Embedding PL/SQL Code in Web Pages (PL/SQL Server Pages)

Coding this procedure as a regular PL/SQL stored procedure would result in convoluted lines with doubled apostrophes, such as this:

htp.print('preamble =

"<img src=''/images/graph_line.gif'' height=''8'' width=''"');

Debugging PL/SQL Server Page Problems

As you begin experimenting with PSP, and as you adapt your first simple pages into more elaborate ones, keep these guidelines in mind when you encounter problems:

■The first step is to get all the PL/SQL syntax and PSP directive syntax right. If you make a mistake here, the file does not compile.

■Make sure you use semicolons to terminate lines where required.

■If a value must be quoted, quote it. You might need to enclose a single-quoted value (needed by PL/SQL) inside double quotes (needed by PSP).

■Mistakes in the PSP directives are usually reported through PL/SQL syntax messages. Check that your directives use the right syntax, that directives are closed properly, and that you are using the right element (declaration, expression, or code block) depending on what goes inside it.

■PSP attribute names are case-sensitive. Most are specified in all lowercase; contentType and errorPage must be specified as mixed-case.

■The next step is to run the PSP file by requesting its URL in a Web browser. At this point, you might get an error that the file is not found.

■Make sure you are requesting the right virtual path, depending on the way the Web gateway is configured. Typically, the path includes the hostname, optionally a port number, the schema name, and the name of the stored procedure (with no .psp extension).

■Remember, if you use the -replace option when compiling the file, the old version of the stored procedure is erased. So, after a failed compilation, you must fix the error or the page is not available. You might want to test new scripts in a separate schema until they are ready, then load them into the production schema.

■If you copied the file from another file, remember to change any procedure name directives in the source to match the new file name.

■Once you get one file-not-found error, make sure to request the latest version of the page the next time. The error page might be cached by the

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browser. You might need to press Shift-Reload in the browser to bypass its cache.

■When the PSP script is run, and the results come back to the browser, use standard debugging techniques to check for and correct wrong output. The tricky part is to set up the interface between different HTML forms, scripts, and CGI programs so that all the right values are passed into your page. The page might return an error because of a parameter mismatch.

■To see exactly what is being passed to your page, use METHOD=GET in the calling form so that the parameters are visible in the URL.

■Make sure that the form or CGI program that calls your page passes the correct number of parameters, and that the names specified by the NAME= attributes on the form match the parameter names in the PSP file. If the form includes any hidden input fields, or uses the NAME= attribute on the Submit or Reset buttons, the PSP file must declare equivalent parameters.

■Make sure that the parameters can be cast from string into the correct PL/SQL types. For example, do not include alphabetic characters if the parameter in the PSP file is declared as a NUMBER.

■Make sure that the query string of the URL consists of name-value pairs, separated by equals signs, especially if you are passing parameters by constructing a hard-coded link to the page.

■If you are passing a lot of parameter data, such as large strings, you might exceed the volume that can be passed with METHOD=GET. You can switch to METHOD=POST in the calling form without changing your PSP file.

■Although the loadpsp command reports line numbers correctly when there is a syntax error in your source file, line numbers reported for runtime errors refer to a transformed version of the source and do not match the line numbers in the original source. When you encounter errors like these, that produce an error trace instead of the expected Web page, you will need to locate the error through exception handlers and by printing debug output.

Putting an Application using PL/SQL Server Pages into Production

When you start developing an application with PSP, you may spend most of your time getting the logic correct in the script. Before putting the application into production, consider other issues such as usability and download speed:

■Pages can be rendered faster in the browser if the HEIGHT= and WIDTH= attributes are specified for all images. You might standardize on picture sizes, or

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store the height and width of images in the database along with the data or URL.

■For viewers who turn off graphics, or who use alternative browsers that read the text out loud, include a description of significant images using the ALT= attribute. You might store the description in the database along with the image.

■Although an HTML table provides a good way to display data, a large table can make your application seem slow. Often, the reader sees a blank page until the entire table is downloaded. If the amount of data in an HTML table is large, consider splitting the output into multiple tables.

■If you set text, font, or background colors, test your application with different combinations of browser color settings:

■Test what happens if you override just the foreground color in the browser, or just the background color, or both.

■Generally, if you set one color (such as the foreground text color), you should set all the colors through the <BODY> tag, to avoid hard-to-read combinations like white text on a white background.

■If you use a background image, specify a similar background color to provide proper contrast for viewers who do not load graphics.

■If the information conveyed by different colors is crucial, consider using some other method instead of or in addition to the color change. For example, you might put a graphic icon next to special items in a table. Some of your viewers may see your page on a monochrome screen, or on browsers that cannot represent different colors. (Such browsers might fit in a shirt pocket and use a stylus for input.)

■Providing context information prevents users from getting lost. Include a descriptive <TITLE> tag for your page. If the user is partway through a procedure, indicate which step is represented by your page. Provide links to logical points to continue with the procedure, return to a previous step, or cancel the procedure completely. Many pages might use a standard set of links that you embed using the include directive.

■In any entry fields, users might enter incorrect values. Where possible, use select lists to present a set of choices. Validate any text entered in a field before passing it to SQL. The earlier you can validate, the better; a JavaScript routine can detect incorrect data and prompt the user to correct it before they press the Submit button and make a call to the database.

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