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Figure 19-29.
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A JDBC Type 4 driver
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Figure 19-30.
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vendor s own client/server networking. Query results flow back across the network, again in vendor-proprietary format, and supplied back to the requesting program. Type 4 drivers preserve one of the important advantages of Type 3 drivers. They can be implemented in pure Java, so like Type 3, they are portable across computer hardware and operating systems. However, unlike Type 3 drivers, they are DBMS-specific, so different client-side code is required for each DBMS brand you want to access. A Type 4 architecture involves less overhead on the server system, and may therefore deliver slightly better performance. In practice, the overhead of the network messaging will almost always swamp this advantage, except in very high transaction rate applications. Figure 19-30 summarizes the four JDBC driver types and shows how they relate to one another. The two columns divide the driver types based on whether they use a vendor-neutral intermediate layer (left column) or translate directly from the JDBC API to a DBMS-proprietary interface. The two rows divide the driver types based on whether the translation to a specific DBMS API occurs on the client side (left column) or on the server side. As the figure shows, each of these two decisions creates trade-offs, and they result in four (2 2) driver types.
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Java is an object-oriented language, so it s probably no surprise that JDBC organizes its API functions around a collection of database-related objects and the methods that they provide: I Driver Manager object. The entry-point to JDBC I Connection objects. Represent individual active connections to target databases I Statement objects. Represent SQL statements to be executed
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I ResultSet objects. Represent the results of a SQL query I MetaData objects. Represent metadata about databases, query results, and statements I Exception objects. Represent errors in SQL statement execution These objects have the logical relationship shown in Figure 19-31, based on which objects provide methods to create other objects. The following sections describe each of these objects, and how their methods are used to connect to databases, execute SQL statements, and process query results. A complete explanation of the JDBC API is beyond the scope of this book, but the concepts described should allow you to make effective use of JDBC and to understand the documentation that is delivered with the package. The DriverManager object is the main interface to the JDBC package. Some of the most important methods that it provides are shown in Table 19-14. After loading the JDBC driver class that you want to use (typically using the Class.forName()
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Figure 19-31.
Key objects used by the JDBC API
SQL: The Complete Reference
Method
getConnection()
Description
Creates and returns a database connection object, given a URL for the datasource, and optionally a user name and password, and connection properties Registers a driver with JDBC driver manager Sets timeout for connection login Obtains login timeout value Enables tracing of JDBC calls
registerDriver() setLoginTimeout() getLoginTimeout() setLogWriter() Table 19-14.
DriverManager Object Methods
method), your program will ask the DriverManager object to connect you to that specific driver and a specific database with the getConnection() method:
// Create a connection to the Oracle JDBC driver String url = will vary depending on OS, etc. String user = Scott ; String pswd = Tiger ; Connection dbconn = DriverManager.getConnection(url, user, pswd);
The getConnection() method returns an object, the Connection object, which embodies the connection that has just been created and the database on the other end of that connection. Other DriverManager methods provide programmatic control over connection timeouts, switch on JDBC call logging for debugging, and other utility functions. If it encounters an error while attempting to make the connection, the DriverManager object will throw an exception. Error handling is described in the Error Handling in JDBC section later in this chapter.
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