JDBC - ODBC Questions......

1. What's the JDBC 2.0 API?

The JDBC 2.0 API is the latest update of the JDBC API. It contains many new features, including scrollable result sets and the new SQL:1999 (formerly SQL 3) data types. There are two parts to the JDBC 2.0 API:

the JDBC 2.0 core API (the java.sql package), which is included in the JavaTM 2 SDK, Standard Edition

the JDBC 2.0 Optional Package API (the javax.sql package), which is available separately or as part of the Java 2 SDK, Enterprise Edition

2. Does the JDBC-ODBC Bridge support the new features in the JDBC 2.0 API?

No, the JDBC-ODBC Bridge that is included in the Java 2 Platform initial release does not support the new features in the JDBC 2.0 API. However, Sun and Merant are working to produce a new version of the Bridge that does support the new features. Note that we do not recommend using the Bridge except for experimental purposes or when you have no other driver available.

3. Can the JDBC-ODBC Bridge be used with applets?

Use of the JDBC-ODBC bridge from an untrusted applet running in a browser, such as Netscape Navigator, isn't allowed. The JDBC-ODBC bridge doesn't allow untrusted code to call it for security reasons. This is good because it means that an untrusted applet that is downloaded by the browser can't circumvent Java security by calling ODBC. Remember that ODBC is native code, so once ODBC is called, the Java programming language can't guarantee that a security violation won't occur. On the other hand, Pure Java JDBC drivers work well with applets. They are fully downloadable and do not require any client-side configuration.

Finally, we would like to note that it is possible to use the JDBC-ODBC bridge with applets that will be run in appletviewer since appletviewer assumes that applets are trusted. It is also possible to use the JDBC-ODBC bridge with applets that are run in the HotJavaTM browser (available from Java Software), since HotJava provides an option to turn off applet security. In general, it is dangerous to turn applet security off, but it may be appropriate in certain controlled situations, such as for applets that will only be used in a secure intranet environment. Remember to exercise caution if you choose this option, and use an all-Java JDBC driver whenever possible to avoid security problems.

4. How do I start debugging problems related to the JDBC API?

A good way to find out what JDBC calls are doing is to enable JDBC tracing. The JDBC trace contains a detailed listing of the activity occurring in the system that is related to JDBC operations.

If you use the DriverManager facility to establish your database connection, you use the DriverManager.setLogWriter method to enable tracing of JDBC operations. If you use a DataSource object to get a connection, you use the DataSource.setLogWriter method to enable tracing. (For pooled connections, you use the ConnectionPoolDataSource.setLogWriter method, and for connections that can participate in distributed transactions, you use the XADataSource.setLogWriter method.)

5. How can I use the JDBC API to access a desktop database like Microsoft Access over the network?

Most desktop databases currently require a JDBC solution that uses ODBC underneath. This is because the vendors of these database products haven't implemented all-Java JDBC drivers.

The best approach is to use a commercial JDBC driver that supports ODBC and the database you want to use. See the JDBC drivers page for a list of available JDBC drivers.

The JDBC-ODBC bridge from Sun's Java Software does not provide network access to desktop databases by itself. The JDBC-ODBC bridge loads ODBC as a local DLL, and typical ODBC drivers for desktop databases like Access aren't networked. The JDBC-ODBC bridge can be used together with the RMI-JDBC bridge , however, to access a desktop database like Access over the net. This RMI-JDBC-ODBC solution is free.

6. Does the JDK include the JDBC API and the JDBC-ODBC Bridge?

Yes, the JDK 1.1 and the Java 2 SDK, Standard Edition (formerly known as the JDK 1.2), contain both the JDBC API and the JDBC-ODBC Bridge. The Java 2 SDK, Standard Edition, contains the JDBC 2.0 core API, which is the latest version. It does not include the JDBC 2.0 Optional Package, which is part of the Java 2 SDK, Enterprise Edition, or which you can download separately.

Note that the version of the JDBC API and the JDBC-ODBC Bridge provided for separate download on the JDBC download page are only for use with the JDK 1.0.2.

7. What JDBC technology-enabled drivers are available?

See our web page on JDBC technology-enabled drivers for a current listing.

8. What documentation is available for the JDBC API?

See the JDBC technology home page for links to information about JDBC technology. This page links to information about features and benefits, a list of new features, a section on getting started, online tutorials, a section on driver requirements, and other information in addition to the specifications and javadoc documentation.

