Essentials, Part 1, Lesson 7: Database Access and Permissions

   
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This lesson converts the application, applet, and servlet examples from Lesson 6 to write to and read from a database using JDBC. JDBC is the Java database connectivity application programming interface (API) available in the Java® 2 Platform software.

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

You need access to a database if you want to run the examples in this lesson. You can install a database on your machine or perhaps you have access to a database at work. Either way, you need a database driver and any relevant environment settings so your program can load the driver and locate the database. The program will also need database login information in the form of a user name and password.

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

Once you have access to a database, create a table in it for the examples in this lesson. You need a table with one text field for storing character data.

TABLE DBA (
        TEXT            varchar2(100),
        primary key (TEXT)
)

Database Access by Applications

This example converts the FileIO program from Lesson 6 to write data to and read data from a database. The top window below appears when you start the Dba application, and the window beneath it appears when you click the Click Me button.

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 JDBC DriverManager class can handle multiple database drivers, and initiates all database communication. To load the driver and connect to the database, the application needs a Connection object and Strings that represent the _driver and _url.

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

The member variables used to establish the database connection above are declared private, and two of those variables are also declared final.

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.

Objects and ClasssThe Java Tutorial

Writing and Reading Data

In the write operation, a Statement object is created from the Connection. The Statement object has methods for executing SQL queries and updates. Next, a String object that contains the SQL update for the write operation is constructed and passed to the executeUpdate method of the Statement object.

Object source = event.getSource();
if(source == button){
  if(_clickMeMode) {
    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);

SQL commands are String objects, and therefore, follow the rules of String construction where the string is enclosed in double quotes (" ") and variable data is appended with a plus (+). The variable theText is a text variable. Single quotes are prepended and appended to comply with SQL syntax.

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("Cannot create SQL statement");
    }
//Display text read from database
    text.setText("Text retrieved from database:");
    button.setText("Click Again");
    _clickMeMode = false;
  } else {
    text.setText("Text to save to database:");
    textField.setText("");
    button.setText("Click Me");
    _clickMeMode = true;
  }
}

Database Access by Applets

The applet version of the example is like the application code described above except for the standard differences between applications and applets described in the Structure and Elements section of Lesson 3.

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

The JDBC driver is used from a program written exclusively in the Java language (Java program). It converts JDBC calls directly into the protocol used by the DBMS. This type of driver is available from the DBMS vendor and is usually packaged with the DBMS software.

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

This error means the DriverManager looked for the JDBC driver in the directory where the applet HTML and class files are and could not find it. To correct this error, copy the driver to the directory where the applet files are, and if the driver is bundled in a zip file, unzip the zip file so the applet can access the driver.

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

  appletviewer DbaAppl.html

Reading a Stack Trace: Assuming the driver is locally available to the applet, if the DbaAppl.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.net.SocketPermission developer resolve)

The first line in the above stack trace tells you access is denied. This means this stack trace was generated because the applet tried to access a system resource without the proper permission. The second line means to correct this condition you need a SocketPermission that gives the applet access to the machine ( developer) where the database is located.

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

You get a stack trace again, but this time it is a different error condition.

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

Now you need a SocketPermission that allows access to the Internet Protocol (IP) address and port on the developer machine where the database is located.

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";
};


Run the applet again. If you use the above policy file with the Socket permissions indicated, it works just fine.

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

JDBC-ODBC Bridge with ODBC Driver

Open DataBase Connectivity (ODBC) is Microsoft's programming interface for accessing a large number of relational databases on numerous platforms. The JDBC-ODBC bridge is built into the Solaris and Windows versions of the Java platform so you can do two things:

  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>

And here is how to start the applet:

  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 )

The first line in the above stack trace tells you access is denied. This means this stack trace was generated because the applet tried to access a system resource without the proper permission. The second line means you need a RuntimePermission that gives the applet access to the sun.jdbc.odbc package. This package provides the JDBC-ODBC bridge functionality to the Java 1 virtual machine (VM).

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

You get a stack trace again, but this time it is a different error condition.

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

The stack trace means the applet needs read permission to the encoded (binary) file. Here is the DbaOdbPol policy file with the permission indicated by the stack trace added to it:

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

Run the applet again. If you use the above policy file with the Runtime and Property permissions indicated, it works just fine.

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

Database Access by Servlets

As you learned in Lesson 6, servlets are under the security policy in force for the web server under which they run. When the database read and write code is added to the FileIOServlet from Lesson 6, the DbaServlet.java servlet for this lesson executes without restriction under Java WebServer 1.1.1.

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

You can find more information on variable access settings in the Objects and Classes trail in The Java Tutorial

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

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