An Introduction to JDBC
In 1996, Sun released a version of the Java Database Connectivity (JDBC) kit. This package allowed programmers to use Java to connect, query and update a database using the Structured Query Language (SQL). The use of Java with JDBC has advantages over other database programming environments. Programs developed with Java and JDBC are platform and vendor independent, i.e., the same Java program can run on a PC, a workstation, or a network computer. Also, the database can be transferred from one database server to another and the same Java programs can be used without alteration. This article discusses how JDBC can be used with the MySQL database. (See “At the Forge” by Reuven Lerner in the September, October and November 1997 issues of LJ.)
A number of technologies have been developed for databases, such as transaction processing, triggers and indexes, which are supported by JDBC. However, these topics are beyond the scope of this article. Since the JDBC package is relatively new, the tools that database developers expect, such as report generators, query builders and form designers are available. In the near future, more tools should be available. Despite the lack of tools, it is possible to develop highly interactive web pages using the JDBC API without much complexity.
The idea behind JDBC is similar to Microsoft's Open Database Connectivity (ODBC). Both ODBC and JDBC are based on the X/Open standard for database connectivity. Programs written using the JDBC API communicate with a JDBC driver manager, which uses the current driver loaded.
Two architectures can be used to communicate with the database (see Figure 1). In the first one, the JDBC driver communicates directly with the database. The driver connects to the database and SQL statements on behalf of the Java program. Results are sent back from the driver to the driver manager and finally to the application.
In the other, the JDBC driver communicates with an ODBC driver via a “bridge”. A single JDBC driver can communicate with multiple ODBC drivers. Each of the ODBC drivers execute SQL statements for specific databases. The results are sent back up the chain as before.
The JDBC/ODBC bridge was developed to take advantage of the large number of ODBC-enabled data sources. The bridge converts JDBC calls to ODBC calls and passes them to the appropriate driver for the backend database. The advantage of this scheme is that applications can access data from multiple vendors. However, the performance of a JDBC/ODBC bridge is slower than a JDBC driver alone would be, due to the added overhead. A database call must be translated from JDBC to ODBC to a native API.
Fewer operations are required to use a JDBC driver without a bridge. In Figure 2, the steps to access a database using a JDBC driver from an applet are shown. The Gwe JDBC driver is used with the MySQL database. The JDBC driver classes are first downloaded from the Gwe host site. Next, the applet logic passes a JDBC call to a driver manager which in turns passes the call to a JDBC driver. The JDBC driver opens a TCP/IP connection with the MySQL database server. Data is transferred back and forth via the connection. When database processing is complete, the connection is closed.
The JDBC API can be used in applets and stand-alone applications. In addition to the usual restrictions for applets, only connections from the same server from which the applet was downloaded are accepted. If the web server and database server are not on the same machine, the web server must run a proxy service to route the database traffic. Stand-alone applications can give access to local information and remote servers.
Installation of the JDBC driver for the MySQL database is simple. The software can be downloaded from Gwe Technologies at http://www.gwe.co.uk/. The copyright statement allows the redistribution of source and binary, but is not identical to the GNU license. Download the file, exgweMysqlJDBC.0.9.2-src.tar.gz. It contains the source and class files for the Gwe MySQL JDBC driver.
The use of JDBC requires access to java.sql classes which were not available in the pre-1.1 Java Development Kit. These classes have been renamed and included in the /exjava directory. Another directory, /exgwe, contains the source and classes for the Gwe MySQL JDBC driver. These classes make use of the classes in the exjava directory. Add the directory in which the tar file was unpacked to the CLASSPATH environment variable, and installation of the JDBC driver is complete.
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