public static synchronized Connection getConnection(String url); public static synchronized Connection getConnection(String url, java.util.Properties info); The third method listed here is by far the most elegant way of connecting to any JDBC driver. An intelligent Java application/applet will use Driver.getPropertyInfo (which will not be covered here) to get a list of all of the required and optional properties for the driver. The Java program can then prompt the user for this information, and then create a java.util.Properties object that contains an element for each of the driver properties to be used for the JDBC connection. The following code shows how to setup the java.util.Properties object: // Create the Properties object. java.util.Properties prop = new java.util.Properties(); // Populate the Properties object with each property to be passed to the // JDBC driver. prop.put(”UID”, “dept12″); prop.put(”PWD”, “Julie”); Connection con = DriverManager.getConnection(”jdbc:odbc:Accounting”, prop); JDBC To ODBC Calls: A Roadmap For all of you ODBC junkies, Tables 5.1 through 5.8 show each JDBC method and the corresponding ODBC call (only JDBC methods that actually make an ODBC call are included). I can hear you now: But isn t this a closely guarded national secret? What if someone takes this information to write another Bridge? First of all, the information provided here can be easily gathered by turning on the JDBC logging facility (DriverManager.setLogStream). The Bridge is nice enough to log every ODBC call as it is made, providing a log stream has been set via the DriverManager (all good JDBC drivers should provide adequate logging to aid in debugging). And second, the Bridge is provided for free. No one could possibly take this information to create a better Bridge at a lower price. It simply can t be done. I provide this information in an effort to help you better understand how the Bridge operates, and, if you are well versed in ODBC, to give you the direct correlation between the Bridge and ODBC. This should enable you to write advanced JDBC applications right off the starting line. Table 5.1 Driver ODBC calls. JDBC Interface Method ODBC Call Comments connect SQLDriverConnect The Bridge creates a connection string using the java.util. Properties
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applets can only access databases on the server from which they were downloaded. Normally, the Java Security Manager will prohibit a TCP connection from being made to an unauthorized hostname; that is, if the TCP connection is being made from within the Java Virtual Machine (JVM). In the case of the Bridge, this connection would be made from within the ODBC driver, outside the control of the JVM. If the Bridge could determine the hostname that it will be connected to, a call to the Java Security Manager could easily check to ensure that a connection is allowed. Unfortunately, it is not always possible to determine the hostname for a given ODBC data source name. For this reason, the Bridge always assumes the worst. An untrusted applet is not allowed to access any ODBC data source. What this means is that if you can t convince the Internet browser in use that an applet is trusted, you can t use the Bridge from that applet. The ODBC URL To make a connection to a JDBC driver, you must supply a URL. The general structure of the JDBC URL is jdbc:: where subprotocol is the kind of database connectivity being requested, and subname provides additional information for the subprotocol. For the Bridge, the specific URL structure is: jdbc:odbc:[;attribute-name=attribute-value]… The Bridge can only provide services for URLs that have a subprotocol of odbc. If a different subprotocol is given, the Bridge will simply tell the JDBC DriverManager that it has no idea what the URL means, and that it can t support it. The subname specifies the ODBC data source name to use, followed by any additional connection string attributes. Here s a code snippet that you can use to connect to an ODBC data source named Accounting, with a user name of dept12 and a password of Julie: // Create a new instance of the JDBC-ODBC Bridge. new jdbc.odbc.JdbcOdbcDriver(); // The JDBC-ODBC Bridge will have registered itself with the JDBC // DriverManager. We can now let the DriverManager choose the right // driver to connect to the given URL. Connection con = DriverManager.getConnection(”jdbc:odbc:Accounting”, “dept12″, “Julie”); An alternative way of connecting to this same data source would be to pass the user name and password as connection string attributes: Connection con = DriverManager.getConnection(”jdbc:odbc:Accounting;UID= dept12;PWD=Julie”); A third, more robust way of connecting would be to use a java.util.Properties object. DriverManager.getConnection is overloaded to support three versions of the interface: public static synchronized Connection getConnection(String url, String user, String password) throws SQLException;
