Internationalizing Messages in Java Source Code

Introduction

Jenkins uses the Localizer library and Maven plugin to implement internationalization.

Its purpose is to generate classes corresponding to groups of Java .properties files during the build process that make the localized texts available in code.

In Jenkins, Localizer is configured to operate on resource files named Messages.properties (and Messages_XX.properties for the localized variants). During the build process of Jenkins core and plugins, it generates Messages classes from each group of Messages.properties files.

Example

As an example, consider the following resource files:

src/main/resources/org/example/package/Messages.properties:

Example.Description = A simple example for localization

src/main/resources/org/example/package/Messages_de.properties:

Example.Description = Ein einfaches Beispiel für Lokalisierung

The first file is the default localization for the key description (used unless a more specific localization is available), while the second file provides the German localization. UTF-8 is supported since Jenkins 2.361.1 (Java 11).

Localizer will generate a class that looks similar to the following from these resource files:

package org.example.package;

public class Messages {
    private final static ResourceBundleHolder holder = ResourceBundleHolder.get(Messages.class); (1)

    public static String Example_Description() { (2)
        return holder.format("Example.Description");
    }

    public static Localizable _Example_Description() { (3)
        return new Localizable(holder, "Example.Description");
    }
}
1 ResourceBundleHolder is an internal utility class wrapping access to the localized .properties files
2 Generated method returning the localized string for the Example.Description property files entry
3 Generated method returning the Localizable instance for the Example.Description property files entry

The generated methods names correspond to the key in the properties files (but don’t match exactly due to Java identifier constraints). Each key in the resource files will result in the generation of a pair of methods: One returning String, the other returning Localizable. Localizable is a type provided by the Localizer library that allows localizing a message to a specific locale. This is rarely needed however, as a call to Messages.Example_Description() will return the localized message for the current locale.

Localized Messages with Parameters

If the values in Messages.properties reference placeholders, then the static methods will be generated with arguments. See the following example for a Messages.properties excerpt and one static method generated from that.

Messages.properties content:

ItemGroupMixIn.may_not_copy_as_it_contains_secrets_and_=May not copy {0} as it contains secrets and {1} has {2}/{3} but not /{4}

Generated source code:

public static String ItemGroupMixIn_may_not_copy_as_it_contains_secrets_and_(Object arg0, Object arg1, Object arg2, Object arg3, Object arg4) {
    return holder.format("ItemGroupMixIn.may_not_copy_as_it_contains_secrets_and_", arg0, arg1, arg2, arg3, arg4);
}

For more advanced usage of parameters in localized messages, see Advanced Localization.

References