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<h1 class="content-header header-section1" id="pgui_config_errorhandling" itemprop="headline">Error handling</h1>
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<div class="page-menu-title">Page Contents</div>
<ul><li><a class="page-menu-link" href="#autoid_43" data-menu-target="autoid_43">The possible exceptions</a></li><li><a class="page-menu-link" href="#autoid_44" data-menu-target="autoid_44">Customizing the behavior regarding
TemplateException-s</a></li><li><a class="page-menu-link" href="#autoid_45" data-menu-target="autoid_45">TemplateException logging</a></li><li><a class="page-menu-link" href="#autoid_46" data-menu-target="autoid_46">Explicit error handling in templates</a></li></ul> </div>
<h2 class="content-header header-section2" id="autoid_43">The possible exceptions</h2>
<p>The exceptions that can occur regarding FreeMarker could be
classified like this:</p>
<ul>
<li>
<p>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.</p>
</li>
<li>
<p>Exceptions occurring when loading and parsing templates:
When you call
<code class="inline-code">Configuration.getTemplate(<em class="code-color">...</em>)</code>,
FreeMarker has to load the template into the memory and parse it
(unless the template is already <a href="pgui_config_templateloading.html#pgui_config_templateloading_caching">cached</a> in
that <code class="inline-code">Configuration</code> object). During this,
these kind of exceptions can occur:</p>
<ul>
<li>
<p><code class="inline-code">TemplateNotFoundException</code> because
the requested template doesn&#39;t exist. Note this extends
<code class="inline-code">IOException</code>.</p>
</li>
<li>
<p><code class="inline-code">freemarker.core.ParseException</code>
because the template is syntactically incorrect according
the rules of the FTL language. Note that this error occurs
when you obtain the <code class="inline-code">Template</code> object
(<code class="inline-code">Configuration.getTemplate(<em class="code-color">...</em>)</code>),
not later when you execute
(<code class="inline-code">Template.process(<em class="code-color">...</em>)</code>)
the template. . Note this extends
<code class="inline-code">IOException</code> (legacy).</p>
</li>
<li>
<p>Any other kind of <code class="inline-code">IOException</code>
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 the <a href="pgui_config_templateloading.html"><code>TemplateLoader</code>
object</a>, which is plugged into the
<code class="inline-code">Configuration</code> object.</p>
</li>
</ul>
</li>
<li>
<p>Exceptions occurring when executing (processing)
templates, that is, when you call
<code class="inline-code">Template.process(<em class="code-color">...</em>)</code>.
Two kind of exceptions can occur:</p>
<ul>
<li>
<p><code class="inline-code">IOException</code> because there was an
error when trying to write into the output writer.</p>
</li>
<li>
<p><code class="inline-code">freemarker.template.TemplateException</code>
because other problem occurred while executing the template.
For example, a frequent error is referring to a variable
that doesn&#39;t exist in the data-model. By default, when a
<code class="inline-code">TemplateException</code> 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
<code class="inline-code">TemplateException</code>, which then you can
catch as
<code class="inline-code">Template.process(<em class="code-color">...</em>)</code>
throws it. This behavior can be customized, and in fact, it
should be; see the recommended configuration <a href="pgui_quickstart_createconfiguration.html">here</a>.
By default FreeMarker also <a href="pgui_misc_logging.html">logs</a>
<code class="inline-code">TemplateException</code>-s.</p>
</li>
</ul>
</li>
</ul>
<h2 class="content-header header-section2" id="autoid_44">Customizing the behavior regarding
TemplateException-s</h2>
<p><code class="inline-code">TemplateException</code>-s thrown during the
template processing are handled by the
<code class="inline-code">freemarker.template.TemplateExceptionHandler</code>
object, which is plugged into the <code class="inline-code">Configuration</code>
object with its
<code class="inline-code">setTemplateExceptionHandler(<em class="code-color">...</em>)</code>
method. These are the <code class="inline-code">TemplateExceptionHandler</code>
implementations with FreeMarker comes with:</p>
<ul>
<li>
<p><code class="inline-code">TemplateExceptionHandler.DEBUG_HANDLER</code>:
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.</p>
</li>
<li>
<p><code class="inline-code">TemplateExceptionHandler.HTML_DEBUG_HANDLER</code>:
Same as <code class="inline-code">DEBUG_HANDLER</code>, but it formats the
stack trace so that it will be readable with Web browsers.
