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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 runtime.

In other words:

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

Reading 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.

  1. Save the following Java code into a file named ReadProperties.java, then compile it using javac:

     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. Build the native executable, passing a system property as a command-line argument. 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 runtime.

  3. To force the class static initializer to run at build time, use the --initialize-at-build-time flag, 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 output similar to the following

     ...
 [1/7] Initializing...                                            (7.7s @ 0.07GB)
 Getting value of static property with key: static_key
 ...

    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 runtime.

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