Overview of MessageFormatter

In ICU4J, the MessageFormatter class is the next iteration of com.ibm.icu.text.MessageFormat. This new version will build on the lessons learned from using MessageFormat for 25 years in various environments, when used directly or as a base for other public APIs.

The effort to design a succesor to MessageFormat will result in a specification referred to as MessageFormat 2.0. The reasoning for this effort is shared in the “Why MessageFormat needs a successor” document.

MessageFormat 2.0 will be more modular and easier to port and backport. It will also provide extension points via interfaces to allow users to supply new formatters and selectors without having to modify the specification. ICU will eventually include support for new formatters, such as intervals, relative time, lists, measurement units, personal names, and more, as well as the ability for users to supply their own custom implementations. These will potentially support use cases like grammatical gender, inflection, markup regimes (such as those require for text-to-speech), and other complex message management needs.

The MessageFormat Working Group, which develops the new data model, semantics, and syntax, is hosted on GitHub. The current specification for the syntax and data model can be found here.

This technical preview implements enough functions for MessageFormatter to be useful in many situations, but the final set of functions and the parameters accepted by those functions is not yet finalized.

Examples

Basic usage

import static org.junit.Assert.assertEquals;
import java.util.Date;
import java.util.HashMap;
import java.util.Locale;
import java.util.Map;

import com.ibm.icu.message2.MessageFormatter;

@Test
public void testMf2() {
    final Locale enGb = Locale.forLanguageTag("en-GB");
    Map arguments = new HashMap<>();
    arguments.put("name", "John");
    arguments.put("exp", new Date(1679971371000L));  // March 27, 2023, 7:42:51 PM

    MessageFormatter mf2 = MessageFormatter.builder()
        .setPattern("{Hello {$name}, your card expires on {$exp :datetime skeleton=yMMMdE}!}")
        .setLocale(enGb)
        .build();

    assertEquals(
        "Hello John, your card expires on Mon, 27 Mar 2023!",
        mf2.formatToString(arguments));
}

Placeholder examples

Plural selection message

@Test
public void testMf2Selection() {
   final String message = "match {$count :plural}\n"
           + " when one {You have one notification.}\n"
           + " when * {You have {$count} notifications.}\n";
   final Locale enGb = Locale.forLanguageTag("en-GB");
   Map arguments = new HashMap<>();

   MessageFormatter mf2 = MessageFormatter.builder()
       .setPattern(message)
       .setLocale(enGb)
       .build();

   arguments.put("count", 1);
   assertEquals(
       "You have one notification.",
       mf2.formatToString(arguments));

   arguments.put("count", 42);
   assertEquals(
       "You have 42 notifications.",
       mf2.formatToString(arguments));
}

Built-in formatter functions

The tech preview implementation comes with formatters for numbers (number), date / time (datetime), plural selectors (plural and selectordinal), and general selector (select), very similar to what MessageFormat offers.

The ICU test code covers most features, and has examples of how to make custom placeholder formatters; you can look for classes that implement com.ibm.icu.message2.FormatterFactory (they are named Custom*Test.java).

Functions currently implemented

These are the functions interpreted right now: