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PART V
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JDBC assumes a driver architecture like that provided by the ODBC standard, on which it is broadly based. Figure 19-31 shows the main building blocks. A Java program connects to the JDBC driver manager via the JDBC API. The JDBC system software is responsible for loading one or more JDBC drivers, typically on demand from Java programs that request them. Conceptually, each driver provides access to one particular DBMS brand, making
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Part V:
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FIGURE 19-31
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whatever brand-specific API calls and sending the SQL statements needed to carry out the JDBC request. The JDBC software is delivered as a Java package, which is imported into a Java program that wants to use JDBC. The JDBC specification does not deal with the specific details of how a JDBC driver is implemented. However, since the introduction of JDBC, developers have tended to characterize JDBC drivers into four driver types. The type descriptions assume a client/ server connection from the JDBC API (on the client system) to a database server. While this is a common enterprise deployment architecture, it s worth noting that JDBC is used to access local databases on systems as small as handheld devices; in this context, the driver types have less meaning. The driver types differ in how they translate JDBC calls (method invocations) into specific actions against the DBMS. A Type 1 driver, also called a JDBC/ODBC bridge, is shown in Figure 19-32. The driver translates JDBC calls into a vendor-neutral API, which in practice is always ODBC. The request passes to a specific ODBC driver for the target DBMS. (Optionally, the ODBC driver manager may be eliminated, since the ODBC API to the driver manager is the same as the API to the driver itself.) Ultimately, the ODBC driver calls the DBMS proprietary API. If the database is on a local system, the DBMS carries out the request. If it s on a remote (server) system, the DBMS code on the client is a network access stub, which translates the request into a network message (proprietary to the DBMS) and sends it to the DBMS server. A Type 1 driver has one significant advantage. Because nearly all popular DBMS products support ODBC, a single Type 1 driver can provide access to dozens of different DBMS brands. Type 1 drivers are widely available, including one that is distributed by Sun.
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FIGURE 19-32 A JDBC Type 1 driver
PART V
A Type 1 driver also has several disadvantages. Each JDBC request passes through many layers on its way to and from the DBMS, so a Type 1 driver typically carries a lot of computing overhead, and its performance suffers as a result. The use of ODBC as an intermediate stage also may limit the functionality provided by the driver features of the underlying DBMS that might be able to be delivered via the JDBC interface directly may not be accessible via ODBC. Finally, the ODBC driver required by a Type 1 driver will be delivered in binary form, not as a Java executable. Thus, any given Type 1 driver is specific to the client computer s hardware and operating system, and will lack the portability of Java. A Type 2 driver is also called a Native API driver. The driver translates JDBC requests directly into the native API of the DBMS, as shown in Figure 19-33. Unlike with the Type 1 driver, no ODBC or other vendor-neutral layer is involved. If the database is located on the same system as the Java program, the Type 2 driver s calls to the native API will go directly
Part V:
Programming with SQL
FIGURE 19-33 A JDBC Type 2 driver
to the DBMS. In a client/server network, the DBMS code on the client is again a network access stub, and the requests flow over the network in a DBMS-proprietary protocol, as in the Type 1 driver. Type 2 drivers present a different set of trade-offs than Type 1 drivers. A Type 2 driver has fewer layers, so performance is typically higher. It still has the disadvantage of requiring binary code to be installed on the client system, so each Type 2 driver will still be specific to a hardware architecture and operating system. Unlike a Type 1 driver, a Type 2 driver is also specific to a DBMS brand. If you want to communicate with several different DBMSs, you will need multiple drivers. Finally, it s worth noting that the Type 1/Type 2 distinction assumes that the native DBMS API is not ODBC. If a DBMS presents a native ODBC interface, then the use of ODBC does not imply an additional layer, and its Type 2 driver will, in fact, use ODBC to access the DBMS. A Type 3 driver is a Network-Neutral driver. The driver translates JDBC requests into network messages in a vendor-neutral format and sends them across the network to the server, as shown in Figure 19-34. On the server, a middleware layer receives the network requests and translates them into calls to the DBMS native API. Query results are passed back across the network, again in a vendor-neutral format.
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