch (Exception e) {

                e.printStackTrace();
                System.exit(1);
            }


                try{
                //Code to write to database
                Statement stmt = c.createStatement();
                String updateString = "INSERT INTO dba " + 
                		"VALUES ('" + DATA + "')";
                int count = stmt.executeUpdate(updateString);
                //Code to read from database
                ResultSet results = stmt.executeQuery(
                		"SELECT TEXT FROM dba ");
                    while(results.next()){
                    String s = results.getString("TEXT");
                    out.println("<BR>
                    <STRONG>Text from database:</STRONG>");
                    out.println(s);
                }

                stmt.close();
                }catch(java.sql.SQLException e){

                System.out.println(e.toString());
            }

            out.println("<P>Return to 
            <A HREF="../dbaHTML.html">Form</A>");
            out.close();
        }

    }

    

More Information

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¹ As used on this web site, the terms "Java virtual machine" or "JVM" mean a virtual machine for the Java platform.