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| <a name="Error Handling"><strong>Error Handling</strong></a> |
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| </td></tr> |
| <tr><td> |
| <blockquote> |
| <p> |
| This is a good one and a fundamental design issue with JSP. The question |
| is: How many different types of errors can one get when using JSP? For |
| example, because the JSP Servlet is auto generated from a .jsp text file |
| and then compiled with a compiler, what happens when there is a |
| generation/parsing error or a compile error? The unnecessary complexity |
| of JSP actually increases the number of ways to get errors! |
| </p> |
| <p> |
| The ugliest aspect of all of this is the fact the errors are reported |
| via two different mechanisms. The parser can throw its own set of errors |
| and the javac compiler can throw a whole different set of errors and |
| as a result of the layers of generation, errors from the compiler generally |
| do not make any sense whatsoever. For example, can you tell me what |
| this error is from? |
| </p> |
| <div align="left"> |
| <table cellspacing="4" cellpadding="0" border="0"> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#ffffff"><pre> |
| org.apache.jasper.JasperException: Unable to compile class: Invalid type |
| expression. |
| out.println("JSP is great!") |
| ^ |
| : Invalid declaration. |
| out.write("\r\n\r\n\r\n"); |
| ^ |
| 2 errors |
| </pre></td> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| </table> |
| </div> |
| <p> |
| If you guessed that the error was a result of a missing <code>;</code> |
| after the first out.println(), you were correct! Now, put yourself in |
| the shoes of someone who has never written or seen a line of Java code. |
| Do you think that person could have figured out the error quickly and |
| easily? Compound that with the fact that if the error had been on a less |
| deterministic part of the file, it is now much harder to find the source |
| of the error because there is a level of abstraction from the original |
| .jsp file and because there is an intermediate .java file that gets |
| generated. |
| </p> |
| <p> |
| Again, Velocity does not suffer from these same problems because there |
| is no intermediate step and no layers of abstraction. |
| </p> |
| <div align="left"> |
| <table cellspacing="4" cellpadding="0" border="0"> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#ffffff"><pre> |
| <%@ page errorPage="/error.jsp" %> |
| </pre></td> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| </table> |
| </div> |
| <p> |
| JSP also allows one to define an error page that is used if an Throwable |
| exception is thrown during the processing of a page. Doesn't this again |
| break the MVC model? In other words, shouldn't the application framework |
| be responsible for dealing with error messages |
| </p> |
| <div align="left"> |
| <table cellspacing="4" cellpadding="0" border="0"> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#ffffff"><pre> |
| <% throw new Exception("oops"); %> |
| </pre></td> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| </table> |
| </div> |
| <p> |
| In order to throw an Exception somewhere in a JSP page, one needs to |
| first embed it within a statement. Note: that in this specific case, if |
| optimizations are turned on in the compiler, chances are that the entire |
| exception would be compiled out. Therefore, a more concrete object must |
| be used instead of the "true". This can actually prove difficult if |
| using a strict MVC model because instantiation of objects breaks the |
| View. |
| </p> |
| <div align="left"> |
| <table cellspacing="4" cellpadding="0" border="0"> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#ffffff"><pre> |
| <% |
| if (true) { |
| throw new Exception("oops"); |
| } |
| %></pre></td> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| </table> |
| </div> |
| <p> |
| The reason is that JSP will generate an additional <code>out.println |
| ("\r\n");</code> after the Exception. When javac attempts to compile the |
| page, another hard to debug error will be reported: |
| </p> |
| <div align="left"> |
| <table cellspacing="4" cellpadding="0" border="0"> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#ffffff"><pre> |
| org.apache.jasper.JasperException: Unable to compile class for |
| JSPC:\engines\jakarta-tomcat\work\localhost_8080%2Fjsp\ |
| _0002ferrorMaker_0002ejsperrorMaker_jsp_3.java:75: |
