Interpolations

Overview

The format of interpolations is ${expression}, where expression can be all kind of expression (e.g. ${100 + x}).

The interpolation is used to insert the value of the expression converted to text (to string). Interpolations can be used only on two places: in text sections (e.g., <h1>Hello ${name}!</h1>) and in string literal expressions (e.g., <#include "/footer/${company}.html">).

The result of the expression must be a string, number or date/time/date-time value, because (by default) only these types are converted to string by interpolation automatically. Values of other types (such as booleans, sequences) must be converted to string "manually" (see some advices later), or an error will stop the template processing.

Warning!

It's a frequent mistake to use interpolations on places where they needn't/shouldn't/can't be used. Interpolations work only in text sections (e.g. <h1>Hello ${name}!</h1>) and in string literals (e.g. <#include "/footer/${company}.html">). A typical WRONG usage is <#if ${big}>...</#if>, which will give syntactical error. You should simply write <#if big>...</#if>. Also, <#if "${big}">...</#if> is WRONG, since it converts the parameter value to string and the if directive wants a boolean value, so it will cause a runtime error.

Automatic escaping

If the interpolation is in a text section (not in a string literal expression), the text that it inserts goes through automatically escaping, if FreeMarker was properly configured. See more about escaping here....

Guide to inserting numerical values

If the expression evaluates to a number then the numerical value will be converted to string according the default number format. This may includes the maximum number of decimals, grouping, and like. Usually the programmer should set the default number format; the template author doesn't have to deal with it (but he can with the number_format setting; see in the documentation of setting directive). Also, you can override the default number format for a single interpolation with the string built-in.

The decimal separator used (and other such symbols, like the group separator) depends on the current locale (language, country), that also should be set by the programmer. For example, this template:

${1.5}

will print something like this if the current locale is English:

1.5

but if the current locale is German then it will print something like:

1,5

since German people use comma as decimal separator.

Warning!

As you can see, interpolations print for human audience (by default at least), as opposed to ''computer audience''. In some cases this is not good, like when you print a database record ID as the part of an URL or as an invisible field value in a HTML form, or when you print CSS/JavaScript numerical literals, because these printed values will be read by computer programs and not by humans. Most computer programs are very particular about the format of the numbers, and understand only a kind of simple US number formatting. For that, use the c (stands for ''computer audience'') built-in, for example:

<a href="/shop/productdetails?id=${product.id?c}">Details...</a>

Guide to inserting date/time/date-time values

If the expression evaluates to a date-like value then that will be transformed to a text according to a default format. Usually the programmer should set the default format; the template author doesn't have to deal with it (but if you care, see the date_format, time_format and datetime_format settings in the documentation of the setting directive). Also, you can override the default formatting for a single interpolation with the string built-in.

Warning!

To display a date-like value as text, FreeMarker must know which parts of it are in use, that is, if only the date part (year, month, day), or only the time part (hour, minute, second, millisecond), or both. Unfortunately, because of the technical limitations of Java platform, for some variables it is not possible to detect this automatically; ask the programmer if the data-model contains such problematic variables. When it's not possible to find out which parts of the date are in use, then you must help FreeMarker with the date, time and datetime built-ins (like ${lastUpdated?datetime}), or it will stop with error.

Guide to inserting boolean values

By default an attempt to print boolean values with interpolation causes an error and aborts template processing. For example this will cause an error: ${a == 2} and will not print ''true'' or something like that. That's because there's no universally useful way of representing booleans (sometimes you want to print yes/no, sometimes enabled/disabled, on/off, etc.).

However, you can convert booleans to strings with the ?string built-in. For example, to print the value of the "married" variable (assuming it's a boolean), you could write ${married?string("yes", "no")}.

FreeMarker can be configured with a default boolean format with the boolean_format setting, then ${married} and such will work. However, in most applications it's not recommended, as boolean should be rendered differently on different places, and leaving the formatting on the default is possibly just an oversight and thus should generate error.

When you want to generate JavaScript or other computer language parts, then ${someBoolean?c} ("c" stands for computer audience) should be used to print true/false. (Remember that ?c was also used to print numbers for computer audience.)

Exact conversion rules

For those who are interested, the exact rules of conversion from the expression value to string (which is then still subject to escaping) are these, in this order:

  1. If the value is a number, then it is converted to string in the format specified with the number_format setting. So this usually formats for human audience, as opposed to computer audience.

  2. Else if the value is date, time or date-time, then it is converted to string in the format specified with the date_format, time_format or datetime_format setting, respectively. If it can't be detected what kind of date-like value it is (date vs time vs date-time), an error will occur.

  3. Else if the value is a string, then there is no conversion.

  4. Else if the engine is in classic compatibility mode:

    1. If the value is a boolean, true values are converted to "true", false values are converted to an empty string.

    2. If the expression is undefined (null or a variable is undefined), it is converted to an empty string.

    3. Else an error will abort the template processing.

  5. Else an error will abort the template processing.