add-logging-to-native-executable.md

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Add Logging to a Native Executable

By default, a native executable produced by Native Image supports logging via the java.util.logging.* API.

Default Logging Configuration

The default logging configuration in a native executable is based on the logging.properties file found in the JDK. This file configures a java.util.logging.ConsoleHandler which will only show messages at the INFO level and above. Custom logging configuration can be loaded either at build time or at run time as described below.

If you require additional logging handlers, you must register the corresponding classes for reflection. For example, if you use java.util.logging.FileHandler, then provide the following reflection configuration in the META-INF/native-image/reachability-metadata.json file:

{
    "name" : "java.util.logging.FileHandler",
    "methods" : [
      { "name" : "<init>", "parameterTypes" : [] }
    ]
  }

For more details, see Reflection Support.

The usage of the logger is shown in the following example:

1. Save the following Java code into a file named LoggerRunTimeInit.java, and compile it:

   import java.io.IOException;
   import java.util.logging.Level;
   import java.util.logging.LogManager;
   import java.util.logging.Logger;
   
   public class LoggerRunTimeInit {
       public static void main(String[] args) throws IOException {
           LogManager.getLogManager().readConfiguration(LoggerRunTimeInit.class.getResourceAsStream("/logging.properties"));
           Logger logger = Logger.getLogger(LoggerRunTimeInit.class.getName());
           logger.log(Level.WARNING, "Danger, Will Robinson!");
       }
   }

   javac LoggerRunTimeInit.java

2. Download the logging.properties resource file and save it in the same directory as LoggerRunTimeInit.java.

3. Build and run the native executable

   native-image LoggerRunTimeInit -H:IncludeResources="logging.properties"

   ./loggerruntimeinit

   It should produce output that looks similar to:

   WARNING: Danger, Will Robinson! [Wed May 18 17:22:40 BST 2022]
   

   In this case, the logging.properties file must be available for runtime processing and therefore needs to be registered in the META-INF/native-image/reachability-metadata.json file. For more details on how to do this, see Use of Resources in a Native Executable.

Related Documentation

• Reachability Metadata: Reflection
• Native Image Build Configuration