This documentation is for an old GraalVM version. See the latest version.

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 executable build time or at runtime 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:

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

For more details, see Reflection Support.

Initializing a Logger at Build Time #

The logger can be initialized at executable build time with a custom logging.properties configuration file, as illustrated in following example.

  1. 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.

  2. Save the following Java code into a file named LoggerBuildTimeInit.java, then compile it using javac:
     import java.io.IOException;
     import java.util.logging.Level;
     import java.util.logging.LogManager;
     import java.util.logging.Logger;
    
     public class LoggerBuildTimeInit {
         private static final Logger LOGGER;
         static {
             try {
                 LogManager.getLogManager().readConfiguration(LoggerBuildTimeInit.class.getResourceAsStream("/logging.properties"));
             } catch (IOException | SecurityException | ExceptionInInitializerError ex) {
                 Logger.getLogger(LoggerBuildTimeInit.class.getName()).log(Level.SEVERE, "Failed to read logging.properties file", ex);
             }
             LOGGER = Logger.getLogger(LoggerBuildTimeInit.class.getName());
         }
    
         public static void main(String[] args) throws IOException {
             LOGGER.log(Level.WARNING, "Danger, Will Robinson!");
         }
     } 
    
  3. Download the logging.properties resource file and save it in the same directory as LoggerBuildTimeInit.java.

  4. Build and run the native executable:
     native-image LoggerBuildTimeInit --initialize-at-build-time=LoggerBuildTimeInit
    
     ./loggerbuildtimeinit
    

    It should produce output that looks similar to:

     WARNING: Danger, Will Robinson! [Wed May 18 17:20:39 BST 2022]
    

    This demonstrates that the logging.properties file is processed at when the executable is built. The file does not need to be included in the native executable and reduces the size of the resulting executable file.

    LoggerHolder.LOGGER is also initialized at build time and is readily available at runtime, therefore improving the startup time. Unless your application needs to process a custom logging.properties configuration file at runtime, this approach is recommended.

Initializing a Logger at Run Time #

The logger can also be initialized at run time, as shown in the following example.

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

     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!");
         }
     }
    
  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 needs to be available for runtime processing and it must be included in the executable via the -H:IncludeResources=logging.properties option. For more details on this option, see Use of Resources in a Native Executable.

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