The Web Server Environment - Database Processing: Fundamentals, Design, and Implementation - page 459

A servlet is a Java program that is invoked via HTTP on the Web server computer. It
responds to requests from browsers. Servlets are interpreted and executed by a Java virtual
machine running on the server.
Because they have a connection to a communications protocol, Type 3 and Type 4 drivers
can be used in either applet or servlet code. Type 2 drivers can be used only in situations where
the Java program and the DBMS reside on the same machine or where the Type 2 driver con-
nects to a DBMS program that handles the communications between the computer running
the Java program and the computer running the DBMS.
Thus, if you write code that connects to a database from an applet (two-tier architecture),
only a Type 3 or Type 4 driver can be used. In these situations, if your DBMS product has a
Type 4 driver, use it; it will be faster than a Type 3 driver.
In three-tier or n -tier architecture, if the Web server and the DBMS are running on the
same machine, you can use any of the four types of drivers. If the Web server and the DBMS are
running on different machines, Type 3 and Type 4 drivers can be used without a problem. Type
2 drivers can also be used if the DBMS vendor handles the communications between the Web
server and the DBMS. The MySQL Connector/J is a Type 4 driver.

Using JDBC
Unlike ODBC, JDBC does not have a separate utility for creating a JDBC data source. Instead,
all of the work to define a connection is done in Java code via the JDBC driver. The coding pat-
tern for using a JDBC driver is as follows:

1. Load the driver.

2. Establish a connection to the database.

3. Create a statement.

4. Do something with the statement.

To load the driver, you must first obtain the driver library and install it in a directory. You
need to ensure that the directory is named in the CLASSPATH for both the Java compiler and
for the Java virtual machine. The name of the DBMS product to be used and the name of the
database are provided at step 2. Figure 11-21 summarizes the JDBC components.
Note that Java is used to create the application shown in the figure, and because Java is
an object-oriented programming language, we see a set of objects in the application that are

MySQL Driver

Driver
Manager

MySQL
Database

Connection

Oracle Database Driver

Statement CallableStatement

Oracle
Database
Database

ResultSet

PreparedStatement

JDBC-ODBC Bridge

ResultSetMetaData

ODBC Driver

Application

Figure 11-21
JDBC Components

SQL Server Database Oracle Database

Database