Using System Properties in Native Image

Assume you have the following Java application that you have compiled 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, for example, whenever you are in the code that is part of your application but run at build time (usually static field initializations and static initializers). Thus if you run the executable above 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 it at executable runtime.

In other words:

Passing -D<key>=<value> as an argument to native-image affects properties seen at executable build time.
Passing -D<key>=<value> as an argument to a native executable affects properties seen at executable runtime.

Using System Properties at Build Time

System Properties can be read at build time and incorporated into the resulting executable file, as shown in the following example.

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();
     } 
 }

Build and run the native executable

 native-image -Dstatic_key=STATIC_VALUE ReadProperties
 ./readproperties -Dinstance_key=INSTANCE_VALUE

You should see output similar to

 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 run at runtime, not at build time.

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 output similar to

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

This confirms that the static initializer was run at build time, not at runtime.

Access Environment Variables at Runtime

NaA native executable can also access environment variables at runtime. Consider the following example.

Save this Java code into the EnvMap.java file:

 import java.util.Map;
 public class EnvMap {
     public static void main (String[] args) {
         var filter = args.length > 0 ? args[0] : "";
         Map<String, String> env = System.getenv();
         for (String envName : env.keySet()) {
             if(envName.contains(filter)) {
                 System.out.format("%s=%s%n",
                                 envName,
                                 env.get(envName));
             }
         }
     }
 }

This code iterates over the environment variables and prints out the ones passing through the filter, passed as the command line argument.

Compile and build a native executable:

 javac EnvMap.java
 native-image EnvMap

Run the resulting native executable and pass some argument, as in the example below. It will correctly print out the values of the environment variable(s). For example:

 ./envmap HELLO
 <no output>
 export HELLOWORLD="Hello World!"
 ./envmap HELLO
 HELLOWORLD=Hello World!

   * [Class Initialization in Native Image](/22.1/reference-manual/native-image/ClassInitialization/)

Using System Properties in Native Image

Using System Properties at Build Time

Access Environment Variables at Runtime

Related Documentation