Native Language Support


Native Language Support (NLS) allows the application to facilitate internationalization. It provides support to manipulate messages and translate them in different languages. Each message to be internationalized is referenced by a key, which can be used in the application code instead of using the message directly.

Messages must be defined in PO files, located in the Classpath of the application (for example in the src/main/resources folder). Here is an example:

msgid ""
msgstr ""
"Language: en_US\n"
"Language-Team: English\n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"

msgid "Label1"
msgstr "My label 1"

msgid "Label2"
msgstr "My label 2"

PO files are declared in Classpath *.nls.list files (and to *.externresources.list for an external resource, see Application Resources).

digraph D {

    internalNLS [shape=diamond, label="internal?"]
    NLSList [shape=box, label="*.nls.list"]
    NLSExt [shape=box, label="*.nls.list +\l*.externresources.list"]
    subgraph cluster_NLS {
        label ="NLS"
        internalNLS -> NLSList [label="yes"]
        internalNLS -> NLSExt [label="no=external"]

The file format is a standard Java properties file, each line represents the Full Qualified Name of a Java interface that will be generated and used in the application. Example:


The message can be accessed with a call to ej.nls.NLS.getMessage(). Example:

import com.mycompany.myapp.Labels;

public class MyClass {

   String label = Labels.NLS.getMessage(Labels.Label1);



For each line, PO files whose name starts with the interface name (Messages and Labels in the example) are retrieved from the Classpath and used to generate:

  • a Java interface with the given FQN, containing a field for each msgid of the PO files
  • a NLS binary file containing the translations

So, in the example, the generated interface com.mycompany.myapp.Labels will gather all the translations from files named Labels*.po and located in the Classpath. PO files are generally suffixed by their locale (Labels_en_US.po) but it is only for convenience since the suffix is not used, the locale is extracted from the PO file’s metadata.

Once the generation is done, the application can use the Java interfaces to get internationalized messages, for example:

import com.mycompany.myapp.Labels;

public class MyClass {

   String label = Labels.NLS.getMessage(Labels.Label1);


The generation is triggered when building the application or after a change done in any PO or *.nls.list files. This allows to always have the Java interfaces up-to-date with the translations and to use them immediately.

The NLS-PO module must be added to the module.ivy of the Application project.

<dependency org="com.microej.library.runtime" name="nls-po" rev="2.2.0"/>

NLS External Loader

The NLS External Loader allows to update the PO files of an application executed on a Virtual Device without rebuilding it. PO files can be dropped in a given location in the Virtual Device folders to dynamically replace the language strings packaged in the application.

This is typically useful when testing or translating an application in order to have a quick feedback when changing the PO files. Once the PO files are updated, a simple restart of the Virtual Device allows to immediately see the result.


To enable the NLS External Loader in the Virtual Device, add the following dependency to the module.ivy file of the Firmware project:

<dependency org="com.microej.tool" name="nls-po-external-loader" rev="2.3.0" transitive="false"/>

Then rebuild the Firmware project to produce the Virtual Device.


Once the project built:

  • unzip the Virtual Device and create a folder named translations in the root folder.

  • copy all the PO files from the project into the translations folder. All PO files found in this folder are processed, no matter their folder level.

  • start the Virtual Device with the launcher. The following logs should be printed if the NLS External Loader has been executed and has found the PO files:

       [mkdir] Created dir: <PATH>\tmp\microejlaunch1307817858\resourcebuffer
    [po-to-nls] *.nls files found in <PATH>\output\<FIRMWARE>\resourceBuffer :
    [po-to-nls]   - com.mycompany.Messages1
    [po-to-nls]   - com.mycompany.Messages2
    [po-to-nls] Loading *.po files for NLS interface com.mycompany.Messages1
    [po-to-nls]   => loaded locales : fr_FR,de_DE,ja_JP,en_US
    [po-to-nls] Loading *.po files for NLS interface com.mycompany.Messages2
    [po-to-nls]   => loaded locales : fr_FR,de_DE,ja_JP,en_US
  • update the languages strings in the PO files of the Virtual Device (the files in the translations/ folder).

  • restart the Virtual Device and check the changes.

It is important to know the following rules about the NLS External Loader:

  • the external PO files names must match with the default PO files names of the application to be processed.
  • when PO files with a given name are loaded, the default translations for these PO files are replaced, there is no merge. It means that:
    • if messages are missing in the new PO files, they are not available anymore for the application and may very probably make it crash.
    • if languages are missing (the application has 3 PO files for English, French and Spanish, and only PO files for English and French are available in the translations folder), the messages of the missing languages are not available anymore for the application and may very probably make it crash.
    • if new messages are added in the PO files, it has no impact, they are ignored by the application.
  • External PO files are loaded at Virtual Device startup, so any change requires a restart of the Virtual Device to be considered

Troubleshooting NLS-PO:S=4

The following error occurs when at least 1 PO file is missing for a language:

[parallel2] NLS-PO:I=6
[parallel2] Exception in thread "main" NLS-PO:S=4 323463627 -1948548092
[parallel2]     at java.lang.Throwable.fillInStackTrace(
[parallel2]     at java.lang.Throwable.<init>(
[parallel2]     at java.lang.Exception.<init>(
[parallel2]     at<init>(
[parallel2]     at com.microej.nls.BinaryNLS.loadBinFile(
[parallel2]     at com.microej.nls.BinaryNLS.<init>(
[parallel2]     at com.microej.nls.BinaryNLS.newBinaryNLS(

Make sure that all PO files are copied in the translations folder.


Crowdin is a cloud-based localization platform which allows to manage multilingual content. The NLS External Loader can fetch translations directly from Crowdin to make the translation process even easier. Translators can then contribute and validate their translations in Crowdin and apply them automatically in the Virtual Device.

A new dependency must be added to the module.ivy file of the Firmware project to enable this integration:

<dependency org="com.microej.tool" name="nls-po-crowdin" rev="1.0.0" transitive="false"/>

Once the module has been built, edit the file platform/tools/crowdin/ to configure the Crowdin connection:

  • set crowdin.token to the Crowdin API token. A token can be generated in the Crowdin in Settings > API > click on New Token.
  • set crowdin.projectsIds to the id of the Crowdin project. The project id can be found in the Details section on a project page. Multiple projects can be set by separating their id with a comma (for example crowdin.projectsIds=12,586,874).

When the configuration is done, the fetch of the Crowdin translations can be done by executing the script crowdin.bat or located in the folder platform/tools/crowdin/. The PO files retrieved from Crowdin are automatically pasted in the folder translations, therefore the new translations are applied after the next Virtual Device restart.