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Use System Properties in a Native Executable

Assume you have compiled the following Java application using javac:

public class App {
    public static void main(String[] args) {
        System.getProperties().list(System.out);
    }
}

If you build a native executable using native-image -Dfoo=bar App, the system property foo will be available at executable build time. This means it is available to the code in your application that is run at build time (usually static field initializations and static initializers). Thus, if you run the resulting executable, it will not contain foo in the printed list of properties.

If, on the other hand, you run the executable with app -Dfoo=bar, it will display foo in the list of properties because you specified property at executable run time.

In other words:

  • Pass -D<key>=<value> as an option to native-image to control the properties seen at build time.
  • Pass -D<key>=<value> as an option to a native executable to control the properties seen at run time.

Read System Properties at Build Time

You can read system properties at build time and incorporate them into the resulting executable file, as shown in the following example.

Prerequisite

Make sure you have installed a GraalVM JDK. The easiest way to get started is with SDKMAN!. For other installation options, visit the Downloads section.

  1. Save the following Java code into a file named ReadProperties.java: 
     public class ReadProperties {
     private static final String STATIC_PROPERTY_KEY = "static_key";
     private static final String INSTANCE_PROPERTY_KEY = "instance_key";
     private static final String STATIC_PROPERTY;
     private final String instanceProperty;
     static {
         System.out.println("Getting value of static property with key: " + STATIC_PROPERTY_KEY);
         STATIC_PROPERTY = System.getProperty(STATIC_PROPERTY_KEY);
     }
    
     public ReadProperties() {
         System.out.println("Getting value of instance property with key: " + INSTANCE_PROPERTY_KEY);
         instanceProperty = System.getProperty(INSTANCE_PROPERTY_KEY);
     }
        
     public void print() {
         System.out.println("Value of instance property: " + instanceProperty);
     } 
        
     public static void main(String[] args) {
         System.out.println("Value of static property: " + STATIC_PROPERTY);
         ReadProperties rp = new ReadProperties();
         rp.print();
     } 
 }
  2. Compile the application: 
     javac ReadProperties.java
  3. Build the native executable, passing a system property as a command-line option. Then run the native executable, passing a different system property on the command line. 
     native-image -Dstatic_key=STATIC_VALUE ReadProperties

     ./readproperties -Dinstance_key=INSTANCE_VALUE

    You should see the following output:

     Getting value of static property with key: static_key
 Value of static property: null
 Getting value of instance property with key: instance_key
 Value of instance property: INSTANCE_VALUE

    This indicates that the class static initializer was not run at build time, but at run time.

  4. To force the class static initializer to run at build time, use the --initialize-at-build-time option, as follows: 
     native-image --initialize-at-build-time=ReadProperties -Dstatic_key=STATIC_VALUE ReadProperties

    In the output from the native-image tool you should see the message like this:

     GraalVM Native Image: Generating 'readproperties' (executable)...
 ==========================================================================
 Getting value of static property with key: static_key
 [1/8] Initializing...                                      (4.0s @ 0.13GB)
 ...
  5. Run the executable again, as follows: 
     ./readproperties -Dinstance_key=INSTANCE_VALUE

    This time you should see the following output, confirming that the static initializer was run at build time, not at run time.

     Value of static property: STATIC_VALUE
 Getting value for instance property key: instance_key
 Value of instance property: INSTANCE_VALUE

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