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Accessing Resources in Native Image

By default, the native-image builder will not integrate any of the resources that are on the classpath into the native executable. To make calls such as Class.getResource() or Class.getResourceAsStream() (or their corresponding ClassLoader methods) return specific resources (instead of null), you must specify the resources that should be accessible at runtime. This can be achieved using a configuration file with the following content:

{
  "resources": {
    "includes": [
      {"pattern": "<Java regexp that matches resource(s) to be included in the executable>"},
      {"pattern": "<another regexp>"},
      ...
    ],
    "excludes": [
      {"pattern": "<Java regexp that matches resource(s) to be excluded from the executable>"},
      {"pattern": "<another regexp>"},
      ...
    ]
  }
}

Provide the configuration file’s path to the native-image tool using the option -H:ResourceConfigurationFiles=/path/to/resource-config.json. Alternatively, you can specify individual resource paths directly to the native-image tool as follows:

native-image -H:IncludeResources="<Java regexp that matches resources to be included in the executable>" -H:ExcludeResources="<Java regexp that matches resources to be excluded from the executable>" ...

You can pass the -H:IncludeResources and -H:ExcludeResources options several times to define more than one regexp to include or exclude resources, respectively.

To see which resources are included in the native executable, use the option -H:Log=registerResource:<log level>. The <log level> argument must be in the range 1 to 5 (from least detailed to most detailed). A log level of 3 provides brief details of the included resources.

Example Usage #

Given this project structure:

app-root
└── src
    └── main
        └── resources
            ├── Resource0.txt
            └── Resource1.txt

Then:

  • All resources can be loaded with ".*/Resource.*txt$", specified as {"pattern":".*/Resource.*txt$"} in a configuration file, or -H:IncludeResources=".*/Resource.*txt$" on the command line.
  • Resource0.txt can be loaded with .*/Resource0.txt$.
  • Resource0.txt and Resource1.txt can be loaded with .*/Resource0.txt$ and .*/Resource1.txt$ (or alternatively with a single .*/(Resource0|Resource1).txt$).
  • Also, if we want to include everything except the Resource2.txt file, we can simply exclude it using -H:IncludeResources=".*/Resource.*txt$" followed by -H:ExcludeResources=".*/Resource2.txt$".

Check this guide which illustrates how to include a resource into a native executable.

Locales #

It is also possible to specify which locales should be included in the executable and which should be the default. For example, to switch the default locale to Swiss German and also include French and English, use the following options:

native-image -Duser.country=CH -Duser.language=de -H:IncludeLocales=fr,en

The locales are specified using language tags. You can include all locales via -H:+IncludeAllLocales, but note that it increases the size of the resulting executable.

Resource Bundles #

Java localization support (java.util.ResourceBundle) enables Java code to load L10N resources and show the user messages suitable for runtime settings such as time, locale, and format.

Native Image needs knowledge ahead-of-time of the resource bundles your application needs so that it can load and store the appropriate bundles for usage in the generated executable. The bundles can be specified in the resource configuration file (see above), in the bundles section:

{
  "bundles": [
    {"name":"your.pkg.Bundle"},
    {"name":"another.pkg.Resource"},
    {"name":"etc.Bundle"}
  ],
  "resources": <see above>
}

Alternatively, bundles can be specified directly as options to the native-image tool as follows:

native-image -H:IncludeResourceBundles=your.pgk.Bundle,another.pkg.Resource,etc.Bundle ...

By default, requested bundles are included for all requested locales. To optimize this, it is possible to use IncludeResourceBundles with a locale-specific substring, for example -H:+IncludeResourceBundles=com.company.bundles.MyBundle_fr-FR will only include the bundle in French.

Resources in Java Modules #

Wherever resources are specified with <Java regexp that matches resources to be included in the image> or resource bundles are specified via bundle name, it is possible to specify the exact modules from which these resources or bundles should be taken. To do so, specify the module name before the resource-regex or bundle name with : as the separator. For example:

{
   "resources": {
      "includes": [
         {
            "pattern": "library-module:^resource-file.txt$"
         }
      ]
   },
   "bundles": [
      {"name":"main-module:your.pkg.Bundle"}
   ]
}

This will cause the native-image tool to only include resource-file.txt from the Java module library-module. If other modules or the classpath contains resources that match the pattern ^resource-file.txt$ only the one in module library-module is registered for inclusion in the executable. Similarly, if other bundles are accessible with the same bundle name your.pkg.Bundle only the one from main-module is included. Native image will also ensure that the modules are guaranteed to be accessible at runtime. That is, the following code pattern:

InputStream resource = ModuleLayer.boot().findModule(moduleName).getResourceAsStream(resourcePath);

will always work as expected for resources registered as described above (even if the module does not contain any code that is considered reachable by static analysis).

Java VM Mode of Localization #

Resource Bundle lookup is a complex and dynamic mechanism which utilizes a lot of Java VM infrastructure. As a result, it causes the size of the executable to increase for smaller applications such as HelloWorld. Therefore, an optimized mode is set by default in which this lookup is simplified utilizing the fact that all bundles are known ahead of build time. For the original Java VM lookup, use the -H:-LocalizationOptimizedMode option.

Further Reading #

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