Freemarker Default Number Formatting

We use Freemarker quite extensively at Betfair, and a question was raised the other day by one of the developers regarding number formatting. He had a numeric value that was being rendered with thousand separators and it wasn’t immediately obvious why. He quite rightly assumed that Freemarker would write the value of the number – unformatted – in the output. After all, this is the default behaviour of pretty much every other language / framework / tool that we use.

Unfortunately, Freemarker sees things differently. And this is where it’s REALLY important to make sure you read the documentation! Freemarker claims to be rendering for a “human audience” by default, but they’ve forgotten that Freemarker is a developer’s tool. Programmers are intelligent enough to know when they want numbers to be formatted. Automatically formatting numbers is not doing us a favour. Unless the programmer asks you to do something… don’t do it!

Scenarios like this can trip up anyone, and could possibly go unnoticed until something breaks in production. Let’s say for example that you were using a number as part of a link that you were rendering in the page. If all your testing used numbers smaller than 1000, you’d never see any problems. It’s only when you go over 1000 that the issue makes itself known. Our test data covers a large number of scenarios that quickly exposed this issue, but I can definitely think of smaller projects I’ve worked on in the past where this would not have been discovered early enough.

In fact, I think this default behaviour is dangerous. It breaks the Principle of Least Astonishment (or Principle of Least Surprise, as we call it). Not only that, but from what I can tell from the docs, it cannot be overridden. So there’s no way to change the default back to what is sensible.

Here’s an example of what happens when you render numbers using Freemarker and how to make sure you get the output you want. The example below assumes an English locale.

<#assign x = 1000>
${x}                 <#-- 1,000 -->
${x?string}          <#-- 1,000 -->
${x?c}               <#-- 1000 -->
${x?string.computer} <#-- 1000 -->

As you can see above, the default rendering of a number is formatted based on locale. If you simply want the raw number, you must explicitly specify that by using ?string.computer or use the shorthand notation ?c.

If you want more information on this, check out the docs.

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