Java Database Connectivity

Java Database Connectivity, or JDBC, is an API for the Java programming language that defines how a client may access a database. (To be strictly correct, JDBC is not an acronym (http://java.sun.com/j2se/1.5.0/docs/guide/jdbc/getstart/intro.html#1018466).) It provides methods for querying and updating data in a database. JDBC is oriented towards relational databases.

The Java 2 Platform, Standard Edition includes the JDBC API together with an ODBC implementation of the API enabling connections to any relational database that supports ODBC. This driver is native code and not Java, and is closed source (http://java.sun.com/products/jdbc/jdbc-3_0-fr-spec-license.html)

Contents

1 Types of Drivers 2 Overview of the API 3 Example 4 Sources of JDBC Drivers 5 Additional Resources

Types of Drivers

There are commercial and free drivers available for most relational database servers. These drivers fall into one of the following types:

• Type 1, the JDBC-ODBC bridge
• Type 2, the Native-API driver
• Type 3, the network-protocol driver
• Type 4, the native-protocol driver

Overview of the API

JDBC allows multiple implementations to exist and be used by the same application. The API provides a mechanism for dynamically loading the correct Java packages and registering them with the JDBC Driver Manager. The DriverManager is used as a connection factory for creating JDBC connections.

JDBC connections support creating and executing statements. These statements may be update statements such as SQL INSERT, UPDATE and DELETE or they may be query statements using the SELECT statement. Additionally, stored procedures may be invoked through a statement. Statements are one of the following types:

• Statement - the statement is sent to the database server each and everytime.
• PreparedStatement - the statement is compiled on the database server allowing it to be executed multiple times in an efficient manner.
• CallableStatement - used for executing stored procedures on the database.

Update statements such as INSERT, UPDATE and DELETE return an update count that indicates how many rows were affected in the database. These statements do not return any other information.

Query statements return a JDBC row result set. The row result set is used to walk over the result set. Individual columns in a row are retrieved either by name or by column number. There may be any number of rows in the result set. The row result set has metadata that describes the names of the columns and their types.

There is an extension to the basic JDBC API that allows for scrollable result sets and cursor support among other things. Refer to the SUN documentation [1] (http://java.sun.com/j2se/1.4.1/docs/api/) for more details.

Example

The method Class.forName() is used to load the jdbc driver class. The line below causes the JDBC driver from some jdbc vendor to be loaded into the application.

Class.forName( "com.somejdbcvendor.TheirJdbcDriver" );

Next, the DriverManager.getConnection() method is used to create a JDBC connection.

Connection conn = DriverManager.getConnection( 
     "jdbc:somejdbcvendor:other data needed by some jdbc vendor",
     "myLogin",
     "myPassword" );

The URL used is dependent upon the particular JDBC driver. It will always begin with "jdbc:", but the rest is upto the particular vendor. Once a connection is established, a statement must be created.

Statement stmt = conn.createStatement();
stmt.executeUpdate( "INSERT INTO MyTable( name ) VALUES ( 'my name' ) " );

Data is retrieved from the database using a database query mechanism. The example below shows creating a statement and executing a query.

Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery( "SELECT * FROM MyTable" );
while ( rs.next() ) {
    int numColumns = rs.getMetaData().getColumnCount();
    for ( int i = 1 ; i <= numColumns ; i++ ) {
       //Column numbers start at 1.
       //Also there are many methods on the result set to return
       // the column as a particular type. Refer to the Sun documentation
       // for the list of valid conversions.
       System.out.println( "COLUMN " + i + " = " + rs.getObject(i) );
    }
}
rs.close();
stmt.close();

Typically, however, it would be rare for a seasoned Java programmer to code in such a fashion. The usual practice would be to abstract the database logic into an entirely different class and to pass preprocessed strings (perhaps derived themselves from a further abstracted class) containing SQL statements and the connection to the required methods.

An example of a PreparedStatement Query. Using conn and class from first example.

PreparedStatement ps = null;
ResultSet rs = null;
try {
ps = conn.prepareStatement( "SELECT i.*, j.* FROM Omega i, Zappa j
     WHERE i = ? AND j = ?" );
// In the prepared statement ps, the question mark denotes variable input,
// which can be passed through a parameter list, for example.

// The following replaces the question marks, 
// with the string or int, before sending it to SQL.
// The first parameter corresponds to the nth occurrence of the ?, 
// the second parameter tells Java to replace it with 
// the second item.
ps.setString(1, "Poor Yorick");
ps.setInt(2, 8008);

// The ResultSet rs, receives the SQL Query response.
rs = ps.executeQuery();
while ( rs.next() ) {
    int numColumns = rs.getMetaData().getColumnCount();
    for ( int i = 1 ; i <= numColumns ; i++ ) {
       //Column numbers start at 1.
       //Also there are many methods on the result set to return
       // the column as a particular type. Refer to the Sun documentation
       // for the list of valid conversions.
       System.out.println( "COLUMN " + i + " = " + rs.getObject(i) );
    }

}
catch (SQLException e) {
 // typical exception handling here
}
finally { // note that these resources need to be closed in the finally clause to avoid 
 try {    // a resource leak since it should always be called
  rs.close();
  ps.close();
 } catch( SQLException e){} // handle errors here or ignore them
}

Here are examples of host database types, Java can convert to with a function. 

setXXX() Methods

Oracle Datatype	setXXX()
CHAR	setString()
VARCHAR2	setString()
NUMBER	setBigDecimal()
	setBoolean()
	setByte()
	setShort()
	setInt()
	setLong()
	setFloat()
	setDouble()
INTEGER	setInt()
FLOAT	setDouble()
CLOB	setClob()
BLOB	setBlob()
RAW	setBytes()
LONGRAW	setBytes()
DATE	setDate()
	setTime()
	setTimestamp()

For the example of a CallableStatement (to call stored procedures in the database), see http://java.sun.com/j2se/1.4.2/docs/guide/jdbc/getstart/callablestatement.html

Sources of JDBC Drivers

• Sun (http://servlet.java.sun.com/products/jdbc/drivers) provides an incomplete list of JDBC drivers and vendors
• OpenLink Software (http://uda.openlinksw.com/jdbc/) ships JDBC Drivers for a number of target databases, including Bridges to other data access mechanisms (e.g., ODBC, JDBC) which can provide more functionality than the targeted mechanism.

Additional Resources

• Sun tutorial (http://java.sun.com/docs/books/tutorial/post1.0/whatsnew/jdbc.html)
• Sun Java 2 1.4.2 API Syntax (http://java.sun.com/j2se/1.4.2/docs/api/)
• Duke's Bakery - A JDBC Order Entry Prototype (http://java.sun.com/developer/technicalArticles//Database/dukesbakery/)
• DBAccessor: A JDBC Wrapper Package (http://java.sun.com/developer/technicalArticles//Database/dbaccessor/)
• List of Java Open Source Databases with JDBC drivers (http://java-source.net/open-source/database-engines)

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