How to escape json strings in freemarker

Question

We are building a restful api using Spring MVC and freemarker as the templating language. We have chosen to build json responses in the freemarker

Example freemarker.ftl:

{
"field1" : "${response.value1}",
"field2" : "${response.value2}"
}

We get a problem when the strings in the values contain quotation marks (or any of the other characters in the JSON syntax).

The question: How can I escape these strings using freemarker?

We have looked at ?xml or ?html but they do not cover all relevant characters (such as \).

EDIT: ?js_string will escape the string to comform with JavaScript. And since JSON is based on JavaScript (JavaScript Object Notation), it will work.

EDIT2: In case a single-quote pops up, ?js_string will escape it which again leads to invalid JSON. The hotfix for it is:

${variable?js_string?replace("\\'", "\'")} 

and if you really want to be picky:

${variable?js_string?replace("\\'", "\'")?replace("\\>",">")}

Alternatively if you use Spring: http://www.springsurf.org/sites/1.0.0.M3/spring-webscripts/spring-webscripts-documentation/reference/html-single/index.html#js-api-index-org.springframework.extensions.webscripts.json.jsonutils

Answer 1

You're looking for the ?js_string operator.

{ "field1" : "${response.value1?js_string}", "field2" : "${response.value2?js_string}" } 

That will take care of escaping quotes, backslashes, et. al in order to make your JS happy.

Edit: I just saw that they introduced a ?json_string operator in Freemarker 2.3.19. See here for exactly how it works. And there was much rejoicing...

Answer 2

Use a FreeMarker macro to combine all of the answers above, while making the template more readable and maintainable:

<#macro json_string string>${string?js_string?replace("\\'", "\'")?replace("\\>", ">")}</#macro>
{
"field1" : "<@json_string "${response.value1}"/>",
"field2" : "<@json_string "${response.value2}"/>"
}

If you want to reuse the macro in multiple templates, put it in its own file and include the file instead of duplicating the macro:

<#include "/path/to/macro.ftl">