9. Are there any ODBC drivers that do not work with the JDBC-ODBC Bridge?

Most ODBC 2.0 drivers should work with the Bridge. Since there is some variation in functionality between ODBC drivers, the functionality of the bridge may be affected. The bridge works with popular PC databases, such as Microsoft Access and FoxPro.

10. Does the JDBC-ODBC Bridge work with Microsoft J++?

No, J++ does not support the JDBC-ODBC bridge since it doesn't implement the Java Native Interface (JNI). Any all-Java JDBC driver should work with J++, however.

11. What causes the "No suitable driver" error?

"No suitable driver" is an error that usually occurs during a call to the DriverManager.getConnection method. The cause can be failing to load the appropriate JDBC drivers before calling the getConnection method, or it can be specifying an invalid JDBC URL--one that isn't recognized by your JDBC driver. Your best bet is to check the documentation for your JDBC driver or contact your JDBC driver vendor if you suspect that the URL you are specifying is not being recognized by your JDBC driver.

In addition, when you are using the JDBC-ODBC Bridge, this error can occur if one or more the the shared libraries needed by the Bridge cannot be loaded. If you think this is the cause, check your configuration to be sure that the shared libraries are accessible to the Bridge.

12. Why isn't the java.sql.DriverManager class being found?

This problem can be caused by running a JDBC applet in a browser that supports the JDK 1.0.2, such as Netscape Navigator 3.0. The JDK 1.0.2 does not contain the JDBC API, so the DriverManager class typically isn't found by the Java virtual machine running in the browser.

Here's a solution that doesn't require any additional configuration of your web clients. Remember that classes in the java.* packages cannot be downloaded by most browsers for security reasons. Because of this, many vendors of all-Java JDBC drivers supply versions of the java.sql.* classes that have been renamed to jdbc.sql.*, along with a version of their driver that uses these modified classes. If you import jdbc.sql.* in your applet code instead of java.sql.*, and add the jdbc.sql.* classes provided by your JDBC driver vendor to your applet's codebase, then all of the JDBC classes needed by the applet can be downloaded by the browser at run time, including the DriverManager class.

This solution will allow your applet to work in any client browser that supports the JDK 1.0.2. Your applet will also work in browsers that support the JDK 1.1, although you may want to switch to the JDK 1.1 classes for performance reasons. Also, keep in mind that the solution outlined here is just an example and that other solutions are possible.

13. Why doesn't calling the method Class.forName load my JDBC driver?

There is a bug in the JDK 1.1.x that can cause the method Class.forName to fail. A workaround is to explicitly call the method DriverManager.registerDriver(new YourDriverClass()). The exact problem in the JDK is a race condition in the class loader that prevents the static section of code in the driver class from executing and registering the driver with the DriverManager.

14. Why do the java.sql and java.math packages fail to download java.* packages? Is there a workaround?

For security reasons, browsers will not download java.* packages. In order to use the JDBC API with browsers that have not been upgraded to JDK1.1 or beyond, we recommend that the java.sql and java.math packages be renamed jdbc.sql and jdbc.math. Most vendors supplying JDBC technology-enabled drivers that are written purely in the Java programming language already provide versions of these renamed packages. When JDK 1.1 support has been added to your browser, you should convert your applets back to the java.* package names.

15. Why is the precision of java.math.BigDecimal limited to 18 digits in the JDK 1.0.2 add-on version of the JDBC API?

In JDK 1.1, java.math.BigInteger is implemented in C. It supports a precision of thousands of digits. The same is true for BigDecigmal.

The version of BigInteger provided with the JDK 1.0.2 add-on version of the JDBC API is a simplified version written in the Java programming language, and it is limited to 18 digits. Because the implementation of BigDecimal is based on BigInteger, it also is limited to this precision.

In the JDBC 2.0 API, you can use a new version of the method ResultSet.getBigDecimal that does not take a scale parameter and returns a BigDecimal with full precision.

16. Can the JDBC API be added to JDK 1.0.2?

Yes. Download the JDBC 1.22 API from the JDBC download page and follow the installation instructions in the release notes.

If you are using any version of the JDK from 1.1 on, the JDBC API is already included, and you should not download the JDBC 1.22 API.

17. How do I retrieve a whole row of data at once, instead of calling an individual ResultSet.getXXX method for each column?

