The exceptions that can occur regarding FreeMarker could be classified like this:
Exceptions occurring when you configure FreeMarker: Typically you configure FreeMarker only once in your application, when your application initializes itself. Of course, during this, exceptions can occur, as it is obvious from the FreeMarker API...
Exceptions occurring when loading and parsing templates: When you call Configuration.getTemplate(...), FreeMarker has to load the template file into the memory and parse it (unless the template is already cached in that Configuration object). During this, two kind of exceptions can occur:
IOException because the template file was not found, or other I/O problem occurred while trying to read it, for example you have no right to read the file, or there are disk errors. The emitter of these errors is the TemplateLoader object, which is plugged into the Configuration object. (For the sake of correctness: When I say ``file'' here, that's a simplification. For example, templates can be stored in a table of a relational database as well. This is the business of the TemplateLoader.)
freemarker.core.ParseException because the template file is syntactically incorrect according the rules of the FTL language. The point is that this error occurs when you obtain the Template object (Configuration.getTemplate(...)), and not when you execute (Template.process(...)) the template. This exception is an IOException subclass.
Exceptions occurring when executing (processing) templates, that is, when you call Template.process(...). Two kind of exceptions can occur:
IOException because there was an error when trying to write into the output writer.
freemarker.template.TemplatException because other problem occurred while executing the template. For example, a frequent error is when a template refers to a variable which is not existing. Be default, when a TemplatException occurs, FreeMarker prints the FTL error message and the stack trace to the output writer with plain text format, and then aborts the template execution by re-throwing the TemplatException, which then you can catch as Template.process(...) throws it. But this behavior can be customized. FreeMarker always logs TemplatException-s.
TemplateException-s thrown during the template processing are handled by the freemarker.template.TemplateExceptionHandler object, which is plugged into the Configuration object with its setTemplateExceptionHandler(...) mehod. The TemplateExceptionHandler contains 1 method:
Whenever a TemplateException occurs, this method will be called. The exception to handle is passed with the te argument, the runtime environment of the template processing is accessible with the env argument, and the handler can print to the output using the out argument. If the method throws exception (usually it re-throws te), then the template processing will be aborted, and Template.process(...) will throw the same exception. If handleTemplateException doesn't throw exception, then template processing continues as if nothing had happen, but the statement that caused the exception will be skipped (see more later). Of course, the handler can still print an error indicator to the output.
In any case, before the TemplateExceptionHandler is invoked, FreeMarker will log the exception.
Let's see how FreeMarker skips ``statements'' when the error handler doesn't throw exception, through examples. Assume we are using this template exception handler:
If an error occurs in an interpolation which is not inside an FTL tag (that is, not enclosed into <#...> or <@...>), then the whole interpolation will be skipped. So this template (assuming that badVar is missing from the data-model):
will print this if we use the MyTemplateExceptionHandler:
This template will print the same (except that the column number will differ...):
since, as it was written, the whole interpolation is skipped if any error occurs inside it.
If an error occurs when evaluating the value of a parameter for a directive call, or if there are other problems with the parameter list, or if an error occurs when evaluating exp in <@exp ...>, or if the value of exp is not an user-defined directive, then the whole directive call is skipped. For example this:
will print this:
Note that the error occurred in the if start-tag (<#if badVar>), but the whole directive call was skipped. Logically, the nested content (Foo) was skipped with this, since the nested content is handled (printed) by the enclosing directive (if).
The output will be the same with this (except that the column number will differ...):
since, as it was written, the whole directive calling will be skipped if any error occurs during the parameter evaluation.
The directive call will not be skipped if the error occurs after the execution of the directive was already started. That is, if an error occurs in the nested content:
or in the macro definition body:
the output will be something like:
FreeMarker comes with these prewritten error handlers:
TemplateExceptionHandler.DEBUG_HANDLER: Prints stack trace (includes FTL error message and FTL stack trace) and re-throws the exception. This is the default handler (that is, it is initially prugged into all new Configuration objects).
TemplateExceptionHandler.HTML_DEBUG_HANDLER: Same as DEBUG_HANDLER, but it formats the stack trace so that it will be readable with Web browsers. Recommended over DEBUG_HANDLER when you generate HTML pages.
TemplateExceptionHandler.IGNORE_HANDLER: Simply suppresses all exceptions (but remember, FreeMarker will still log them). It does nothing to handle the event. It does not re-throw the exception.
TemplateExceptionHandler.RETHROW_HANDLER: Simply re-throws all exceptions, it doesn't do anything else. This handler can be good for Web applications (assuming you don't want to continue template processing after exception), because it gives the most control to the Web application over page generation on error conditions (since FreeMarker doesn't print anything to the output about the error). For more information about handling errors in Web applications see the FAQ.
Although it has nothing to do with the FreeMarker configuration (the topic of this chapter), for the sake of completeness it is mentioned here that you can handle errors directly in templates as well. This is usually a bad practice (try keep templates simple and non-technical), but nonetheless necessary sometimes:
Handling missing/null variables: Template Author's Guide/The Template/Expressions/Handling missing values
Surviving malfunctioning ``portlets'' and such expendable page sections: Reference/Directive Reference/attempt, recover