The possible exceptions
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.
-
Exceptions occurring when loading and parsing templates: When you call
Configuration.getTemplate(...)
, FreeMarker has to load the template into the memory and parse it (unless the template is already cached in thatConfiguration
object). During this, these kind of exceptions can occur:-
TemplateNotFoundException
because the requested template doesn't exist. Note this extendsIOException
. -
freemarker.core.ParseException
because the template is syntactically incorrect according the rules of the FTL language. Note that this error occurs when you obtain theTemplate
object (Configuration.getTemplate(...)
), not later when you execute (Template.process(...)
) the template. . Note this extendsIOException
(legacy). -
Any other kind of
IOException
because an error has occurred while reading an existing template. For example you have no right to read the file, or the connection through which you read the template is broken. The emitter of these is theTemplateLoader
object, which is plugged into theConfiguration
object.
-
-
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.TemplateException
because other problem occurred while executing the template. For example, a frequent error is referring to a variable that doesn't exist in the data-model. By default, when aTemplateException
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 theTemplateException
, which then you can catch asTemplate.process(...)
throws it. This behavior can be customized, and in fact, it should be; see the recommended configuration here. By default FreeMarker also logsTemplateException
-s.
-
Customizing the behavior regarding TemplateException-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(...)
method. These are the TemplateExceptionHandler
implementations with FreeMarker comes with:
-
TemplateExceptionHandler.DEBUG_HANDLER
: Prints stack trace (includes FTL error message and FTL stack trace) and re-throws the exception. This is the default handler, however, you should be careful not using it in production environment, as it shows technical information about your system. -
TemplateExceptionHandler.HTML_DEBUG_HANDLER
: Same asDEBUG_HANDLER
, but it formats the stack trace so that it will be readable with Web browsers. Recommended overDEBUG_HANDLER
when you generate HTML pages, but it should only be used for development as it shows technical information about your system. -
TemplateExceptionHandler.IGNORE_HANDLER
: Simply suppresses all exceptions (though FreeMarker will still log them ifConfiguration.getLogTemplateExceptions
istrue
). 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 should be used in most applications today. It doesn't print anything to the output about the error, which makes it safe, and the developers can still get the error details from the logs. It's not as convenient during template development asHTML_DEBUG_HANDLER
orDEBUG_HANDLER
though. For more information about handling errors in Web applications see the FAQ.
You can also write a custom
TemplateExceptionHandler
by implementing that
interface, which contains this method:
void handleTemplateException(TemplateException te, Environment env, Writer out) throws TemplateException;
Whenever a TemplateException
occurs, this
method will be called. The exception to handle is in the
te
argument, the runtime environment of the
template processing is in the env
argument, and
the handler can print to the output using the out
argument. If this 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.
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:
class MyTemplateExceptionHandler implements TemplateExceptionHandler { public void handleTemplateException(TemplateException te, Environment env, java.io.Writer out) throws TemplateException { try { out.write("[ERROR: " + te.getMessage() + "]"); } catch (IOException e) { throw new TemplateException("Failed to print error message. Cause: " + e, env); } } } ... cfg.setTemplateExceptionHandler(new MyTemplateExceptionHandler());
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):
a${badVar}b
will print this if we use the
MyTemplateExceptionHandler
:
a[ERROR: Expression badVar is undefined on line 1, column 4 in test.ftl.]b
This template will print the same (except that the column number will differ...):
a${"moo" + badVar}b
because 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:
a<#if badVar>Foo</#if>b
will print this:
a[ERROR: Expression badVar is undefined on line 1, column 7 in test.ftlh.]b
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...):
a<#if "foo${badVar}" == "foobar">Foo</#if>b
because 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:
a <#if true> Foo ${badVar} Bar </#if> c
or in the macro definition body:
a <@test /> b <#macro test> Foo ${badVar} Bar </#macro>
the output will be something like:
a Foo [ERROR: Expression badVar is undefined on line 4, column 5 in test.ftlh.] Bar c
TemplateException logging
By default FreeMarker logs all
TemplateException
-s under the
freemarker.runtime
log category, even when it
will throw it at you from its public API. As logging has become
common practice in Java applications, this usually leads to double
logging of exceptions now, so it's recommended to disable this
legacy behavior by
cfg.setLogTemplateExceptions(false)
(or
log_template_exceptions=false
) where you
configure FreeMarker.
Explicit error handling in templates
Although it has nothing to do with the FreeMarker configuration (the topic of this chapter), for the sake of completeness it's mentioned here that you can handle errors directly inside the templates as well:
-
Handling missing/null variables: Template Author's Guide/The Template/Expressions/Handling missing values
-
Substituting failing but expendable page sections: Template Language Reference/Directive Reference/attempt, recover