The ResultSet.getXXX methods are the only way to retrieve data from a ResultSet object, which means that you have to make a method call for each column of a row. It is unlikely that this is the cause of a performance problem, however, because it is difficult to see how a column could be fetched without at least the cost of a function call in any scenario. We welcome input from developers on this issue.

18. Why does the ODBC driver manager return 'Data source name not found and no default driver specified Vendor: 0'

This type of error occurs during an attempt to connect to a database with the bridge. First, note that the error is coming from the ODBC driver manager. This indicates that the bridge-which is a normal ODBC client-has successfully called ODBC, so the problem isn't due to native libraries not being present. In this case, it appears that the error is due to the fact that an ODBC DSN (data source name) needs to be configured on the client machine. Developers often forget to do this, thinking that the bridge will magically find the DSN they configured on their remote server machine

19. Are all the required JDBC drivers to establish connectivity to my database part of the JDK?

No. There aren't any JDBC technology-enabled drivers bundled with the JDK 1.1.x or Java 2 Platform releases other than the JDBC-ODBC Bridge. So, developers need to get a driver and install it before they can connect to a database. We are considering bundling JDBC technology- enabled drivers in the future.

20. Is the JDBC-ODBC Bridge multi-threaded?

No. The JDBC-ODBC Bridge does not support concurrent access from different threads. The JDBC-ODBC Bridge uses synchronized methods to serialize all of the calls that it makes to ODBC. Multi-threaded Java programs may use the Bridge, but they won't get the advantages of multi-threading. In addition, deadlocks can occur between locks held in the database and the semaphore used by the Bridge. We are thinking about removing the synchronized methods in the future. They were added originally to make things simple for folks writing Java programs that use a single-threaded ODBC driver.

21. Does the JDBC-ODBC Bridge support multiple concurrent open statements per connection?

No. You can open only one Statement object per connection when you are using the JDBC-ODBC Bridge.

22. Does the JDBC-ODBC Bridge developed by Merant and Sun support result sets that contain Japanese Characters (DBCS)?

Yes, but we haven't tested this ourselves. The version of the Bridge in the Java 2 SDK, Standard Edition, and Java 2 SDK, Enterprise Edition, also supports a new charSet Connection property for specifying the character encoding used by the underlying DBMS.

23. Why can't I invoke the ResultSet methods afterLast and beforeFirst when the method next works?

You are probably using a driver implemented for the JDBC 1.0 API. You need to upgrade to a JDBC 2.0 driver that implements scrollable result sets. Also be sure that your code has created scrollable result sets and that the DBMS you are using supports them.

24. How can I retrieve a String or other object type without creating a new object each time?

Creating and garbage collecting potentially large numbers of objects (millions) unnecessarily can really hurt performance. It may be better to provide a way to retrieve data like strings using the JDBC API without always allocating a new object.

We are studying this issue to see if it is an area in which the JDBC API should be improved. Stay tuned, and please send us any comments you have on this question.

25. There is a method getColumnCount in the JDBC API. Is there a similar method to find the number of rows in a result set?

No, but it is easy to find the number of rows. If you are using a scrollable result set, rs, you can call the methods rs.last and then rs.getRow to find out how many rows rs has. If the result is not scrollable, you can either count the rows by iterating through the result set or get the number of rows by submitting a query with a COUNT column in the SELECT clause.

26. I would like to download the JDBC-ODBC Bridge for the Java 2 SDK, Standard Edition (formerly JDK 1.2). I'm a beginner with the JDBC API, and I would like to start with the Bridge. How do I do it?

The JDBC-ODBC Bridge is bundled with the Java 2 SDK, Standard Edition, so there is no need to download it separately.

27. If I use the JDBC API, do I have to use ODBC underneath?

No, this is just one of many possible solutions. We recommend using a pure Java JDBC technology-enabled driver, type 3 or 4, in order to get all of the benefits of the Java programming language and the JDBC API.

28. Once I have the Java 2 SDK, Standard Edition, from Sun, what else do I need to connect to a database?

You still need to get and install a JDBC technology-enabled driver that supports the database that you are using. There are many drivers available from a variety of sources. You can also try using the JDBC-ODBC Bridge if you have ODBC connectivity set up already. The Bridge comes with the Java 2 SDK, Standard Edition, and Enterprise Edition, and it doesn't require any extra setup itself. The Bridge is a normal ODBC client. Note, however, that you should use the JDBC-ODBC Bridge only for experimental prototyping or when you have no other driver available.