How to format dates and numbers

A frequently asked question is how best to display dates and numbers using a specified format. There are a number of approaches for this, the most naive of which would be to add a method to your action class to do the formatting for you. This method would take in a Date (or subclass) object as a parameter, and return a formatted String.

That approach however suffers from a number of flaws. For example, it is not i18n aware. The date format specified is rigid, and will not adapt to different locales easily (assuming you're not using a default formatter that is). It also clutters up your actions with code that has nothing to do with the action itself.

Instead, the recommended approach is to use Java's built-in date formatting features via use of the s:text tag.

The s:text tag should be used for all i18n values. It will look up the properties file for your action, and from that select the value for the key that you specify. This is best illustrated in an example:

<!-- display the number of items in a cart -->
<s:text name="'cart.items'" value0="cartItems" />

The above tag will work as follows. value0 will result in a call to getCartItems() on your action class. The cart.items name is escaped, so it is treated as a literal key into the actions' properties file. Your MyAction.properties file will contain the following:

cart.items=You have {0} items in your cart.

Normal Java MessageFormat behaviour will correctly substitute {0} with the value obtained from getCartItems.

Needless to say, this can get a lot more elaborate, with the ability to specify both date and number formatting. Let us consider another example. The goal here is to display a greeting to the user, as well as the date of their last visit.

<s:text name="'last.visit'" value0="userName" value1="lastVisit(userName)" />

MyAction.java contains:

public String getUserName() { ... };
public Date getLastVisit(String userName) { ... };

Your MyAction.properties file will then contain:

last.visit=Welcome back {0}, your last visit was at {1,date,HH:mm dd-MM-yyyy}

As you can see, this is a very powerful mechanism and allows you to easily display numbers and dates using any formatting rules that Java allows.

value0 interface deprecated

The examples above pass in the values as:

<s:text name="'text.message'" value0="userName"/>

These values should now (>2.1.7?) be passed as params:

<s:text name="'text.message'"> 
    <s:param value="'userName'"/>
</s:text>

Some message format examples

Here are some examples of formatting in the properties file:

format.date = {0,date,MM/dd/yy}
format.time = {0,date,MM/dd/yy ha}
format.percent = {0,number,##0.00'%'}
format.money = {0,number,$##0.00}