Recommended over <code class="inline-code">DEBUG_HANDLER</code> when you
generate HTML pages, but it should only be used for development
as it shows technical information about your system.</p>
</li>
<li>
<p><code class="inline-code">TemplateExceptionHandler.IGNORE_HANDLER</code>:
Simply suppresses all exceptions (though FreeMarker will still
log them if
<code class="inline-code">Configuration.getLogTemplateExceptions</code> is
<code class="inline-code">true</code>). It does nothing to handle the event.
It does not re-throw the exception.</p>
</li>
<li>
<p><code class="inline-code">TemplateExceptionHandler.RETHROW_HANDLER</code>:
Simply re-throws all exceptions; it doesn&#39;t do anything else.
This should be used in most applications today. It doesn&#39;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&#39;s not as convenient during template development as
<code class="inline-code">HTML_DEBUG_HANDLER</code> or
<code class="inline-code">DEBUG_HANDLER</code> though. For more information
about handling errors in Web applications <a href="app_faq.html#misc.faq.niceErrorPage">see the FAQ</a>.</p>
</li>
</ul>
<p>You can also write a custom
<code class="inline-code">TemplateExceptionHandler</code> by implementing that
interface, which contains this method:</p>
<div class="code-block role-unspecified">
<pre class="code-block-body">void handleTemplateException(TemplateException te, Environment env, Writer out)
throws TemplateException;</pre> </div>
<p>Whenever a <code class="inline-code">TemplateException</code> occurs, this
method will be called. The exception to handle is in the
<code class="inline-code">te</code> argument, the runtime environment of the
template processing is in the <code class="inline-code">env</code> argument, and
the handler can print to the output using the <code class="inline-code">out</code>
argument. If this method throws exception (usually it re-throws
<code class="inline-code">te</code>), then the template processing will be
aborted, and
<code class="inline-code">Template.process(<em class="code-color">...</em>)</code>
will throw the same exception. If
<code class="inline-code">handleTemplateException</code> doesn&#39;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.</p>
<p>Let&#39;s see how FreeMarker skips statements when the error
handler doesn&#39;t throw exception, through examples. Assume we are
using this template exception handler:</p>
<div class="code-block role-unspecified">
<pre class="code-block-body">class MyTemplateExceptionHandler implements TemplateExceptionHandler {
public void handleTemplateException(TemplateException te, Environment env, java.io.Writer out)
throws TemplateException {
try {
out.write(&quot;[ERROR: &quot; + te.getMessage() + &quot;]&quot;);
} catch (IOException e) {
throw new TemplateException(&quot;Failed to print error message. Cause: &quot; + e, env);
}
}
}
<em>...</em>
cfg.setTemplateExceptionHandler(new MyTemplateExceptionHandler());</pre> </div>
<p>If an error occurs in an interpolation which is not inside an
FTL tag (that is, not enclosed into
<code class="inline-code">&lt;#<em class="code-color">...</em>&gt;</code> or
<code class="inline-code">&lt;@<em class="code-color">...</em>&gt;</code>), then
the whole interpolation will be skipped. So this template (assuming
that <code class="inline-code">badVar</code> is missing from the
data-model):</p>
<div class="code-block role-template">
<div class="code-block-label">Template</div><pre class="code-block-body">a${badVar}b</pre> </div>
<p>will print this if we use the
<code class="inline-code">MyTemplateExceptionHandler</code>:</p>
<div class="code-block role-output">
<div class="code-block-label">Output</div><pre class="code-block-body">a[ERROR: Expression badVar is undefined on line 1, column 4 in test.ftl.]b</pre> </div>
<p>This template will print the same (except that the column
number will differ...):</p>
<div class="code-block role-template">
<div class="code-block-label">Template</div><pre class="code-block-body">a${&quot;moo&quot; + badVar}b</pre> </div>
<p>because the whole interpolation is skipped if any error occurs
inside it.</p>
<p>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
<code class="inline-code"><em class="code-color">exp</em></code> in
<code class="inline-code">&lt;@<em class="code-color">exp</em>
<em class="code-color">...</em>&gt;</code>, or if the value of
<code class="inline-code"><em class="code-color">exp</em></code> is not an
user-defined directive, then the whole directive call is skipped.