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An ODBC Driver Manager (such as the one provided by Microsoft for Win95/NT); do not confuse this with the JDBC DriverManager class Any ODBC drivers to be used from the Bridge (from vendors such as Intersolv, Microsoft, and Visigenic) Before actually attempting to use the Bridge, save yourself lots of headaches be sure to test the ODBC drivers that you will be using! I have pursued countless reported problems that ended up being nothing more than an ODBC configuration issue. Make sure you setup your data sources properly, and then test them to make sure you can connect and perform work. You can accomplish this by either using an existing tool or writing your own sample ODBC application. Most vendors include sample source code to create an ODBC application, and Microsoft provides a tool named Gator (a.k.a ODBCTE32.EXE) which can fully exercise ODBC data sources on Win95/NT. The Bridge Is Great, But… All looks good for the Bridge; it gives you access to any ODBC data source, and it s free! But wait, there are a few limitations that I need to make you aware of before you start. First, as I mentioned before, a lot of software must be installed and configured on each system that will be using the Bridge. In today s environment, this feat cannot be accomplished automatically. Unfortunately, this task can be a major limitation, not only from the standpoint of getting the software installed and configured properly, but ODBC drivers may not be readily available (or may be quite costly) for the operating system that you are using. Second, understand the limitations of the ODBC driver that you will be using. If the ODBC driver can t do it, neither can the Bridge. The Bridge is not going to add any value to the ODBC driver that you are using other than allowing you to use it via JDBC. One of the most frequently asked questions I get is: If I use the Bridge, can I access my data over the Internet? If the ODBC driver that you are using can, then the Bridge can; if it can t, then neither can the Bridge. Third, keep in mind the quality of the ODBC driver. In order for the Bridge to properly use an ODBC driver, it must be ODBC version 2.0 or higher. Also, if there are bugs in the ODBC driver, they will surely be present when you use it from JDBC. Finally, there are Java security considerations. From the JDBC API specification, all JDBC drivers must follow the standard security model, most importantly: JDBC should not allow untrusted applets access to local database data An untrusted applet will normally only be allowed to open a database connection back to the server from which it was downloaded For trusted applets and any type of application, the Bridge can be used in any fashion to connect to any data source. For untrusted applets, the prognosis is bleak. Untrusted
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Don t forget, if you have problems finding the class file or the driver, set the CLASSPATH. See Chapter 3 for more help on this topic. Coming Up Next In the next chapter, we ll explore the bridge between ODBC and JDBC. You ll see how easy it is to use existing ODBC drivers with JDBC, and learn some of the fine points of the relation, similarity, and difference between the two database connectivity standards. You won t want to miss this one; the author, Karl Moss, is also the author of the Sun/Intersolv ODBC-JDBC bridge included in the JDBC package. Chapter 5 Accessing ODBC Services Using JDBC One of JavaSoft s first tasks in developing the JDBC API was to get it into the hands of developers. Defining the API specification was a major step, but JDBC drivers must be implemented in order to actually access data. Because ODBC has already established itself as an industry standard, what better way to make JDBC usable by a large community of developers than to provide a JDBC driver that uses ODBC. JavaSoft turned to Intersolv to provide resources to develop a bridge between the two, and the resulting JDBC driver the Bridge is now included with the Java Developer s kit. The Bridge works great, but there are some things you need to understand before you can implement it properly. In this chapter, we ll cover the requirements necessary to use the Bridge, the limitations of the Bridge, and the most elegant way to make a connection to a JDBC URL. I ll also provide you with a list of each JDBC method and the corresponding ODBC call (broken down by the type of call). Bridge Requirements One thing to note about the JDBC-ODBC Bridge is that it contains a very thin layer of native code. This library s sole purpose is to accept an ODBC call from Java, execute that call, and return any results back to the driver. There is no other magic happening within this library; all processing, including memory management, is contained within the Java side of the Bridge. Unfortunately, this means that there is a library containing C code that must be ported to each of the operating systems that the Bridge will execute on. This is obviously not an ideal situation, and invalidates one of Java s major advantages portability. So, instead of being able to download Java class files and execute on the fly, you must first install and configure additional software in order to use the Bridge. Here s a short checklist of required components: The Java Developer s Kit The JDBC Interface classes (java.sql.*) The JDBC-ODBC Bridge classes (jdbc.odbc.* or sun.jdbc.odbc.* for JDBC version 1.1 and higher)
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Listing 4.10 HTML code to call the interactive query applet.

