JDBC

Packages

java.sql JDBC 2.1 Core API

javax.sql JDBC 2.0 Optional package

Use: J2SE (1.0; restructured to JDBC 2.1 in 1.2)

Overview

Databases are everywhere; it’s hard to imagine a big enterprise application without some kind of persistence. To access the data in those databases from Java, you can use Java Database Connectivity (JDBC). JDBC is a generic SQL database access framework that provides a uniform interface on top of a variety of different database connectivity modules. JDBC provides a way to manipulate the data in a database independent from any particular DBMS.

One of the challenges with this kind of framework is that each database can have its own SQL version, with minor but important differences. The framework had to be flexible as well as simple. This results in an API with only a few interfaces, and only a few methods in each interface. The consequence is that JDBC is reasonably easy of use.

To communicate with the database, you need a driver that understands and speaks the databases’ protocol. You can get this drive with the database from the vendor, or from some third party. The driver contains implementations for the interfaces specific for this protocol.

Every driver has a class that implements the Driver interface. When the class is loaded, it creates an instance of itself and registers with the DriverManager. The DriverManager keeps a list of drivers it can use. When a connection is requested the DriverManager tries to locate a suitable driver. DriverManager checks its list of drivers and starts with the first driver specified at creation time (reading from the jdbc.drivers property) and

The Statement object is the object that receives a SQL statement as a String from the client and executes the query on the database to change or retrieve information. Depending on the type of query, either executeUpdate or exceuteQuery is called. The Statement returns a ResultSet when information is requested (SELECT).

The ResultSet object is a representation of a table of data that encapsulates the result of the executed SELECT. Every time you call the next method on the result set, to iterate through the data table, the cursor is set to the next line in the results. When the cursor is moved to the next line, you can retrieve the values in the columns of that specific line. To read other lines, you call the next method multiple times.

Databases tend to grow fairly large so the results could be big, as well. To prevent memory problems from occurring, the ResultSet

fetches only a limited number of rows in batches. When the end of the current batch is reached, the ResultSet requests a new batch from the database. This is transparent for the user.Typical use could be as shown in Example 7.2:

Example 7.2 Obtaining results from a database

Connection con = DriverManager.getConnection("some url"); Statement stmt = con.createStatement();

String query = "SELECT * FROM students WHERE " +

" iq GREATER THAN 140 AND sociallife='non-existent'"; ResultSet nerds = stmt.executeQuery(query);

while (nerds.next()) {

String name = nerds.getString(1); int iq = nerds.getInt(2);

//read entries from the resultset

}

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