For example this:</p>
<div class="code-block role-template">
<div class="code-block-label">Template</div><pre class="code-block-body">a&lt;#if badVar&gt;Foo&lt;/#if&gt;b</pre> </div>
<p>will print this:</p>
<div class="code-block role-output">
<div class="code-block-label">Output</div><pre class="code-block-body">a[ERROR: Expression badVar is undefined on line 1, column 7 in test.ftlh.]b</pre> </div>
<p>Note that the error occurred in the <code class="inline-code">if</code>
start-tag (<code class="inline-code">&lt;#if badVar&gt;</code>), but the whole
directive call was skipped. Logically, the nested content
(<code class="inline-code">Foo</code>) was skipped with this, since the nested
content is handled (printed) by the enclosing directive
(<code class="inline-code">if</code>).</p>
<p>The output will be the same with this (except that the column
number will differ...):</p>
<div class="code-block role-template">
<div class="code-block-label">Template</div><pre class="code-block-body">a&lt;#if &quot;foo${badVar}&quot; == &quot;foobar&quot;&gt;Foo&lt;/#if&gt;b</pre> </div>
<p>because whole directive calling will be skipped if any error
occurs during the parameter evaluation.</p>
<p>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:</p>
<div class="code-block role-template">
<div class="code-block-label">Template</div><pre class="code-block-body">a
&lt;#if true&gt;
Foo
${badVar}
Bar
&lt;/#if&gt;
c</pre> </div>
<p>or in the macro definition body:</p>
<div class="code-block role-template">
<div class="code-block-label">Template</div><pre class="code-block-body">a
&lt;@test /&gt;
b
&lt;#macro test&gt;
Foo
${badVar}
Bar
&lt;/#macro&gt;</pre> </div>
<p>the output will be something like:</p>
<div class="code-block role-output">
<div class="code-block-label">Output</div><pre class="code-block-body">a
Foo
[ERROR: Expression badVar is undefined on line 4, column 5 in test.ftlh.]
Bar
c</pre> </div>
<h2 class="content-header header-section2" id="autoid_45">TemplateException logging</h2>
<p>By default FreeMarker <a href="pgui_misc_logging.html">logs</a> all
<code class="inline-code">TemplateException</code>-s under the
<code class="inline-code">freemarker.runtime</code> 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&#39;s recommended to disable this
legacy behavior by
<code class="inline-code">cfg.setLogTemplateExceptions(false)</code> (or
<code class="inline-code">log_template_exceptions=false</code>) where you
configure FreeMarker.</p>
<h2 class="content-header header-section2" id="autoid_46">Explicit error handling in templates</h2>
<p>Although it has nothing to do with the FreeMarker
configuration (the topic of this chapter), for the sake of
completeness it&#39;s mentioned here that you can handle errors directly
inside the templates as well:</p>
<ul>
<li>
<p>Handling missing/null variables: <a href="dgui_template_exp.html#dgui_template_exp_missing">Template Author&#39;s Guide/The Template/Expressions/Handling missing values</a></p>
</li>
<li>
<p>Substituting failing but expendable page sections: <a href="ref_directive_attempt.html">Template Language Reference/Directive Reference/attempt, recover</a></p>
</li>
</ul>
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