Interactive JDBC SQL Query Applet

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The Final Product Figure 4.3 shows a screen shot of the completed applet, and Figure 4.4 shows the applet running. Not too shabby for our first try. We ve covered a lot of ground in creating this applet, so let s take some time to recap the important details. We learned how to: Figure 4.3 The completed Interactive Query applet. Figure 4.4 The Interactive Query applet running. Open a connection to a data source Connect a Statement object to the data source via the connection Execute a query Get MetaData information about the result of the query Use the MetaData information to properly get the results row-by-row, column-by-column Close the connection To use the applet, you can load the HTML file in a Java-enabled Web browser, or you can start the applet from the command line: bash$ appletviewer IQ.html &
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// of its assignee, in this case the String variable Output. // We simply get each “cell” and add a space to it, then append it onto // the Output variable. } // End for loop (end looping through the columns for a specific row ). Output+=”n”; // For each row that we fetch, we need to add a carriage return so that // the next fetched row starts on the next line. } // End while loop ( end fetching rows when no more rows are left ). stmt.close(); // Clean up, close the stmt, in effect, close the input-output query // connection streams, but stay connected to the data source. } catch( Exception e ) { e.printStackTrace(); Output=e.getMessage(); } // We have to catch any exceptions that were thrown while we were // querying or retrieving the data. Print the exception // to the console and return it so it can be shown to the user // in the applet. return Output; // Before exiting, return the result that we got. } Wrapping It Up The last part of the applet, shown in Listing 4.9, involves terminating the connection to the data source. This is done in the destroy method of the applet. We have to catch an exception, if one occurs, while the close method is called on the connection. Listing 4.9 Terminating the connection. public void destroy() { try {con.close();} catch( Exception e ) { e.printStackTrace(); System.out.println(e.getMessage()); } } // end destroy } // end applet The HTML File That Calls The Applet We need to call this applet from an HTML file, which is shown in Listing 4.10. We don t pass in any properties, but we could easily include a default data source URL and user name that the applet would read in before initializing the user interface, and then set the appropriate TextField to show these defaults. Note that we set the width and height carefully in the tag. This is to make sure that our applet s user interface has enough room to be properly laid out.
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Tuesday, September 25th, 2007

because we only want to pipe all of the results into the OutputField TextArea. We cast all of the returned results into a String however, if the database contains binary data, we could get some weird output, or even cause the program to break. When I tested the applet, the data source that I queried contained numerical and strings only. In Chapter 7, I ll show you how to deal with different data types in the ANSI SQL-2 specification, upon which the data types for the JDBC are based. Listing 4.7 Executing a statement. public String Select(String QueryLine) { // This is the method we called above in Listing 4.5. // We return a String, and use a String parameter for the entered query. String Output=”"; int columns; int pos; try { // Several of the following methods can throw exceptions if there was a // problem with the query, or if the connection breaks, or if // we improperly try to retrieve results. Statement stmt = con.createStatement(); // First, we instantiate a Statement class that is required to execute // the query. The Connection class returns a Statement object in its // createStatement method, which links the opened connection to // the passed-back Statement object. This is how the stmt instance // is linked to the actual connection to the data source. ResultSet rs = stmt.executeQuery(QueryLine); // The ResultSet in turn is linked to the connection to the data source // via the Statement class. The Statement class contains the executeQuery // method, which returns a ResultSet class. This is analagous to a // pointer that can be used to retrieve the results from the JDBC // connection. columns=(rs.getMetaData()).getColumnCount(); // Here we use the getMetaData method in the result set to return a // Metadata object. The MetaData object contains a getColumnCount // method which we use to determine how many columns of data // are present in the result. We set this equal to an integer // variable. Listing 4.8 Getting the Result and MetaData Information. while(rs.next()) { // Now, we use the next method of the ResultSet instance rs to fetch // each row, one by one. There are more optimized ways of doing // this–namely using the inputStream feature of the JDBC driver. // I show you an example of this in Chapter 9. for( pos=1; pos<=columns; pos++) { // Now let's get each column in the row ( each cell ), one by one. Output+=rs.getObject(pos)+" "; // Here we've used the general method for getting a result. The // getObject method will attempt to caste the result in the form
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Thursday, September 20th, 2007