| Statement not reached. |
| out.write("\r\n"); |
| ^</pre></td> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| </table> |
| </div> |
| <p> |
| Taking a direct quote out of Jason's book (I couldn't say it better myself): |
| </p> |
| <div align="left"> |
| <table cellspacing="4" cellpadding="0" border="0"> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#ffffff"><pre> |
| In fact, there are many such "gotchas" when using scriptlets with JSP. |
| If you accidentally write a scriptlet instead of an expression (by |
| forgetting the equal sign), declare a static variable inside a scriptlet |
| (where statics aren't allowed), forget a semi-colon (they're not needed |
| in expressions but are needed in scriptlets), or write anything but |
| perfect Java code, you're likely to get a confusing error message |
| because the compiler is acting on the generated Java code, not on the |
| JSP file. To demonstrate the problem, picture if <%= name %> were |
| replaced by <% name %> in errorTaker.jsp. Tomcat generates this error: |
| |
| org.apache.jasper.JasperException: Unable to compile class for |
| JSPC:\engines\jakarta-tomcat\work\localhost_8080%2Fjsp\ |
| _0002ferrorTaker_0002ejsperrorTaker_jsp_6.java:91: |
| Class name not found. |
| name |
| ^ |
| |
| Debugging an error like this often requires a programmer to look at the |
| generated code to reconstruct what caused the error.</pre></td> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| </table> |
| </div> |
| <p> |
| Velocity does not have of these same problems because it does not allow |
| the author to place any Java code within a template. The only things allowed |
| in the template are Velocity Template Language (VTL) and method calls. |
| </p> |
| <p> |
| Everything else is considered 'text' for literal output by the parser. |
| The only place where one could run into trouble within Velocity is if |
| there is a call to a method which throws an exception during runtime. |
| For example, this VTL defines a String <code>$foo</code> and then |
| attempts to call its <code>substring()</code> method on it would throw |
| an <code>IndexOutOfBoundsException</code>: |
| </p> |
| <div align="left"> |
| <table cellspacing="4" cellpadding="0" border="0"> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#ffffff"><pre> |
| #set ($foo = "bar") |
| |
| #set ($bar = $foo.substring(0,10)) |
| </pre></td> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| <tr> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
| </table> |
| </div> |
| <p> |
| When the exception is thrown, the parser will stop processing and throw |
| that exception up the stack tree where it can be caught in the method |
| that caused the parser to execute. At that point, the exception can be |
| handled gracefully. Yes, this is something that is probably not easily |
| debugged by a Designer without Java knowledge, it is easily debugged by |
| a Template Engineer who has at least limited Java knowledge. |
| </p> |
| <p> |
| This is one of the benefits of using Turbine combined with Velocity |
| because of Turbine's design it easy to deal with Exceptions in a |
| consistent manner. It is also possible to get this same functionality |
| with by using Velocity's included VelocityServlet. The Exception will |
| contain the line number and column number in the .vm file of where the |
| error happened. Because there is no abstraction like with JSP, the line |
| number and column matches up to the error. Also, the only tool that will |
| throw the exception is the parser and that exception will contain the |
| location and error information pertinent to your actual template, not an |
| intermediary file. No need to try to debug the cryptic javac messages |
| which are a result of generated .java code. Note: one commercial |
| application server offers better handling of matching the line number in |
| the JSP file to the error. |
| </p> |
| <p> |
| You make the decision. |
| </p> |
| <p> |
| <strong>[ <a href="ymtd-generation.html">Generation?</a> <- Previous | |
| Next -> <a href="./ymtd-javabeans.html">JavaBeans</a> ] |
| </strong></p> |
| </blockquote> |
| </p> |
| </td></tr> |
| <tr><td><br/></td></tr> |
| </table> |
| </td> |
| </tr> |
| |
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| <div align="center"><font color="#525D76" size="-1"><em> |
| Copyright © 1999-2004, The Apache Software Foundation |
| </em></font></div> |
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