Log4J – Runtime variable substitution

Question

Log4J appears to have an annoying restriction – at runtime, variable substitution does not appear to work.

In this example

File: Log4j.properties

file_pattern=%d{ISO8601} %-5p %m%n

log4j.rootLogger=DEBUG, FileAppender

log4j.appender.FileAppender=org.apache.log4j.FileAppender log4j.appender.FileAppender.layout=org.apache.log4j.PatternLayout log4j.appender.FileAppender.layout.ConversionPattern=${file_pattern} log4j.appender.FileAppender.File=log4jtest1.log

log4j.appender.FileAppender.Threshold=ERROR

The FileAppender configured in the log4j.properties file produces the correct output:

File: log4jtest1.log

ERROR Sample error message FATAL Sample fatal message

If I attempt to create a FileAppender at runtime

import org.apache.log4j.FileAppender;
import org.apache.log4j.Level;
import org.apache.log4j.Logger;
import org.apache.log4j.PatternLayout;

public class Main {
    static final Logger logger = Logger.getLogger(Main.class);

    public static void main(String[] args) throws Exception {

        FileAppender appender = new FileAppender();
        appender.setFile("log4test2.log");

        PatternLayout pl = new PatternLayout("${file_pattern}");

        appender.setLayout(pl);
        appender.setName("log4jtest2");
        appender.setThreshold(Level.ERROR);
        appender.activateOptions(); 
        logger.addAppender(appender);

        logger.trace("Sample trace message");
        logger.debug("Sample debug message");
        logger.info("Sample info message");
        logger.warn("Sample warn message");
        logger.error("Sample error message");
        logger.fatal("Sample fatal message");
    }
}

Te output is

File: log4jtest2.log

${file_pattern}${file_pattern}

Can anyone explain what is the problem and how can it be fixed?

Related question: Can an application access the ResourceBundle in order to read variables intended to be substituted?

Answer 1

Variable substitution is a feature of PropertyConfigurator not of PatternLayout. If you look at your code, you never define what file_pattern should be. But why would you need variable substitution in code? Just do this:

 PatternLayout pl = new PatternLayout("%d{ISO8601} %-5p %m%n");

If you want to reference that string somewhere else, just make it a constant.

Edit: You will have to read the properties object, but PropertyConfigurator can take a properties object instead a file, so you could load that, do what you need to do and pass it on to the PropertiesConfigurator, so you only have one configuration path.

Answer 2

You can load it manually:

Properties props = new Properties();
InputStream fis = new FileInputStream(new File("/somewhere/log4j.properties")); 
props.load(fis);
fis.close();
PatternLayout layout = new PatternLayout(props.getProperty("file_pattern"));