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 tonative-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.
-
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 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.
-
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