} Opening The Connection Our next step is to connect to the database that will process the user s query, as shown in Listing 4.6. Listing 4.6 Opening a database connection. if (evt.target == ConnectBtn) { // If the user clicks the “Connect” button, connect to the database // specified in the DBurl TextArea and the user name specified in the // NameField TextArea. url=DBurl.getText(); Name=NameField.getText(); try { new imaginary.sql.iMsqlDriver(); // This creates a new instance of the Driver we want to use. There are a // number of ways to specify which driver you want to use, and there is // even a way to let the JDBC DriverManager choose which driver it thinks // it needs to connect to the data source. con = DriverManager.getConnection(url, Name, “”); // Actually make the connection. Use the entered URL and the entered // user name when making the connection. We haven’t specified a password, // so just send nothing (”"). ConnectBtn.setLabel(”Reconnect to Database”); // Finally, change what the ConnectBtn to show “Reconnect to Database”. } catch( Exception e ) { e.printStackTrace(); OutputField.setText(e.getMessage()); } // The creation of the connection throws an exception if there was a // problem connecting using the specified parameters. We have to enclose // the getConnection method in a try-catch block to catch any // exceptions that may be thrown. If there is a problem and an exception // thrown, print it out to the console, and to the OutputField. return true; } return false; } // handleEvent() end No Guts, No Glory: Executing Queries And Processing Results Now that we have opened the connection to the data source (Listing 4.6), it s time to set up the mechanism for executing queries and getting the results, as shown in Listings 4.7 and 4.8. The parameter that we need in this method is a String containing the SQL query the user entered into the QueryField. We will return the results of the query as a string
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Wednesday, September 19th, 2007

} //init Everything has been added to the user interface, so let s show it! We also don t need to do anything else as far as preparation, so that ends the init method of our applet. Now we can move on to handling events. Handling Events We want to watch for four events when our applet is running: the user pressing the Enter key in the DBurl, NameField, and QueryField TextAreas, and the user clicking on the Connect button. Earlier in the chapter, we saw how to watch for events, but now we get to see what we do once the event is trapped, as shown in Listing 4.5. The event handling code is contained in the generic handleEvent method. Listing 4.5 Handling events. public boolean handleEvent(Event evt) { // The standard format for this method includes the Event class where // all the properties are set. if (evt.target == NameField) {char c=(char)evt.key; // Look for the Enter key pressed in the NameField. if (c == ‘n’) { Name=NameField.getText(); // Set the global Name variable to the contents in the NameField. return true; } else { return false; } } if (evt.target == DBurl) {char c=(char)evt.key; // Look for the enter key pressed in the DBurl TextArea. if (c == ‘n’) { url=DBurl.getText(); // Set the global url variable to the contents of the DBurl TextArea. return true; } else { return false; } } if (evt.target == QueryField) {char c=(char)evt.key; // Look for the Enter key pressed in the QueryField. if (c == ‘n’) { OutputField.setText(Select(QueryField.getText())); // Get the contents of the QueryField, and pass them to the Select // method that is defined in Listing 4.7. The Select method executes the // entered query, and returns the results. These results are shown in the // OutputField using the setText method. return true; } else { return false; }
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Tuesday, September 18th, 2007

REMAINDER to signal that any component assigned the GridBagContstraint Con takes up the rest of the space on a row. Similarly, we can set gridheight to REMAINDER so that a component assigned this constraint takes up the remaining vertical space. The last detail associated with GridBagLayout involves assigning the properties to the component. This is done via the setConstraints method in GridBagLayout. Listing 4.4 shows how we do this. Notice that we assign properties for the TextArea, but not for the Labels. Because we re positioning the Labels on the right side of the screen (the default), there is no need to assign constraints. There are more properties you can set with GridBagLayout, but it s beyond the scope of this book. Listing 4.4 Assigning properties to components. add(new Label(”Name”)); gridbag.setConstraints(NameField, Con); add(NameField); // Note that we did not setConstraints for the Label. The GridbagLayout // manager assumes they carry the default constraints. The NameField is // assigned to be the last component on its row via the constraints Con, // then added to the user interface. add(new Label(”Database URL”)); gridbag.setConstraints(DBurl, Con); add(DBurl); gridbag.setConstraints(ConnectBtn, Con); add(ConnectBtn); // Here, we only want the ConnectBtn button on a row, by itself, so we // set the constraints, and add it. add(new Label(”SQL Query”)); gridbag.setConstraints(QueryField, Con); add(QueryField); Label result_label = new Label(”Result”); result_label.setFont(new Font(”Helvetica”, Font.PLAIN, 16)); result_label.setForeground(Color.blue); gridbag.setConstraints(result_label, Con); add(result_label); // Here we add a label on its own line. We also set the colors for it. Con.weighty=1.0; gridbag.setConstraints(OutputField, Con); OutputField.setForeground(Color.white); OutputField.setBackground(Color.black); add(OutputField); // This is what we were talking about before. We want the large OutputField to // take up as much of the remaining space as possible, so we set the // weighty=1 at this point. This sets the field apart from the previously // added components, and gives it more room to exist in. show();
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